TP5
Session assets
Discussion on Slack
Orals: Thu, 22 Sep | Room Manuel de Falla
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, https://doi.org/10.5194/epsc2022-1024, 2022.
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, https://doi.org/10.5194/epsc2022-1052, 2022.
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].
In this work, we focus on the Kodiak butte (Fig. 1b) situated ~1 km away from the current main delta front. This butte exhibits Gilbert-type structures of bottomsets, foresets and topsets characteristic of the deltaic suite, and as therefore been identified as being a remnant of a past distal extension of the western delta [1; 2]. This butte was imaged from the east during the earliest part of the mission (Fig. 2a), but we also took advantage of the “Rapid Traverse” route to the delta foot to gather images of its northern-facing side (Fig. 2b), giving us data to study the 3D geometry of the sedimentary rocks that make this butte. Here, we provide an overview of the facies and stacking pattern of the deltaic series observed at Kodiak.
We use both 2D high-resolution image data acquired by Mastcam-Z and SuperCam’s RMI instruments, but also 3D Digital Outcrop Model (DOM) reconstructed from RMI remote observations (e.g., [2], visible online at: https://skfb.ly/o89yU). While both views help us in determining the stratigraphic architecture of the butte, the later being critical in assessing with more precision the sediment transport directions, and especially their temporal variations within distinct units [2]. That is, we are able to describe at least two main depositional episodes recorded at Kodiak, with Delta Units 1 and 2 (Fig. 2). Both units are characterized by a well-evidenced Gilbert-type succession of bottomsets, foresets and topsets, of a similar size and extension. The lower elevation and position further north of Delta Unit 1 (Fig. 2) indicates that it predates Delta Unit 2. An overall direction of transport toward the south-west is observed for this unit from the dip of foresets observed from both east and north faces.
Delta Unit 2 was observed in the first part of the mission, when the rover was positioned about 2 km away to the east, and was first characterized in [1]. High-resolution RMI images [3] allow us to characterize a sedimentary succession made of rocks whose texture ranges from coarse sandstones to (boulder) conglomerates, which is common for deltaic systems. A closer observation of the sedimentary architecture was possible using a 3D DOM made from RMI images [2] (Fig. 3). On this model, we were able to precisely delimits the different bottomsets, foresets and topsets (Fig. 3a), but also to provide measurements of the individual bed thicknesses within each interval with an average of 15 cm, 17 cm and 19 cm for bottomsets, foresets and topsets, respectively. After close observation, it appears that the topsets of Unit 2 also show that at least three sets of oblique stratifications do not follow the same transport direction than the other beds, evidencing a transport clearly toward the east (green lines in Fig. 3b). Also, individual beds exhibit a reduced thickness with an average of 13.5 cm per bed (Fig. 3b). While the overall unit 2 follows a usual stacking pattern for the deposition of a prograding delta, it appears that local and recurring variations in the energy and direction of transport occurs during the late-stages of the delta’s deposition, probably following onset of meandering beds or varying direction braided bars, an observation not dissimilar to the “curvilinear unit” observed on the main delta front at equivalent elevations [4].
Finally, large-scale boulders are observed to irregularly and unconformably overlie both Delta Units 1 and 2 (Fig. 4) on top of the butte. These clasts have been measured on top of Unit 2 have an average long-axis size of ~52 cm, and range from 22 to 104 cm (Fig. 4). These poorly sorted and angular clasts are part of a clast-supported unit that seem to have deposited episodically a certain amount of time after the deposition of the main delta. This implies a strong decoupling between both settings, implying a complex history of the area, whose timely relations are yet to be ascertained.
While the exploration of the main delta of Jezero crater is just beginning, the characterization of the Kodiak butte, a remote distal remnant of the past deltaic fan, is already a “gold mine” of geologic information about the aqueous past of this area. The two different episodes in the delta’s history, represented by Delta Units 1 and 2, their inner variations, but also the presence of the later boulder conglomerate unit, are informing us about the late-stage evolution of the hydrological history of the Jezero crater. This is notably important since we are observing the foresets/topsets transition to occur at ~-2490 m in elevation, that is about 100 m lower than the modeled lake level from the craters’ outlet valley [1]. Following work to precisely characterize Delta Unit 1 and the (co-)relations to the main delta units [5, 6] will therefore be paramount in assessing the paleoenvironmental evolution of the basin, its fluvial activity, and its link to the regional to global climate of ancient Mars.
References: [1] Mangold et al., 2021, Science, 374, 6568. [2] Caravaca et al., 2022, LPSC, Abstract #1189. [3] Gasnault et al., 2021, LPSC¸ Abstract #2248. [4] Stack et al., 2020, Space Science Reviews, 216, 127. [5] Mangold et al., 2022, LPSC, Abstract #1814. [6] Gupta et al., 2022, LPSC, Abstract #2295.
How to cite: Caravaca, G., Dromart, G., Mangold, N., Gupta, S., Le Mouélic, S., Gasnault, O., Maurice, S., and Wiens, R. C.: Overview of the facies and stratigraphy of a distal Delta remnant at the Kodiak butte (Jezero crater, Mars), Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-345, https://doi.org/10.5194/epsc2022-345, 2022.
Introduction
We developed a simple model to simulate Ma_MISS data acquisition and visualization and to support the development of acquisition strategies.
The Ma_MISS instrument
Ma_MISS (Mars Multispectral Imager for Subsurface Studies) is the miniaturized VNIR spectrometer embedded in the drill system of the ExoMars rover[1]. Ma_MISS will perform spectral reflectance measurements inside holes drilled into the surface of Mars up to a depth of 2 m. It will characterize the mineralogy and stratigraphy of the borehole walls and will allow the in-situ study of the subsurface environment, before any samples are collected and extracted. Taking advantage of the finely controllable drill tool motion, Ma_MISS can map the borehole walls. In addition to single point acquisitions, Ma_MISS can also be programmed to acquire “Ring” and “Column” scans, where the spectrometric acquisitions are interleaved with step rotations or translations of the drill tool.
Borehole stratigraphy model
To easily simulate Ma_MISS surveys, we first developed a simple model of a borehole. As further described in [2], six different simulant samples were prepared. Reflectance spectra were acquired on each of the samples with DAVIS-MOT, the new Ma_MISS laboratory model[3]. The reflectance spectra of these samples are shown in Figure 1. Six different regions were drawn as polylines on a 2D projection of the cylindrical borehole walls. Out of the six simulants, a different one was assigned to each region. As shown in Figure 2, this model was made to represents a 250 mm deep section of a borehole, exhibiting a varied stratigraphy with 5 different layers and a vertical vein.
Figure 1: Reflectance spectra of samples.