Europlanet Science Congress 2022
Palacio de Congresos de Granada, Spain
18 – 23 September 2022
Europlanet Science Congress 2022
Palacio de Congresos de Granada, Spain
18 September – 23 September 2022

TP5

Mars Science and Exploration

The aim of this session is to share the knowledge and experience gained by all Mars exploration programmes, both in Europe and worldwide, to promote synergies among the various missions in operations and development.

We welcome contributions from any field of Mars science and exploration, in particular mission and instrument overviews of latest scientific results and technical developments. These include latest discoveries and new challenges for all operational orbiters (Mars Express, ExoMars TGO, Odyssey, MRO, MAVEN, Mangalyaan/MOM, Tianwen-1, Hope), surface assets (MSL, Insight, Mars2020, Tianwen-1), and preparations for the new missions (ExoMars RSP, MMX, Mars Sample Return, etc).

Co-organized by MITM
Convener: Alejandro Cardesin-Moinelo | Co-conveners: Lucie Riu, Eleni Bohaceck, Elliot Sefton-Nash, Colin Wilson, Csilla Orgel
Orals
| Thu, 22 Sep, 10:00–11:30 (CEST), 17:30–18:30 (CEST)|Room Manuel de Falla, Fri, 23 Sep, 10:00–13:20 (CEST)|Room Machado
Posters
| Attendance Thu, 22 Sep, 18:45–20:15 (CEST)|Poster area Level 1

Orals: Thu, 22 Sep | Room Manuel de Falla

Chairpersons: Eleni Bohaceck, Elliot Sefton-Nash
Mars Surface and Subsurface
10:00–10:10
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EPSC2022-1024
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MI
Elena Pettinelli, Sebastian Lauro, Elisabetta Mattei, Barbara Cosciotti, David Stillman, Graziella Caprarelli, and Roberto Orosei

Introduction

Bright basal reflections detected by MARSIS at Ultimi Scopuli (Orosei et al., 2018; Lauro et al., 2021) started a wide scientific debate on the possible materials capable to generate such strong echoes from the base of the South Polar Layers Deposits (SPLD). Different interpretations were suggested: some involving the presence of briny water at the base of the SPLD (Orosei et al., 2018; Lauro et al., 2021; Mattei et al., 2022; Stillman et al., 2022) and others the existence of conductive materials, like saline ice and hydrated clays (Bierson et al., 2021; Smith et al., 2021) or ilmenite-rich basaltic rocks (Grima et al., 2022).

The original study (Orosei et al., 2018) was based on an inversion approach of MARSIS data (Lauro et al., 2019) from which the basal permittivities were retrieved. Such permittivity values are estimated from the amplitude of the reflected signal (Orosei et al., 2018), which does not allow to separately compute real and imaginary parts of the complex permittivity but only the apparent permittivity (ea) (Mattei et al., 2022). This is a real single quantity (to not be confused with the real part of permittivity, e’) that accounts for both polarization and conductive processes and fully describes the dielectric property of a material. In other words, the apparent permittivity is the physical quantity associated to a material lying below the SPLD that MARSIS measure.

The analysis of MARSIS data at Ultimi Scopuli defined the presence of two distinct distributions of apparent permittivity values.  A distribution with high values, inside the so-called bright area, which were interpreted as evidence of basal salty liquid water and a distribution with low values typical of dry rocks/soil, outside the bright area (Orosei et al., 2018). The presence of other wet areas was subsequently confirmed applying a different analysis based on a signal processing approach commonly used in terrestrial Radar Echo Sounding (RES) studies to discriminate between wet and dry subglacial basal conditions (Lauro et al., 2021). Moreover, other indirect evidence supports the existence of liquid water below the ice at Ultimi Scopuli (Carrer and Bruzzone, 2021).

 

Results and discussions

The main argument against the possible presence of basal briny water is the very low temperature inferred from thermal models at the base of the SPLD (~180K), which was believed to require a large amount of salt to maintain the water in a liquid state (e.g., Sori and Bramson, 2019). Based on laboratory measurements, however, recent papers have discarded such requirement showing that few hundreds of mM of perchlorate salts are capable to maintain the water liquid at temperature lower than 200K (Mattei et al., 2022; Stillman et al., 2021; Stillman et al., 2022). Moreover, neither dielectric theory nor extensive experimental data support the hypothesis that saline ices or hydrated salts and clays can produce the bright basal reflections detected by MARSIS at the base of the SPLD (Mattei et al., 2022; Stillman et al., 2022). On the other hand, the largest amount of ilmenite content detected so far on Mars is £5% (e.g., Morris et al., 2006) which is largely insufficient to create strong radar basal reflections (Hansen et al., 1973). Another puzzling aspect in this controversy, is the presence of other bright areas detected by MARSIS below the South polar cap, sometime where the ice is thinner than 1.5 km (Khuller and Plaut, 2021). It should be notice, however, that the data analyzed by Khuller and Plaut (2021) are not the same (on-board standard mode) as those used in Orosei et al. (2018) and Lauro et al. (2021) (super frame and flash memory mode).

We present here the results of a large literature review on the dielectric properties of the materials suggested to be present at the base of the SPLD, as a function of temperature and composition. For these materials we computed the apparent permittivity which we compared to the apparent permittivity values retrieved by MARSIS (Fig.1). The results are discussed in the framework of the thermal state at the base of the SPLD and show that only perchlorates solutions can generate the basal bright reflections detected by MARSIS at Ultimi Scopuli.

Fig.1 Box plot of the apparent permittivity. The plot indicates the basal apparent permittivity retrieved inside the main bright area (blue) and outside the bright areas (red). Color bars indicate a range of apparent permittivity values for several lithologies potentially present at the base of the SPLD, measured mostly at MARSIS frequencies and 200 K.

References

Bierson, C. et al.  Geophysical Research Letters, 48(13), doi.org/10.1029/2021GL093880, (2021). 

Carrer L. and L. Bruzzone,  IEEE Transactions on Geoscience and Remote Sensing, vol. 60, pp. 1-15, 2022, Art no. 4600915, doi: 10.1109/TGRS.2021.3111814.

Grima, C.,  et al. (2022). Geophysical Research Letters, 49(2), e2021GL096518, doi.org/10.1029/2021GL096518.  

Khuller, A. R., & Plaut, J. J. (2021). Geophysical Research Letters, 48(13), e2021GL093631, doi.org/10.1029/2021GL093631 

Lauro, S. E., et al. (2019). Remote Sensing, 11(20), 2445. Remote Sens. 2019, 11(20), 2445, doi.org/10.3390/rs11202445.

Lauro, S.E., et al.

Mattei, E., et al. (2022). Earth and Planetary Science Letters, 579, 117370, doi.org/10.1016/j.epsl.2022.117370.

Morris, R. V., et al. (2006). Journal of Geophysical Research: Planets, 111(E2).

Orosei, R., et al. (2018). Science, 361(6401), 490-493, doi: 10.1126/science.aar7268.

Smith, I. B., et al. (2021).

Sori, M. M., & Bramson, A. M. (2019). Geophysical Research Letters, 46(3), 1222-1231, doi.org/10.1029/2018GL080985.

Stillman, D. E., et al. (2022). LPI Contributions, 2678, 2133.

Stillman, D. E., et al.(2021).LPI Contributions, 2614, 6028.

How to cite: Pettinelli, E., Lauro, S., Mattei, E., Cosciotti, B., Stillman, D., Caprarelli, G., and Orosei, R.: The search for liquid water below the South Polar Layer Deposits: where we stand?, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-1024, 2022.

10:10–10:20
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EPSC2022-1052
Sebastian Lauro, Elena Pettinelli, Graziella Caprarelli, Jamaledin Baniamerian, Elisabetta Mattei, Barbara Cosciotti, David Stillman, Katie Primm, Francesco Soldovieri, and Roberto Orosei

Introduction 

The presence of liquid water at the base of the Southern polar cap of Mars has been inferred from exceptionally strong radar echoes detected by MARSIS aboard the MEX spacecraft [1]. The identification of water from strong radar reflections is based on the high value of the dielectric permittivity of water-bearing materials compared to that of dry rocks. This identification has been challenged based on thermal models of the Martian polar cap, which could not produce basal temperatures compatible with the presence of liquid water, and alternative interpretations have been proposed like wet clays and iron-rich basalts [2-4]. However, a quantitative determination of which hypothesis best explains the strong radar echoes detected by MARSIS requires a careful modelling of electromagnetic propagation within the Martian South Polar Layered Deposits (SPLD), the dust-laden ice sheet covering most of the Martian polar regions. In addition to the dielectric properties of the material beneath the Martian ice cap, the main factor determining the strength of basal radar echoes is the attenuation experienced by the radio waves as they propagate within the SPLD. Here we estimate the bulk loss tangent (ratio of imaginary to real part of the complex dielectric permittivity) of the SPLD from differential attenuation of basal echoes detected by MARSIS at different frequencies.  

 

Data and Methods  

The used MARSIS dataset consists of 132 radar observations collected at 3MHz and 4MHz or 4MHz and 5MHz, acquired at Ultimi Scopuli between 2010 and 2019 (Fig.1): 36 at 3 MHz, 132 at 4MHz and 96 at 5MHz. Such observations have been collected on a large region, were both bright and non-bright areas were detected [5]. The basal reflectivity is lower at higher frequencies, with a systematic difference between each frequency pair (3/4 MHz and 4/5 MHz) regardless the acquisition inside or outside the bright area (Fig.2). This behavior can be ascribed to different causes: the attenuation in the SPLD; the scattering generated by the basal interface which, in turns, depends on the interface roughness and the dielectric contrast between the SPLD and the underlying material. From data analysis, it is possible to ascribe the frequency behavior of the MARSIS observations mostly to the signal attenuation in the SPLD.  Under these assumptions, loss tangent is computed from the measurements of the normalized basal echo power observed at different depths and frequencies. 

                                                                                               (Pb/Ps)dB ≃ R0−𝜉 𝜈 tan𝛿 𝜏, 

where Pb is the basal echo power, Ps is the surface echo power,  𝜉=2𝜋 10log10(⁡e), and R0 is a constant which depends on surface and basal Fresnel reflection coefficients, 𝜈 is the frequency and 𝜏 is the two-way travel time. 

Fig.1 Mars Orbiter Laser Altimeter topographic map of the investigated area at Ultimi Scopuli. The white lines represent the MARSIS observations in the region. The gray region indicates the main bright area studied in [1]. Black lines are the observations illustrated in Fig.2. 

 

Fig. 2 The plots refer to observations collected inside (a) and outside the bright areas (b) of Fig. 1, after applying an along track average.  

Results 

In the entire investigated region, the estimated loss tangent value is of the order of 10−3 . This value implies an attenuation of several dB's over the thickness of the SPLD in the area where strong echoes were detected by MARSIS, thus increasing the value of the dielectric permittivity of the basal material required to produce such echoes. The observed frequency behavior of basal echoes requires the presence of a significant amount of dust within the SPLD, similar to what has been deduced from gravity measurements, and puts an upper limit to the basal temperature of the SPLD. Furthermore, the extrapolation of the observed attenuation at higher frequencies explains why SHARAD, the other radar sounder at Mars, is unable to detect the strong basal echoes found by MARSIS. The upper limit on basal temperature retrieved in this analysis, when compared with literature data about the electrical conductivity of geomaterials at low temperature, rules out the possibility that clays or other dry minerals can produce the strong echoes detected by MARSIS. The most likely explanation for such echoes thus remains the presence of perchlorate brines at the base of the SPLD. 

References 

Orosei, R., et al., (2018). Science, doi: 10.1126/science.aar7268. 

Sori, M. M., & Bramson, A. M. (2019). GRL, doi.org/10.1029/2018GL080985. 

Smith, I. B. et al., (2021). GRL, doi.org/10.1029/2021GL093618. 

Grima, C., et al., (2022). GRL, doi.org/10.1029/2021GL096518.   

Lauro, S.E., et al.. Nat Astron, doi.org/10.1038/s41550-020-1200-6 

How to cite: Lauro, S., Pettinelli, E., Caprarelli, G., Baniamerian, J., Mattei, E., Cosciotti, B., Stillman, D., Primm, K., Soldovieri, F., and Orosei, R.: Using MARSIS signal attenuation to constrain SPLD basal temperature and composition  , Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-1052, 2022.

10:20–10:30
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EPSC2022-457
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ECP
Léopold Desage, Alain Herique, Wlodek Kofman, and Sonia Zine

Abstract

The SHAllow RADar (SHARAD) is a synthetic aperture radar (SAR) onboard Mars Reconnaissance Orbiter,  in Martian orbit since 2006 [1]. With its 20MHz frequency, it can fathom the first hundreds of meters of the subsurface with a range resolution of about 10m in typical Martian materials. In order to detect subsurface interfaces with radars, we need to eliminate the echoes coming from the surface : as the design of most radars in orbit gives them a large antenna lobe, off-nadir echoes could arrive at the same delay as a subsurface nadir reflection. The classical method to remove this so called “clutter” consists in comparing the radar signal to simulations of the surface echoes using Digital Terrain Models (DTMs). Our aim is to study the first tens of meters of the Martian subsurface with SHARAD. To do so, we need high resolution DTMs because in theory, the higher we get in resolution, the more detailed the simulation will be and the clearer the discrimination between surface and subsurface features will be. We will present results on high resolution simulations performed with SPRATS, our coherent simulator. We will show that while being high resolution, models obtained by photogrammetry sometimes contain artifacts that can be misleading for radar data interpretation.  

 

Simulations with SPRATS : first study with MOLA DTMs

SPRATS is a toolset developed at IPAG that allows to perform both coherent radar simulations of surfaces and 3D SAR processing of them [2], [3]. Those capabilities enable the simulation of the actual signal sensed by SHARAD, with the same processing applied to it, in order to get as close as possible from the instrument results. It  allows for a direct power comparison and thus finer analysis. To begin, we performed simulations with DTMs generated with MOLA, a laser altimeter [4]. While having a relatively low resolution, the nature of the acquisition method give those DTMs a high accuracy and precision, resulting in very low artifacts. To study deep reflectors in areas of relatively low rugosity (i.e. the northern plains [5]), those models are sufficient. But the low resolution is a limiting factor when studying close subsurface, or simply to reproduce surface roughness effects on the radar signal. To improve the simulations, higher resolution  models are necessary.

 

HRSC models, higher resolution but sometime containing artifacts

With a resolution of 50 to 100m, HRSC DTMs [6] yield better results in simulating smaller details. It allows to confirm or discard reflectors identified with MOLA [7]. However, these models are acquired by photogrammetry, a technique that is an estimation of the surface topography, compared to MOLA which is a direct measurement. Photogrammetry introduces artifacts that are not easy to estimate, because they depend on the actual topography. We will show a comparative study of simulations with HRSC and MOLA models on a region of interest located in Terra Cimmeria, following a study made by [8]. The amplitude of the artifacts on the HRSC models is too high to study the first tens of meters of the subsurface with SHARAD. Following the idea of getting as close as possible to the actual SHARAD data, we need models that describe the surface at a resolution better that the radar’s wavelength.

 

Simulations with models at wavelength-scale resolution (CTX)

We will present a comparative study of simulations using CTX models — with a resolution of 12m — and HRSC models. We will also show that the scale of the artifacts on these DTMs being below the SHARAD’s wavelength, CTX DTMs yield near perfect surface echoes simulation,  allowing for a fine detail comparative analysis of the SHARAD data. However, given their acquisition method [9], CTX DTMs have a relatively poor surface coverage compared to HRSC, so we used photoclinometry with CTX images on lower resolution models [10] to keep the high resolution information. Comparing high resolution simulations using these models to SHARAD data allowed to highlight small-scale artifacts on the CTX DTMs, as they introduce noise in the radargram.

 

Conclusions

This study shows that wavelength-scale or smaller artifacts on DTMs are needed to perform shallow subsurface analysis of SHARAD data.  It also showed that high resolution models acquired by photogrammetry are prone to artifacts, which can perturb the simulated signal. This artifacts issue can prove to be helpful for DTM quality estimation, especially for missions where no laser altimeter is present to validate the altimetry measurements.

 

References

[1] R. Seu et al., « SHARAD sounding radar on the Mars Reconnaissance Orbiter », 2007, doi: 10.1029/2006JE002745.

[2] Y. Berquin, A. Herique, W. Kofman, et E. Heggy, « Computing low-frequency radar surface echoes for planetary radar using Huygens-Fresnel’s principle: COMPUTING RADAR SURFACE ECHOES », oct. 2015, doi: 10.1002/2015RS005714.

[3] J.-F. Nouvel, A. Herique, W. Kofman, et A. Safaeinili, « Radar signal simulation: Surface modeling with the Facet Method: RADAR SIGNAL SIMULATION » , febr. 2004, doi: 10.1029/2003RS002903.

[4] M. T. Zuber et al., « The Mars Observer laser altimeter investigation », 1992, doi: 10.1029/92JE00341.

[5] C. M. Stuurman et al., « SHARAD detection and characterization of subsurface water ice deposits in Utopia Planitia, Mars: SHARAD DETECTION OF ICE UTOPIA PLANITIA », sept. 2016, doi: 10.1002/2016GL070138.

[6] G. Neukum et R. Jaumann, « HRSC: the High Resolution Stereo Camera of Mars Express », 2004.

[7] C. W. Cook et al., « Sparse subsurface radar reflectors in Hellas Planitia, Mars », sept. 2020, doi: 10.1016/j.icarus.2020.113847.

[8] S. Adeli, E. Hauber, G. G. Michael, P. Fawdon, I. B. Smith, et R. Jaumann, « Geomorphological Evidence of Localized Stagnant Ice Deposits in Terra Cimmeria, Mars », juin 2019, doi: 10.1029/2018JE005772.

[9] M. C. Malin et al., « Context Camera Investigation on board the Mars Reconnaissance Orbiter », 2007, doi: 10.1029/2006JE002808.

[10] S. Doute et C. Jiang, « Small-Scale Topographical Characterization of the Martian Surface With In-Orbit Imagery », janv. 2020, doi: 10.1109/TGRS.2019.2937172.

How to cite: Desage, L., Herique, A., Kofman, W., and Zine, S.: SHARAD Data Analysis with High Resolution Digital Terrain Models, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-457, 2022.

10:30–10:40
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EPSC2022-345
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ECP
Gwénaël Caravaca, Gilles Dromart, Nicolas Mangold, Sanjeev Gupta, Stéphane Le Mouélic, Olivier Gasnault, Sylvestre Maurice, and Roger C. Wiens

For more than a year now, the Perseverance rover of the Mars2020 mission has been exploring the Jezero crater in the northern hemisphere of Mars. This landing area was notably selected due to the presence of a large-scale fan-shaped sedimentary edifice in its western part at the mouth of Neretva Vallis (NV, Fig. 1a, e.g., [1]), that was interpreted to be a fossil deltaic system. Upon arrival in the crater in February 2021, remote observation from the ground were performed, allowing to answer this question with long-distance imaging using Mastcam-Z and SuperCam’s Remote Micro-Imager. Indeed, these observations allowed to identify with certainty structures characteristic of a Gilbert-type delta, confirming the fluvio-deltaic origin of this western fan [1].