Poster presentations and abstracts

The Mercurian Gamma-ray and Neutron Spectrometer (MGNS) is a scientific instrument developed to study the elementary composition of the Mercury’s sub-surface by measurements of neutron and gamma-ray emission of the planet. MGNS measures neutron fluxes in a wide energy range from thermal energy up to 10 MeV and gamma-rays in the energy range of 300 keV up to 10 MeV with the energy resolution of 5% FWHM at 662 keV and of 2% at 8 MeV. The innovative crystal of CeBr3 is used for getting such a good energy resolution for a scintillation detector of gamma-rays.

During the BC long cruise to Mercury, it is planned that the MGNS instrument will operate practically continuously to perform measurements of neutrons and gamma-ray fluxes for achieving two main goals of investigations.

The first goal is monitoring of the local radiation background of the prompt spacecraft emission due to bombardment by energetic particles of Galactic Cosmic Rays. This data will be taken into account at the mapping phase of the mission on the orbit around Mercury. Detailed knowledge of the spacecraft background radiation during the cruise will help to derive the data for neutron and gamma-ray emission of the planet at the mapping stage of the mission because many elements, like Mg, Na, O and others, the abundance of which at the uppermost layer of the planet is studied, are also present in the material of the spacecraft. Indeed, the nuclear lines of Al, Mg and O are well-pronounced in the spectrum, which are also expected to be detectable in the gamma-ray spectrum of the Mercury emission.

The second goal of MGNS cruise operations is the participation in the Inter Planetary Network (IPN) program for the localization of sources of Gamma-Ray Bursts in the sky. In fact, the localization accuracy by the interplanetary triangulation technique is inversely proportional to the distance between the spacecrafts that jointly detected a GRB. Before the launch of BepiColombo, the IPN network included a group of spacecrafts in the near-to-Earth orbit (e.g. Konus-Wind, Fermi-GBM, INTEGRAL, Insight-HXMT) and the Mars Odyssey spacecraft on the orbit around Mars. Now, MGNS provides another interplanetary location, potentially increasing the accuracy of GRBs localization. During the first 13 months of continuous operation, MGNS detected 24 GRB's. Pre-set value of time resolution for continuous measurements of profiles of GRBs is 20 seconds. Since of November 14, 2019, the BC Mission Operation Centre has allocated downlink resources to run MGNS continuously in a 1 sec time resolution for GRB measurements. The GRB detection rate, based on data with a time resolution of 1 sec is about 2-3 GRB's per month.

Gamma-rays of solar flares are also detectable by MGNS. Solar flares are nonstationary and anisotropic processes and the ability to observe them from different directions in the Solar system is crucial for further understanding their developments and propagation, as it was demonstrated in the case of HEND instrument on board Mars Odyssey. The Sun cycle is presently around its minimum, and MGNS has not detected any solar events during its first 7 months of the cruise, but the flight to Mercury is long enough and many future flares are expected to be detected.

The MGNS instrument will also perform special sessions of measurements during flybys of Earth, Venus and Mercury with the objective to measure neutron and gamma-ray albedo of the upper atmosphere of Earth and Venus and of the surface of Mercury. Another objective is to test the computational model of the local background of the spacecraft using the data measured at different orbital phases of flyby trajectories. The low altitude flybys (such as the 700 km flyby for Venus and three 200 km flybys for Mercury) would be the most useful for such tests being BC maximally shadowed for cosmic radiation by the actual planet. Neutron and gamma-ray measurements during Earth flybys enable investigation of interaction between solar wind and Earth environments as well as studies of spacecraft neutron and gamma-ray background upon its passage through the Earth's radiation belts.

How to cite: Kozyrev, A., Litvak, M., Sanin, A., Malakhov, A., Mitrofanov, I., Owens, A., Schulz, R., and Quarati, F.: MGNS experiment science investigations during BepiColombo cruise, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-389, https://doi.org/10.5194/epsc2020-389, 2020.

Planetary Space Weather is the discipline that studies the state of the Sun and how it interacts with the interplanetary and planetary environments. It is driven by the Sun’s activity, particularly through large eruptions of plasma (known as coronal mass ejections, CMEs), solar wind stream interaction regions (SIR) formed by the interaction of high-speed solar wind streams with the preceding slower solar wind, and bursts of solar energetic particles (SEPs) that form radiation storms. This is an emerging topic, whose real-time forecast is very challenging because among other factors, it needs a continuous coverage of its variability within the whole heliosphere as well as of the Sun’s activity to improve forecasting.
The long cruise of BepiColombo constitutes an exceptional opportunity for studying the Space Weather evolution within half-astronomical unit (AU), as well as in certain parts of its journey, can be used as an upstream solar wind monitor for Venus, Mars and even the outer planets. This work will present preliminary results of the Space Weather conditions encountered by BepiColombo since its launch until mid-2020, which includes data from the solar minimum of activity and few slow solar wind structures. Data come from three of its instruments that are operational for most of the cruise phase, i.e., the BepiColombo Radiation Monitor (BERM), the Mercury Planetary Orbiter Magnetometer (MPO-MAG), and the Solar Intensity X-ray and particle Spectrometer (SIXS). Modelling support for the data observations will be also presented with the so-called solar wind ENLIL simulations.

How to cite: Sanchez-Cano, B., Moissl, R., Heyner, D., Huovelin, J., Mays, M. L., Odstrcil, D., Lester, M., Bunce, E. J., James, M. K., Witasse, O., and Benkhoff, J.: Preliminary results of Space Weather conditions encountered by BepiColombo during the first phase of its cruise, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-689, https://doi.org/10.5194/epsc2020-689, 2020.

BepiColombo (ESA/JAXA), Solar Orbiter (ESA/NASA) and Parker Solar Probe (NASA), will be all traveling in the inner heliosphere for 5 years, between the launch of Solar Orbiter (10th February 2020) and the end of the cruise phase of BepiColombo (2018 - 2025). During the five years to come, these three spacecraft missions will cover different radial distances: BepiColombo will evolve between the orbit of Mercury up to Earth (0.3 AU and 0.9 AU), while Solar Orbiter’s highly elliptical orbit will cover distances from 1.02 AU to 0.28 AU, and Parker Solar Probe from about 0.7 AU to 0.04 AU. In addition to these missions, previous and future spacecraft missions will be taking measurements in our solar system (from Venus ~ 0.7 AU till Jupiter 5 AU). The exceptional and complementary plasma instrumental payloads and magnetometers on-board the different satellites will allow us to make unique multi-point measurements in the solar wind. Hence, in this work, we present the potential coordinated observations between at least BepiColombo, Solar Orbiter and Parker Solar Probe (including also other satellites such as probes at L1, or around Venus, Mars and Jupiter). More specifically, in the first part we focus on the different scientific topic that could be studied during the cruise phase of BepiColombo, in the second part we present the related potential operational instruments and in the last part we discuss the different windows of opportunities we have investigated based on different spacecraft geometries.

How to cite: Hadid, L. and the BepiColombo coordinated observations working group: Coordinated measurements during the Cruise phase of BepiColombo: science cases, operational instruments and windows of opportunities, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-633, https://doi.org/10.5194/epsc2020-633, 2020.

Abstract:

Our tectonic mapping as part of a larger morphostratigraphic mapping effort of the H13, Neruda Quadrangle (1) has led us to recognise the “Neruda-Paramour” thrust system. The system appears to extend from the southern limits of H13 north through H09 and H08 and simplifies at its northern end into Paramour Rupes (Figure 1). Mercury’s tectonic evolution is dominated by global scale contractional structures of which the Neruda-Paramour system is part of (2). These structures are believed to have formed either by secular cooling of the planet (3), tidal despinning (4), mantle overturn (5), true polar wander (6) or a combination of these processes. Regardless of the process, Mercury’s surface exhibits abundant evidence of global contraction in the form of shortening structures such as lobate scarps, high relief ridges and wrinkles ridges (7,8). These features are widely accepted as the surface expressions of thrust faulting and folding, produced by lithospheric horizontal compression. Often, these contractional features comprise major thrust systems as linked segments with a consistent trend (8). In order to understand the development of the Neruda-Paramour system and to ascertain if there is any sequence of movements, we are first mapping the system in its entirety followed by age dating of each thrust segment’s last movement using the buffered crater counting (BCC) technique.

Methods:

Tectonic lineament mapping

Primary basemap: Global ~166 mpp v1.0 BDR tiles with moderate (~74°) solar incidence angles.

Secondary basemaps: low (~45°) and high (~78°) incidence angle basemaps, ~665mpp enhanced colour mosaic; MLA- and stereo-derived DEMs.

Scale: 1:3M scale with digitisation at 1:300k.

Software: Esri ArcGIS 10.5.1 GIS software.

Buffered crater counting

Software: Esri ArcGIS 10.5.1 GIS software with CraterTools extension (9). CraterStats 2.0 (10).

We use the BCC technique by (8,11) which counts craters that are unfaulted and undeformed by the structures under investigation to derive absolute model ages of linear landforms. The BCC technique uses a buffer zone around the linear feature to include more craters to be taken into consideration for the crater size frequency distribution (CSFD) count. This addition of included craters produces a more robust measurement, which is important due to the restricted surface area that the structures occupy (12). We have chosen to consider all craters that directly intersect the structures ≥2 km. A fault buffer width of 2R (R=radius of the crater) for buffer generation is used. CSFD data is exported to CraterStats 2.0 where it is plotted in a log N_cum (cumulative crater frequency) vs log D  (diameter) and using the production function (PF) a best fit of the CSFD data is made, giving an absolute model age value. The Neukum Production Function and Le Feuvre and Wieczorek Production Function are used and the results from each are compared. Method after (8).

Results:

We will present our analysis of the mutual age relationships between elements of the Neruda-Paramour thrust system and discuss the implications for the tectonics of this part of the globe.

Acknowledgements:

Mapping is in association with Planmap, funded through the EU Horizon 2020 research and innovation programme under grant agreement No. 776276. Ben Man is supported by STFC and the Open University’s Space Strategic Research Area.

References:

How to cite: Man, B., Rothery, D. A., Balme, M. R., Conway, S. J., and Wright, J.: Investigating the Neruda-Paramour thrust system, Mercury, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-794, https://doi.org/10.5194/epsc2020-794, 2020.

Introduction

Boulders, meters-size blocks of rock, are seen in great numbers in high-resolution images of the Moon, Mars, and small bodies. Boulders and their geological associations provide information about surface processes on these bodies. Here we report on rarity of boulders on Mercury, and discuss possible implications of this.

Rarity of boulders on Mercury

We studied boulders on Mercury with high-resolution images acquired by MDIS NAC camera onboard MESSENGER. We screened all ~3000 images of the highest resolution (<2.5 m/pix sampling) and acceptable quality, and found discernable boulders only in 14 of them (see examples in Fig. 1,2) [1]. The highest resolution images are small (0.25 Mpix), have a considerable smear, low signal-to-noise ratio, and are separated by large distances; they can be considered as random samples of surface morphology in a region limited by 40 – 70°N and 210 – 320°E, which is occupied mostly by the intercrater plains. The majority of the found boulders are associated with a single large young impact crater (Fig. 1), and a few boulders are associated with small impact craters (Fig. 2). With the available images, it is impossible to distinguish between intact boulders and debris piles of disintegrated or partly disintegrated boulders, therefore such (partly) disintegrated boulders are included in our boulder count.

To compare Mercury with the Moon, we extracted similar random 0.25 Mpix samples from LROC NAC images that have the same sampling, the same solar illumination incidence angle and are situated in highlands. We artificially degraded image quality to mimic MDIS NAC quality. Then we screened these image samples searching for boulders and found them at ~15% of the random sample images. Thus, boulders on Mercury are extremely rare in comparison to the Moon (~ 30x less abundant).

Possible causes

Boulder population on the Moon is dynamic: the characteristic boulder lifetime is ~100 Ma [2], and the observed population represents equilibrium between boulder formation and obliteration. The same obviously occurs on Mercury. Therefore, much sparce boulder population can occur due to a lower production rate, a shorter lifetime, or both.

Production rate. Boulders on the Moon occur in association with (1) fresh impact craters of different sizes and (2) hilltops, upper edges of rilles, and other convex relief forms. Respectively, there are two mechanisms of boulder formation: (1) bedrock fragmentation and excavation by impacts, and (2) progressive exposure of pre-existing blocks and fractured bedrock by removal of regolith layer from convex relief by diffusive creep. On Mercury we observe boulders in the first type of settings (craters) only. The intercrater plains on Mercury are significantly smoother than the lunar highlands [3], therefore suitable hilltops may be less abundant and might not be sampled in our limited image set. Thus, the general smoothness of Mercury in comparison to the Moon [3] may contribute to a lower boulder production rate.

Regolith on Mercury is thought to be thicker than on the Moon [3,4]. The smallest craters are forming in regolith and cannot excavate boulders. Due to the thicker regolith, the onset crater size for boulder formation is larger. Since the crater formation rate on Mercury is on the same order of magnitude, the larger onset crater size means a lower rate of boulder-forming impacts, and therefore, a lower boulder formation rate. However, these two factors seem insufficient to explain the huge difference in boulder occurrence between the Moon and Mercury.

Lifetime. Boulders are destroyed by small meteoritic impacts, ground by micrometeoritic impacts [2], and cracked and disintegrated by thermal stresses [5,6]. The relative role of these processes on the Moon is controversial. Since meteoritic flux is on the same order of magnitude on both bodies, it cannot account for the observed huge difference.

Peak daytime temperature at a flat monolith bedrock surface induces tensile stress equal to E α ΔT/(1-ν), where E is the Young’s modulus (~60 GPa for basalts [7]), ν is Poisson ratio (~0.25), α is linear thermal expansion coefficient (~10^-5 K^-1), and ΔT is the peak temperature excess above the mean temperature. For Mercury, this stress would regularly exceed 100 MPa. This value gives an order of magnitude estimate for peak tensile stress experienced by a boulder, if it is larger or comparable to the diurnal thermal skin depth for solid rock, which is ~3 m for the long day on Mercury. For smaller boulders the peak stress is lower; detailed modeling has been done in [5]. The boulders we observe are larger than 3 m, therefore, the peak stress is on the order of 100 MPa, much higher than a typical tensile strength of intact basalts (~14 MPa [7]). Therefore, all boulders acquire boulder-scale fractures at their first exposure at the surface. The difference Δα between thermal expansion coefficients of rock-forming mineral grains leads to thermal stress at grain scale [6]. They are scaled as C E Δα ΔT, where C depending on ν and grain geometry is on the order of unity. These stresses would also routinely exceed tensile strength; they would lead to formation of microfractures at grain scale. Thus, thermal stresses cause extensive fracturing of rocks on Mercury, which likely contributes to the short boulder life time.

The micrometeoritic flux on Mercury is significantly higher than on the Moon [8,9]. If micrometeoritic grinding is the dominant boulder obliteration mechanism on the Moon, then the difference in the micrometeoritic flux is likely to be sufficient to account for the entire observed difference in boulder abundance.

References

[1] Zharkova A. et al. (2019) LPSC 50, 1162.

[2] Basilevsky A.T. et al. (2013) PSS 89, 118-126.

[3] Kreslavsky M. et al. (2014) GRL 41, 8245-8251.

[4] Zharkova A. et al. (2015) AGU Fall, P53A-2099.

[5] Molaro J. et al. (2017) Icarus 294, 247-261.

[6] Molaro J. et al. (2015) JGR 120, 255-277.

[7] Schultz R. (1993) JGR 98, 10883-10895.

[8] Cintala M. (1992) JGR 97, 947.

[9] Borin P. et al. (2009) Astron. Astrophys. 503, 259.

How to cite: Kreslavsky, M. A., Zharkova, A., and Gritsevich, M.: Rarity of Boulders on Mercury: Possible Causes, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-958, https://doi.org/10.5194/epsc2020-958, 2020.

The optical constant of the material, meaning the complex refractive index m=n+i k, is an essential parameter when considering the reflection and absorption properties of that material. The refractive index is a function of wavelength of the light, and usually the imaginary part k is what governs the reflection or transmission spectral behavior of the material.

The knowledge of the complex refractive index as a function of wavelength, m(λ), is needed for light scattering simulations. On the other hand, rigorous scattering simulations can be used to invert the refractive index from measured or observed reflection spectra. We will show how the combination of geometric optics and radiative transfer codes can be used in this task.

In this work, the possible application is with the future visual-near infrared observations of Mercury by the ESA BepiColombo mission. That application in mind, we have used four particulate igneous glassy materials with varying overall albedo and in several size fractions in reflectance spectra measurements (hawaiitic basalt, two gabbronorites, anorthosite, see details from Carli et al, Icarus 266, 2016). The grounded material consist of particle with clear edges and quite flat facets, and we choose to model the particle shapes by geometries resulting from Voronoi division of random seed points in 3D space (see Fig. 1).