Polar Deposits Research Articles

Images of surface volatiles in Mercury’s polar craters acquired by the MESSENGER spacecraft

Images acquired by NASA's MESSENGER spacecraft have revealed the morphology of frozen volatiles in Mercury's permanently shadowed polar craters and provide insight into the mode of emplacement and evolution of the polar deposits. The images show extensive, spatially continuous regions with distinctive reflectance properties. A site within Prokofiev crater identified as containing widespread surface water ice exhibits a cratered texture that resembles the neighboring sunlit surface except for its uniformly higher reflectance, indicating that the surficial ice was emplaced after formation of the underlying craters. In areas where water ice is inferred to be present but covered by a thin layer of dark, organic-rich volatile material, regions with uniformly lower reflectance extend to the edges of the shadowed areas and terminate with sharp boundaries. The sharp boundaries indicate that the volatile deposits at Mercury's poles are geologically young, relative to the time scale for lateral mixing by impacts, and either are restored at the surface through an ongoing process or were delivered to the planet recently.

Characterization of the morphometry of impact craters hosting polar deposits in Mercury's north polar region

Earth‐based radar images of Mercury show radar‐bright material inside impact craters near the planet's poles. A previous study indicated that the polar‐deposit‐hosting craters (PDCs) at Mercury's north pole are shallower than craters that lack such deposits. We use data acquired by the Mercury Laser Altimeter on the MESSENGER spacecraft during 11 months of orbital observations to revisit the depths of craters at high northern latitudes on Mercury. We measured the depth and diameter of 537 craters located poleward of 45°N, evaluated the slopes of the northern and southern walls of 30 PDCs, and assessed the floor roughness of 94 craters, including nine PDCs. We find that the PDCs appear to have a fresher crater morphology than the non‐PDCs and that the radar‐bright material has no detectable influence on crater depths, wall slopes, or floor roughness. The statistical similarity of crater depth‐diameter relations for the PDC and non‐PDC populations places an upper limit on the thickness of the radar‐bright material (<170 m for a crater 11 km in diameter) that can be refined by future detailed analysis. Results of the current study are consistent with the view that the radar‐bright material constitutes a relatively thin layer emplaced preferentially in comparatively young craters.

Bright and Dark Polar Deposits on Mercury: Evidence for Surface Volatiles

Measurements of surface reflectance of permanently shadowed areas near Mercury's north pole reveal regions of anomalously dark and bright deposits at 1064-nanometer wavelength. These reflectance anomalies are concentrated on poleward-facing slopes and are spatially collocated with areas of high radar backscatter postulated to be the result of near-surface water ice. Correlation of observed reflectance with modeled temperatures indicates that the optically bright regions are consistent with surface water ice, whereas dark regions are consistent with a surface layer of complex organic material that likely overlies buried ice and provides thermal insulation. Impacts of comets or volatile-rich asteroids could have provided both dark and bright deposits.

Reading the climate record of the martian polar layered deposits

The martian polar regions have layered deposits of ice and dust. The stratigraphy of these deposits is exposed within scarps and trough walls and is thought to have formed due to climate variations in the past. Insolation has varied significantly over time and caused dramatic changes in climate, but it has remained unclear whether insolation variations could be linked to the stratigraphic record. We present a model of layer formation based on physical processes that expresses polar deposition rates of ice and dust in terms of insolation. In this model, layer formation is controlled by the insolation record, and dust-rich layers form by two mechanisms: (1) increased summer sublimation during high obliquity, and (2) variations in the polar deposition of dust modulated by obliquity variations. The model is simple, yet physically plausible, and allows for investigations of the climate control of the polar layered deposits (PLD). We compare the model to a stratigraphic column obtained from the north polar layered deposits (NPLD) (Fishbaugh, K.E., Hvidberg, C.S., Byrne, S., Russel, P.S., Herkenhoff, K.E., Winstrup, M., Kirk, R. [2010a]. Geophys. Res. Lett., 37, L07201) and show that the model can be tuned to reproduce complex layer sequences. The comparison with observations cannot uniquely constrain the PLD chronology, and it is limited by our interpretation of the observed stratigraphic column as a proxy for NPLD composition. We identified, however, a set of parameters that provides a chronology of the NPLD tied to the insolation record and consistently explains layer formation in accordance with observations of NPLD stratigraphy. This model dates the top 500m of the NPLD back to ∼1millionyears with an average net deposition rate of ice and dust of 0.55mma−1. The model stratigraphy contains a quasi-periodic ∼30m cycle, similar to a previously suggested cycle in brightness profiles from the NPLD (Laskar, J., Levrard, B., Mustard, F. [2002]. Nature, 419, 375–377; Milkovich, S., Head, J.W. [2005]. J. Geophys. Res. 110), but here related to half of the obliquity cycles of 120 and 99kyr and resulting from a combination of the two layer formation mechanisms. Further investigations of the non-linear insolation control of PLD formation should consider data from other geographical locations and include radar data and other stratigraphic datasets that can constrain the composition and stratigraphy of the NPLD layers.

Reactive oxygen species form part of a regulatory pathway initiating trans-differentiation of epidermal transfer cells in Vicia faba cotyledons.

Various cell types can trans-differentiate to a transfer cell (TC) morphology characterized by deposition of polarized ingrowth walls comprised of a uniform layer on which wall ingrowths (WIs) develop. WIs form scaffolds supporting amplified plasma membrane areas enriched in transporters conferring a cellular capacity for high rates of nutrient exchange across apo- and symplasmic interfaces. The hypothesis that reactive oxygen species (ROS) are a component of the regulatory pathway inducing ingrowth wall formation was tested using Vicia faba cotyledons. Vicia faba cotyledons offer a robust experimental model to examine TC induction as, on being placed into culture, their adaxial epidermal cells rapidly (hours) form ingrowth walls on their outer periclinal walls. These are readily visualized by electron microscopy, and epidermal peels of their trans-differentiating cells allow measures of cell-specific gene expression. Ingrowth wall formation responded inversely to pharmacological manipulation of ROS levels, indicating that a flavin-containing enzyme (NADPH oxidase) and superoxide dismutase cooperatively generate a regulatory H2O2 signature. Extracellular H2O2 fluxes peaked prior to the appearance of WIs and were followed by a slower rise in H2O2 flux that occurred concomitantly, and co-localized, with ingrowth wall formation. De-localizing the H2O2 signature caused a corresponding de-localization of cell wall deposition. Temporal and epidermal cell-specific expression profiles of VfrbohA and VfrbohC coincided with those of extracellular H2O2 production and were regulated by cross-talk with ethylene. It is concluded that H2O2 functions, downstream of ethylene, to activate cell wall biosynthesis and direct polarized deposition of a uniform wall on which WIs form.

Predictions of MESSENGER Neutron Spectrometer measurements for Mercury's north polar region

From radar images of Mercury's poles and MESSENGER Neutron Spectrometer (NS) measurements obtained during the spacecraft's flybys of Mercury, predictions of neutron count rates and their uncertainties are calculated for Mercury's north polar region as of the end of the MESSENGER primary orbital mission. If Mercury's poles contain large amounts of water ice, as has been suggested on the basis of the radar data, then during the one-year-long orbital mission the NS should detect signals indicative of excess polar hydrogen with a significance of at least 4σ, where σ is the standard deviation derived from Poisson counting statistics. If the polar deposits are not enriched with hydrogen, but are dominated by other elements, such as sulfur, then the MESSENGER neutron measurements should be able to confirm the absence of deposits having surface concentrations in excess of 50 wt% H2O on permanently shadowed floors of craters near Mercury's north pole. Because of the large spatial footprint of the NS data, individual polar deposits will not be spatially resolved, but longitudinal asymmetries may be detected if residual systematic uncertainties are sufficiently low.

Large asymmetric polar scarps on Planum Australe, Mars: Characterization and evolution

Numerous scarps with similar characteristics have been observed in the polar layered deposits of Planum Australe, Mars. They are referred to as LAPSs (for Large Asymmetric Polar Scarps) because of their typical cross-section featuring a trough between a straight slope on one side with outcrops of layered deposits and a convex slope on the other side without any outcrops. These LAPSs are restricted to the outlying region of Ultimi Lobe. Topographic data, optical images, and subsurface radar observations have been analyzed and compared to produce a complete morphologic and stratigraphic description of these scarps. In all, 167 LAPS-like features have been identified. All have similar dimensions and characteristics and appear to be deep depressions in the ice. The polar deposits have an average thickness of 1km in this region and the LAPS depressions commonly reach half of that thickness. Subsurface data indicate that the depressions could reach bedrock at certain locations. Many surface features of the polar deposits of Mars are considered to be consequences of depositional and/or erosion processes. We propose a mechanical failure of the ice for the LAPSs origin, given the striking similarities in shape and size they show with rollover anticlines above listric faults commonly observed as a crustal extension mode on Earth. This tectonic scenario would imply a substantial outward sliding of the polar deposits in the region of Ultimi Lobe and a low basal shear stress. No information is available to determine whether such a system could be active at present. Confirmation of the “mechanical failure” hypothesis of these LAPSs on Mars is of major importance as it could be a macro-expression of fundamental differences between ice-sheet behavior under martian and terrestrial conditions.

Arabidopsis exocyst subunits SEC8 and EXO70A1 and exocyst interactor ROH1 are involved in the localized deposition of seed coat pectin

• Polarized deposition of cell wall pectins is a key process in Arabidopsis thaliana myxospermous seed coat development. The exocyst, an octameric secretory vesicle tethering complex, has recently been shown to be involved in the regulation of cell polarity in plants. Here, we used the Arabidopsis seed coat to study the participation of the exocyst complex in polarized pectin delivery. • We characterized the amount of pectinaceous mucilage and seed coat structure in sec8 and exo70A1 exocyst mutants. Using a yeast two-hybrid screen, we identified a new interactor of the exocyst subunit Exo70A1, termed Roh1, a member of the DUF793 protein family. • T-DNA insertions in SEC8, EXO70A1 caused considerable deviations from normal seed coat development, in particular reduced pectin deposition and defects in the formation of the central columella of seed epidermal cells. A gain-of-function mutation of ROH1 also caused reduced pectin deposition. Interestingly, we observed a systematic difference in seed coat development between primary and secondary inflorescences in wild-type plants: siliques from secondary branches produced seeds with thicker seed coats. • The participation of exocyst subunits in mucilage deposition provides direct evidence for the role of the exocyst in polarized cell wall morphogenesis.

Onset and migration of spiral troughs on Mars revealed by orbital radar

The landscape of the north polar layered deposits of Mars (NPLD) is dominated by a pinwheel array of enigmatic spiral troughs. The troughs have intrigued planetary scientists since the Mariner 9 spacecraft returned the first close-up image in 1972, but conclusive evidence of their origin has remained elusive. Debate continues regarding all aspects of the troughs, including the possibility that they have migrated, their age in relation to the current NPLD surface, and whether they are fundamentally erosional or constructional features. The troughs are probably related to climatic processes, yet the nature of this relationship has remained a mystery. Previous data characterizing only the exposed NLPD surface were insufficient to test these hypotheses. Here we show that the central spiral troughs initiated after deposition of three-quarters of the NPLD, quickly reached a stable morphology and migrated approximately 65 kilometres poleward and 600 metres in altitude over the past two million years or so. Our radar stratigraphy rules out hypotheses of erosional incision post-dating deposition, and instead largely validates an early hypothesis for constructional trough migration with wind transport and atmospheric deposition as dominant processes. These results provide hard constraints for palaeo-climate models and a new context for evaluating imagery, spectral data, and now radar sounding data, the better to understand the link between orbital parameters and climate, the role of climate in shaping the polar ice of Mars, and eventually, the age of the polar deposits themselves.

MAIC-2, a latitudinal model for the Martian surface temperature, atmospheric water transport and surface glaciation

The Mars Atmosphere-Ice Coupler MAIC-2 is a simple, latitudinal model, which consists of a set of parameterisations for the surface temperature, the atmospheric water transport and the surface mass balance (condensation minus evaporation) of water ice. It is driven directly by the orbital parameters obliquity, eccentricity and solar longitude ( L s) of perihelion. Surface temperature is described by the Local Insolation Temperature (LIT) scheme, which uses a daily and latitude-dependent radiation balance. The evaporation rate of water is calculated by an expression for free convection, driven by density differences between water vapor and ambient air, the condensation rate follows from the assumption that any water vapour which exceeds the local saturation pressure condenses instantly, and atmospheric transport of water vapour is approximated by instantaneous mixing. Glacial flow of ice deposits is neglected. Simulations with constant orbital parameters show that low obliquities favour deposition of ice in high latitudes and vice versa. A transient scenario driven by a computed history of orbital parameters over the last 10 million years produces essentially monotonically growing polar ice deposits during the most recent 4 million years, and a very good agreement with the observed present-day polar layered deposits. The thick polar deposits sometimes continue in thin ice deposits which extend far into the mid latitudes, which confirms the idea of “ice ages” at high obliquity.

A photohabitable zone in the martian snowpack? A laboratory and radiative-transfer study of dusty water–ice snow

Dusty water–ice snowpacks on Mars may provide a habitable zone for DNA based photosynthetic life. Previous work has over estimated the depths and thicknesses of such photohabitable zones by not considering the effect of red dust within the snowpack. For the summer solar solstice, at 80°N and a surface albedo of 0.45, there is a calculated photohabitable zone in the snowpack between depths of 5.5 and 7.5 cm. For an albedo of 0.62, there is a calculated photohabitable zone in the snowpack between depths of 8 and 11 cm. A coupled atmosphere–snow radiative-transfer model was set to model the Photosynthetic Active Radiation and DNA dose rates through water–ice snow at the north polar region of Mars. The optical properties of the polar caps were determined by creating a laboratory analogue to the Mars north polar deposits, and directly measuring light penetration and albedo. It is important for future exobiology missions to the polar regions of Mars to consider the implications of these findings, as drilling to depths of ∼11 cm should be sufficient to determine whether life exists within the martian snows, whether it is photosynthetic or otherwise, as at this depth the snow cover will provide a permanent protection from DNA damaging UV radiation.

Enhanced CO 2 trapping in water ice via atmospheric deposition with relevance to Mars

It has been suggested that inclusions of CO 2 or CO 2 clathrate hydrates may comprise a portion of the polar deposits on Mars. Here we present results from an experimental study in which CO 2 molecules were trapped in water ice deposited from CO 2/H 2O atmospheres at temperatures relevant for the polar regions of Mars. Fourier-Transform Infrared spectroscopy was used to monitor the phase of the condensed ice, and temperature programmed desorption was used to quantify the ratio of species in the generated ice films. Our results show that when H 2O ice is deposited at 140–165 K, CO 2 is trapped in large quantities, greater than expected based on lower temperature studies in amorphous ice. The trapping occurs at pressures well below the condensation point for pure CO 2 ice, and therefore this mechanism may allow for CO 2 deposition at the poles during warmer periods. The amount of trapped CO 2 varied from 3% to 16% by mass at 160 K, depending on the substrate studied. Substrates studied were a tetrahydrofuran (C 4H 8O) base clathrate and Fe–montmorillonite clay, an analog for Mars soil. Experimental evidence indicates that the ice structures are likely CO 2 clathrate hydrates. These results have implications for the CO 2 content, overall composition, and density of the polar deposits on Mars.

Investigation of snow cover and an air of atmosphere in vicinities of the North Pole using the pollen analysis method

This paper presents the results of pollen analysis from the samples of atmospheric air and samples of a freshly fallen snow taken from the surface of the ice-block drifted station ‘Barneo-2005’ (Russia) is where, about the vicinities of located North Pole in 2005. New data received and the data that has been obtained by us previously (Ukraintseva and Sokolov 2003) allow as to conclusion that bacteria, pollen, spores of plants, certain fragments of their tissues, and organs are brought by air currents to the high latitude regions of the Earth, where they form specific spore–pollen spectra. These data have a high theoretical–practical application continuum, especially during the study of mineral sequences of the polar basin's deposits. Taking into account the importance of the data, which we have obtained, we suggest below the methods of sampling during such this of investigation.

Subsurface structure of Planum Boreum from Mars Reconnaissance Orbiter Shallow Radar soundings

We map the subsurface structure of Planum Boreum using sounding data from the Shallow Radar (SHARAD) instrument onboard the Mars Reconnaissance Orbiter. Radar coverage throughout the 1,000,000-km 2 area reveals widespread reflections from basal and internal interfaces of the north polar layered deposits (NPLD). A dome-shaped zone of diffuse reflectivity up to 12 μs (∼1-km thick) underlies two-thirds of the NPLD, predominantly in the main lobe but also extending into the Gemina Lingula lobe across Chasma Boreale. We equate this zone with a basal unit identified in image data as Amazonian sand-rich layered deposits [Byrne, S., Murray, B.C., 2002. J. Geophys. Res. 107, 5044, 12 pp. doi:10.1029/2001JE001615; Fishbaugh, K.E., Head, J.W., 2005. Icarus 174, 444–474; Tanaka, K.L., Rodriguez, J.A.P., Skinner, J.A., Bourke, M.C., Fortezzo, C.M., Herkenhoff, K.E., Kolb, E.J., Okubo, C.H., 2008. Icarus 196, 318–358]. Elsewhere, the NPLD base is remarkably flat-lying and co-planar with the exposed surface of the surrounding Vastitas Borealis materials. Within the NPLD, we delineate and map four units based on the radar-layer packets of Phillips et al. [Phillips, R.J., and 26 colleagues, 2008. Science 320, 1182–1185] that extend throughout the deposits and a fifth unit confined to eastern Gemina Lingula. We estimate the volume of each internal unit and of the entire NPLD stack (821,000 km 3), exclusive of the basal unit. Correlation of these units to models of insolation cycles and polar deposition [Laskar, J., Levrard, B., Mustard, J.F., 2002. Nature 419, 375–377; Levrard, B., Forget, F., Montmessin, F., Laskar, J., 2007. J. Geophys. Res. 112, E06012, 18 pp. doi:10.1029/2006JE002772] is consistent with the 4.2-Ma age of the oldest preserved NPLD obtained by Levrard et al. [Levrard, B., Forget, F., Montmessin, F., Laskar, J., 2007. J. Geophys. Res. 112, E06012, 18 pp. doi:10.1029/2006JE002772]. We suggest a dominant layering mechanism of dust–content variation during accumulation rather than one of lag production during periods of sublimation.

Surface-enhanced spectroscopic investigation of the adsorption properties of hydroxybenzoic acid isomers onto metallic surfaces

Surface-enhanced Raman Spectroscopy (SERS), surface-enhanced infrared absorption spectroscopy (SEIRA), temperature-programmed desorption (TPD), and density functional theory were used to characterize the adsorption properties of the hydroxybenzoic acid (HBA) isomers including ortho-hydroxybenzoic acid (OHA), meta-hydroxybenzoic acid (MHA), and para-hydroxybenzoic acid (PHA) using various deposition solvents with different polar properties. SERS typically enhances the Raman shifts of the monolayer, while SEIRA is a longer range effect, often providing vibrational enhancement to both the monolayer and multilayer. TPD analysis showed that OHA adsorption to silver is weaker than MHA/PHA, most likely as a result of the strong OHA intramolecular hydrogen-bonding. SERS and SEIRA demonstrated that OHA ionized efficiently in the monolayer and multilayer independent of the solvent polarity because of OHA's low p K a (2.98). MHA/PHA ionized better than OHA in the multilayer in less polar deposition solvents, and a decrease in the polarity of the deposition solvent created additional ordering in the MHA monolayer while inducing stronger adsorption in the PHA monolayer. It is believed that a lower level of solvation with less polar deposition solvents allowed for more adsorbate/substrate interaction and more intermolecular attraction. The addition of more MHA to a multilayer resulted only in stronger SEIRA peaks. As a PHA multilayer thickened there was significant structural changes represented by new bands and spectral peak shifts with greater intermolecular attraction as the multilayer approached bulk properties. Due to the range of applications involving HBA isomers, these studies could find significant applications in biochemistry, medicine, nanotechnology and environmental science.

The Polar Deposits of Mars

The tantalizing prospect of a readable record of martian climatic variations has driven decades of work toward deciphering the stratigraphy of the martian polar layered deposits and understanding the role of the residual ice caps that cover them. Spacecraft over the past decade have provided a massive infusion of new data into Mars science. Polar science has benefited immensely due to the near-polar orbits of most of the orbiting missions and the successful landing of the Phoenix spacecraft in the northern high latitudes. Topographic, thermal, radar, hyperspectral, and high-resolution imaging data are among the datasets that have allowed characterization of the stratigraphy of the polar layered deposits in unprecedented detail. Additionally, change within the residual ice caps has been monitored with spacecraft instruments for several years. These new data have provided a golden opportunity to understand the interplay between the martian orbit, climate, and polar ice.

True Polar Wander driven by late-stage volcanism and the distribution of paleopolar deposits on Mars

The areal centroids of the youngest polar deposits on Mars are offset from those of adjacent paleopolar deposits by 5–10°. We test the hypothesis that the offset is the result of True Polar Wander (TPW), the motion of the solid surface with respect to the spin axis, caused by a mass redistribution within or on the surface of Mars. In particular, we consider the possibility that TPW is driven by late-stage volcanism during the Late Hesperian to Amazonian. There is observational and qualitative support for this hypothesis: in both north and south, observed offsets lie close to a great circle 90° from Tharsis, as expected for polar wander after Tharsis formed. We calculate the magnitude and direction of TPW produced by mapped late-stage lavas for a range of lithospheric thicknesses, lava thicknesses, eruption histories, and prior polar wander events. We find that if Tharsis formed close to the equator, the stabilizing effect of a fossil rotational bulge located close to the equator leads to predicted TPW of < 2°, too small to account for observed offsets. If, however, Tharsis formed far from the equator, late-stage TPW driven by low-latitude, late-stage volcanism would be 6–33°, similar to that inferred from the location of paleopolar deposits. A volume of 4.4 ± 1.3 × 10 19 kg of young erupted lava can account for the offset of the Dorsa Argentea Formation from the present-day south rotation pole. This volume is consistent with prior mapping-based estimates and would imply a mass release of CO 2 by volcanic degassing similar to that in the atmosphere at the present time. The South Polar Layered Deposits are offset from the present rotation pole in a direction that is opposite to the other paleopolar deposits. This can be explained by either a sequential eruption of late-stage lavas, or an additional contribution from a plume beneath Elysium. We predict that significant volcanic activity occurred during the time interval represented by the Basal Unit/Planum Boreum unconformity; Planum Boreum postdates the Promethei Lingula Lobe; and that the north polar deposits span a substantial fraction of Solar System history. If the additional contribution to TPW from plumes is small, then we would also predict that Tharsis Montes Formation postdates the Promethei Lingula Lobe of the South Polar Layered Deposits. We conclude with a list of observational tests of the TPW hypothesis.

North polar deposits of Mars: Extreme purity of the water ice

The polar layered deposits are the largest reservoir of water on the surface of Mars. The physical properties of the ice and their spatial distribution are largely unknown. 140,000 data points from the sounding radar SHARAD on the Mars Reconnaissance Orbiter were analyzed over the Gemina Lingula region, one‐fourth of the north polar layered deposits area. Maps of the dielectric properties of the bulk ice were drawn up. There is no basal melting signature. A drop of the dielectric constant in north‐west of Gemina Lingula could be explained by an abrupt 250‐meter uplift of the base. The bulk ice of the studied region has an average dielectric constant of 3.10 (σ = 0.12) and a loss tangent &lt;0.0026 (σ = 0.0005). Analytic interpretations shown the volume of ice is pure at ≥95%. The impurities have a radial distribution, with higher concentrations at margins.

Storms, polar deposits and the methane cycle in Titan's atmosphere

In Titan's atmosphere, the second most abundant constituent, methane, exists as a gas, liquid and solid, and cycles between the atmosphere and the surface. Similar to the Earth's hydrological cycle, Titan sports clouds, rain and lakes. Yet, Titan's cycle differs dramatically from its terrestrial counterpart, and reveals the workings of weather in an atmosphere that is 10 times thicker than the Earth's atmosphere, that is two orders of magnitude less illuminated, and that involves a different condensable. While ongoing measurements by the Cassini-Huygens mission are revealing the intricacies of the moon's weather, circulation, lake coverage and geology, knowledge is still limited by the paucity of observations. This review of Titan's methane cycle therefore focuses on measured characteristics of the lower atmosphere and surface that appear particularly perplexing or alien.

Shallow radar (SHARAD) sounding observations of the Medusae Fossae Formation, Mars

The SHARAD (shallow radar) sounding radar on the Mars Reconnaissance Orbiter detects subsurface reflections in the eastern and western parts of the Medusae Fossae Formation (MFF). The radar waves penetrate up to 580 m of the MFF and detect clear subsurface interfaces in two locations: west MFF between 150 and 155° E and east MFF between 209 and 213° E. Analysis of SHARAD radargrams suggests that the real part of the permittivity is ∼3.0, which falls within the range of permittivity values inferred from MARSIS data for thicker parts of the MFF. The SHARAD data cannot uniquely determine the composition of the MFF material, but the low permittivity implies that the upper few hundred meters of the MFF material has a high porosity. One possibility is that the MFF is comprised of low-density welded or interlocked pyroclastic deposits that are capable of sustaining the steep-sided yardangs and ridges seen in imagery. The SHARAD surface echo power across the MFF is low relative to typical martian plains, and completely disappears in parts of the east MFF that correspond to the radar-dark Stealth region. These areas are extremely rough at centimeter to meter scales, and the lack of echo power is most likely due to a combination of surface roughness and a low near-surface permittivity that reduces the echo strength from any locally flat regions. There is also no radar evidence for internal layering in any of the SHARAD data for the MFF, despite the fact that tens-of-meters scale layering is apparent in infrared and visible wavelength images of nearby areas. These interfaces may not be detected in SHARAD data if their permittivity contrasts are low, or if the layers are discontinuous. The lack of closely spaced internal radar reflectors suggests that the MFF is not an equatorial analog to the current martian polar deposits, which show clear evidence of multiple internal layers in SHARAD data.