The dispersal of pyroclasts from Apollinaris Patera, Mars: Implications for the origin of the Medusae Fossae Formation

The Medusae Fossae Formation (MFF) has long been thought to be of Amazonian age, but recent studies propose that a significant part of its emplacement occurred in the Hesperian and that many of the Amazonian ages represent modification (erosional and redepositional) ages. On the basis of the new formational age, we assess the hypothesis that explosive eruptions from Apollinaris Patera might have been the source of the Medusae Fossae Formation. In order to assess the likelihood of this hypothesis, we examine stratigraphic relationships between Apollinaris Patera and the MFF and analyze the relief of the MFF using topographic data. We predict the areal distribution of tephra erupted from Apollinaris Patera using a Mars Global Circulation Model (GCM) combined with a semi-analytical explosive eruption model for Mars, and compare this with the distribution of the MFF. We conclude that Apollinaris Patera could have been responsible for the emplacement of the Medusae Fossae Formation.     

Mapping Medusae Fossae Formation materials in the southern highlands of Mars

The Medusae Fossae Formation (MFF) is an extensive deposit (2.2 × 10⁶ km², Bradley, B.A., Sakimoto, S.E.H., Frey, H., Zimbelman, J.R. [2002]. J. Geophys. Res. 107, 5058) of wind-eroded material of widely debated origin, which unconformably overlies a considerable area of the crustal dichotomy boundary on Mars. The MFF shows a variety of layering patterns, erosional styles and channel-like forms and has been mapped into five main outcrops and three geological members according to exposure and stratigraphy (Scott, D.H., Tanaka, K.L., 1986. USGS Map I-1802-A; Greeley, R., Guest, J.E., 1987. Map I-1802-B; Zimbelman, J.R., Crown, D., Jenson, D., 1996. Lunar Planet. Sci. XXVII. Abstract #1748.). Away from the three main lobes are numerous outliers of MFF materials. These have mainly been reported in the northern lowlands regions (Keszthelyi, L., Jaeger, W.L., and HiRISE team, 2008. Lunar Planet. Sci. XXXIX. Abstract #2420.) but few studies have examined the possibility of MFF outliers on high ground south of the dichotomy boundary. We have searched Mars Orbiter Camera Narrow Angle (MOC NA) images for outliers in this region. Our observations show that there are many MFF outliers on the southern highlands. The characteristics of the outliers indicate materials which overlie the underlying terrain for they appear widely in dips, craters and topographic lows. The surfaces are typified by yardang fields and have a similar patchy and discontinuous nature to materials of the upper member of the MFF. Most have consistent lineation orientations across the wider area which match the dominant orientation of yardangs in the main MFF outcrops. Furthermore, elevation data shows that the maximum, minimum and mean elevations of these newly discovered outliers are closest to those of the upper member of the MFF. We therefore conclude that these deposits are MFF outliers and that they probably represent remnant upper member material. We suggest that there might be two possible explanations for these outliers: (1) the MFF had a much greater pre-erosional extent than previously estimated, or (2) materials from the main outcrops were eroded and then blown south to accrue in the highland areas, where they were subsequently reworked. We suggest that the topography of the region favors the first option. We outline an “overflowing” layer-cake deposition model, in which layers of sediment stacked up against the dichotomy boundary until they reached the topographic level of the highlands. Further materials (that went onto become upper-member MFF material and outliers) were then deposited across a wider area, including south of the dichotomy boundary. Severe erosion subsequently removed much of this material.     

Distinct erosional progressions in the Medusae Fossae Formation, Mars, indicate contrasting environmental conditions

The form of erosional remnants of the Medusae Fossae Formation (MFF) on Mars provide evidence of their development progression and implicate two spatially distinct environments in the equatorial regions of Mars. Ubiquitous yardangs are clearly the product of strong unidirectional winds acting over time on variably indurated deposits. Yardang orientation is used as a proxy to map regional and local wind direction at meso-scale resolution. In other, more limited areas not subjected to strong unidirectional winds, randomly oriented kilometer-scale mesas and buttes are found to be remnants of progressive cliff recession through mass wasting as support is lost from within the MFF lithology at its margins. The differing processes that dominate the formation of the distinctive landforms have implications for meso-scale variations in climate that remain unresolved by current modeling efforts. Additionally, the variability of erosional forms within the deposit emphasizes the overall complexity of this extensive formation.     

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.     

Tectonic histories between Alba Patera and Syria Planum, Mars

Syria Planum and Alba Patera are two of the most prominent features of magmatic-driven activity identified for the Tharsis region and perhaps for all of Mars. In this study, we have performed a Geographic Information System-based comparative investigation of their tectonic histories using published geologic map information and Mars Orbiter Laser Altimetry (MOLA) data. Our primary objective is to assess their evolutional histories by focusing on their extent of deformation in space and time through stratigraphic, paleotectonic, topographic, and geomorphologic analyses. Though there are similarities among the two prominent features, there are several distinct differences, including timing deformational extent, and tectonic intensity of formation. Whereas Alba Patera displays a major pulse of activity during the Late Hesperian/Early Amazonian, Syria Planum is a long-lived center that displays a more uniform distribution of simple graben densities ranging from the Noachian to the Amazonian, many of which occur at greater distances away from the primary center of activity. The histories of the two features presented here are representative of the complex, long-lived evolutional history of Tharsis.     

HiRISE images of yardangs and sinuous ridges in the lower member of the Medusae Fossae Formation, Mars

HiRISE images of the lower member of the Medusae Fossae Formation (MFF) were used to identify characteristics of two specific landforms that are well expressed in this particular geologic unit; yardangs and sinuous ridges. Yardangs are wind-eroded ridges that are usually confined to arid environments where the bedrock materials can be easily eroded by windblown sand. Yardangs are common in the lower member of MFF, where many individual yardangs show evidence of a caprock unit overlying a more friable unit, most consistent with an ignimbrite origin for these MFF deposits. Heights of the yardangs in the lower member materials are generally less than a few tens of meters, in contrast to yardangs in the thicker middle member MFF materials to the east of the study area. The yardangs may form in materials comprised of discrete depositional units, and there is good evidence that at least a dozen such depositional events contributed to the emplacement of the lower member of MFF. The lower member yardang heights indicate aeolian erosion has removed at least 19,000 km³ of lower member MFF materials. Sinuous ridges are elongate, positive-relief landforms that have been attributed to a variety of possible fluvial flow processes on Mars. Sinuous ridges are very common within exposures of the lower member of MFF. Multiple ridge types are present, but all forms seen at HiRISE scale are most consistent with some form of aqueous channel flow rather than other possible origins. The results from this initial examination of HiRISE images indicate the potential utility of comparing yardangs and sinuous ridges in the lower member to other members of MFF, although it remains to be determined if sinuous ridges are abundant in the younger MFF members.     

The age of the Medusae Fossae Formation: Evidence of Hesperian emplacement from crater morphology, stratigraphy, and ancient lava contacts

The Medusae Fossae Formation (MFF), covering about 2.1 × 10⁶ km² (with an estimated volume of 1.4 × 10⁶ km³) and straddling the equatorial region of Mars east of Tharsis, has historically been mapped and dated as Amazonian in age. Analysis of the MFF using a range of new observations from recent mission data at multiple resolutions reveals evidence that the formation is older than previously hypothesized, with parts of the MFF having formed in the Hesperian and parts having been reworked and reformed throughout the Amazonian, up to the present. Ancient outcroppings of the MFF, edged with jagged yardangs, became a “mold” for embaying Hesperian-aged lavas. The erosion of the MFF left solidified lava “casts” in the embaying lava unit. This lava edge morphology permits the identification of ancient contacts between the MFF and Hesperian-aged lava terrain. Additionally, the flanking fan of the Hesperian-aged Apollinaris Patera volcano embays the formation at its foot, indicating that parts of the MFF were formed in the Hesperian. Erosion has erased and inverted many of the superposed craters in the region, showing that very young Amazonian ages derived from impact crater size-frequency distributions are resurfacing ages, and not emplacement ages. We find abundant evidence that the formation is extremely mobile and continuously reworked. We conclude that a significant part of the MFF may have originally been emplaced in the Hesperian. These observations place new constraints on the mode of origin of the MFF.     

East-west trending magnetic anomalies in the Southern Hemisphere of Mars: Modeling analysis and interpretation

Maps of the vector components of the martian crustal magnetic field over the strongly magnetized Terra Cimmeria/Sirenum region are constructed using Mars Global Surveyor magnetometer data. Although pronounced east-west trending anomalies are present on the radial and north field component maps at the mapping altitude (∼360-380 km), these trends are much less prominent at the lower aerobraking altitude (∼90-150 km). Comparisons with similar maps produced using artificial data at the aerobraking altitude indicate that elongated sources in this region may have maximum lengths along the martian surface of ∼500 km and maximum aspect ratios of ∼2. Iterative forward modeling of several relatively isolated anomalies in the mapped region yields paleomagnetic pole positions consistent with those estimated in previous studies of other anomalies using mapping phase and science phasing orbit data. On this basis, it is inferred that sources in the studied region are most probably magnetized primarily in northward or southward directions. Using this additional constraint, iterative forward modeling is then applied to determine a magnetization distribution that is consistent with data at both the aerobraking altitude and the mapping altitude. The model magnetization distribution, which includes 41 discrete sources, again indicates no highly elongated sources. An examination of surface geology in the region as well as a consideration of the global distribution of anomalies suggests that magmatic intrusions (e.g., subsurface dike swarms), cooling in the presence of water, are the most likely sources of the magnetic anomalies.     

Deep versus shallow origin of gravity anomalies, topography and volcanism on Earth, Venus and Mars

The relation between gravity anomalies, topography and volcanism can yield important insights about the internal dynamics of planets. From the power spectra of gravity and topography on Earth, Venus and Mars we infer that gravity anomalies have likely predominantly sources below the lithosphere up to about spherical harmonic degree l=30 for Earth, 40 for Venus and 5 for Mars. To interpret the low-degree part of the gravity spectrum in terms of possible sublithospheric density anomalies we derive radial mantle viscosity profiles consistent with mineral physics. For these viscosity profiles we then compute gravity and topography kernels, which indicate how much gravity anomaly and how much topography is caused by a density anomaly at a given depth. With these kernels, we firstly compute an expected gravity-topography ratio. Good agreement with the observed ratio indicates that for Venus, in contrast to Earth and Mars, long-wavelength topography is largely dynamically supported from the sublithospheric mantle. Secondly, we combine an empirical power spectrum of density anomalies inferred from seismic tomography in Earth’s mantle with gravity kernels to model the gravity power spectrum. We find a good match between modeled and observed gravity power spectrum for all three planets, except for 2?l?4 on Venus. Density anomalies in the Venusian mantle for these low degrees thus appear to be very small. We combine gravity kernels and the gravity field to derive radially averaged density anomaly models for the Martian and Venusian mantles. Gravity kernels for l?5 are very small on Venus below ≈800 km depth. Thus our inferences on Venusian mantle density are basically restricted to the upper 800 km. On Mars, gravity anomalies for 2?l?5 may originate from density anomalies anywhere within its mantle. For Mars as for Earth, inferred density anomalies are dominated by l=2 structure, but we cannot infer whether there are features in the lowermost mantle of Mars that correspond to Earth’s Large Low Shear Velocity Provinces (LLSVPs). We find that volcanism on Mars tends to occur primarily in regions above inferred low mantle density, but our model cannot distinguish whether or not there is a Martian analog for the finding that Earth’s Large Igneous Provinces mainly originate above the margins of LLSVPs.     

Predicted and observed magnetic signatures of martian (de)magnetized impact craters

The current morphology of the martian lithospheric magnetic field results from magnetization and demagnetization processes, both of which shaped the planet. The largest martian impact craters, Hellas, Argyre, Isidis and Utopia, are not associated with intense magnetic fields at spacecraft altitude. This is usually interpreted as locally non- or de-magnetized areas, as large impactors may have reset the magnetization of the pre-impact material. We study the effects of impacts on the magnetic field. First, a careful analysis is performed to compute the impact demagnetization effects. We assume that the pre-impact lithosphere acquired its magnetization while cooling in the presence of a global, centered and mainly dipolar magnetic field, and that the subsequent demagnetization is restricted to the excavation area created by large craters, between 50- and 500-km diameter. Depth-to-diameter ratio of the transient craters is set to 0.1, consistent with observed telluric bodies. Associated magnetic field is computed between 100- and 500-km altitude. For a single-impact event, the maximum magnetic field anomaly associated with a crater located over the magnetic pole is maximum above the crater. A 200-km diameter crater presents a close-to-1-nT magnetic field anomaly at 400-km altitude, while a 100-km diameter crater has a similar signature at 200-km altitude. Second, we statistically study the 400-km altitude Mars Global Surveyor magnetic measurements modelled locally over the visible impact craters. This approach offers a local estimate of the confidence to which the magnetic field can be computed from real measurements. We conclude that currently craters down to a diameter of 200 km can be characterized. There is a slight anti-correlation of −0.23 between magnetic field intensity and impact crater diameters, although we show that this result may be fortuitous. A complete low-altitude magnetic field mapping is needed. New data will allow predicted weak anomalies above craters to be better characterized, and will bring new constraints on the timing of the martian dynamo and on Mars’ evolution.     

The influence of mantle melting on the evolution of Mars

We present a parameterized convection model of Mars by incorporating a new heat-flow scaling law for stagnant-lid convection, to better understand how the evolution of Mars may be affected by mantle melting. Melting in the mantle during convection leads to the formation of a compositionally buoyant lithosphere, which may also be intrinsically more viscous by dehydration. The consequences of these melting effects on the evolution of terrestrial planets have not been explored before. The temporal evolution of crust and lithospheric mantle is modeled in a self-consistent manner considering mantle melting, convective instability, and the rewetting of dehydrated lithosphere from below by hydrogen diffusion. Though the effect of compositional buoyancy turns out to be minimal, the introduction of viscosity contrast between wet and dry mantle can considerably slow mantle cooling and sometimes lead to non-monotonic core cooling. Furthermore, with or without dehydration stiffening, our model predicts that the martian mantle must have been degassed more extensively (>80%) than previously suggested (<10%); the loss of such a large amount of water from the mantle to surface has significant implications about the role of water in the early surface and climate evolution of Mars.     

Magnetic storms on Mars

Based on data from the Mars Global Surveyor magnetometer we examine periods of significantly enhanced magnetic disturbances in the martian space environment. Using almost seven years of observations during the maximum and early declining phase of the previous solar cycle the occurrence pattern and typical time profile of such periods is investigated and compared to solar wind measurements at Earth. Typical durations of the events are 20-40 h, and there is a tendency for large events to last longer, but a large spread in duration and intensity are found. The large and medium intensity events at Mars are found to occur predominantly in association with interplanetary sector boundaries, with solar wind dynamic pressure enhancements being the most likely interplanetary driver. In addition it is found that, on time scales of months to several years, the dominant cause of global variability of the magnetic field disturbance at Mars is solar wind dynamic pressure variations associated with the eccentricity of the martian orbit around the Sun.     

Mars magnetic field: Sources and models for a quarter of the southern hemisphere

The heavily-cratered southern hemisphere of Mars encompasses the planet’s strongest, most widespread magnetization. Our study concentrates on this magnetized region in the southern hemisphere within 40° of latitude 40°S, longitude 180°W. First we rotate the coordinates to position the center at −40°, 180° and treat these new latitudes and longitudes as if they were Cartesian coordinates. Then, using an ordinary two-dimensional Fourier analysis for downward continuation, the MGS (MAG/ER) magnetic field data at satellite mapping elevation of ∼400 km are extrapolated to 100 km, sources are estimated and used to model the fields. Quantitative comparison of the downward continued field with the aerobraking field for bins having angular deviation within ±30° gives correlation of .947, .868, and .769 for the components, respectively. This agreement of the fields may result from most of the power in the magnetization resting in wavelengths ∼400 km, with comparatively little at ∼100 km. Over this region, covering nearly an octant of the planet, just a dozen sources can account for 94% of the variance of the magnetic field at the surface. In these models for the field an obvious asymmetry in polarity exists, with majority of the sources being positive. The locations of strongest surface magnetization appear to be near - but not actually within - ancient multi-ringed basins. We test the likelihood of this association by comparing the observed sources found within and near basins for two alternative basin location scenarios with random distributions. For both alternatives we find the observed distributions to be low-probability occurrences. If contemporaneous, this would establish that Mars’ magnetic field extended to the time of impacts causing these basins.     

Aram Chaos and its constraints on the surface heat flux of Mars

The surface heat flux of a planet is an important parameter to characterize its internal activity and to determine its thermal evolution. Here we report on a new method to constrain the surface heat flux of Mars during the Hesperian. For this, we explore the consequences for the martian surface heat flux from a recently presented new hypothesis for the formation of Aram Chaos (Zegers, T.E., Oosthoek, J.H.P., Rossi, A.P., Blom, J.K., Schumacher, S. [2010]. Earth Planet. Sci. Lett. 297, 496-504. doi:10.1016/j.epsl.2010.06.049.). In this hypothesis the chaotic terrain is thought to have formed by melting of a buried ice sheet. The slow sedimentation and burial of the ice sheet led to an increased thermal insulation of the ice and subsequently to a temperature increase high enough to trigger melting and the formation of the subsurface lake. As these processes highly depend on the thermal properties of the subsurface and especially on the surface heat flux, it is possible to constrain the latter by using numerical simulations. Based on the hypothesis for the formation of Aram Chaos, we conducted an extensive parameter study to determine the parameter settings leading to sufficient melting of the buried ice sheet. We find that the surface heat flux in the Aram Chaos region during the Hesperian was most likely between 20 and 45 mW m⁻² with a possible maximum value of up to 60 mW m⁻².     

Early martian dynamo generation due to giant impacts

During late-stage planet formation, giant impacts produce localized mantle melt regions within which impactor iron droplets settle to the bottom near a permeability horizon. After accumulation, iron heated by the impact migrates downward to the core through colder, mostly solid mantle. The degree of thermal equilibration and partitioning of viscous heating between impactor iron and silicates depends on the mechanism of iron transport to the core. Simple estimates suggest that, following a giant impact, the temperature difference between iron delivered to the core and the mantle outside the impact heated region can be ∼10³ K. Hot impactor iron mergers with the core where it may be efficiently mixed or remain stratified due to thermal buoyancy. In either case, collisional energy carried to the core by impactor iron helps establish conditions favorable for early core cooling and dynamo generation. In this study, we consider the end-member scenario in which impactor iron forms a layer at the top of the core. Energy transfer from the impactor iron layer to the mantle is sufficient to power a dynamo for up to ∼30 Myr even in the limit of a very viscous mantle and heat flux limited by conduction. Using two-dimensional finite element calculations of mantle convection, we show that large-scale mantle flow driven by the buoyancy of the impact thermal anomaly focuses plumes in the impact region and increases both dynamo strength and duration. Melting within the mantle thermal boundary layer likely leads to formation of a single superplume in the location of the impact anomaly driven upwelling. We suggest that formation of magnetized southern highland crust may be related to spreading and differentiation of an impact melt region during the impact-induced dynamo episode.     

Flow patterns of lobate debris aprons and lineated valley fill north of Ismeniae Fossae, Mars: Evidence for extensive mid-latitude glaciation in the Late Amazonian

A variety of Late Amazonian landforms on Mars have been attributed to the dynamics of ice-related processes. Evidence for large-scale, mid-latitude glacial episodes existing within the last 100 million to 1 billion years on Mars has been presented from analyses of lobate debris aprons (LDA) and lineated valley fill (LVF) in the northern and southern mid-latitudes. We test the glacial hypothesis for LDA and LVF along the dichotomy boundary in the northern mid-latitudes by examining the morphological characteristics of LDA and LVF surrounding two large plateaus, proximal massifs, and the dichotomy boundary escarpment north of Ismeniae Fossae (centered at 45.3°N and 39.2°E). Lineations and flow directions within LDA and LVF were mapped using images from the Context (CTX) camera, the Thermal Emission Imaging Spectrometer (THEMIS), and the High Resolution Stereo Camera (HRSC). Flow directions were then compared to topographic contours derived from the Mars Orbiter Laser Altimeter (MOLA) to determine the down-gradient components of LDA and LVF flow. Observations indicate that flow patterns emerge from numerous alcoves within the plateau walls, are integrated over distances of up to tens of kilometers, and have down-gradient flow directions. Smaller lobes confined within alcoves and superposed on the main LDA and LVF represent a later, less extensive glacial phase. Crater size-frequency distributions of LDA and LVF suggest a minimum (youngest) age of 100 Ma. The presence of ring-mold crater morphologies is suggestive that LDA and LVF are formed of near-surface ice-rich bodies. From these observations, we interpret LDA and LVF within our study region to result from formerly active debris-covered glacial flow, consistent with similar observations in the northern mid-latitudes of Mars. Glacial flow was likely initiated from the accumulation and compaction of snow and ice on plateaus and in alcoves within the plateau walls as volatiles were mobilized to the mid-latitudes during higher obliquity excursions. Together with similar analyses elsewhere along the dichotomy boundary, these observations suggest that multiple glacial episodes occurred in the Late Amazonian and that LDA and LVF represent significant reservoirs of non-polar ice sequestered below a surface lag for hundreds of millions of years.     

Origin of the structure and planform of small impact craters in fractured targets: Endurance Crater at Meridiani Planum, Mars

We present observations and models that together explain many hallmarks of the structure and growth of small impact craters forming in targets with aligned fractures. Endurance Crater at Meridiani Planum on Mars (diameter ≈ 150 m) formed in horizontally-layered aeolian sandstones with a prominent set of wide, orthogonal joints. A structural model of Endurance Crater is assembled and used to estimate the transient crater planform. The model is based on observations from the Mars Exploration Rover Opportunity: (a) bedding plane orientations and layer thicknesses measured from stereo image pairs; (b) a digital elevation model of the whole crater at 0.3 m resolution; and (c) color image panoramas of the upper crater walls. This model implies that the crater’s current shape was mostly determined by highly asymmetric excavation rather than long-term wind-mediated erosion. We show that modal azimuths of conjugate fractures in the surrounding rocks are aligned with the square component of the present-day crater planform, suggesting excavation was carried farther in the direction of fracture alignments. This was previously observed at Barringer Crater in Arizona and we show the same relationship also holds for Tswaing Crater in South Africa. We present models of crater growth in which excavation creates a “stellate” transient cavity that is concave-cuspate in planform. These models reproduce the “lenticular-crescentic” layering pattern in the walls of some polygonal impact craters such as Endurance and Barringer Craters, and suggest a common origin for tear faults and some crater rays. We also demonstrate a method for detailed error analysis of stereogrammetric measurements of bedding plane orientations.     

Analysis of phyllosilicate deposits in the Nili Fossae region of Mars: Comparison of TES and OMEGA data

Diverse phyllosilicate deposits discovered previously in the Nili Fossae region with near infrared reflectance data are a window into the complex history of aqueous alteration on Mars. In this work, we used thermal infrared data from the Thermal Emission Spectrometer (TES) in combination with near infrared data from the Observatoire pour la Minéralogie, l’Eau, les Glaces, et l’Activité (OMEGA) to better constrain the mineralogy and geologic origin of these deposits. We developed a TES spectral index for identification of clay minerals, which correctly identifies the phyllosilicates in the Nili Fossae area and points to several other interesting deposits in the Syrtis Major region. However, detailed inspection of the TES spectral features of Nili Fossae phyllosilicates shows a feature at low wavenumbers (350-550 cm⁻¹) that is not an exact match to any specific Fe³⁺-, Al-, or Mg-rich phyllosilicate phase. Instead, the feature is more similar to basaltic glass and may indicate that the phyllosilicates in this region are: (1) rich in Fe²⁺ (based on similarity to trends seen in laboratory data of clay minerals), (2) poorly crystalline/extremely disordered, and/or (3) present within a matrix of actual basalt glass. This feature is similar to spectral features seen in altered rocks in the Columbia Hills region of Gusev Crater by previous authors. By calibrating measured spectral index values against mathematical spectral mixtures of typical martian dark surfaces and known abundances of alteration minerals, we are able to estimate an enrichment in abundance of alteration minerals in the altered surfaces. Many dark, Noachian deposits in the Nili Fossae area are enriched phyllosilicates by 20-30% (±10-15%) relative to dark, volcanic surfaces in the same region. The distribution and abundance of these phases indicates that alteration in the region was pervasive, but did not completely erase the original mineralogy of what was likely an Fe-rich basalt protolith. As a group, the Nili Fossae phyllosilicate deposits are fundamentally different from those found in the Mawrth Vallis region. Nili Fossae deposits have strong thermal infrared features related to admixed pyroxene, plagioclase, and occasionally olivine, whereas the Mawrth Vallis deposits contain no mafic minerals. Comparison of TES and OMEGA data also illustrates some more general differences between the datasets, including the impact of physical character of the martian surface on detectability of minerals in each spectral range.     

Localized ionization patches in the nighttime ionosphere of Mars and their electrodynamic consequences

Using an electron transport model, we calculate the electron density of the electron impact-produced nighttime ionosphere of Mars and its spatial structure. As input we use Mars Global Surveyor electron measurements, including an interval when accelerated electrons were observed. Our calculations show that regions of enhanced ionization are localized and occur near magnetic cusps. Horizontal gradients in the calculated ionospheric electron density on the night side of Mars can exceed 10⁴ cm⁻³ over a distance of a few tens of km; the largest gradients produced by the model are over 600 cm⁻³ km⁻¹. Such large gradients in the plasma density have several important consequences. These large pressure gradients will lead to localized plasma transport perpendicular to the ambient magnetic field which will generate horizontal currents and electric fields. We calculate the magnitude of these currents to be up to 10 nA/m². Additionally, transport of ionospheric plasma by neutral winds, which vary in strength and direction as a function of local time and season, can generate large (up to 1000 nA/m²) and spatially structured horizontal currents where the ions are collisionally coupled to the neutral atmosphere while electrons are not. These currents may contribute to localized Joule heating. In addition, closure of the horizontal currents and electric fields may require the presence of vertical, field-aligned currents and fields which may play a role in high altitude acceleration processes.     

Concentric crater fill in the northern mid-latitudes of Mars: Formation processes and relationships to similar landforms of glacial origin

Hypotheses accounting for the formation of concentric crater fill (CCF) on Mars range from ice-free processes (e.g., aeolian fill), to ice-assisted talus creep, to debris-covered glaciers. Based on analysis of new CTX and HiRISE data, we find that concentric crater fill (CCF) is a significant component of Amazonian-aged glacial landsystems on Mars. We present mapping results documenting the nature and extent of CCF along the martian dichotomy boundary over −30 to 90°E latitude and 20-80°N longitude. On the basis of morphological analysis we classify CCF landforms into “classic” CCF and “low-definition” CCF. Classic CCF is most typical in the middle latitudes of the analysis area (∼30-50°N), while a range of degradation processes results in the presence of low-definition CCF landforms at higher and lower latitudes. We evaluate formation mechanisms for CCF on the basis of morphological and topographic analyses, and interpret the landforms to be relict debris-covered glaciers, rather than ice-mobilized talus or aeolian units. We examine filled crater depth-diameter ratios and conclude that in many locations, hundreds of meters of ice may still be present under desiccated surficial debris. This conclusion is consistent with the abundance of “ring-mold craters” on CCF surfaces that suggest the presence of near-surface ice. Analysis of breached craters and distal glacial deposits suggests that in some locations, CCF-related ice was once several hundred meters higher than its current level, and has sublimated significantly during the most recent Amazonian. Crater counts on ejecta blankets of filled and unfilled craters suggests that CCF formed most recently between ∼60 and 300 Ma, consistent with the formation ages of other martian debris-covered glacial landforms such as lineated valley fill (LVF) and lobate debris aprons (LDA). Morphological analysis of CCF in the vicinity of LVF and LDA suggests that CCF is a part of an integrated LVF/LDA/CCF glacial landsystem. Instances of morphological continuity between CCF, LVF, and LDA are abundant. The presence of formerly more abundant CCF ice, coupled with the integration of CCF into LVF and LDA, suggests the possibility that CCF represents one component of the significant Amazonian mid-latitude glaciation(s) on Mars.