Polygonal terrain results from thermal contraction, is commonly found in periglacial environments, and serves as a valuable proxy for interpreting subsurface ice distribution and climate history on Earth and Mars. In this study, we investigate the morphology of polygons in Beacon Valley, Antarctica, to assess the relationship between polygon size and ice table depth and to evaluate whether existing models for ice-cemented soils apply to polygons overlying massive ice. Using high-resolution remote sensing imagery and lidar datasets, we mapped over 1500 polygons and compared their sizes to measured ice table depth. Statistical analyses indicate that polygons formed over massive ice are generally smaller than those formed on ice-cemented soils. This observation contradicts current models, which predict larger polygons on massive ice due to a reduction in stress through viscous flow. We propose that sediment within the glacial ice inhibits viscous deformation and lowers tensile strength, thereby reducing the size of the observed polygons. Our statistical analysis also reveals two distinct subpopulations of polygons, for both polygons on ice-cemented soil and massive ice, which we interpret as evidence of polygon time evolution. This observation is supported by a crack propagation model that illustrates how polygon size decreases with time. Our findings challenge prevailing models of polygon formation and evolution and offer a revised conceptual framework with implications for interpreting subsurface ice availability on both Earth and Mars.

Ground-Penetrating Radar (GPR) is widely used for subsurface imaging to support investigations in geology, hydrology, agriculture, engineering, mineral resources, unexploded ordnance, and archaeology. These diverse applications require investigation depths spanning several centimeters to kilometers, with corresponding frequencies of several GHz to a few MHz, respectively. Early GPR development began in the mid-20th century, but the first major application was the Surface Electrical Properties (SEP) experiment on the Apollo 17 lunar landing. We review key previous work in lunar GPR and discuss several approaches to future exploration.

This paper presents a field study conducted in Haines Junction, Yukon, utilizing Spectral Induced Polarization (SIP) to investigate the subsurface properties of a pingo site and specifically identify its ice core.The effectiveness of SIP analysis was demonstrated using a FUCHS frequency domain instrument, which measured electrical impedance magnitude and phase shift angle at multiple frequencies (1.46 Hz-40 kHz).The main focus of the analysis centered on the results obtained from electrical impedance magnitude and phase shift angle inversion at 40 kHz and 1.46 Hz.The inversion results revealed the presence of high resistivity layers within the subsurface, similar to results that would be expected with electrical resistivity tomography (ERT).Additionally, the SIP data revealed that some areas with high resistivity also had negative phase shift angle values, suggesting the presence of materials with polarizing properties.The analysis of the imaginary part of electrical impedance at 40 kHz for these areas highlighted the contribution of polarization, indicating the presence of ice.Furthermore, the electrical impedance magnitude at 1.46 Hz exhibited similarities to the 40 kHz analysis, but with higher resistivity.This pattern is another indicator of the presence of ice within the subsurface of the study area and was expressed using the Resistivity Frequency Effect (RFE) equation.The RFE analysis and the patterns of polarization confirmed the presence of the pingo ice core and identified its distinctive signature compared to other layers.Subsequent drilling confirmed the presence of interbedded ice and clay from 2.4-3.6 m depth and massive ice from 3.6-8.3m depth.1

Slope streaks (SS) are enigmatic linear features characterized by relatively low-albedo features that appear and fade on high-albedo slopes on Mars. Despite numerous hypotheses proposed to explain their formation, the primary mechanism behind SS remains elusive. Here, we examine 702 SS features using 32 multitemporal Context Imager (CTX) images and mesoscale modeling data obtained from a site (centered at 31.230°N, 216.281°E) in the Olympus Mons Aureole region. Our investigation revealed several key findings that shed light on the dynamics of SS formation and fading. We discovered a significant preference for SS formation on south-facing (equator-facing) slopes compared to north-facing slopes, with SS being over seven times more likely to occur on the former. Furthermore, SS formation was found to be seasonal with significantly enhanced by a factor of ~6 near the equinoxes (from solar longitude Ls 337°-42° and 136°-227°) compared to other times of the year. Our analysis also revealed a correlation between the rates of SS formation and fading, with scree slopes exhibited the fastest-fading SS also experiencing the highest rates of newly-formed SS. Additionally, we measured the median starting and stopping slopes of SS to be 23.4° and 14.9°, respectively, significantly below the angle of repose of sand. These low slopes suggest the necessity for an energetic trigger mechanism to initiate SS formation. Infrared spectroscopy revealed that the principal distinction between the material inside and outside of a SS lies in the reduced abundance of dust within the streak. Notably, this site demonstrates the highest rates of SS formation (a 29.6% increase in new SS per Mars year) and fading (a 12% fading of SS per Mars year) ever quantified. These elevated rates may be attributed to the site's topography, which facilitates calm surficial nighttime winds throughout the year, leading to widespread dust deposition. Daytime downhill winds near the ridgelines of S-facing slopes may then trigger movement of newly deposited dust aggregates or alternatively, a Knudsen pump phenomenon could serve as a potential trigger for SS. We propose that SS is inhibited during the northern summer solstice season due to the relatively clear aphelion atmosphere, which limits dust deposition. Similarly, the lack of triggering activity during the winter solstice is likely due to more moderate daytime winds. Overall, our findings are consistent with the wind-triggered dry avalanche hypothesis as a plausible explanation for SS formation.

<p>There are several active geologic processes on Mars today one of which is the formation of slope streaks. Slope streaks are a widespread and relatively common process that were first observed as dark fan-shaped features with lobed ends in Viking Orbiter images taken in 1977 (Morris, 1982; Ferguson and Lucchitta, 1984). Investigation of repeat images identified slope streaks as relatively low-albedo features that vary in width (up to 200 m wide) and length (up to a few kilometers long) (Sullivan et al., 2001). Although it was assumed that the slope streaks formed on steep slopes >20°, the slopes were not resolved due to the resolution limit of the data. Slope streaks have been found to form in high-albedo dusty regions on Mars, concentrated around the equator between 39°N and 28°S (Sullivan et al., 2001; Schorghofer and King, 2011; Heyer et al., 2019). Additionally, slope streaks have been observed to fade over decades and high-albedo slope streaks have also been observed (interpreted to be faded slope streaks) (Schorghofer et al., 2007). The formation of slope streaks has previously been observed to be inconsistent spatially and temporally (Schorghofer and King, 2011); however, more recent research has identified seasonal variations of formation, with the highest rates of formation occurring in the fall (near Ls 190) (Heyer et al., 2019). There are many proposed formation mechanisms for slope streaks that fall into either a dry or wet mechanism category. The dry mechanism involves a granular flow triggered by a disturbance mechanism (e.g. dust devil or meteorite impact), while a wet mechanism would indicate a debris flow triggered by a phase change of H<sub>2</sub>O (e.g. melting of ice to trigger groundwater discharge). Research presented here investigates the slope profiles of identified slope streaks to further understand and constrain the formation mechanism. We investigated 13 well-monitored slope streak sites. Using Arcmap we identified slope streaks within each site with a polyline. For each site we identified CTX stereopairs, processed each image using the Integrated Software for Imagers and Spectrometers (ISIS3), and then used Ames Stereo Pipeline (ASP) to create digital elevation models (DEM) for each site. In Arcmap using the DEMs and the polylines for each slope streak we extracted the slope profiles to determine the starting and stopping slope of each slope streak and then average slope of the entire slope streak. Results indicate that on average slope streaks starts at a slope of 24° and end on a slope of 16° with the ending slope decreasing with increasing flow distance. Also, the majority of slope streaks start on a slope <30°, which is near the dynamic angle of repose. The low start angle and the decreasing stop angle with flow distances indicates an energetic triggering mechanism may be necessary to create a slope streak. Recent research from Heyer et al. (2020) identified dust devil tracks that appear to have triggered slope streaks, supporting our results that are most consistent with a dry and energetic triggering mechanism.</p>

It has recently been suggested that clay minerals, which are widespread on the Martian surface, could be the possible source of the basal bright reflections detected by MARSIS at Ultimi Scopuli, instead of briny water. This hypothesis is based on dielectric measurements on a wet Ca-Montorillonite (STx-1b) sample conducted at 230 K, which reported permittivity values (apparent permittivity of 39 at 4 MHz) compatible with the median value of 33 retrieved by MARSIS 4 MHz data inversion in the high reflectivity area. These experimental results are, however, incompatible with well-established dielectric theory and with laboratory measurements on clays, at MARSIS frequency and Martian temperatures, reported in the literature. Here, we replicate the experiment using a setup to precisely control the rate of cooling/warming and the temperature inside and outside the clay sample. We found that the rate of cooling, the position of the temperature sensor and, consequently, the thermal equilibrium between the sample and the sensor play a fundamental role in the reliability of the measurements. Our results indicate that even for a large water content in the clay sample, at 230 K and 4 MHz, the apparent permittivity is only 8.4, dropping to 4.1 at 200 K, ruling out clays as a possible source of the bright reflections detected by MARSIS at the base of the SPLD.

Martian slope streaks are large surface features with lower albedo than their surroundings that are one of the few active geologic processes occurring on Mars today. We investigated the slope streaks’ spectral properties using images from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) at nine sites, including three sites with observations over multiple years to enable time-series analysis. For each individual slope streak within each image, we determined the average spectra of each streak and of the slope immediately adjacent to account for changes in geology across a CRISM image or within slopes containing streaks. We find a trend where the visible spectral continuum (0.4–0.6 μ m) is strongly negative for the darkest slope streaks, and the spectral continuum increases as the slope streaks fade, consistent across all sites and over multiple observations of the same site. We do not find absorption at 1.4 or 1.9 μ m associated with hydration. In some slope streaks, we see evidence of Fe-bearing minerals, but these signatures are also found in the streak-free slope directly adjacent, suggesting that the streaks have a similar mineralogy to their surroundings. The spectral changes that we see are most consistent with changes in the fine dust component and support a dry flow mechanism whereby slope streaks are formed by processes that trigger dust removal from the slope in particular locations, with the streaks slowly fading as dust resettles on the surface.

Anomalously bright basal reflections detected by MARSIS at Ultimi Scopuli have been interpreted to indicate the presence of water-saturated materials or ponded liquid water at the base of the South Polar Layered Deposits (SPLD). Because conventional models assume basal temperatures (<= 200K) much lower than the melting point of water, this interpretation has been questioned and other explanations for the source of the bright basal reflections have been proposed, involving clay, hydrated salts, and saline ices. Combining previous published data, simulations, and new laboratory measurements, we demonstrate that the dielectric properties of these materials do not generate strong basal reflections at MARSIS frequencies and Martian temperatures. Plausible candidates remain perchlorates and chlorides brines that exhibit a strong dielectric response at much lower temperatures than other materials. This explanation might require that metastability could be maintained for a long period of time on a geological scale. (c) 2022 Elsevier B.V. All rights reserved.

When a brine mixed with particles is frozen, some liquid water persists due to the freezing point depression caused by the solute impurity, surface energy, and disjoining pressure (wetting forces). This unfrozen water forms a complex "liquid vein network" (LVN). However, details of the freezing process are still not fully understood, including the permeability/tortuosity of the LVN, and the unfrozen water content at a given temperature. Here, we have applied nuclear magnetic resonance (NMR) relaxation, self-diffusion measurements and magnetic resonance imaging (MRI) to investigate the distribution and structure of LVNs. Magnesium chloride (MgCl2) salt concentrations of 15, 30, and 60 mM were investigated with and without poly-methyl methacrylate (PMMA) particles of diameter 0.4, 9.9, and 102.2 mu m, allowing us to quantify unfrozen water content and the structure of the LVN as a function of temperature, MgCl2 concentration, and PMMA particle size. The results of magnetic resonance imaging (MRI) and self-diffusion confirm that the inhibition of ice recrystallization is a function of particle size. To gain information on LVN structure, we compared NMR results to Monte Carlo simulations of freezing in brine-particle systems. Comparisons between laboratory and simulation data suggest that, for our experimental range of temperature (-17.4 degrees C to-0.9 degrees C +/- 0.5 degrees C), solutes make the dominant contribution to the unfrozen liquid fraction for particle sizes larger than a few microns, whereas in the finest grained porous media we tested, the unfrozen liquid fraction is controlled primarily by the films that wet particle-ice interfaces.

Strong radar reflections have been previously mapped at the base of the Martian South Polar Layered Deposits. Here, we analyze laboratory measurements of dry and briny samples to determine the cause of this radar return. We find that liquid vein networks consisting of brines at the grain boundaries of ice crystals can greatly enhance the electrical conductivity, thereby causing strong radar reflections. A brine concentration of 2.7–6.0 vol% in ice is sufficient to match the electrical properties of the basal reflection as observed by Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS). When brine is mixed with sediments, the brine‐ice mixture in the pores must be 2–5 times more concentrated in salt, increasing the brine concentration to 6.3–29 vol%. Our best fit of the median observed MARSIS value suggests a salt‐bulk sample concentration of ∼6 wt%. Thus, salt enhancement mechanisms on the order of a magnitude greater than the Phoenix landing site are needed. To form brine, the basal reflector must reach a temperature greater than the eutectic temperature of calcium perchlorate of 197.3 ± 0.2 K, which may be possible if more complex thermal modeling is assumed. Colder metastable brines are possible, but stability over millions of years remains unclear. Conversely, gray hematite with a concentration of 33.2–59.0 vol% possess electrical properties that could cause the observed radar returns, but require concentrations 2–3 times larger than anywhere currently detected. We also argue that brines mixed with high‐surface‐area sediments, or dry red hematite, jarosite, and ilmenite cannot create the observed radar returns at low temperatures.

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.

Knowledge of the physical and thermal properties of the South Polar Layer Deposits (SPLD) is key to constrain the source of bright basal reflections at Ultimi Scopuli detected by the MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) radar sounder. Here we present a detailed analysis of attenuation, based on data acquired by MARSIS at 3, 4, and 5 MHz. We show that attenuation is frequency dependent, and that its behavior is consistent throughout the entire region. This suggests that the SPLD are compositionally homogeneous at Ultimi Scopuli, and our results are consistent with dust contents of 5 to 12%. Using these values as input, and plausible estimates of surface temperature and heat flux, we inferred basal temperatures around 200 K: these are consistent with perchlorate brines within liquid vein networks as the source of the reflections. Furthermore, extrapolation of the attenuation to higher frequencies explains why SHARAD (Shallow Radar) has thus far not detected basal reflections within the SPLD at Ultimi Scopuli.

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

We provide an up-to-date review on the different landscapes and landforms that have been attributed to the action of liquid water in the Amazonian epoch on Mars and the current state of the art regarding their interpretation. This chapter accompanies the chapter by Dundas, where the counterarguments are presented. The Amazonian epoch is thought to be dominated by hyper-arid climate conditions hostile to surface liquid water, and our review reveals that this steady state is likely to be punctuated by episodic appearances of liquid water at the surface. The proposed sources of liquid water are varied: groundwater, thawing of surface-ice, deliquescence, ground-ice or glaciers, with triggers as variable as microclimates, climate-shifts, geothermal anomalies, and impact cratering. Our review covers recently active surface processes in the form of slope streaks, dark dune flows, and recurring slope lineae, all hypothesized at one point or another to be seeps of liquid water. We then cover landscapes and landforms that are proposed to be a result freeze–thaw cycles in the recent past, including gullies, lobate forms on hillslopes, pingo-like mounds, low-centered polygons, patterned ground, and ice-loss landscapes. We present evidence that liquid water has been produced at the base of glacial landforms during the Amazonian, resulting in sinuous ridges (eskers), and enhanced crater-wall erosion. Liquid water brought up from depth is thought to have produced mud-volcanoes and other features related to sedimentary volcanism in the Amazonian and small fluvial channels among other features are thought to be related to melting induced by periodic impact events. We end by summarizing the importance of the search evidence of liquid water on Mars and by proposing solutions to the current impasses where progress is hindered due to limitations in data or our understanding.

One objective of a lander mission to Jupiter's icy moon Europa is to detect liquid water within 30 km as well as characterizing the subsurface ocean. In order to satisfy this objective, water within the ice shell must also be identified. Inductive electromagnetic (EM) methods are optimal for water detection on Europa because even a small fraction of dissolved salts will make water orders of magnitude more electrically conductive than the ice shell. Compared to induction studies by the Galileo spacecraft, measurements of higher-frequency ambient EM fields are necessary to resolve the shallower depths of intrashell water. Although these fields have been mostly characterized by prior missions, their unknown source structures and plasma properties do not allow EM sounding using a single surface magnetometer or the orbit-to-surface magnetic transfer function, respectively. Instead, broadband EM sounding can be accomplished from a single surface station using the magnetotelluric (MT) method, which measures horizontal electric fields as well as the three-component magnetic field. We have developed a prototype Europa Magnetotelluric Sounder (EMS) to meet the measurement requirements in the relevant thermal, vacuum, and radiation environment. EMS comprises central electronics, a fluxgate magnetometer on a mast, and three ballistically deployed electrodes to measure differences in surface electric potential. In this paper, we describe EMS development and testing as well as providing supporting information on the concept of operations and calculations on water detectability. EMS can uniquely determine the occurrence of intrashell water on Europa, providing important constraints on habitability.

Recurring slope lineae (RSL) are dark narrow features that incrementally lengthen and fade at least once each Mars year. Their origin has remained enigmatic. We report quantitative modeling of three hypotheses related to triggering of over-steepened sand caches that result in dry-grain flow. First, sand could be supplied by either wind blowing upslope from within the crater or by wind blowing into the crater from the outside. Without cementation, grain flows should be directly correlated with temporal variations in potential sediment transport. Second, deliquesced perchlorate salts could increase soil cohesion; with the loss of water this soil cohesion may be reduced. The timing of accumulation versus release can be distinct from each other. Third, seasonal water frost could act in a similar cementation capacity. We assess these hypotheses using atmospheric modeling to determine wind speed, wind direction, temperature, and relative humidity at three craters (Rauna, Krupac, and Palikir) that host confirmed RSL and that cover a large latitudinal range. Overall, we find no convincing support for any of these hypotheses. Deliquescence is predicted to occur only at Rauna crater and its formation does not correlate with RSL activity. The occurrence of frost is inconsistently correlated with RSL activity among the three craters. Upslope winds at Palikir crater transport a significant amount of sediment only when RSL are active. However, the largest sediment flux into the crater is also during periods of RSL activity, thus supporting both internal and external contributions. Sediment transport into Krupac crater to 5- and SW-facing slopes is also maximized when RSL are active, but there is no correlated upslope transport. This supports the hypothesis that RSL formation is dominated by external sediment supply alone. On the other hand, W- and NW-facing Krupac and all RSL at Rauna show no correlations with any directional sediment transport. Nonetheless, we suggest that the mixed success of the external sediment transport model is still quantitatively better than any competitor (including water), and that we simply lack the model and data resolution to treat RSL at the required meter scales. In all of the aeolian models, favorably-directed potential sediment flux greatly exceeds the volume of sand that must be displaced to form RSL. We conclude that RSL may require a particular combination of favorable strong winds, local sediment traps, and a supply of sand grains that easily saltate on Mars (similar to 100 mu m in diameter).