Oral presentations and abstracts

JAXA’s Hayabusa2 is a sample-return mission was launched on Dec. 3, 2014 for bringing back first samples from a C-complex asteroid [1,2]. It arrived at asteroid Ryugu on June 27, 2018 and left for Earth on Nov. 13, 2019 after conducting global remote-sensing observations, two touchdown sampling operations, rover deployments, and an artificial impact experiment. We review our science results and update the mission status of Hayabusa2 in this presentation. 

The global observations revealed that Ryugu has a top-shaped body with very low density (1.19±0.02 g/cc) [3], spatially uniform Cb-type spectra without strong Fe-rich serpentine absorption at 0.7-um [4], and a weak but significant OH absorption at 2.7 um [5]. Based on these observations, we proposed that Ryugu materials may have experienced aqueous alteration and subsequent thermal metamorphism due to radiogenic heating [4]. However, other scenarios, such as impact-induced thermal metamorphism and extremely primitive carbonaceous materials before extensive alteration, were also considered because there were many new properties of Ryugu whose origins are unclear. Also, numerical calculations show that impact heating can raise the temperatures high enough to dehydrate serpentine at typical collision speed in the asteroid main belt [6].  

Further analysis using high-resolution data obtained at low-altitude descents for both rehearsal and actual touchdown operations as well as the artificial impact experiment by small carryon impactor (SCI) and landers observations the Ryugu surface on allowed us to find out what caused the properties of Ryugu. For example, subtle but distinct latitudinal variation of spectral slope in optical wavelengths found in the initial observations [4] turned out be caused by solar heating or space weathering during orbital excursion toward the Sun and subsequent erosion of the equatorial ridge owing to slowdown in Ryugu’s spin rate [7]. The SCI impact created a very large (~17 m in crest diameter) crater consistent with gravity-controlled scaling showing that Ryugu surface has very low intra-boulder cohesion and the Ryugu surface is very young and well mixed [8].

Furthermore, the MASCOT lander also showed that typical boulders on Ryugu is not covered with a layer of fine regolith [9] and yet possess very low thermal inertia (282+93/-35 MKS) consistent with highly porous structure [10]. This value is consistent with the global values or Ryugu [4, 11], suggesting that the vast majority of boulders on Ryugu are very porous. However, thermal infrared imager (TIR) also found that Ryugu has a number of “dense boulders” with high thermal inertia (>600 MKS) consistent with typical carbonaceous chondrites, showing that Ryugu’s parent body must have had a large enough gravity and pressure to compress the constituent materials [11]. This observation supports that Ryugu originated from a large parent body, such as proto-Polana and proto-Eulalia, which are estimated to be ~100 km in diameter.

Some of the dense boulders were also covered by multi-band images of optical navigation camera (ONC-T) and turned out to have C-type spectra with albedos much higher than the Ryugu average [12]. These spectra and albedos are similar to carbonaceous chondrites heated at low temperatures. Although the total mass of these high-albedo boulders on Ryugu is estimated to be very small (< 1%), the spectral and albedo varieties are much greater than the bulk Ryugu surface and approximately follow the dehydration track of carbonaceous chondrites [12]. These spectral match supports that Ryugu materials experienced aqueous alteration and subsequent thermal metamorphism. The dominance of a high-temperature component and scarcity of lower temperature components are consistent with radiogenic heating in a relatively large parent body because large bodies would have only thin low-temperature thermal skin and large volume of high-temperature interior. 

If radiogenic heating is really responsible for Ryugu’s moderate dehydration, this may place a very important constraint on the timing of the formation of Ryugu’s parent body. Because the radiogenic heat source for most meteorite parent bodies are likely extinct species, such as 26Al, the peak temperature is chiefly controlled by the timing of accretion [13]. Thus, high metamorphism temperatures (several hundred degrees in Celsius) of Ryugu’s bulk materials inferred from spectral comparison with laboratory heated CM and CI meteorites [4, 12] require Ryugu’s parent body formed early in the Solar System. Because Ryugu’s parent body contained substantial amount of water at the time of formation, it must have been formed outside the snowline. Thus, the birth place of Ryugu’s parent body would be a high-accretion-rate location outside the snowline.

Recent high-precision measurements of stable isotopes of meteorites have found that there is a major dichotomy between carbonaceous chondrites (CCs) and some iron meteorites, which formed outside Jupiter’s orbit, and non-carbonaceous meteorites (NCs), which formed inside Jupiter’s orbit [e.g., 14]. If Ryugu belongs to CCs, then Ryugu materials could be form near Jupiter, where accretion could occur early. Thus, measurements of stable isotopes of elements, such as Cr, Ti and Mo, of Ryugu samples to be returned to Earth by the end of 2020 would be highly valuable for constraining the original locations of Polana or Eulalia, among the largest C-complex asteroids in the inner main belt. 

Acknowledgements: This study was supported by JSPS Core-to-Core program “International Network of Planetary Sciences”, CNES, and Univ. Co?te d’Azur. 

References:  [1] Watanabe et al., SSR, 208, 3-16, 2017. [2] Tsuda et at., Acta Astronaut. 91, 356-363, 2013. [3] Watanabe et al., Science, 364, 268-272, 2019. [4] Sugita et al., Science, 364, eaaw0422, 2019. [5] Kitazato et al., Science, 364, 272-275, 2019. [6] Michel et al., Nature Comm., 11, 5184, 2020. [7] Morota et al., Science, 368, 654-659, 2020. [8] Akarawa et al. Science, 368, 67-671, 2020. [9] Jaumann et al. Science, 365, 817-820, 2019.  [10] Grott et al., Nature Astron. 3, 971-976, 2019.  [11] Okada et al., Nature, 579, 518-522, 2020. [12] Sugimoto et al. 51st LPSC, #1770, 2020.  [13] Grimm and McSween, Science, 259, 653-655, 1993.  [14] Kruijer et al., PNAS, 114, 6712-6716, 2017. 

How to cite: Sugita, S., Honda, R., Morota, T., Kameda, S., Tatsumi, E., Tachibana, S., Kitazato, K., Okada, T., Namiki, N., Arakawa, M., Michel, P., Domingue, D., Tanaka, S., Yoshikawa, M., Watanabe, S., and Tsuda, Y.: Mission Status of Hayabusa2, Science Highlights, and Outlook for Sample Analyses, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-995, https://doi.org/10.5194/epsc2020-995, 2020.

Thermal imaging, or thermography, has revealed the surface physical state of the C-type near-Earth asteroid 162173 Ryugu (Okada et al., 2020). The asteroid is the target body of JAXA Hayabsua2 asteroid sample return mission, and it has been characterized through remote sensing and surface experiments, and will be deeply and accurately investigated by analysis of returned sample. Thermal observations are among such multi-scale observations, providing a new insight into understanding planetary evolution process.

Thermal infrared imager TIR (Okada et al., 2017; 2020) was used to take one-rotation global thermal images of Ryugu at every 6° step, from the home position (20 km altitude) or from the Mid-Altitude (5 km altitude). There were two big surprises contrary to the predictions before arrival at Ryugu: i) flat diurnal temperature profiles compared to the case of non-rough surface, and ii) non-cold spots identified for most of boulders. The flat diurnal temperature profiles and its maximum temperature in a day indicate that Ryugu must have very rough surfaces made of highly porous materials, derived from the thermal inertia of 300 ± 100 J K^-1s^-0.5m^-2 (hereafter, tiu). Non-cold boulders indicate that boulders are less consolidated or compacted than typical carbonaceous chondrite meteorites, and shows the same thermophysical properties as the surroundings. TIR was also used to take close-up thermal images during the descent operations, and to have proven that the surface of asteroid is covered with fragments of porous rocks, larger than several centimeters in diameter. The typical size of fragments larger than thermal skin depth (~35 mm) results in similar thermal properties between the boulders and their surroundings. We also consider the surface roughness effect (Shimaki et al., 2020) to obtain the maps of thermal inertia ( 225 ± 45 tiu) and the roughness (0.41 ± 0.05) at the same time, corresponding to very rough surfaces made of highly-porous materials. This thermal inertia is basically consistent with the value (282 +93/-35 tiu) by in situ measurement using a thermal radiometer MARA on MASCOT lander (Grott et al., 2019). Furthermore, in the close-up thermal images, there were found boulders colder by 20 °C or more, indicating the thermal inertia of typical carbonaceous chondrite meteorites.

Considering these results, we proposed a formation scenario of Ryugu: fluffy cosmic dusts gathered to form porous planetesimals, and then much larger sized but still porous bodies. A low degree of consolidation and alteration has occurred at most of the body, while a higher degree of consolidation or alteration proceeded at the deep interior. Huge meteoritic impacts destroyed and fragmented the bodies, and part of those fragments were re-accreted to form the next generation, rubble-pile bodies (asteroids). Boulders found on Ryugu might have originated from the deep interior of parent bodies, so that most of them are very porous and less consolidated but some of them are relatively dense materials similar to carbonaceous chondrites, which might have originated from the interior. Due to YORP effect, the rotation rate decreased to current one, and the current shape of a spinning top-shape were formed. Analysis of returned sample will make progress in our knowledge of the planetary formation process.

How to cite: Okada, T., Tanaka, S., Shimaki, Y., Sakatani, N., Arai, T., Senshu, H., Demura, H., Kouyama, T., Sekiguchi, T., and Fukuhara, T. and the Hayabusa2 TIR Team: Highly porous nature of C-type asteroid 162173 Ryugu revealed by thermal imaging from Hayabusa2, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-12, https://doi.org/10.5194/epsc2020-12, 2020.

Thermophysical analyses of planetary bodies such as the Moon, Mars, and numerous asteroids have allowed for remote estimates of regolith physical properties, such as particle size and packing density, as well as the relative spatial abundance of boulders.  Here we define “regolith” as a particulate assemblage where most particles are comparable to or smaller than the length scale of the diurnal skin depth (the e-folding depth of the diurnal thermal wave).

Until recently, regolith and boulders were believed to be thermally quite distinct; regolith on the Moon, Mars, and most asteroids was usually known or suspected to be fine (i.e., ranging from fine dust to sand), meaning that it was known or assumed to have a thermal inertia much lower than that of boulders and bedrock. Upon the arrival of NASA OSIRIS-REx at asteroid Bennu and JAXA Hayabusa2 at asteroid Ryugu and the subsequent thermophysical analyses of the respective asteroid surfaces, this preconceived notion of thermophysically distinct regolith and boulders/bedrock was found to be flawed (DellaGiustina and Emery et al., 2019; Sugita et al., 2019). Boulders cover the vast majority of both asteroids’ surfaces, yet the thermal inertia values determined for these boulder-rich surfaces fall within a range that was previously believed to exclusively represent coarse, sand-to-pebble–sized regolith. Recent work has been devoted to the analysis of the boulders’ thermal inertia and the physical interpretation thereof (Grott et al., 2019; Rozitis et al., in revision); the general conclusion so far is that the boulders have very low thermal conductivity and density owing to the presence of numerous pores and fractures, more so than most (or perhaps all) carbonaceous chondrite meteorites in Earth’s collections. As such, these boulders are likely to be structurally distinct from all known meteorite specimens.

Although fine-particulate regolith is rare on the surface of Bennu, it is present in some locations. It is still of great interest to probe the physical properties of this regolith by means of thermal analysis so as to make predictions about the properties of the samples that will be returned by OSIRIS-REx and to learn about the evolution of the asteroid surface and the mechanisms by which regolith is produced and lost or destroyed. There remains the enigmatic question: although we suspect that the boulders on Bennu are distinct from the meteorite collection, will the returned samples of particulate material share these distinct properties? At what scales can we define the physical thermally relevant properties of  boulders and regolith particles on Bennu? In other words, if the boulders are indeed highly fractured and porous, are the regolith particles also fractured and porous or are their dimensions below the relevant length scales?

It is very challenging to estimate the physical properties of the regolith on Bennu using thermal data, even in regions where it appears to be abundant, due to the coarseness of the regolith. Commonly used planetary thermophysical models rely on the assumption that the material on the surface, be it regolith or rock, can be approximated as a continuous, non-discretized material with physical properties that are either constant with depth or are allowed to vary with depth in some well-defined way (e.g. an exponential density increase, or up to a few layers of physically distinct material, such as regolith on bedrock or dust coatings). This assumption of material continuity is valid when regolith particles are smaller, or perhaps even much smaller, than the diurnal skin depth of the thermal wave. However, high-resolution images of the Nightingale Crater on Bennu, which is the OSIRIS-REx mission’s primary sample collection site, revealed a particle size frequency distribution (SFD) that crosses this threshold; i.e., there are particles present that are smaller than, comparable to, and larger than the diurnal skin depth (~1­–5 cm) present within a single, meters-wide observation footprint. The thermophysical behavior of such a regolith configuration has never  been comprehensively studied and likely cannot be properly approximated with standard 1D thermal modeling methods.

We will present preliminary results using a 3D regolith model where we render hundreds of regolith particles and rocks, approximated as spheres, in a finite element mesh framework. The model is heated diurnally with a solar source to study the thermal response of such skin depth–crossing SFDs under Bennu surface-like conditions. The SFD of the particles is informed by particle size counts in Nightingale Crater from high-resolution visible images. With the SFD partially constrained, we are able to focus our efforts on exploring the material property parameter space, namely to estimate the thermal conductivity and density of individual regolith particles. Given the coarseness of the regolith on Bennu, we find that the model is more sensitive than one might expect to the thermal conductivity of the individual regolith particles owing to the effects of particle non-isothermality (Ryan et al., 2020). Although this present modeling work is focused on analyzing thermal emission data of Bennu obtained by the OSIRIS-REx Thermal Emission Spectrometer (OTES, Christensen et al., 2018), we aim to expand our efforts to study the more general thermal behavior of coarse regoliths and regoliths with wide SFDs under a range of solar heating conditions.

Acknowledgements

This material is based upon work supported by NASA under Contract NNM10AA11C issued through the New Frontiers Program. We are grateful to the entire OSIRIS-REx Team for making the encounter with Bennu possible.

References:

Christensen, P.R. et al. (2018). The OSIRIS-REx Thermal Emission Spectrometer (OTES) Instrument. Space Science Reviews 214, 87.

DellaGiustina, D.N., Emery, J.P., et al. (2019) Properties of rubble-pile asteroid (101955) Bennu from OSIRIS-REx imaging and thermal analysis, Nature Astronomy, DOI:10.1038/s41550-019-0731-1.

Grott, M., Knollenberg, J., et al. (2019) Low thermal conductivity boulder with high porosity identified on C-type asteroid (162173) Ryugu, Nature Astronomy, DOI:10.1038/s41550-019-0832-x.

Rozitis, B., Ryan, A.J., Emery, J.P., et al. (in revision) Asteroid (101955) Bennu’s Weak Boulders and Thermally Anomalous Equator, Science Advances, submitted March 2020.

Ryan, A., Pino Muñoz, D., Bernacki, M., Delbo, M. (2020) Full-Field Modeling of Heat Transfer in Asteroid Regolith: Radiative Thermal Conductivity of Polydisperse Particulates, JGR:Planets. DOI:10.1029/2019JE006100.

Sugita, S., Honda, R., et al. (2019) The geomorphology, color, and thermal properties of Ryugu: Implications for parent-body processes, Science. DOI:10.1126/science.aaw0422

How to cite: Ryan, A., Pino Muñoz, D., Rozitis, B., Bernacki, M., Delbo, M., Emery, J., Burke, K., Bennett, C., Siegler, M., Cambioni, S., Hamilton, V., Christensen, P., and Lauretta, D.: Thermophysical Analysis of Regolith on (101955) Bennu: The Coarse Regolith Conundrum, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-172, https://doi.org/10.5194/epsc2020-172, 2020.

Introduction:  The Japanese spacecraft Hayabusa2 made the rendezvous with a C/F-type asteroid Ryugu from June 2018 to November 2019 [1]. After the global mapping and the first touch-down operation, Small Carry-on Impactor (SCI) operation was successfully conducted on 5 April 2019 to make an artificial crater (SCI crater) [2]. This operation aimed to reveal cratering mechanism on asteroids with very small gravity. Mass of the impactor is ~2 kg and the impact speed was estimated as ~2 km/s. Moreover, this operation gave us the precious opportunity to uncover the internal material of Ryugu and observe it by the remote-sensing instruments. The observation of SCI crater might give us the insights of space weathering, heating, and layering on Ryugu. Hayabusa2 is equipped with a telescopic multiband camera, ONC-T with seven color filters in UV to NIR wavelength; ul: 0.40 µm, b: 0.48 µm, v: 0.55 µm, Na: 0.59 µm, w: 0.70 µm, x: 0.86 µm, p: 0.95 µm [3].

We investigate spectral slope, UV down/up-turn, and 0.7-µm band absorption around the SCI crater. In the global observation, the spectral slope was the most prominent variation in visible spectra [3]. The bluer regions, such as the equatorial ridge and the pole regions, of Ryugu correspond to the higher gravitational potential regions, suggesting the mass wasting from higher to lower potential regions leads to expose the fresh bluer subsurface material on Ryugu [3]. Recently more detailed mapping on the pole regions were conducted and possible 0.7-µm band absorption and UV less upturn (flat UV) were suggested [4]. These two features could be indicative of hydrated minerals, such as Fe-bearing phyllosilicates. This observational result leaded a scenario of space weathering by solar wind in the past [4]. The observational facts from the SCI crater could be also strong constraints on space weathering effect on Ryugu. In this study, we report the visible color observation results of the SCI crater and discuss the processes occurred on the surface of Ryugu in the context of both the global observation and the SCI crater observation.

Data processing:  Before (8 March 2019) and after (16 May 2019) the SCI operation, ONC-T multi-band observations at the SCI target site were conducted. The spatial resolution of before and after the SCI observations are 13 and 5.5 cm/pixel, and the phase angles are 17˚ and 30˚, respectively. After the conversion to radiance factor (I/F) [5,6] and co-registration to v-band images, we measured the spectral indexes, such as spectral slope (0.48 - 0.86 µm), 0.7-µm band depth, and UV-index. UV-index measures the excess from the spectral slope.

Color of the SCI crater:  Figure 1 shows the spectral slope map for the SCI crater. The floor of the SCI crater is found to be bluer than surrounding region. The clear color difference is observed between inside and outside of the SCI crater. The floor material might be excavated sublayer of the impact site. It should be also noted that the possible impact site between two boulders, Mobile boulder and Stable boulder, are redder in comparison with the global average. We also see the relatively red part on the eastern wall of the crater. Moreover, after the SCI operation, Mobile boulder was excavated and we see the bluer color for the part which had been lied underground. The UV-index value of inside the SCI crater is slightly smaller than surrounding area. However, the 0.7-µm band depth seems to be similar for inside and outside of the SCI crater.

Discussions: Based on the global observation, we had expected to see blue material inside of the SCI crater. Even we see the relatively bluer material inside of the SCI crater than its vicinity, the degree of blueness of the SCI crater is similar to the equatorial ridge and is far redder than the material on both north and south poles which has negative value of spectral slope. Relatively blue material inside of the SCI crater supports the hypothesis of the fresh bluer material under the red surface material as suggested by [3,7]. However, since we see the bluest material on the pole regions, the sublayer of the SCI target region might have experienced space weathering process in the past. Moreover, absence of 0.7-µm band feature also suggests that the space weathering had affected not only the outermost surface but also meter-scaled substrates. Thus, we hypothesize mass wasting by the YORP spin-up on the formation of the equatorial ridge [8], gardening by impacts, or granular convection [9] could form the space weathered subsurface layer.

Another interesting color feature is the redder material in the middle of the SCI crater, at the impact point. We can see the similar feature for some of the natural craters of several tens meter scale. This could be explained by the metamorphism by high pressure and temperature process [10], breaking up of material just at the impact site which can be sometimes observed by the impact experiments, or excavating another layer underneath.

Acknowledgments: We are grateful to the entire Hayabusa2 team for making the encounter with Ryugu possible. This study is supported by the JSPS core-to-core program “International Network of Planetary Sciences”. ET acknowledges financial support from the project ProID2017010112 under the Operational Programs of the European Regional Development Fund and the European Social Fund of the Canary Islands (OP-ERDF-ESF), and the Canarian Agency for Research, Innovation and Information Society (ACIISI). 

References: [1] Watanabe, S. et al. (2019) Science 364, 268. [2] Arakawa, M. et al.(2020) Science 368, 67. [3] Sugita, S. et al. (2019) Science 364, eaaw0422. [4] Tatsumi, E. et al. (2019) Asteroid Science in the Age of Hayabusa2 and OSIRIS-Rex, #2091. [5] Tatsumi, E. et al. (2019) Icarus 325, 153. [6] Tatsumi, E. et al. (2019) LPSC 50, Abstract #1745. [7] Morota, T. et al. (2020), Science 368, 654. [8] Sugiura, K. et al. (2019) Asteroid Science in the Age of Hayabusa2 and OSIRIS-Rex, #2012. [9] Tsuchiyama, A. et al. (2011) Science 333, 1125. [10] Hiroi, T. et al. (2020) LPSC 51.

How to cite: Tatsumi, E. and the Hayabusa2 team: Visible color of the artificial crater on Ryugu created by Small Carry-on Impactor, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-331, https://doi.org/10.5194/epsc2020-331, 2020.

Abstract

In September 2019, the OSIRIS-REx Camera Suite (OCAMS; Rizk et al., 2018) globally mapped asteroid (101955) Bennu in four broadband filters (b′, v, w, x) covering 0.44 to 0.89 microns. The multispectral images show complex relationships between color and morphology on Bennu’s surface. These data indicate that Bennu’s color has been influenced by primordial heterogeneity and space weathering.

Introduction

To evaluate relationships between color and morphology on Bennu, we radiometrically and photometrically corrected OCAMS images acquired by the MapCam instrument (Golish et al., 2020a, Golish et al., 2020b). Calibrated images were subsequently mosaicked to develop band-ratio and principal component analysis (PCA) maps. To establish statistically meaningful trends between color, reflectance, and morphological features, we mapped ~1600 boulders and ~700 craters, and extracted their average MapCam color indices.

Bennu’s global average spectrum is blue (–1% per 0.1 μm) in MapCam data (0.44 to 0.89 μm), but spectral slopes can vary from blue (negative) to red (positive) at the meter scale. The diverse color and reflectance of boulders suggests primordial heterogeneity inherited from Bennu’s parent body and exogenic impactors (DellaGiustina and Kaplan et al., accepted). Spectral changes in craters as a function of radius indicate that color may also be influenced by space weathering on Bennu.  

Boulders

On the basis of reflectance and color, we categorized Bennu’s boulders into four types: 1) High-reflectance boulders (>4.9% normal albedo) are brighter than units having the average color of Bennu, texturally smooth, exhibit angular morphology, and have blue spectral slopes across the MapCam v to x bands. 2) Dark boulders (≤4.9% normal albedo) are subangular and have rougher, more undulating surface textures compared to the bright boulders. They encompass a wide range of sizes and include the largest boulders on the asteroid (25 to 100 m in diameter). 3) Boulders with very high reflectance (up to 0.26; ~1% in number) show evidence of an absorption at 1 μm (downturn in the x band). These boulders were spectroscopically identified to contain pyroxene using data acquired by the OSIRIS-REx Visible and InfraRed Spectrometer (OVIRS; Reuter et al., 2018). 4) About 2% of boulders surveyed have an absorption feature that is detectable above the radiometric uncertainty of OCAMS at 0.7 μm (absorption depth of 2 to 10%). This absorption has been observed in spectra of primitive asteroids and carbonaceous meteorites and has been attributed to the Fe²⁺-Fe³⁺ intervalence charge transfer associated with hydrated clay-bearing phyllosilicates.

Craters

The color of the largest craters (>100 m) on Bennu is indistinguishable from that of the average terrain. However, many small (≤25 m) craters are redder than average across MapCam b′ to x filters, resulting in neutral to red spectral slopes. The size distribution of these small reddish craters implies that they are the youngest component of the global crater population.

References

Lauretta, D. S., et al. “OSIRIS-REx: sample return from asteroid (101955) Bennu.” Space Science Reviews 212.1-2 (2017): 925-984.

Rizk, B., et al. “OCAMS: the OSIRIS-REx camera suite.” Space Science Reviews 214.1 (2018): 26.

Golish, D. R., et al. “Ground and in-flight calibration of the OSIRIS-REx camera suite.” Space Science Reviews 216.1 (2020): 12.

Golish, D. R., et al. “Disk-resolved photometric modeling and properties of asteroid (101955) Bennu.” Icarus (2020): 113724.

DellaGiustina, D. N., and Kaplan, H. H., et al. “Exogenic basalt on asteroid (101955) Bennu”. Nature Astronomy, in revision.

Reuter, D.C., et al.  “The OSIRIS-REx Visible and InfraRed Spectrometer (OVIRS): Spectral Maps of the Asteroid Bennu”. Space Science Reviews 214.1 (2018): 54. 

How to cite: DellaGiustina, D. and the OSIRIS-REx color mapping and interpretation team: Relationships Between Color & Morphology on Bennu, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-481, https://doi.org/10.5194/epsc2020-481, 2020.

1. Introduction

We present visible to near infrared (VNIR) and thermal infrared (TIR) spectral data for asteroid (101955) Bennu collected by the OSIRIS-REx Visible and InfraRed Spectrometer (OVIRS) [1, 2] and the OSIRIS-REx Thermal Emission Spectrometer (OTES) [3]. The data discussed here were collected during the 12:30 pm Equatorial Station of the Detailed Survey mission phase and Reconnaissance A (varying local times). Constraints applied to the selection of data are described by [4-6].

2. OVIRS results

Early results [7] revealed that an unambiguous “3-µm” band is present, consistent with the presence of hydrated (phyllo-)silicates. The specific position of this band in OVIRS spectra, 2.74 µm ± 0.01, is consistent with the positions observed in low petrologic subtype CM2 meteorites [8]. The global distribution of this feature is described by [9].

Since the acquisition of global mapping data at ~20-30 m/spot, we have identified a complex of features in the 3.2–3.6 µm region that we attribute to the presence of C-bearing compounds (organics and carbonate minerals) [4, 5]. Absorption band positions, widths, and relative strengths appear to be associated with a variable mixture of organics and multiple carbonate minerals. The varying shape and depth of a 3.4-µm absorption feature across Bennu’s surface spans the range seen among disk-averaged spectra of main-belt carbonaceous asteroids. Bennu’s distribution of carbon-bearing materials does not correlate with the distribution of hydrated minerals, surface brightness, or geologic features. Carbonate features identified in this spectral region are interpreted as having a variety of cation compositions. The organic features are consistent with aromatic and aliphatic C-H bonds like those of insoluble organic matter in meteorites and other primitive objects [10, 11]. The deepest 3.4-micron absorptions occur on individual boulders, and surface variation may be attributable to differences in abundance, fresh exposure by processes such as thermal fracturing, or differences in space weathering. There is no definitive spectral evidence of either organics or carbonates outside of the 3.2–3.6 μm region.

Several weak absorption bands also have been observed [4]. These are consistent with phyllosilicates (e.g., the 1.4-µm region), Fe-bearing phases (e.g., 1.05-µm region), and magnetite (0.55 µm).

3. OTES results

OTES spectra acquired during the Preliminary Survey mission phase are broadly consistent with carbonaceous chondrites (CCs) in the CI/CM groups [7]. Potential evidence of magnetite is present in features at 555 and 346 cm^–1 [1] and is consistent with aqueous alteration. Detailed Survey measurements at ~40 m/spot exhibit spectral variability, primarily in the shape of the silicate stretching feature and the depth of the silicate bending feature. These variations can be described by two endmember spectral types, T1 and T2, which appear to primarily represent differences in the amount of fine particulate (<~65–100 µm) dust across the surface of Bennu [6]. The dust appears to be up to ~10–15 microns thick and does not exhibit the spectral characteristics of typical fine-particulate materials. Thermal inertia data also constrain the thickness of any dust layer to <50 µm [12]. The locations of T2 spectra, inferred to have a slightly greater proportion of dust cover, tend to correspond with those of boulders having rough surfaces, suggesting that dust may be trapped preferentially on these rocks.

Analysis of the shape of the silicate bending feature complex centered at ~440 cm^–1 (~22.7 µm) indicates that anhydrous silicates are likely to comprise less than ~10 vol.% of the bulk silicate mineralogy.

A CC (C1) clast, 91A_1, from the Almahata Sitta meteorite (Ur-ung) exhibits a strong similarity to the T1 spectrum, particularly in the silicate stretching region. This sample exhibits signs of mild heating [13], with ~10 vol% recrystallized olivine.

We cannot rule out that portions of Bennu’s surface lithology(-ies) have been heated but such heating is likely to be limited to temperatures <400°C [e.g., 14].

4. Implications for the returned sample

The material collected from Bennu’s surface will be constrained by the design of the sampling mechanism to particles less than ~2 cm in diameter [15]. Imaging [16] and spectral data suggest that particles smaller than the maximum sampleable size are present and should be collected. It is probable that the sample will be dominated volumetrically by phyllosilicate minerals, with anhydrous silicates unlikely to comprise more than ~10 vol.% of the silicate mineralogy. Magnetite is expected to be in the returned sample, as are carbonates and organics. VNIR and TIR spectral features imply that at least some of Bennu’s surface materials may differ from those of typical CC meteorites.

Acknowledgements

This material is based upon work supported by NASA under Contract NNM10AA11C issued through the New Frontiers Program. We are grateful to the entire OSIRIS-REx Team for making the encounter with Bennu possible.

References

[1] Reuter, D. C. et al. Space Science Reviews, 214, 54, 2018.

[2] Simon A. A. et al. Remote Sensing, 10, 1486, 2018.

[3] Christensen, P. R. et al. Space Science Reviews, 214, 87, 2018.

[4] Simon, A. A. et al. LPSC 51, #1046, 2020.

[5] Kaplan, H. H. et al. LPSC 51, #1050, 2020.

[6] Hamilton, V. E. et al.: Evidence for limited compositional and particle size variation on asteroid (101955) Bennu from thermal infrared spectroscopy, in prep.

[7] Hamilton, V. E. et al. Nature Astronomy, 3, 332-340, 2019.

[8] Takir, D. et al. Meteoritics & Planetary Science, 48, 1618-1637, 2013.

[9] Praet, A. et al., this meeting.

[10] Kaplan, H. H. et al., Composition of organics on asteroid (101955) Bennu, in prep.

[11] Simon, A. A. et al.: Global Mineralogy of (101995) Bennu from the OSIRIS-REx Visible and InfraRed Spectrometer, in prep.

[12] Rozitis, B. et al.: Asteroid (101955) Bennu’s weak boulders and thermally anomalous equator, Science Advances, in revision.

[13] Goodrich, C. A. et al. Meteoritics & Planetary Science, 54, 2769 – 2813, 2019.

[14] Hanna, R. D. et al., Icarus, 346, 113760, 2020.

[15] Bierhaus, E. B. et al. Space Science Reviews, 214, 107, 2018.

[16] DellaGiustina, D. N. and J. P. Emery et al. Nature Astronomy, 3, 341-351, 2019.

How to cite: Hamilton, V., Simon, A., Kaplan, H., Goodrich, C., Lauretta, D., and Spectral Analysis Working Group, T. O.-R.: VNIR-TIR spectroscopy of (101955) Bennu, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-166, https://doi.org/10.5194/epsc2020-166, 2020.

In early December 2018, NASA’S Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS-REx) [1] spacecraft arrived at the Near-Earth asteroid (101955) Bennu, [2,3] with the objective of globally characterizing the asteroid and returning a sample to Earth in 2023. Given this context, we probe the spectral nature of craters on Bennu, using spectral data acquired by the OSIRIS-REx Visible and InfraRed Spectrometer (OVIRS) [4].

     2. Data and methods

We selected 46 craters from the crater catalogue of Bennu [5], depending on the availability of spectral data. The spectral data are calibrated as detailed in [6,7] and follow the photometric correction of [8], which results in reflectance factor (REFF) spectra corrected to the geometry of i=30°, e=0°, α =30°. We then derive an average spectrum of a crater from all the spectra whose footprints fall in its interior. From this average spectrum, we compute the following four spectral parameters to characterize the spectral properties of each crater.

1) Depth of the absorption band at 2.7 μm

2) Position of the 2.7-μm band minimum

3) Normalized spectral slope from 0.55 to 2.0 μm

4) Photometrically corrected REFF at 0.55 μm

For comparison with the crater spectra, we derive an average global spectrum from the Equatorial Station 3 (EQ3) data set [1] (acquired at phase angle α ∼ 8°), which has an almost complete spatial coverage of Bennu.

     3. Results and Discussion

• Detection of a shortward shift of the 2.7-μm absorption band minimum

The minimum position of the 2.7-μm absorption band in some craters shifts towards wavelengths shortward of 2.74 μm, the measured band position in Bennu’s global average spectrum [9]. This absorption band is associated with hydrated phyllosilicates, resulting from aqueous alteration of Bennu in the past. The minimum position of this band is related to Mg/Fe ratio of the phyllosilicates [10,11], which depends on the extent of the alteration. This minimum position is therefore an indication of the alteration the material experienced. We measure this shift to be up to 25 nm. The presence of such a shift of this band can be translated into the presence of fresher and/or less altered material inside the craters, in the context of space weathering for C-complex asteroids [12, 13].

• Anti-correlation between spectral slopes and REFF suggests the presence of fine grains

The relation between spectral slopes and REFF varies across craters. Particularly, it varies even among different scans of the same crater. To better understand this variation, Pearson’s correlation coefficient (r) was estimated. We considered a series of spectra acquired as the spectrometer scans from the rim of a crater, across its center, and towards the rim again. The REFF and spectral slope of each spectrum of this scan were then compared using r. We find an anti-correlation between REFF and spectral slopes (increasing spectral slopes versus decreasing REFF) within some craters (Fig. 1). Laboratory experiments conducted on carbonaceous chondrite meteorites have shown that redder slopes are associated with finer grains [14]. We then checked image data of the corresponding footprints and found the presence of unresolved material at cm to sub-cm scales[LDS-(1] . As such, we suggest that the observed anti-correlation is an indication of the presence of fine grains inside craters.

Fig. 1. Top (a): Crater ID 3 (Osprey) without and with footprints of an OVIRS scan. The rightmost graph shows the spectral slopes and REFF variation across the crater as the spectrometer scans through the crater, as seen on the rendering shown to its left. The x-axis represents the spectrum number of the scan and error bars are also given for the uncertainty of each parameter of a spectrum. r is evaluated at -0.84 for these two parameters, indicating a strong anti-correlation. Values of the global average spectrum are also given for comparison. Bottom (b): Similarly, a view of the crater ID 1 without and with the rendering of another OVIRS scan, where r is -0.79.

.

• Spectral heterogeneity detected in the largest equatorial crater on Bennu

We observe spectral heterogeneity in one of the oldest geological features on Bennu, the large equatorial crater located at -8°, 269° measuring 187 m in diameter [15]. We attribute this heterogeneity to mass movement [16]. We propose that such local spectral heterogeneities could be used as a tracer of mass movement across Bennu.

•  Smaller craters are brighter, with deeper hydration and redder spectral slopes

By examining the spectral parameters of all the 46 craters, we independently confirm the results of [13] that smaller (presumably younger) craters are redder and brighter. In addition, the OVIRS data indicate they have deeper 2.7-μm bands (Fig. 2). Comparing global average spectral values of Bennu and crater frequency distributions as a function of the chosen spectral parameters, we find that, given the time, craters will evolve to assume the global average spectral properties of Bennu.

Fig. 2 Variations of normalized spectral slope, REFF and 2.7 μm band depth with the crater diameter are given in a, b, and c panels respectively. Variation of REFF with 2.7-μm band depth is plotted in the panel d. The sizes of data points are scaled to corresponding crater sizes and crater IDs are embedded next to respective data points.

References

[1] Lauretta, D. S. et al. 2017, Space Science Reviews, 212, 925. [2] DeMeo, F. E. et al. 2009, Icarus, 202, 160. [3] Clark, B. E et al. 2011, Icarus, 216, 462. [4] Reuter D. C. et al. 2017, Space Sci. Rev. 214, 54. [5] Bierhaus, E. et al. 2019, in EPSC–DPS2019–1134. [6] Simon, A. A. et al. 2018, Remote Sensing, vol. 10, issue 9, p. 1486 [7] Simon et al. (submitted) [8] Zou et al. (submitted). [9] Hamilton, V. E. et al. 2019, Nature Astronomy, [10] Farmer, V. C. 1974, in The Infrared Spectra of Minerals [11] Beck, P. et al. 2010, Geochimica et Cosmochimica Acta, 74, 4881. [12] Lantz, C. et al. 2017, Icarus, 285, 43. [13] DellaGiustina et al. – this conference [14] Cloutis, E. A. et al. 2018, Icarus, 305, 203. [15] Walsh, K. J. et al. 2019, Nature Geoscience. [16] Jawin et al. (submitted).

How to cite: Deshapriya, J. D. P., Barucci, M. A., and Bierhaus, E. B. and the the OSIRIS-REx team: Spectral properties of craters on (101955) Bennu, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-112, https://doi.org/10.5194/epsc2020-112, 2020.

The Hera mission has been approved for development and launch in the new ESA Space Safety Programme by the ESA Council at Ministerial Level, Space19+, in November 2019. Hera will both offer a high science return and contribute to the first deflection test of an asteroid, in the framework of the international NASA- and ESA-supported Asteroid Impact and Deflection Assessment (AIDA) collaboration.

The impact of the NASA DART (Doube Asteroid Redirection Test) spacecraft on the natural satellite of Didymos in October 2022 will change its orbital period around Didymos. As Didymos is an eclipsing binary, and close to the Earth on this date, the change can be detected by Earth-based observers. ESA’s Hera spacecraft will rendezvous Didymos four years after the impact. Hera’s instruments will perform the measurements necessary to understand the effect of the DART impact on Didymos’ secondary, in particular its mass, its internal structure, the direct determination of the momentum transfer and the detailed characterization of the crater left by DART. This new knowledge will also provide unique information on many current issues in asteroid science.

From small asteroid internal and surface structures, through rubble-pile evolution, impact cratering physics, to the long-term effects of space weathering in the inner Solar System, Hera will have a major impact on many fields. For instance, collisions play a fundamental role in our Solar System history, from planet formation by collisional accretion to cratering of solid surfaces and asteroid family formation by collisional disruption. The fully documented hypervelocity impact experiment provided by DART and Hera will feed collisional models with information obtained at actual asteroid scale and for an impact speed (~6 km/s) that is close to the average impact speed between asteroids in the main belt. Moreover, Hera will perform the first rendezvous with an asteroid binary, characterize the smallest object ever visited (165 m in diameter) and provide the first direct measurement of an asteroid interior. Additionally, studies using Hera data will in turn affect our understanding of the asteroid population as a whole. The scientific legacy of the Hera mission will extend far beyond the core aims of planetary defense.

Acknowledgment: The authors acknowledge funding support from ESA and from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 870377 (project NEO-MAPP), from the European Space Agency and from the French space agency CNES.

How to cite: Michel, P. and Kueppers, M. and the Hera Investigation Team: The science return of the ESA Hera mission to the binary asteroid Didymos, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-88, https://doi.org/10.5194/epsc2020-88, 2020.

Martian Moon eXploration (MMX) mission by Japan Aerospace Exploration Agency (JAXA) is the third Japanese sample return mission.  One of the main mission goals is to decipher the origin of these moons, which will provide important clues on planetary formation and how water is delivered to inner planets.

MMX will be launched in September 2024 to Martian system to bring back samples from Phobos conducting detailed observations of Phobos and Deimos, and monitoring Mars’s climate. The mission is five-year round trip with return sample on Earth on July 2029. The spacecraft will arrive to Mars system on August 2025, stay three years, and have QSO (Quasi Satellite Orbits) around Phobos at different altitudes to select the landing sampling sites. The spacecraft will land for several hours on the Phobos surface to collect at least 10g of Phobos regolith using a corer going down to a depth of at least 2cm. MMX may collect Phobos samples in two different sites. After three years the spacecraft will leave the Martian system and return the samples to Earth, completing the first round-trip to the Martian system.

The principal objectives of the mission are:

• To settle the controversy on the origin of the Martian moons by close-up observations and return sample analysis
• To constrain processes for planetary formation and material transport in the region connecting the inner and outer solar systems
• To reveal evolutionary processes of the Martian system in the circum-Martian environments

A set of mission instruments are defined and under development to achieve the major mission goals. The included instruments are: the wide-angle camera OROCHI (Optical RadiOmeter composed of CHromatic Imagers), the telescopic (narrow-angle) camera TENGOO (TElescopic Nadir imager for GeOmOrphology), the laser altimeter LIDAR (Light Detection and Ranging), CMDM (Circum-Martian Dust Monitor), MSA (Mass Spectrum Analyzer), MEGANE (Gamma rays and Neutrons Spectrometer) provided by NASA, a near-infrared spectrometer MIRS (MMX InfraRed Spectrometer) provided by CNES, SMP sampling device and the sample return capsule.  A small Rover (total weight less than 30kg) developed by CNES and DLR is also a part of the mission. The Rover payload includes four scientific instruments: a IR radiometer (miniRAD), a Raman spectrometer (RAX), a stereo pair of cameras looking forward (NavCAM) and two cameras looking at the interface wheel-surface (WheelCAM) and consequent Phobos’ regolith mechanical properties. ESA will participate to the mission assisting with deep space communication equipment.

   1. MIRS

MIRS instrument is built at LESIA-Paris Observatory in collaboration with four other French laboratories (LAB, LATMOS, LAM, IRAP-OMP), with collaboration and financial support of CNES and close collaboration with JAXA and MELCO. MIRS is an imaging spectrometer in the 0.9 - 3.6 microns spectral band with spectral resolution better than 20 nm. The IFOV is 0.35 mrad and FOV of +/-1.65°. The SNR is higher than 100 up to 3.2 µm in a maximum integration time less than 2s. The detector has dimension of 256 x 256 pixels with pixel pitch of 30 microns. The total mass is lower than 11.9 kg (including 20% margin), and the volume of complete instrument is about 320 x 150 x 400 mm³.

   2. MIRS science objectives

MIRS is expected to characterize Phobos and Deimos surfaces and Mars atmospheric composition by remotely identifying diagnostic features in the near-infrared range.  MIRS is used to achieve some of the mission-requirements, in particular:

1: To grasp the surface distribution of the constituent materials of Phobos. Hydrous minerals and other related minerals should be identified and characterized spectroscopically for main parts of the full body in correspondence with its topography (at horizontal spatial resolutions of 20 m or better) and in a radius of 50 m or more around the sampling point (at spatial resolutions of 1 m or better).

For the global areas, MIRS is expected to spectroscopically measure water (ice) (absorption band at 3.0-3.2 μm) and hydrous silicate minerals (features at 2.7-2.8 μm) at the wavelength resolutions (wavelength width) of 20 nm, S/N ratios > 100 and a spatial resolution of 20 m (for +/-30° latitude). The spectral radiometric absolute accuracy is expected to be of 10%, and the relative accuracy of 1%. If possible, also to measure organic matter (3.3.-3.5 μm) and hydrous/anhydrous silicate minerals (1.0 μm). An area within 50 m from a selected sampling spot at 1 m resolution will be also mapped.

2: To grasp the distribution of constituent materials of Deimos, from spectroscopic information, clarify the surface distribution of hydrous minerals and other related minerals corresponding to its topography at characteristic parts of the moon with a horizontal spatial resolution of 100 m or better.

MIRS is expected to spectroscopically map major regions of Deimos at a spatial resolution better than 100 m for major absorption bands as observed in Phobos.

3: To constrain transport processes for dust and water near the Martian surface, continuous observations of the mid- to low-latitude distributions of dust storms, ice clouds, and water vapor in the Martian atmosphere are performed from high altitudes equatorial orbit in different seasons to within 1-hour time resolutions.

MIRS is expected to perform observations of distributions of total amount of water vapor columns at 10 km spatial resolutions and spectral radiometric absolute accuracy of 10%, and spectral radiometric relative accuracy of 1% with temporal resolution less than 1-hour for the mid- to low-latitude selected areas. These observations are expected to be performed over several successive days in different seasons.               

   3. Conclusions

MIRS will allow compositional characterization of Phobos, Deimos and temporal characterization of particular phenomena of Mars atmosphere. It will be also a fundamental instrument to evaluate sampling site candidates and support the selection of the two sampling sites on the Phobos surface.

The mission will be able to clarify the origin of the Martian moons and may also be able to elucidate the process of the evolution of the Mars environment.

Acknowledgements

MMX is under developed and built by JAXA, with contributions from CNES, DLR and NASA. We thank the MMX JAXA teams for their efforts in defining and building the mission. The MIRS team thanks CNES for the financial support and collaboration to build MIRS instrument.

How to cite: Barucci, M. A., Reess, J.-M., Bernardi, P., Nakagawa, H., Iwata, T., Nakamura, T., Doressoundiram, A., Fornasier, S., Le Du, M., Sawyer, E., Gautier, T., Leyrat, C., Jorda, L., Merlin, F., Canalias, E., Rocard, F., Kuramoto, K., and Kawakazu, Y. and the MIRS MMX Team: MMX JAXA Mission and MIRS Imaging Spectrometer, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-111, https://doi.org/10.5194/epsc2020-111, 2020.

The Japan Aerospace Exploration Agency, JAXA, Martians Moons eXploration (MMX) mission will investigate the Martian Moons Phobos and Deimos, and return samples from Phobos to Earth. As part of this mission a small (~25 kg) rover, contributed by the Centre National d’Etudes Spatiales (CNES) and the German Aerospace Center (DLR), with additional contributions from INTA (Spain) and JAXA, will be delivered to the surface of Phobos. The rover will demonstrate the technology of locomotion on a regolith-covered, low gravity planetary surface. In addition, the rover will provide scientific data on the regolith properties (mechanical, mineralogical and thermal), provide ground truth for the MMX orbiter instruments, give context information for the returned samples, and contribute to reducing the risk of the landing and sampling operations of the MMX mission.

In order to achieve these goals, the rover has a small suite of scientific instruments: a Raman spectrometer (RAX) to measure the mineralogical composition of the surface material, a radiometer (miniRAD) to measure the surface brightness temperature and determine thermal properties of both regolith and rocks (if in the field of view), a stereo pair of navigation cameras looking forwards (NAVCam) that will place constraints on the level of heterogeneity of the regolith both in terms of composition and space weathering alteration, and two cameras looking at the interface between wheel and surface (WheelCam). The WheelCams will observe the properties of the regolith compaction and flow around the wheels, and the resulting trenches in order to characterise the mechanical properties of the regolith itself.

The MMX rover will be deployed from the main spacecraft from an altitude of less than 100 m above the surface of Phobos. The uprighting and deployment (legs/wheels and solar panels) sequences will be performed automatically once the rover comes to rest on the surface. The rover will then operate for 100 days covering a total distance of several meters to hundreds of meters.

The MMX launch is currently planned for late 2024 with the Mars orbit insertion occurring in 2025, and the rover delivery and operations in 2026 or 2027.

This presentation will provide an overview of the MMX rover and the expected science return from each of the four instruments.

How to cite: Murdoch, N., Michel, P., Ulamec, S., Grott, M., Böttger, U., Vernazza, P., Arrat, D., Chalon, M., Tardivel, S., Biele, J., Grundmann, J. T., and Miyamoto, H. and the MMX rover team: The Martians Moons eXploration (MMX) Rover to Phobos, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-247, https://doi.org/10.5194/epsc2020-247, 2020.

Abstract

In this study, we present several ways in which a rover wheel can be used as a tool to study regolith dynamics. We demonstrate specific analysis methods by conducting numerical simulations of a simple rover wheel traversing a bed of regolith. The simulation data is used to analyze the general flow behavior of granular material around a wheel in a reduced-gravity environment.

Introduction

In the mid-2020s, the Martian Moons eXploration mission (MMX) will visit Phobos and Deimos, the moons of Mars. The key science objectives of the Japanese mission are to clarify the origins of the Martian moons and to investigate the mechanisms behind the evolution of these bodies. During the MMX mission, the French Space Agency (CNES) and the German Aerospace Center (DLR) will deploy a small, 25 kg, wheeled rover to the surface of Phobos [4, 6]. Wheeled rovers have been used to explore the Moon and Mars, but they have never been used to traverse small-body surfaces. Like the Moon and Mars, Phobos is thought to be covered by a layer of loose regolith. Its low-gravity environment, however, is comparable to that of a large asteroid. As such, the MMX rover will serve primarily as a technology demonstration. It will be used to evaluate the performance of a wheeled locomotion system on a granular surface in milli-gravity conditions.

The MMX rover provides a unique opportunity to make in-situ observations on the surface of Phobos. Among other instruments, the rover will be equipped with two navigation cameras (NavCams) and two wheel cameras (WheelCams). The WheelCams will capture the direct interaction between the rover’s wheels and surface of Phobos. When combined with images and engineering data, a rover’s wheels can become powerful tools for studying the surface mechanical properties of soil. For instance, an extensive study on the cohesion and frictional properties of the Martian surface was conducted by observing trenches created by Spirit and Opportunity, the Mars Exploration Rovers (MER) [2]. In another example, the strength and stiffness properties of lunar soil were estimated using lander images and wheel sinkage estimates from China’s Yutu rover on the Cheng’E-3 mission [1].

The objective of this work is to present different types of regolith-focused studies that can be made by treating a rover wheel as an excavation tool. We begin by providing a survey of techniques that have been employed in the past. Then, we demonstrate how numerical modeling can be used to augment in-situ observations from actual planetary missions.

Simulations

Wheel-regolith simulations are conducted using the soft-sphere DEM implementation in the open-source code Chrono [3, 5]. For simplicity, we consider a generic wheel design that consists of a cylindrical hub and paddle-like grousers. The rover wheel is constructed in Chrono using two different methods. The first method consists of representing the wheel as an assembly of cylinders and boxes. In the second method, the wheel is imported into the simulations as a custom triangular mesh. Comparing the results for the two different wheel models helps to validate the meshing technique so that more complex wheel designs can be used in future simulations.

Preliminary regolith beds consist of approximately 150,000 mono-disperse spherical particles.  The particles have material strength properties comparable to quartz sand and are settled in a rectangular container with random-loose packing. After the particles settle, the wheel is added to the simulations so that it sits just above the surface of the bed. We let the wheel sink into the granular material under some gravity level and then rotate the wheel at a low and constant speed. The wheel makes a quarter of a turn and is free to translate in the vertical and horizontal directions. Position, velocity, force, and torque information is reported for the wheel and all of the particles.

Results

Simulations are conducted for Earth and reduced-gravity levels and for varied magnitudes of particle cohesion. Preliminary results for wheel sinkage and slip are reported, with particular focus on the motion and behavior of the grains directly beneath the wheel. The simulation results are discussed in the larger context of how to use wheel-regolith modeling to augment surface-science experiments performed by actual planetary rovers.

Acknowledgements

We would like to thank ISAE-SUPAERO and the Centre National d’Études Spatiales (CNES) for financially supporting this research effort.

References

[1] Gao, Yang, et al. "Lunar soil strength estimation based on Chang’E-3 images." Advances in Space Research 58.9 (2016): 1893-1899.

[2] Sullivan, R, et al. "Cohesions, friction angles, and other physical properties of Martian regolith from Mars Exploration Rover wheel trenches and wheel scuffs." Journal of Geophysical Research: Planets 116.E2 (2011).

[3] Sunday, C., et al. “Validation of Chrono for granular DEM simulations in reduced-gravity environments.” (2020) [submitted].

[4] Tardivel, S., et al. “The MMX rover: An innovative Design enabling Phobos in-situ Exploration.” In LCPM Low-Cost Planetary Missions Conference, IAA, 2019.

[5] Tasora, Alessandro, et al. “Chrono: An open source multi-physics dynamics engine.” International Conference on High Performance Computing in Science and Engineering, Springer, Cham , 2015.

[6] Ulamec, S., et al., “A rover for the JAXA MMX mission to Phobos.” In 70th International Astronautical Congress (IAC), 2019.

How to cite: Sunday, C., Murdoch, N., Tardivel, S., Imperatore, N., Michel, P., and Ulmec, S.: Using rover wheels to study regolith dynamics, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-234, https://doi.org/10.5194/epsc2020-234, 2020.

The Lucy Mission is a NASA Discovery class mission to send a highly capable and robust spacecraft to investigate seven Jupiter Trojan asteroids; a class of stable, primitive bodies near both the L4 and L5 Lagrange points with Jupiter. It is believed that Jupiter Trojan asteroids are leftover planetesimals from the outer planetary system that have been preserved since early in Solar System history, and represent the last of all of the stable populations of the Solar System to be visited by spacecraft.

Lucy is slated to launch in October 2021, reach its first Trojan asteroid in 2027, and have its final encounter in 2033. During its lifetime, Lucy will perform five Trojan encounters closely studying at least seven objects (one encounter is of a nearly equal mass binary and another is an asteroid with a known satellite). The science goals include determining the surface composition, assessing the geology, determining the bulk properties and searching for satellites around all of Lucy’s targets. The payload suite consists of a color camera and infrared imaging spectrometer, a high resolution panchromatic imager, and a thermal infrared spectrometer. Additionally, two spacecraft subsystems will also contribute to the science investigations: the terminal tracking cameras and the telecommunication subsystem to measure the mass of the Trojan asteroids.

Lucy’s Trojan targets include one C-type (Eurybates, 64 km in diameter), three P-types (Menoetius, Patroclus, and Polymele; 105, 114, 21 km in diameter, respectively), and two D-types (Leucus and Orus; 41 and 52 km in diameter, respectively), thereby covering a wide range of spectral types and sizes. Lucy will be the first spacecraft to observe the largest remnant of a catastrophic collision up close (Eurybates), and the first to visit a near-equal mass binary (Patroclus and Menoetius). In addition, on its way to L4, Lucy will fly by DonaldJohanson in 2025, a 4 km in diameter Main Belt asteroid named in honor of the discoverer of the Lucy fossil. In this talk, recent results from an international observational campaign of some of the Lucy’s targets will be presented, including the detection of a small satellite (1-2 km) orbiting Eurybates, and a detailed characterization of Leucus’s shape and rotational axis orientation.

How to cite: Marchi, S., Levison, H., Olkin, C., and Noll, K.: The NASA Lucy Mission: Surveying the Diversity of Trojan Asteroids, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-163, https://doi.org/10.5194/epsc2020-163, 2020.

1. Introduction

The Psyche mission began development in January 2017 when it was selected by NASA as the 14^th Discovery Class mission. Having successfully completed its critical design review, the spacecraft is being assembled by MAXAR and the Jet Propulsion Laboratory under the leadership of the PI, Linda Elkins-Tanton, at Arizona State University. The mission plans to explore for the first time the main-belt asteroid (16) Psyche. Launch is planned for August 2022 leading to rendezvous with Psyche in early 2026.

Early radar [1] and spectroscopic measurements [2] of Psyche indicated a metallic composition leading to speculation that Psyche was the remnant core of a larger differentiated asteroid. Continued observing over decades has resulted in varying estimates for key physical parameters. Current expectations are that Psyche is a mix of rock and metal and that a complete understanding of its composition and origin awaits closer inspection by the Psyche science instruments [3]. The mission concept is designed to distinguish between the range of possible compositions. The science objectives of the Psyche mission are to:

   1. Determine whether Psyche is a core, or if it is unmelted material.

   2. Determine the relative ages of regions of its surface.

   3. Determine whether small metal bodies incorporate the same light elements as are expected in the Earth’s high-pressure core.

   4. Determine whether Psyche was formed under conditions more oxidizing or more reducing than Earth’s core.

   5. Characterize Psyche’s topography.

2. Mission Overview

The Psyche mission operations concept is inherited from the Dawn mission to Vesta and Ceres. The Psyche spacecraft will orbit the asteroid at a series of four progressively lower orbits (Orbits A–D) that provide increasingly higher resolution science measurements. Each orbit phase is designed to address specific science objectives and provide refinements to Psyche’s shape and mass that enable the descent to the next orbit. Like Dawn, the Psyche mission uses solar electric propulsion to travel to Psyche as well as enter orbit and transfer between the science orbits. Figure 1 shows the spacecraft interplanetary trajectory and key mission dates.

3. Science Investigations

The Psyche spacecraft plans to carry three science instruments and conduct the gravity science investigation using the X-band telecommunication system.

The magnetometer consists of two sensors mounted on a 2-m boom in a gradiometer configuration. The instrument is developed by the Technical University of Denmark with heritage from SWARM [4] whereas the science investigation is managed at the Massachusetts Institute of Technology. Figure 2 shows the magnetometer sensors and electronics units. The primary objective of the magnetometer is to search for the existence of a remanent magnetic field that would confirm that Psyche was once a planetary core. The magnetometer is operated continuously starting shortly after launch until the end of the mission with its science objectives achieved in Orbit C.

The multispectral imager consists of two redundant imagers with a panchromatic and seven narrowband filters. The filters were selected to detect minerals such as oldhamite, olivine, and pyroxene that would help discriminate between Psyche formation scenarios. The imaging campaign also provides the Psyche shape model and data to make a geologic map. Figure 3 shows the imager hardware being developed by Malin Space Science Systems that is managed and operated by Arizona State University with heritage from MSL MastCam and MCO MARCI [5]. Orbits A and B are the primary science orbits for the imager providing global maps up to 20 m/pixel. Imaging will also continue throughout the remainder of the mission.

The gamma-ray and neutron spectrometer (GRNS) consists of two separate units mounted on their own 2-m boom (Fig 4). GRNS is developed by the Johns Hopkins Applied Physics Laboratory with heritage from MESSENGER and Lunar Prospector [6]. The GRNS provides elemental composition measurements for key elements such as nickel, iron, sulfur, silicon, potassium, and others. Nickel content is an indicator of whether Psyche is a remnant core or unmelted material. GRNS will be able to map the distribution of metals and silicates across the surface and measure nickel content to 2 wt% in Orbit D.

The X-band telecommunication system is capable of providing two-way coherent Doppler and ranging data via Deep Space Network (DSN) tracking that will be used to map the gravity field of Psyche and to probe its interior structure.

The Deep Space Optical Communications (DSOC) technology demonstration is planned as part of the mission payload. It does not play a role in the science investigations.

4. Summary

The Psyche mission development is proceeding on schedule. Meanwhile, the Psyche asteroid is becoming a more complex and compelling target.

Acknowledgements

This work was, in part, carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.

References

[1] Ostro, S. J. et al. (1985) Science, 229, 442-446.

[2] Binzel, et al. (1995) Icarus, 117, 443-445.

[3] Elkins-Tanton, L. T. et al. (2020) JGR Planets, 125, doi: 10.1029/2019JE006296.

How to cite: Polanskey, C., Elkins-Tanton, L., Bell, J., Binzel, R., Lawrence, D., Merayo, J., Park, R., Weiss, B., and Williams, D.: Mission to (16) Psyche, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-988, https://doi.org/10.5194/epsc2020-988, 2020.

The Destiny+ mission (Demonstration and Experiment of Space Technology for Interplanetary voyage Phaethon fLyby and dUst Science) has been selected as part of its M-class Space Science Program by the Japanese space agency JAXA/ISAS and is set to launch in 2023/2024. The mission target is the active asteroid (3200) Phaethon with a projected flyby in early 2028. The scientific payload consists of two cameras (the Telescopic Camera for Phaethon, TCAP, and the Multi-band Camera for Phaethon, MCAP), and the Destiny+ Dust Analyzer (DDA). DDA is the technological successor to the Cosmic Dust Analyzer (CDA) aboard Cassini-Huygens, which prominently investigated the dust environment of the Saturnian system. The DDA sensor is designed as a combination of impact ionization time-of-flight mass spectrometer and trajectory sensor, which will allow for the analysis of sub-micron and micron sized dust particles with respect to their composition (mass resolution m/Δm ≈ 100-150), mass, electrical charge, velocity (about 10% accuracy), and impact direction (about 10° accuracy).

Besides attempting to sample the impact-generated dust cloud around Phaethon during the flyby, DDA will be actively observing the interplanetary & interstellar dust environment over the roughly four years spanning cruise phase from the Earth-Moon system through interplanetary space. After launch into a GTO-like orbit, Destiny+ will first employ its solar-electric propulsion system to spiral up to the lunar orbit within about 18 months, followed by a series of lunar swingbys and interim coasting phases in distant cislunar space, accumulating momentum to leave the Earth-Moon system at high excess velocity. The subsequent roughly 2-year interplanetary transfer to intercept Phaethon will be characterized by moderate orbital eccentricity of up to 0.1 and largely unpowered coasting phases.

During these four years, the DDA sensor will benefit from a maximum pointing coverage range enabled by its dual-axis pointing mechanism and spacecraft attitude flexibility (during times of unpowered flight). This will allow for exhaustive mapping and analysis of the different interplanetary dust populations, as well as interstellar dust encountered in the region between 0.9-1.1 AU.

Here, we give a progress report on the science planning efforts for the 4-year transfer phase. We present a tentative observation timeline that assigns scientific campaigns to different phases of the mission, taking into account results of various dust models, as well as operational and technical constraints.

How to cite: Sommer, M., Krüger, H., Srama, R., Hirai, T., Kobayashi, M., Arai, T., Sasaki, S., Kimura, H., Moragas-Klostermeyer, G., Strub, P., and Lohse, A.-K.: Destiny+ Dust Analyzer – Campaign & timeline preparation for interplanetary & interstellar dust observation during the 4-year transfer phase from Earth to Phaethon, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-342, https://doi.org/10.5194/epsc2020-342, 2020.

How have planets formed in the Solar System? And what chemical composition they inherited from their natal environment? Is the chemical composition passed unaltered from the earliest stages of the formation of the Sun to its disk and then to the planets which assembled in the disk? Or does it reflects chemical processes occurring in the disk and/or during the planet formation process? And what was the role of comets in the delivery of volatiles and prebiotic compounds to early Earth?

A viable way to answer these questions is to observe protoplanetary disks around young Sun-like stars and compare their chemical composition with that of the early Solar System, which is imprinted in comets. The impacting images recently obtained by millimetre arrays of antennas such as ALMA provided the first observational evidence of ongoing planet formation in 0.1-1 million years old disks, through rings and gaps in their dust and gas distribution. The chemical composition of the forming planets and small bodies clearly depends on the location and timescale for their formation and is intimately connected to the spatial distribution and abundance of the various molecular species in the disk. The chemical characterisation of disks is therefore crucial.

This field, however, is still in its infancy, because of the small sizes of disks (~100 au) and to the low gas-phase abundance of molecules (abundances with respect to H₂ down to 10^-12), which requires an unprecedented combination of angular resolution and sensitivity. I will show the first pioneering results obtained as part of the ALMA chemical survey of protoplanetary disks in the Taurus star forming region (ALMA-DOT program). Thanks to the ALMA images at ~20 au resolution, we recovered the radial distribution and abundance of diatomic molecules (CO and CN), S-bearing molecules (CS, SO, SO₂, H₂CS), as well as simple organics (H₂CO and CH₃OH) which are key for the formation of prebiotic compounds. Enhanced H₂CO emission in the cold outer disk, outside the CO snowline, suggests that organic molecules may be efficiently formed in disks on the icy mantles of dust grain. This could be the dawn of ice chemistry in the disk, producing ices rich of complex organic molecules (COMs) which could be incorporated by the bodies forming in the outer disk region, such as comets. 

The next step is the comparison of the molecules radial distribution and abundance in disks with the chemical composition of comets, which are the leftover building blocks of giant planet cores and other planetary bodies. The first pioneering results in this direction have been obtained thanks to the ESA’s Rosetta mission, which allowed obtaining in situ measurements of the COMs abundance on the comet 67P/Churyumov-Gerasimenko. The comparison with three protostellar solar analogs observed on Solar System scales has shown comparable COMs abundance, implying that the volatile composition of comets and planetesimals may be partially inherited from the protostellar stage. The advent of new mission, devoted to sample return such as AMBITION will allow us to do a step ahead in this direction.

How to cite: Podio, L., Garufi, A., Codella, C., Fedele, D., Rygl, K., Favre, C., Bacciotti, F., Bianchi, E., Ceccarelli, C., Mercimek, S., Teague, R., and Testi, L.: The chemical content of planet-forming disks: towards a comparison with comets to unveil the origin of the Solar System, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-628, https://doi.org/10.5194/epsc2020-628, 2020.

Comets are the most pristine objects in our Solar System. Having spent most of their life at large distance from the Sun, where they remained mostly unaffected by solar radiation, comets are the most unaltered remnants from the era of planet formation. In June 2019, a multi-spacecraft project – Comet Interceptor – was selected by the European Space Agency (ESA) as its next planetary mission, and the first in its new class of Fast (F) projects [1]. The mission’s primary science goal is to characterise, for the first time, a long-period comet – preferably one which is dynamically new – or an interstellar object. An encounter with a comet approaching the Sun for the first time will provide valuable data to complement that from all previous comet missions: the surface of such an object would be being heated to temperatures above the its constituent ices’ sublimation point for the first time since its formation.

A mission to an unknown target: As a comet’s trajectory needs to be very well known in order to send a spacecraft to it, past missions to comets (e.g. Giotto to comets 1P/Halley in 1986 [2] and 26P/Grigg-Skejllerup in 1992 [3] and Rosetta to 67P/Churyumov-Gerasimenko in 2014-2016 [4]) have, by necessity, been sent to short-period comets with well-characterised orbits. A consequence of this is that all past missions have encountered comets that have evolved from their original condition during their time orbiting near the Sun. Comet Interceptor will take a different approach: it will be delivered to Sun-Earth Lagrange Point L2 with the ESA Ariel mission, planned for launch in 2028. At L2, it will be in a relatively stable location in space, where a ‘parking’ orbit can be maintained through occasional station-keeping, waiting for later injection onto an interplanetary trajectory to intersect the path of its target. This allows a relatively rapid response to the appearance of a suitable target, which will need to traverse the ecliptic plane in an annulus which contains Earth’s orbit. In addition to having a spacecraft capable of being targeted at relatively short notice, this mission to a “new” comet is possible because large sky survey observatories are now finding incoming comets with greater warning times, of a few years at least. With the advent of powerful facilities such as the Vera Rubin Observatory’s Legacy Survey of Space and Time, LSST, under construction at the time of writing in Chile [5], the prospects of finding a suitable dynamically new comet nearing the Sun for the first time are very promising. The enticing possibility also exists for the spacecraft to encounter an interstellar object if one is found on a suitable trajectory. Simulations of LSST performance, based on the best current understanding of the underlying population of Oort cloud comets from the Pan-STARRS survey, suggest that  about 5 years between discovery and interception is likely, and the target comet may be found before the mission is launched. A short period comet will serve as a backup destination in case a suitable target is not found within a period of approximately 3 years post-launch. An important consequence of the mission design is that the spacecraft must be as flexible as possible, i.e. able to cope with a wide range of target activity levels, flyby speeds, and encounter geometries. This flexibility has significant impacts on the spacecraft solar power input, thermal design, and dust shielding that can cope with dust impact speeds ranging from around 10 to 80 km/s, depending on the target comet’s orbital path.

A Multi-Spacecraft Architecture: Comet Interceptor will comprise three spacecraft. When approaching the target, the two sub-spacecraft – one (B2) provided by ESA, the other (B1) by the Japanese space agency, JAXA, will be released from the primary craft. The main spacecraft, which will act as the primary communication point for the whole constellation, will be targeted to pass outside the hazardous inner coma, making remote and in situ observations on the sunward side of the comet. The two sub-spacecraft will be targeted closer to the nucleus and inner coma region. These two platforms will perform valuable complementary observations to those of the primary spacecraft, venturing into a region of the coma that presents a higher risk to their safety. Data will be transmitted from the two sub-spacecraft to the primary spacecraft in real time, for later transmission to Earth. Dust shields are included on all three spacecraft, to protect them from high speed dust impacts.

Scientific Goals and Observations: Measurements of the target include its surface composition, shape, and structure, its dust environment, and the composition of the gas coma. A unique, multi-point ‘snapshot’ measurement of the comet-solar wind interaction region will also be obtained, complementing single spacecraft observations made at other comets. The mission’s instrument complement will be provided by consortia of institutions in Europe and Japan.

Acknowledgments: The Comet Interceptor team comprises over 200 scientists and engineers from 23 countries. We thank the whole team, and the study team at ESA, and colleagues at JAXA, for their continuing invaluable contributions to the mission, as well as to the national agencies that will fund the mission’s instrumentation and scientific team.

References: [1] Snodgrass, C. and Jones, G. (2019) Nature Comms. 10, 5418. [2] Reinhard R. (1986) Nature 321, 313–318. [3] Grensemann, M. G. and Schwehm, G. (1993) J. Geophys. Res. 98, A12, 20907. [4] Taylor, M. G. G. T., et al. (2017) Philos. Trans. R. Soc. A 375, 20160262. [5] Filacchione, G. et al. (2019) Space Sci. Rev. 215, 19.

How to cite: Tubiana, C., Jones, G., and Snodgrass, C. and the the Comet Interceptor Team: Comet Interceptor: A mission to an ancient world, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-574, https://doi.org/10.5194/epsc2020-574, 2020.

The last thirty years of cosmochemistry and planetary science have shown that one major Solar System reservoir is vastly undersampled in the available suite of extra-terrestrial materials, namely small bodies that formed in the outer Solar System (>10AU). Because various dynamical evolutionary processes have modified their initial orbits (e.g., giant planet migration, resonances), these objects can be found today across the entire Solar System as P/D near-Earth and main-belt asteroids, Jupiter and Neptune Trojans, comets, Centaurs, and small (diameter <200km) trans-Neptunian objects. This reservoir is of tremendous interest, as it is recognized as the least processed since the dawn of the Solar System and thus the closest to the starting materials from which the Solar System formed. Some of the next major breakthroughs in planetary science will come from studying outer Solar System samples (volatiles and refractory constituents) in the laboratory. Yet, this can only be achieved by an L-class mission that
directly collects and returns to Earth materials from this reservoir. It is thus not surprising that two white papers advocating a sample return
mission of a primitive Solar System small body (ideally a comet) were submitted to ESA in response to its call for ideas for future L-class
missions in the 2035-2050 time frame. I will present an overview of the ideas listed in one of these two white papers and discuss how such a
mission would be complementary to current and future ground based observations of primitive Solar System small bodies.

How to cite: Vernazza, P. and Beck, P.: Sample return of primitive matter from the outer Solar System, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-10, https://doi.org/10.5194/epsc2020-10, 2020.

Since the 1980s, ESA has played a leading role in cometary space science with Giotto and the Rosetta/Philae missions to comets 1P/Halley and 67P/Churyumov-Gerasimenko, respectively. These missions have greatly improved our knowledge of comets and placed Europe in an ideal position for the preparation of future cometary missions. After a cometary flyby mission (Giotto) and an orbiter (Rosetta) with a nucleus lander (Philae), and the selection of F-class Comet Interceptor ESA mission, the next major step in cometary science would be a cryogenic sample return mission as studied in the AMBITION proposal [1] which was submitted to ESA in the context of the future Voyage 2050 program. Despite the success of previous missions, a number of high priority scientific questions are still debated within the cometary community:

The best approach to answer these questions is to return a pristine and volatile rich cometary sample to Earth. Such a sample, preserved at cryogenic temperature, will be transferred to a state-of-the-art curation facility, where it will be investigated in great detail.

In this scenario part of the sample will be studied by means of different analytical techniques in laboratory-controlled conditions while the remaining sample will be stored at cryogenic temperature for future investigations. Laboratory analyses of the returned sample can achieve much higher precision and resolution measurements as compared to in-situ measurements.

The AMBITION study has considered different mission targets (see Figure 1), including Centaurs, Jupiter family comets and its extinct members, returning Oort cloud comets, dynamically new comets, main belt comets and interstellar comets as possible candidates. Among them, Jupiter family comets are the ones which offer the best characteristics, in terms of orbital parameters, evolutionary history, composition and activity, to be to be selected for an L-class cryogenic sample return mission. The other targets could be explored by more consolidated (orbiters, landers) small and medium class missions.

The selection, collection and storage of the sample at controlled cryogenic temperature and pressure during the return flight, Earth re-entry and landing phases will require significant technological advancement to guarantee maintenance of the optimal environmental conditions for the collected sample. Several options are identified for pristine sample collection from the cometary nucleus, including subsurface corers able to extract carrots up to 3 m depth as well as grabbers and manipulator arms to collect ice-rich, semi-buried boulders from the surface.

The AMBITION mission is the “Holy Grail” in the field of cometary exploration and Solar System formation. For its implementation it will need a strong commitment from the scientific community, industrial partners, ESA and national space agencies.

Figure 1: Approximate mission classes for different mission and comet types, in increasing complexity from left to right, and covering varying evolution stages of comets from the four possible reservoirs (Kuiper Belt, the Oort cloud, the Main Belt, and other planetary systems). Shading indicates approximate cost from yellow (F-class) through orange (M-class) to red (L-class or multiagency flagship missions). Hatched boxes indicate that such a combination is unfeasible, mostly due to excessive ∆v requirements. Past and planned missions are shown. From [1].

[1] Bockelée-Morvan, D. et al., AMBITION, Comet Nucleus Cryogenic Sample Return (2019), eprint, 2019arXiv190711081B

How to cite: Filacchione, G. and Bockelée-Morvan, D. and the AMBITION Team: AMBITION, the Comet Nucleus Cryogenic Sample Return mission for ESA Voyage 2050 program, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-25, https://doi.org/10.5194/epsc2020-25, 2020.

GAUSS (Genesis of Asteroids and EvolUtion of the Solar System) is a mission concept for the future exploration of Ceres. As both the largest resident of the main asteroid belt and the only dwarf planet in the inner Solar System, Ceres holds critical information for probing the evolution and habitability of our Solar System. NASA’s DAWN mission performed the by far most comprehensive investigation of Ceres during its over three year in-orbit operation around this unique world. Data collected by remote sensing instruments revealed an amazingly diverse landscape comprising different types of geological features. Beneath its volatile- and organic-rich surface, Ceres might have once possessed a global ocean, the remnants of which possibly still exist today as pockets of brine between the mantle and the crust. Hydrothermal activities that took place in recent geological time transferred materials deep inside Ceres to its surface, forming several outstanding surface features that are optimal for future sampling. Similar processes could occur on other ocean worlds in the Solar System, making Ceres a benchmark case for studying the evolution and habitability of these objects in general.

To fully understand the physical and chemical evolution of Ceres, high resolution analyses of samples are necessary. With cryogenic sample return as its final step, the GAUSS project aims to answer the following key questions:

• What is the origin of Ceres and the origin and transfer of water and other volatiles in the inner solar system?
• What are the physical properties and internal structure of Ceres? What do they tell us about the evolutionary and aqueous alteration history of icy dwarf planets?
• What are the astrobiological implications of Ceres? Was it habitable in the past and is it still today?
• What are the mineralogical connections between Ceres and our current collections of primitive meteorites?

How to cite: Shi, X. and the GAUSS Project Team: GAUSS: Towards Sample Return from Dwarf Planet Ceres, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-766, https://doi.org/10.5194/epsc2020-766, 2020.

Janus is a NASA SIMPLEx mission currently in Phase B. The SIMPLEx program is designed around the idea of using secondary launch opportunities to explore interplanetary destinations. The Janus mission concept plans to take advantage of the NASA Psyche launch to send two spacecraft to fly by Near Earth Objects of interest. A specific point design has been developed that sends two spacecraft to two binary asteroid systems, (175706) 1996 FG3 and (35107) 1991 VH, both of which have been observed repeatedly with photometry, spectrometry and radar. 

The Janus mission sends light-weight, low-cost spacecraft built by Lockheed Martin to encounter these high-science value small body targets. The science instruments are a visible and IR imager, from Malin Space Science Systems. The spacecraft will perform a rigorous remote sensing campaign when the object is a point source, and when resolved. The spacecraft will track the binary asteroid systems through closest approach, allowing for a combination of absolute surface resolution, relative resolution across the target asteroids and phase angle coverage unparalleled in previous asteroid flyby missions. 

Janus science will combine flyby observations of the target binary asteroids with ground-based observations, enabling the high resolution imaging and thermal data to be placed into a global context and leveraging all available data to construct an accurate topographical and morphological model of these bodies. Based on these measurements, the formation and evolutionary implications for small rubble pile asteroids will be studied. 

The science team members all have experience on asteroid missions or have made extensive ground based observations of NEAs. The industry team has extensive experience in the design, fabrication and operation of interplanetary spacecraft and instrumentation.

Acknowledgements: The Janus mission is supported by NASA under a contract from the SIMPLEx Program Office. Part of this research was conducted at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA.

How to cite: Scheeres, D., McMahon, J., Bierhaus, E. B., Wood, J., Benner, L., Hartzell, C., Hayne, P., Hopkins, J., Jedicke, R., Le Corre, L., Naidu, S., Pravec, P., and Ravine, M.: Janus: A NASA SIMPLEx mission to explore two NEO Binary Asteroids, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-930, https://doi.org/10.5194/epsc2020-930, 2020.

The past twenty years have seen an evolution in the definition and categorization of small bodies in the Solar System. While new types of bodies are being discovered at an increasing pace, objects familiar to us have been rediscovered with traits previously unknown, resulting in “hybrid” bodies like “Main-belt comets” or “active asteroids” [1]. New knowledges of small bodies are essential to further our understanding of the solar system as they directly shed light on planetary formation and evolution scenarios, the distribution and migration of water, and the emergence of life. To get a full picture of these small bodies, it is necessary to carry out detailed and comprehensive investigations, especially with dedicated space missions. As demonstrated by the success of a number of such missions recently completed and ongoing, a growing consensus is emerging that future missions should: 1) cover a diversity of targets, especially those never visited before; 2) characterize the structure and composition of the target body with highest possible resolution. The first Chinese small body mission is designed to take on both challenges by performing sample return from a quasi-satellite of the Earth—2016 HO3 and visiting for the first time a “main-belt comet”--133P/Elst-Pizarro.

In April 2019, CNSA released an open call of onboard opportunity for an asteroid exploration mission [2]，which encourages international cooperation. This asteroid exploration mission is characterized by multi-task, multi-target and multi-mode (e. g. joint exploration by multiple devices, landing and sampling etc.). On the basis of feasibility demonstration, design research and key techniques research, various work of the mission is currently in progress, such as the scientific research of small celestial particles, that is, combining remote sensing and surface in-situ measurement data and features of different scales (sub-millimeter to decimeter) to obtain clues of composition and evolution of small bodies.

[1] Hsieh, Henry H., David C. Jewitt, and Yanga R. Fernández. The Astronomical Journal 127(5):2997. (2004).

[2] http://www.cnsa.gov.cn/n6758823/n6758839/c6805886/part/6780392.pdf

How to cite: Huang, J., Zhang, X., Wang, T., Huo, Z., Shi, X., and Meng, L.: Small body exploration in China, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-1126, https://doi.org/10.5194/epsc2020-1126, 2020.