Europa Subsurface Studies: The Europa Clipper is a NASA mission to study Europa, the ice-covered moon of Jupiter characterized by a global sub-ice ocean overlying a silicate mantle, through a series of fly-by observations from a spacecraft in Jovian orbit. The science goal is to “explore Europa to investigate its habitability”. The Radar for Europa Assessment and Sounding: Ocean to Near-surface (REASON) is one of the primary instruments of the scientific payload. REASON is an active dual-frequency (9/60 MHz) instrument led by the University of Texas Institute for Geophysics (UTIG). It is designed to achieve multi-disciplinary measurements to investigate subsurface waters and the ice shell structure (Sounding), the surface elevation and tides (Altimetry), near-surface physical properties (Reflectometry), and the ionospheric environment including plume activity (Plasma/Particles). REASON will play a critical role in achieving the mission’s habitability driven science objectives, which include characterizing the distribution of any shallow subsurface water, searching for an ice-ocean interface and evaluating a broad spectrum of ice-ocean-atmosphere exchange hypotheses. 

Terrestrial Analogs: The development of successful measurement approaches and data interpretation techniques for exploring Europa and understanding its habitability will need to leverage knowledge of analogous terrestrial environments and processes. Towards this end, we are investigating, and considering for future investigations, a range of terrestrial radio glaciological analogs for hypothesized physical, chemical, and biological processes on Europa and present airborne data collected with the UTIG/University of Kansas dual-frequency radar system over a variety of terrestrial targets relevant to Europa’s potential exchange processes and habitability.  These targets include water filled fractures, brine rich ice, subglacial lakes, accreted marine ice, and ice roughness ranging from porous ice regolith (firn) to extensive crevasse fields. Our goal is to provide context for understanding and optimizing the observable signature of these processes in future radar data collected at Europa with implications for its habitability.

How to cite: Blankenship, D. D., Young, D. A., Chan, K., Wolfenbarger, N. S., Gerekos, C., and Steinbrügge, G. B.: Exploring Europa, Jupiter’s Ocean World: A View from Earth, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17429, https://doi.org/10.5194/egusphere-egu23-17429, 2023.

The exploration of ocean worlds in the outer Solar System, for example, the Jovian moon Europa and the Saturnian moon Enceladus, are of particular interest for the search for extraterrestrial life. Direct in situ exploration of moons harbouring significant amounts of liquid water beneath their ice surface poses many challenges and requires a sophisticated technological approach. The TRIPLE project (Technologies for Rapid Ice Penetration and Subglacial Lake Exploration) initiated by the German Space Agency at DLR forms a national consortium to work on robotic technologies for sub-ice exploration. The planned system consists of the fully autonomous, untethered miniature submersible robot, called nanoAUV, the IceCraft, a melting probe for penetrating the ice with the nanoAUV as payload, and an astrobiology in-situ laboratory, the AstroBioLab, to study fluid and sediment samples.

Beneath a several kilometre-thick ice-shell of the moons considered here, global oceans are well hidden and not easily accessible, posing extreme challenges for any robotic exploration as it is addressed in the TRIPLE project. Therefore, ice drilling and state-of-the-art technologies need to be developed to meet the manifold requirements. In view of future missions to icy moons, in TRIPLE, an analogue terrestrial demonstration is intended for first time exploration of a subglacial lake at the Dome-C region in Antarctica. The Dome-C mission requires a retrievable melting probe that can penetrate a 4-kilometre-thick layer of ice. It is essential for the mission that the melting probe is able to detect and avoid obstacles along its trajectory and to anchor itself at the ice-water interface for release and support of the nanoAUV into the water. The AstroBioLab concept provides an automated sample analysis laboratory for habitability investigations. It shall not only be able to detect various biosignatures in samples taken from the subglacial habitats, but shall also provide unequivocal evidence of life. For the field test in a terrestrial analogue setting, portable and robust devices using fast analysis methods are particularly suitable, which, as far as possible, should not require time-consuming sample preparation. In this contribution, we give an overview of the TRIPLE project and report on its current status.

How to cite: Kowalski, J., Boxberg, M. S., Grundmann, J. T., de Vera, J.-P. P., Heinen, D., and Funke, O. and the TRIPLE consortium: The TRIPLE project – Towards technology solutions for life detection missions, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17371, https://doi.org/10.5194/egusphere-egu23-17371, 2023.

Recently, it has become evident that icy moons in our solar system might constitute excellent targets for the search for life beyond Earth. Both Europa and Enceladus are of high interest for the detection of biosignatures, mainly due to the putative presence of all ingredients required to form life (as we know it), i.e., liquid water, an energy source, and the required chemical ingredients. If life is indeed present on these two bodies, molecular biosignatures may be preserved and protected from the radiative environment in the near surface ice. In situ instrumentation on board of a payload could perform compound identification and biosignature detection facilitating better limits of detection and more specific compound detection compared to spectroscopic measurements from orbit.

Several (groups of) compounds are listed as molecular biosignatures, including certain amino acids and lipids.¹ However, reliable in situ detection of molecular biosignatures is challenging. Not only does the instrumentation need to be flight-capable, it should also be sensitive enough to detect trace abundances, while simultaneously covering a high dynamic range, so as to not exclude highly abundant compounds. Additionally, instrumentation should preferably be capable of detecting many different classes of molecules and not be limited to a single compound or group of molecules.

ORIGIN (ORganics Information Gathering INstrument) is a space-prototype laser ablation ionisation mass spectrometer (LIMS) operated in desorption mode and designed for in situ detection of molecular biosignatures for space exploration missions. The simplistic and compact design make it a lightweight and robust system, which meets the requirements of space instrumentation. Currently, the setup consists of a nanosecond pulsed laser system and a miniature reflectron-type time-of-flight (RTOF) mass analyser (160 mm x Ø 60 mm). Biomolecules are desorbed and ionised by the laser pulse, after which the positive ions are separated based on their mass-to-charge ratio (TOF principle) by the mass analyser.

The molecular biosignature detection capabilities of ORIGIN have been recently demonstrated for amino acids, polycyclic hydrocarbons, and lipids ^2–4. In this contribution, our envisioned concept of going from obtained ice samples to the detection of molecular biosignatures using LIMS will be discussed. In addition, we will show results of lipid biosignature detection using ORIGIN, covering sensitivity and dynamic range², implying the future applicability for the detection of life on Icy Moons. Additionally, future projects of analogue ice studies with the ORIGIN space-prototype will be covered.

1. Hand, K. P. et al. Report of the Europa Lander Science Definition Team. (Jet Propulsion Laboratory, 2017).

2. Boeren, N. J. et al. Detecting Lipids on Planetary Surfaces with Laser Desorption Ionization Mass Spectrometry. Planet. Sci. J. 3, 241 (2022).

3. Kipfer, K. A. et al. Toward Detecting Polycyclic Aromatic Hydrocarbons on Planetary Objects with ORIGIN. Planet. Sci. J. 3, 43 (2022).

4. Ligterink, N. F. W. et al. ORIGIN: a novel and compact Laser Desorption – Mass Spectrometry system for sensitive in situ detection of amino acids on extraterrestrial surfaces. Sci. Rep. 10, 9641 (2020).

How to cite: Boeren, N. J., Keresztes Schmidt, P., de Koning, C. P., Kipfer, K. A., Ligterink, N. F. W., Tulej, M., Wurz, P., and Riedo, A.: Molecular Biosignature Detection on Ocean Worlds using a Prototype Laser-Desorption Ionisation Mass Spectrometer, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14516, https://doi.org/10.5194/egusphere-egu23-14516, 2023.

The production of knowledge on how planetary worlds work is still mainly driven by remote observations that offer fragmented insights at the surface processes at meters scales and a hollowed vision on the near-surface structure down to few decameters deep. The latter statement also holds for remote polar environments on Earth where in-situ investigations do not necessarily sample exhaustively the vast extents of the cryosphere.

Yet, those superficial portions of planetary bodies hold signatures of outstanding processes related to the regional depositional and erosional history. They also host structures relevant to future in-situ exploration such as surface roughness and porosity for landing site reconnaissance, snow deposits, buried ice lenses and putative accessible aquifers.

Because of its meter-scale wavelengths, the surface echo strength recorded by air- and space-born radar sounders convolves many information on the (near-)surface structure and composition. The Radar Statistical Reconnaissance (RSR) is a technique developed over the last decade to disentangle those signatures, essentially extending the capability of a nadir radar sounder to be used as both a surface reflectometer and scatterometer. We review some recent application strategies of the RSR in the Terrestrial cryosphere and in the solar system. Future advancements and targets will also be presented to highlight the interplanetary development and challenges of this technique.

How to cite: Grima, C.: Deciphering the (Near-)Surface of Planets with Nadir-pointing Radar Statistics, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2134, https://doi.org/10.5194/egusphere-egu23-2134, 2023.

Between 2006 and 2016, the Cassini spacecraft carried out 13 bistatic radar observations of the surface of Saturn's largest moon, Titan. Unmodulated right circularly polarized radio signals were transmitted by the spacecraft to the moon’s surface. Cassini’s high gain antenna was pointed so that specular reflections from Titan’s surface were received on Earth. Proper processing of right (RCP) and left circularly polarized (LCP) echoes from the moon can provide information about surface roughness and near-surface relative dielectric constant (ɛ_r) of the illuminated terrains.

During Titan flybys T101, T102, T106, and T124, the track of the bistatic observations crossed the main stable liquid bodies of the north pole of Titan: Ligeia, Kraken, Punga Mare, and their estuaries. Strong and narrowband X-band (λ=3.6 cm) echoes were successfully detected from the seas at the Deep Space Network 70-meter station in Canberra.

Reflected spectra feature Dirac-like shapes, with a spectral broadening around 1 Hz and lower bounded by the processing time resolution. Compared to bistatic observations of other planets, this implies unprecedentedly low RMS slope values for Titan’s seas on an effective length-scale of a few meters. Profiles of reflected LCP and RCP power are in general consistent with purely coherent reflections from the Fresnel area around the moving specular point, indicating a very flat surface.

In addition, from the circular polarization power ratio, the surface dielectric constant can be derived. This can enrich our current understanding of the chemistry of Titan’s liquid hydrocarbon seas, further constraining their methane-ethane mixing ratio. From Cassini RADAR, VIMS, and ISS, Titan’s seas are expected to be ternary mixtures of methane, ethane and nitrogen (ɛ_r ≈ 1.6-1.9). From bistatic radar data, significant relative variations in liquid hydrocarbon composition are seen, and an unexpected correlation between the dielectric constant and incidence angle of observation seems to arise. The absolute values of permittivity are somewhat lower than expected.

From the computed dielectric constant values, physical optics models are used to constrain the RMS height of the surface. This analysis provides meaningful insights into the presence of small capillary waves in the order of millimeters over the liquid surfaces of Titan, as already detected by Cassini monostatic RADAR.

How to cite: Brighi, G., Poggiali, V., Tortora, P., Zannoni, M., Hayes, A., Lalich, D., Bonnefoy, L., MacKenzie, S., D Nicholson, P., Oudrhiri, K., D Lorenz, R., and M Soderblom, J.: Cassini Bistatic Radar Observations of Titan's Seas: Results about Dielectric Properties and Capillary Waves Detection, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10075, https://doi.org/10.5194/egusphere-egu23-10075, 2023.

Orbital Synthetic Aperture Radar (SAR) interferometry (InSAR) and tomography (TomoSAR) are key techniques for the exploration of terrestrial ice sheets that are used operationally. However, in the context of planetary exploration, these approaches are rather exotic and have not been used yet. In the frame of DLR’s Enceladus Explorer (EnEx) initiative, we propose a multi-modal, multi-frequency orbital radar mission, operating -among others- in a SAR interferometric and tomographic mode capable of delivering high-accuracy and high-resolution topography, tidal deformation, and composition measurements as well as 3-D metric-resolution imaging of the ice crust along tens of kilometers wide swaths. The ice penetration capability of radar signals allows for the exploration of both surface and subsurface features down to hundreds of meters, depending on the used carrier frequency.

Multiple SAR acquisitions of the same area are needed to form interferometric and tomographic products. These acquisitions are collected successively following a repeat-pass concept using so-called periodic orbits with repeating trajectories. For the available observation geometries, the baselines between the repeat trajectories need to lie within a few hundreds of meters (i.e., the radar needs to fly within a tube of hundreds of meters). Unfortunately, the low Enceladus mass and its proximity to Saturn commonly lead to instabilities for highly inclined science orbits. We find that published orbit solutions do not exhibit sufficient stability for providing the necessary repeat passes. However, through a grid-search approach in a high-fidelity gravitational model, we identified highly stable periodic orbits that sustain the required repeat characteristic up to hundreds of days. The short repeat periods in the order of 1 to 4 days allow for a fast acquisition of InSAR observations and the formation of tomographic stacks within several days.

Based on a representative system, we present global performance simulations for both InSAR and TomoSAR products with a focus on the prominent south polar plume region of Enceladus. The performance of these products depends on several factors, including the system being used, the orbital geometry, the accuracy of the guidance, navigation, and control (GNC), the accuracy of the orbit determination, and the structure and composition of the ice crust, which affects the backscatter characteristics and potential decorrelation effects in the SAR acquisitions. We use an End-to-End (E2E) simulator developed at DLR for generating realistic SAR, InSAR, and TomoSAR products. The E2E is capable of accommodating the designed orbits, the Enceladus topography, deformation models, representative backscatter maps, and decorrelation effects, as well as any relevant instrument, baseline, and attitude errors.

How to cite: Benedikter, A., Rodriguez-Cassola, M., Krieger, G., Hussmann, H., Stark, A., Wickhusen, K., Stelzig, M., and Vossiek, M.: Radar Interferometry and Tomography for the Exploration of Enceladus’ Surface and Subsurface, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17280, https://doi.org/10.5194/egusphere-egu23-17280, 2023.

In this work the radiation environment around Ganymede is investigated. We apply a single-particle Monte Carlo model to obtain 3-D distribution maps of the H⁺, O⁺⁺, and S⁺⁺⁺ populations at the altitude of ~500 km and to deduce surface precipitation maps. We perform these simulations for three distinct configurations between Ganymede’s magnetic field and Jupiter’s plasma sheet (JPS), characterized by magnetic and electric field conditions similar to those during the NASA Galileo G2, G8, and G28 flybys (i.e., when the moon was above, inside, and below the centre of Jupiter’s plasma sheet). Our results provide a reference frame for future studies of planetary space weather phenomena in the near-Ganymede region and surface evolution mechanisms. For ions with energies up to some tens of iloelectronvolts, we find an increased and spatially extended flow in the anti-Jupiter low-latitude and equatorial regions above Ganymede’s leading hemisphere. Our results also show that the ion flux incident at 500 km altitude is not a good approximation of the surface’s precipitating flux. To study, therefore, Ganymede’s surface erosion processes it may be best to consider also low-altitude orbits as part of future space missions. This study is relevant to the ESA JUpiter ICy moons Explorer mission, which will allow a detailed investigation of the Ganymede environment and its implications on the moon’s surface evolution.

How to cite: Plainaki, C., Massetti, S., Jia, X., Mura, A., Roussos, E., Milillo, A., and Grassi, D.: The energetic ion environment around Ganymede to be investigated with JUICE, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4509, https://doi.org/10.5194/egusphere-egu23-4509, 2023.

Polygonal impact craters (PIC) are impact craters that have at least one straight rim segment in planform [1-8]. Among all impact craters, PICs represent a small percentage. They exist on both rocky and icy planetary bodies [9]. To our knowledge no studies on PICs have been carried out for Ganymede. Here we are examining the straight segments of PICs and their relationship with adjacent lineaments or fractures. We use the global mosaic prepared by [10], which combines the best high-resolution images from Voyager 1, Voyager 2, Galileo and Juno spacecrafts. Despite the resolution limits and different illumination angles, we identified and mapped 459 PICs across Ganymede whose diameter range from 5 km to 153 km.  PICs, which were superimposed by other craters or terrains are not considered for this study. The number of straight segments possessed by PICs ranges from 1 to 9 with quadrangular, hexagonal and octagonal shapes being most common. Most of these PICs exhibit a central peak or a pit, with a minor fraction of them showing a dome. Straight rim segments of PICs align with the linear features adjacent to them and indicate that such lineaments are not exclusively surface features but lead to a localization of deformation and influence the cratering process. Straight rim segments of PICs in the dark cratered terrain (dc) are oriented along fractures and furrows. For instance, Galileo Regio have many PICs because of the NW-SE trending furrows and a high density of faults and fractures.  Here, most of the PICs have hexagonal shape with two of the straight segments parallel to the orientation of furrows and rest of the segments are at approximately perpendicular angle. Also, the presence of PICs suggests that they formed after formation of the linear features. The majority of linear features on anti-Jovian hemisphere trends in NW-SE direction while the preferred orientation of linear features on sub-Jovian hemisphere is in NE-SW direction [11]. However, the preliminary orientation analysis of straight segments of PICs using rose diagrams does not show a preferred orientation for the anti-Jovian and sub-Jovian hemispheres.

REFERENCES: [1] Fielder, G. (1961) PSS. 8(1), 1-8. [2] Kopal, Z. (2013) Springer. [3] Shoemaker, E.M. (1962) Physics and Astronomy of the Moon, Academic Press, New York, pp. 283-359. [4] Roddy, D.J. (1978) Lunar Planet. Sci. Conf. Proc. 9, 3891-3930. [5] Öhman et al. (2005) Impact Tectonics. Springer, Berlin, pp. 131–160. [6] Öhman et al. (2008) Meteorit. Planet. Sci. 43, 1605–1628. [7] Beddingfield et al. (2016) Icarus 274, 163-194. [8] Beddingfield and Cartwright (2020) Icarus 343, 113687. [9] Öhman et al. (2010) Geolog. Soc. Am. Special Papers 465, 51–65. [10] Kersten et al. (2022) pp. EPSC2022-450. [11] Rossi et al (2020) Journal of Maps, 16(2), 6-16.

How to cite: Baby, N. R., Kenkmann, T., Stephan, K., and Wagner, R. J.: Polygonal impact craters on Ganymede, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-354, https://doi.org/10.5194/egusphere-egu23-354, 2023.