Do young martian ray craters have ages consistent with the crater count system?

McEwen et al. (McEwen, A.S., Preblich, B.S., Turtle, E.P., Artemieva, N.A., Golombek, M.P., Hurst, M., Kirk, R.L., Burr, D.M., Christensen, P. [2005]. Icarus 176, 351-381) developed a useful test for the internal consistency of crater-count chronometry systems. They argued that certain multi-kilometer, fresh-looking martian craters with prominent rays should be the youngest or near-youngest craters in their size range. The “McEwen et al. test” is that the ages determined from crater densities of the smallest superimposed craters (typically diameter D ∼ 5-20 m) should thus be comparable to the expected formation intervals of the host primary. McEwen et al. concluded from MOC data that crater chronometry failed this test by factors of 700-2000. We apply HiRISE and other imagery to eight different young craters in order to re-evaluate their arguments. We use existing crater chronology systems as well as the reported observed production rate of 16 m craters (Malin, M.C., Edgett, K., Posiolova, L., McColley, S., Noe Dobrea, E. [2006]. Science 314, 1573-1557; Hartmann, W.K., Quantin, C., Mangold, N. [2007]. Icarus 186, 11-23; Kreslavsky [2007]. Seventh International Conference on Mars, 3325). Every case passes the McEwen et al. test. We conclude that the huge inconsistencies suggested by McEwen et al. are spurious. Many of these craters show evidence of impact into ice-rich material, and appear to have ice-flow features and sublimation pits on their floors. As production rate data improve, decameter-scale craters will provide a valuable way of dating these young martian geological formations and the processes that modify them.     

Recent low-latitude freeze-thaw on Mars

Outside polar latitudes, features corresponding to surface thaw have yet to be identified on Mars. The youthful gully landforms observed at mid-high latitude [Malin, M., Edgett, K., 2000. Science 288, 2330-2335] are the nearest candidate, but the source (and nature) of the gully carving agent remains controversial [e.g., Musselwhite, D.S., Swindle, T.D., Lunine, J.I., 2001. Geophys. Res. Lett. 28, 1283-1285; Mellon, M.T., Phillips, R.J., 2001. J. Geophys. Res. 106, 1-15; Knauth, L.P., Burt, D.M., 2002. Icarus 158, 267-271; Costard, F., Forget, F., Mangold, N., Peulvast, J.P., 2002. Science 295, 110-113; Christensen, P.R., 2003. Nature 422, 45-48; Treiman, A.H., 2003. J. Geophys. Res. 108]. At higher obliquity than the present epoch, near-surface ground ice should be present globally [Mellon, M.T., Jakosky, B.M., 1995. J. Geophys. Res. 100 (E6), 11781-11799], populated by condensation of atmospheric water vapour in the top few metres of the regolith, or emplaced as dusty ice sheets reaching down towards the equator. The latitudinal restriction of these gullies to regions poleward of ±30° appears to argue against a thaw component to their formation—since ground ice is present and stable at all latitudes at high obliquity, the current (low) obliquity regime should result in ground ice thaw at low latitudes, where insolation and daytime temperatures are currently greatest, and this is not observed. A previously undescribed meltwater sequence in the Cerberus plains, at 20° N/187° E, shows that comparable, but much more continuous, and mappable melting and surface runoff have occurred in the geologically recent past at near-equatorial latitudes on Mars. Polygonal ground in the Cerberus plains is seen by the Mars Global Surveyor Mars Orbiter Camera (MOC) to suffer sequential, regional-scale volatile-loss consistent with thaw of near-surface ground ice under periglacial conditions. This degradation is continuously sampled by a single MOC strip, showing an icy landscape undergoing thaw modification and collapse, and may form the first evidence of equatorial wet-based glaciation during late Amazonian time, with indications of melting within the last million years. The dissolution and re-formation of polygonal ground links this landform to freeze-thaw processes, providing the conclusion to a question that has been the subject of debate for three decades—whether Mars' polygonal grounds require ice to form—and a consistent explanation for the fate of the water that carved the great outflow channels, much of which may still reside as ground ice in the regolith. This thaw occurs in the Cerberus Formation; deposits that are considered to be magmatic in origin, and the type formation for late-stage, “plains-style” volcanism on Mars [Keszthelyi, L., McEwen, A.S., Thordarson T., 2000. J. Geophys. Res. 105, 15027-15049]. By superposing large numbers of small impact craters, polygonal ground in the Cerberus plains sustains previous suggestions of a non-magmatic origin for this and other landforms in the region [Page, D.P., Murray, J.B., 2006. Icarus 183, 46-54]. Together, these periglacial landforms document evidence of climate change much younger than is currently recognised by crater counts, with important implications for age constraints on young surfaces and absolute age determinations by this method. It is tentatively suggested that this melting may be occurring today, with a striking correspondence between permafrost thaw in the Cerberus plains, the high atmospheric methane flux currently observed over this region [Mumma, M.J., Novak, R.E., DiSanti, M.A., Bonev, B.P., Dello Russo, N., 2004. Bull. Am. Astron. Soc. 36, 1127; Krasnopolsky, V.A., Maillard, J.P., Owen, T.C., 2004. Icarus 172, 537-547; Formisano, V., Atreya, S., Encrenaz, T., Ignatiev, N., Giuranna, M., 2004. Science 306, 1758-1761], and the only latitude zone on Mars—equatorward of 30° N—where melting of ground ice is thought possible in the current climate [Haberle, R.M., McKay, C.P., Schaeffer, J., Cabrol, N.A., Grin, E.A., Zent, A.P., Quinn, R., 2001. J. Geophys. Res. 106 (E10), 23317-23326; Lobitz, B., Wood, B.L., Averner, M.M., McKay, C.P., 2001. Proc. Natl. Acad. Sci. 98, 2132-2137]. Low-latitude polygonal ground as transient, and hydrologically active over wide areas transforms our understanding of the recent climatic evolution of Mars, supporting models of atmospheric water-ice migration [Mischna, M., Richardson, M.I., Wilson, R.J., McCleese, D.J., 2003. J. Geophys. Res. 108 (E6). 5062], complex, volatile stratigraphies [Clifford, S.M., Parker, T.J., 2001. Icarus 154, 40-79], and hypothesised, geologically recent ‘ice ages’ [Head, J.W., Mustard, J.F., Kreslavsky, M.A., Milliken, R.E., Marchant, D.R., 2003. Nature 426, 797-802]. The temporal coincidence of glacial epochs on the Earth and Mars during the Quaternary and latest Amazonian would suggest a coupled system linking both [Sagan, C., Young, A.T., 1973. Nature 243, 459].     

Spectral heterogeneity on Phobos and Deimos: HiRISE observations and comparisons to Mars Pathfinder results

The High-Resolution Imaging Science Experiment (HiRISE) onboard Mars Reconnaissance Orbiter (MRO) has been used to observe Phobos and Deimos at spatial scales of around 6 and 20 m/px, respectively. HiRISE (McEwen et al., JGR, 112, CiteID E05S02, DOI: 10.1029/2005JE002605, 2007) has provided, for the first time, high-resolution colour images of the surfaces of the Martian moons. When processed, by the production of colour ratio images for example, the data show considerable small-scale heterogeneity, which might be attributable to fresh impacts exposing different materials otherwise largely hidden by a homogenous regolith. The bluer material that is draped over the south-eastern rim of the largest crater on Phobos, Stickney, has been perforated by an impact to reveal redder material and must therefore be relatively thin. A fresh impact with dark crater rays has been identified. Previously identified mass-wasting features in Stickney and Limtoc craters stand out strongly in colour. The interior deposits in Stickney appear more inhomogeneous than previously suspected. Several other local colour variations are also evident.Deimos is more uniform in colour but does show some small-scale inhomogeneity. The bright “streamers” (Thomas et al., Icarus, 123, 536–556,1996) are relatively blue. One crater to the south-west of Voltaire and its surroundings appear quite strongly reddened with respect to the rest of the surface. The reddening of the surroundings may be the result of ejecta from this impact.The spectral gradients at optical wavelengths observed for both Phobos and Deimos are quantitatively in good agreement with those found by unresolved photometric observations made by the Imager for Mars Pathfinder (IMP; Thomas et al., JGR, 104, 9055–9068, 1999). The spectral gradients of the blue and red units on Phobos bracket the results from IMP.     

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.     

Spectral evidence of volcanic cryptodomes on the northern plains of Mars

The composition and detailed morphology of dome-shaped features located in western Arcadia Planitia and just west of Utopia Planitia were examined in this study utilizing data from Mars Reconnaissance Orbiter and Mars Odyssey sensors. The domes have diameters averaging 1.5 km and heights averaging 160 m, and are generally dark-toned, although some are lighter toned or have split dark and light-toned surfaces. The domes are surrounded by annular deposits comprising, with increasing distance from the domes, dark-toned aprons, light-toned aureoles, and dark-toned aureoles. Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) data over several areas in the western Arcadia region show that spectra from the flanks of several domes have 1 and 2 μm absorption features consistent with the presence of olivine and a high-Ca pyroxene, nominally augite. Modified Gaussian Model (MGM) analysis of these spectra indicates Fe-rich olivine compositions. The tops of domes and the aprons surrounding many domes have negative sloping flat spectra in the near infrared, which is consistent with tachylite-rich, glassy compositions. High Resolution Imaging Science Experiment (HiRISE) images over several domes indicate that relatively high thermal inertia values associated with the tops of domes can be attributed to boulder strewn surfaces. HiRISE images also reveal that light-toned aureoles around domes consist of crenulated ground resembling “brain terrain” textures previously described for ice-rich concentric crater fill elsewhere on the northern plains. The plains surrounding the domes also display lineations that are interpreted to be lava channels or tubes. The combination of volcanic and ice-related features are consistent with the domes having formed as cryptodomes in the near sub-surface. We suggest that the domes could be basaltic in composition if the magmas were degassed and/or highly crystallized, and thus more viscous than typical basaltic magmas. The intrusion of these magmas into an ice-rich horizon would have produced a pervasively jointed chilled margin on the domes, which, once the domes were exposed, would have mechanically weathered to form the dark aprons. The domes could have served as local centers for ice accumulation during periods of high orbital obliquity, which ultimately would have led to the formation of the “brain terrain” surrounding the features. The domes represent late stage volcanic products on the northern plains of Mars and associated features provide more evidence for the role that ice accumulation and modification has played in recent martian history.     

The formation and evolution of youthful gullies on Mars: Gullies as the late-stage phase of Mars’ most recent ice age

Gullies are extremely young erosional/depositional systems on Mars that have been carved by an agent that was likely to have been comprised in part by liquid water [Malin, M.C., Edgett, K.S., 2000. Evidence for recent groundwater seepage and surface runoff on Mars. Science 288, 2330-2335; McEwen, A.S. et al., 2007. A closer look at water-related geologic activity on Mars. Science 317, 1706-1709]. The strong latitude and orientation dependencies that have been documented for gullies require (1) a volatile near the surface, and (2) that insolation is an important factor for forming gullies. These constraints have led to two categories of interpretations for the source of the volatiles: (1) liquid water at depth beneath the melting isotherm that erupts suddenly (“groundwater”), and (2) ice at the surface or within the uppermost layer of soil that melts during optimal insolation conditions (“surface/near-surface melting”). In this contribution we synthesize global, hemispheric, regional and local studies of gullies across Mars and outline the criteria that must be met by any successful explanation for the formation of gullies. We further document trends in both hemispheres that emphasize the importance of top-down melting of recent ice-rich deposits and the cold-trapping of atmospherically-derived H₂O frost/snow as important components in the formation of gullies. This provides context for the incorporation of high-resolution multi-spectral and hyper-spectral data from the Mars Reconnaissance Orbiter that show that (1) cold-trapping of seasonal H₂O frost occurs at the alcove/channel-level on contemporary Mars; (2) gullies are episodically active systems; (3) gullies preferentially form in the presence of deposits plausibly interpreted as remnants of the Late Amazonian emplacement of ice-rich material; and (4) gully channels frequently emanate from the crest of alcoves instead of the base, showing that alcove generation is not necessarily a product of undermining and collapse at these locations, a prediction of the groundwater model. We interpret these various lines of evidence to mean that the majority of gullies on Mars are explained by the episodic melting of atmospherically emplaced snow/ice under spin-axis/orbital conditions characteristic of the last several Myr.     

Stratigraphical evidence of late Amazonian periglaciation and glaciation in the Astapus Colles region of Mars

Recent modeling of the meteorological conditions during and following times of high obliquity suggests that an icy mantle could have been emplaced in western Utopia Planitia by atmospheric deposition during the late Amazonian period [Costard, F.M., Forget, F., Madeleine, J.B., Soare, R.J., Kargel, J.S., 2008. Lunar Planet. Sci. 39. Abstract 1274; Madeleine, B., Forget, F., Head, J.W., Levrard, B., Montmessin, F., 2007. Lunar Planet. Sci. 38. Abstract 1778]. Astapus Colles (ABa) is a late Amazonian geological unit — located in this hypothesized area of accumulation — that comprises an icy mantle tens of meters thick [Tanaka, K.L., Skinner, J.A., Hare, T.M., 2005. US Geol. Surv. Sci. Invest., Map 2888]. For the most part, this unit drapes the early Amazonian Vastitas Borealis interior unit (ABv_i); to a lesser degree it overlies the early Amazonian Vastitas Borealis marginal unit (ABv_m) and the early to late Hesperian UP plains unit HBu₂ [Tanaka, K.L., Skinner, J.A., Hare, T.M., 2005. US Geol. Surv. Sci. Invest., Map 2888]. Landscapes possibly modified by late-Amazonian periglacial processes [Costard, F.M., Kargel, J.S., 1995. Icarus 114, 93-112; McBride, S.A., Allen, C.C., Bell, M.S., 2005. Lunar Planet. Sci. 36. Abstract 1090; Morgenstern, A., Hauber, E., Reiss, D., van Gasselt, S., Grosse, G., Schirrmeister, L., 2007. J. Geophys. Res. 112, doi:10.1029/2006JE002869. E06010; Seibert, N.M., Kargel, J.S., 2001. Geophys. Res. Lett. 28, 899-902; Soare, R.J., Kargel, J.S., Osinski, G.R., Costard, F., 2007. Icarus 191, 95-112; Soare, R.J., Osinski, G.R., Roehm, C.L., 2008. Earth Planet. Sci. Lett. 272, 382-393] and glacial processes [Milliken, R.E., Mustard, J.F., Goldsby, D.L., 2003. J. Geophys. Res. 108 (E6), doi:10.1029/2002JE002005. 5057; Mustard, J.F., Cooper, C.D., Rifkin, M.K., 2001. Nature 412, 411-414; Tanaka, K.L., Skinner, J.A., Hare, T.M., 2005. US Geol. Surv. Sci. Invest., Map 2888] have been reported within the region. Researchers have assumed that the periglacial and glacial landscapes occur within the same geological unit, the ABa [i.e., Morgenstern, A., Hauber, E., Reiss, D., van Gasselt, S., Grosse, G., Schirrmeister, L., 2007. J. Geophys. Res. 112; doi:10.1029/2006JE002869. E06010; Tanaka, K.L., Skinner, J.A., Hare, T.M., 2005. US Geol. Surv. Sci. Invest., Map 2888]. In this study we use HiRISE (High Resolution Image Science Experiment, Mars Reconnaissance Orbiter) imagery to identify the stratigraphical separation of the two landscapes and show that periglacial landscape modification has occurred in the geological units that underlie the ABa, not in the ABa itself. Moreover, we suggest that the periglacial landscape extends well beyond the perimeter of the ABa and could be the product of “wet” cold-climate processes. These processes involve freeze-thaw cycles and intermittently stable liquid-water at or near the surface. By contrast, we propose that the ABa is a very recent late-Amazonian geological unit formed principally by “dry” cold-climate processes. These processes comprise accumulation (by atmospheric deposition) and ablation (by sublimation).     

Investigating gully flow emplacement mechanisms using apex slopes

The origin of the martian gullies has been much debated since their discovery by the Mars Orbiter Camera (MOC, Malin, M.C., Edgett, K.S. [2000]. Science 288, 2330-2335). Several previous studies have looked at slope gradients in and around gullies, but none have used Digital Elevation Models (DEMs) from the High Resolution Imaging Science Experiment (HiRISE, McEwen, A.S., and 14 colleagues [2007]. J. Geophys. Res. 112 (E05), E0505S02), which has a pixel scale down to 25 cm/pixel. We use five 1 m/post HiRISE DEMs to measure gully apex slopes, the local channel gradient at the upslope extent of the gully debris apron, which marks a shift from erosion to deposition. The apex slope provides information about whether a flow was likely a typical dry granular flow (begins depositing on slopes ∼21°) or fluidized by some extra mechanism (depositing on shallower slopes). We find that 72% of the 75 gully fans studied were likely emplaced by fluidized flows. Relatively old gullies appear more likely to have hosted fluidized flows than relatively fresh gullies. This suggests a time and location dependent fluidizing agent, possibly liquid water produced in a different climate as previously proposed. Our results do not provide evidence for water-rich flows in gullies today.     

Principal components analysis of Mars in the near-infrared

Principal components analysis and target transformation are applied to near-infrared image cubes of Mars in a study to disentangle the spectra into a small number of spectral endmembers and characterize the spectral information. The image cubes are ground-based telescopic data from the NASA Infrared Telescope Facility during the 1995 and 1999 near-aphelion oppositions when ice clouds were plentiful [ [Clancy et al., 1996] and [56]], and the 2003 near-perihelion opposition when ice clouds are generally limited to topographically high regions (volcano cap clouds) but airborne dust is more common [Martin, L.J., Zurek, R.W., 1993. J. Geophys. Res. 98 (E2), 3221-3246]. The heart of the technique is to transform the data into a vector space along the dimensions of greatest spectral variance and then choose endmembers based on these new “trait” dimensions. This is done through a target transformation technique, comparing linear combinations of the principal components to a mineral spectral library. In general Mars can be modeled, on the whole, with only three spectral endmembers which account for almost 99% of the data variance. This is similar to results in the thermal infrared with Mars Global Surveyor Thermal Emission Spectrometer data [Bandfield, J.L., Hamilton, V.E., Christensen, P.R., 2000. Science 287, 1626-1630]. The globally recovered surface endmembers can be used as inputs to radiative transfer modeling in order to measure ice abundance in martian clouds [Klassen, D.R., Bell III, J.F., 2002. Bull. Am. Astron. Soc. 34, 865] and a preliminary test of this technique is also presented.     

Volcanism on Io: New insights from global geologic mapping

We produced the first complete, 1:15 M-scale global geologic map of Jupiter’s moon Io, based on a set of monochrome and color Galileo-Voyager image mosaics produced at a spatial resolution of 1 km/pixel. The surface of Io was mapped into 19 units based on albedo, color and surface morphology, and is subdivided as follows: plains (65.8% of surface), lava flow fields (28.5%), mountains (3.2%), and patera floors (2.5%). Diffuse deposits (DD) that mantle the other units cover ∼18% of Io’s surface, and are distributed as follows: red (8.6% of surface), white (6.9%), yellow (2.1%), black (0.6%), and green (∼0.01%). Analyses of the geographical and areal distribution of these units yield a number of results, summarized below. (1) The distribution of plains units of different colors is generally geographically constrained: Red-brown plains occur >±30° latitude, and are thought to result from enhanced alteration of other units induced by radiation coming in from the poles. White plains (possibly dominated by SO₂ + contaminants) occur mostly in the equatorial antijovian region (±30°, 90-230°W), possibly indicative of a regional cold trap. Outliers of white, yellow, and red-brown plains in other regions may result from long-term accumulation of white, yellow, and red diffuse deposits, respectively. (2) Bright (possibly sulfur-rich) flow fields make up 30% more lava flow fields than dark (presumably silicate) flows (56.5% vs. 43.5%), and only 18% of bright flow fields occur within 10 km of dark flow fields. These results suggest that secondary sulfurous volcanism (where a bright-dark association is expected) could be responsible for only a fraction of Io’s recent bright flows, and that primary sulfur-rich effusions could be an important component of Io’s recent volcanism. An unusual concentration of bright flows at ∼45-75°N, ∼60-120°W could be indicative of more extensive primary sulfurous volcanism in the recent past. However, it remains unclear whether most bright flows are bright because they are sulfur flows, or because they are cold silicate flows covered in sulfur-rich particles from plume fallout. (3) We mapped 425 paterae (volcano-tectonic depressions), up from 417 previously identified by Radebaugh et al. (Radebaugh, J., Keszthelyi, L.P., McEwen, A.S., Turtle, E.P., Jaeger, W., Milazzo, M. [2001]. J. Geophys. Res. 106, 33005-33020). Although these features cover only 2.5% of Io’s surface, they correspond to 64% of all detected hot spots; 45% of all hot spots are associated with the freshest dark patera floors, reflecting the importance of active silicate volcanism to Io’s heat flow. (4) Mountains cover only ∼3% of the surface, although the transition from mountains to plains is gradational with the available imagery. 49% of all mountains are lineated and presumably layered, showing evidence of linear structures supportive of a tectonic origin. In contrast, only 6% of visible mountains are mottled (showing hummocks indicative of mass wasting) and 4% are tholi (domes or shields), consistent with a volcanic origin. (5) Initial analyses of the geographic distributions of map units show no significant longitudinal variation in the quantity of Io’s mountains or paterae, in contrast to earlier studies. This is because we use the area of mountain and patera materials as opposed to the number of structures, and our result suggests that the previously proposed anti-correlation of mountains and paterae (Schenk, P., Hargitai, H., Wilson, R., McEwen, A., Thomas, P. [2001]. J. Geophys. Res. 106, 33201-33222; Kirchoff, M.R., McKinnon, W.B., Schenk, P.M. [2011]. Earth Planet. Sci. Lett. 301, 22-30) is more complex than previously thought. There is also a slight decrease in surface area of lava flows toward the poles of Io, perhaps indicative of variations in volcanic activity. (6) The freshest bright and dark flows make up about 29% of all of Io’s flow fields, suggesting active emplacement is occurring in less than a third of Io’s visible lava fields. (7) About 47% of Io’s diffuse deposits (by area) are red, presumably deriving their color from condensed sulfur gas, and ∼38% are white, presumably dominated by condensed SO₂. The much greater areal extent of gas-derived diffuse deposits (red + white, 85%) compared to presumably pyroclast-bearing diffuse deposits (dark (silicate tephra) + yellow (sulfur-rich tephra), 15%) indicates that there is effective separation between the transport of tephra and gas in many Ionian explosive eruptions. Future improvements in the geologic mapping of Io can be obtained via (a) investigating the relationships between different color/material units that are geographically and temporally associated, (b) better analysis of the temporal variations in the map units, and (c) additional high-resolution images (spatial resolutions ∼200 m/pixel or better). These improvements would be greatly facilitated by new data, which could be obtained by future missions.     

Soluble ferric iron as an effective protective agent against UV radiation: Implications for early life

Some recent MER Rover Opportunity results on ancient sedimentary rocks from Mars describe sandstones originated from the chemical weathering of olivine basalts by acidic waters [Squyres, S.W., Knoll, A.H., 2005. Earth Planet. Sci. Lett. 240, 1-10]. The absence of protective components in early Mars atmosphere forced any possible primordial life forms to deal with high doses of UV radiation. A similar situation occurred on the primitive Earth during the development of early life in the Archean [Berkner, L.V., Marshall, L.C., 1965. J. Atmos. Sci. 22 (3), 225-261; Kasting, J.F., 1993. Science 259, 920-926]. It is known that some cellular and/or external components can shield organisms from damaging UV radiation or quench its toxic effects [Olson, J.M., Pierson, B.K., 1986. Photosynth. Res. 9, 251-259; García-Pichel, F., 1998. Origins Life Evol. B 28, 321-347; Cockell, C., Rettberg, P., Horneck, G., Scherer, K., Stokes, M.D., 2003. Polar Biol. 26, 62-69]. The effectiveness of iron minerals for UV protection has also been reported [Phoenix, V.R., Konhauser, K.O., Adams, D.G., Bottrell, S.H., 2001. Geology 29 (9), 823-826], but nothing is known about the effect of iron in solution. Here we demonstrate the protective effect of soluble ferric iron against UV radiation on acidophilic photosynthetic microorganisms. These results offer an interesting alternative means of protection for life on the surface of early Mars and Earth, especially in light of the geochemical conditions in which the sedimentary minerals, jarosite and goethite, recently reported by the MER missions, were formed [Squyres, S.W., Arvidson, R.E., Bell III, J.F., Brückner, J., Cabrol, N.A., Calvin, W., Carr, M.H., Christensen, P.R., Clark, B.C., Crumpler, L., Des Marais, D.J., d'Uston, C., Economou, T., Farmer, J., Farrand, W., Folkner, W., Golombek, M., Gorevan, S., Grant, J.A., Greeley, R., Grotzinger, J., Haskin, L., Herkenhoff, K.E., Hviid, S., Johnson, J., Klingelhöfer, G., Knoll, A.H., Landis, G., Lemmon, M., Li, R., Madsen, M.B., Malin, M.C., McLennan, S.M., McSween, H.Y., Ming, D.W., Moersch, J., Morris, R.V., Parker, T., Rice Jr., J.W., Richter, L., Rieder, R., Sims, M., Smith, M., Smith, P., Soderblom, L.A., Sullivan, R., Wänke, H., Wdowiak, T., Wolff, M., Yen, A., 2004. Science 306, 1698-1703; Klingelhöfer, G., Morris, R.V., Bernhardt, B., Schröder, C., Rodionov, D.S., de Souza Jr., P.A., Yen, A., Gellert, R., Evlanov, E.N., Zubkov, B., Foh, J., Bonnes, U., Kankeleit, E., Gütlich, P., Ming, D.W., Renz, F., Wdowiak, T., Squyres, S.W., Arvidson, R.E., 2004. Science 306, 1740-1745].     

Stratigraphical and morphological evidence for pingo genesis in the Cerberus plains

‘Rootless’ debris cones (or pseudocraters) occur in platy, patterned ground throughout the Cerberus plains of Mars and are thought to represent the products of explosive magma-ice interaction [Lanagan et al., 2001. Geophys. Res. Lett. 28, 2365-2368; Fagents et al., 2002. In: Smellie, J.L., Chapman, M.G. (Eds.), Volcano-Ice Interaction on Earth and Mars. In: Geol. Soc. Spec. Publ., vol. 202, pp. 295-317]. Requiring lava and water interspersed, they are central to theories of multiple magmatic and aqueous flood events [Burr et al., 2002. Icarus 159, 53-73; Berman, D.C., Hartmann, W.K., 2002. Icarus 159, 1-17] and widespread sheet volcanism [Keszthelyi et al., 2000. J. Geophys. Res. 105, 15027-15049] in the region during the late Amazonian (a region reported to have been occupied by water bodies ranging from lakes to oceans [Scott et al., 1995. Map of Mars showing channels and possible paleolake basins. USGS Miscellaneous Investigations Series, Map I-2461 (1:30,000,000)]). The nature of the platy substrate is the subject of debate, with evidence given for lava [Keszthelyi et al., 2000. J. Geophys. Res. 105, 15027-15049; Plescia, J.B., 2003. Icarus 164, 79-95] and ice [Brakenridge, G.R., 1993. Lunar Planet. Sci. XXIV (Part 1), 175-176; Rice et al., 2002. Lunar Planet. Sci. XXXIII. Abstract #2026; Murray et al., 2005. Nature 434, 352-355]. The superposition relationships of cones and platy deposits in the channels of the Athabasca Valles precludes a magmatic origin, indicating later formation as permafrost mounds (or ‘pingos’), with implications for geologically recent flood volcanism, age constraints on young surfaces and recent climate change on Mars.     

Morphology, temperature, and eruption dynamics at Pele

The Pele region of Io has been the site of vigorous volcanic activity from the time of the first Voyager I observations in 1979 up through the final Galileo ones in 2001. There is high-temperature thermal emission from a visibly dark area that is thought to be a rapidly overturning lava lake, and is also the source of a large sulfur-rich plume. We present a new analysis of Voyager I visible wavelength images, and Galileo Solid State Imager (SSI) and Near Infrared Mapping Spectrometer (NIMS) thermal emission observations which better define the morphology of the region and the intensity of the emission. The observations show remarkable correlations between the locations of the emission and the features seen in the Voyager images, which provide insight into eruption mechanisms and constrain the longevity of the activity. We also analyze an additional wavelength channel of NIMS data (1.87 μm) which paradoxically, because of reduced sensitivity, allows us to estimate temperatures at the peak locations of emission. Measurements of eruption temperatures on Io are crucial because they provide our best clues to the composition of the magma. High color temperatures indicative of ultramafic composition have been reported for the Pillan hot spot and possibly for Pele, although recent work has called into question the requirement for magma temperatures above those expected for ordinary basalts. Our new analysis of the Pele emission near the peak of the hot spot shows color temperatures near the upper end of the basalt range during the I27 and I32 encounters. In order to analyze the observed color temperatures we also present an analytical model for the thermal emission from fire-fountains, which should prove generally useful for analyzing similar data. This is a modification of the lava flow emission model presented in Howell (Howell, R.R. [1997]. Icarus 127, 394-407), adapted to the fire-fountain cooling curves first discussed in Keszthelyi et al. (Keszthelyi, L., Jaeger, W., Milazzo, M., Radebaugh, J., Davies, A.G., Mitchell, K.L. [2007]. Icarus 192, 491-502). When applied to the I32 observations we obtain a fire-fountain mass eruption rate of 5.1 × 10⁵ kg s⁻¹ for the main vent area and 1.4 × 10⁴ kg s⁻¹ for each of two smaller vent regions to the west. These fire-fountain rates suggest a solution to the puzzling lack of extensive lava flows in the Pele region. Much of the erupted lava may be ejected at high speed into the fire-fountains and plumes, creating dispersed pyroclastic deposits rather than flows. We compare gas and silicate mass eruption rates and discuss briefly the dynamics of this ejection model and the observational evidence.     

Ionospheric plasma acceleration at Mars: ASPERA-3 results

The Analyzer of Space Plasma and Energetic Atoms (ASPERA) on-board the Mars Express spacecraft (MEX) measured penetrating solar wind plasma and escaping/accelerated ionospheric plasma at very low altitudes (250 km) in the dayside subsolar region. This implies a direct exposure of the martian topside atmosphere to solar wind plasma forcing leading to energization of ionospheric plasma. The ion and electron energization and the ion outflow from Mars is surprisingly similar to that over the magnetized Earth. Narrow “monoenergetic” cold ion beams, ion beams with broad energy distributions, sharply peaked electron energy spectra, and bidirectional streaming electrons are particle features also observed near Mars. Energized martian ionospheric ions (O⁺, O⁺₂, CO⁺₂, etc.) flow in essentially the same direction as the external sheath flow. This suggests that the planetary ion energization couples directly to processes in the magnetosheath/solar wind. On the other hand, the beam-like distribution of the energized plasma implies more indirect energization processes like those near the Earth, i.e., energization in a magnetized environment by waves and/or parallel (to B) electric fields. The general conditions for martian plasma energization are, however, different from those in the Earth's magnetosphere. Mars has a weak intrinsic magnetic field and solar wind plasma may therefore penetrate deep into the dense ionospheric plasma. Local crustal magnetization, discovered by Acuña et al. [Acuña, M.J., Connerey, J., Ness, N., Lin, R., Mitchell, D., Carlsson, C., McFadden, J., Anderson, K., Rème, H., Mazelle, C., Vignes, D., Wasilewski, P., Cloutier, P., 1999. Science 284, 790-793], provide some dayside shielding against the solar wind. On the other hand, multiple magnetic anomalies may also lead to “hot spots” facilitating ionospheric plasma energization. We discuss the ASPERA-3 findings of martian ionospheric ion energization and present evidences for two types of plasma energization processes responsible for the low- and mid-altitude plasma energization near Mars: magnetic field-aligned acceleration by parallel electric fields and plasma energization by low frequency waves.     

Emplacement of the youngest flood lava on Mars: A short, turbulent story

Recently acquired data from the High Resolution Imaging Science Experiment (HiRISE), Context (CTX) imager, and Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) onboard the Mars Reconnaissance Orbiter (MRO) spacecraft were used to investigate the emplacement of the youngest flood-lava flow on Mars. Careful mapping finds that the Athabasca Valles flood lava is the product of a single eruption, and it covers 250,000 km² of western Elysium Planitia with an estimated 5000-7500 km³ of mafic or ultramafic lava. Calculations utilizing topographic data enhanced with MRO observations to refine the dimensions of the channel system show that this flood lava was emplaced turbulently over a period of only a few to several weeks. This is the first well-documented example of a turbulently emplaced flood lava anywhere in the Solar System. However, MRO data suggest that this same process may have operated in a number of martian channel systems. The magnitude and dynamics of these lava floods are similar to the aqueous floods that are generally believed to have eroded the channels, raising the intriguing possibility that mechanical erosion by lava could have played a role in their incision.     

Distribution and evolution of scalloped terrain in the southern hemisphere, Mars

Scalloped depressions are a unique martian surface morphology found in the northern and southern hemisphere latitude-dependent dust and ice-rich surface mantles. These features exhibit a distinct asymmetric north-south slope profile, characterized by steep pole-facing scarps, flat floors and gentle equator-facing slopes. We examined High Resolution Stereo Camera (HRSC) images of the southern hemisphere to determine their longitudinal distribution, which revealed that a majority of scalloped terrain is located in the region of the southern wall of the Hellas Basin and northern Malea Planum. A detailed map of this area was produced where scallops were found to contour the southern wall of the basin, and where the ice-rich mantle was seen to be thickest. Scalloped terrain is concentrated along the topographic highs near the Amphitrites and Peneus Paterae and areal extent and depth decreases with increasing depth into the basin. We also examined existing hypothesis for the formation and evolution of scalloped depressions using High Resolution Imaging Science Experiment (HiRISE) images and data from the Thermal Emission Imaging System-Infrared (THEMIS-IR) and the Thermal Emission Spectrometer (TES). Our approach provides regional context for the development of scalloped terrains within the southern hemisphere, and offers detailed evidence of scallop depressions forming around small cracks, presumably caused by thermal contraction. Morphometric measurements show that scalloped depressions can be as much as 40 m deep, with typical depths of between 10 and 20 m. Our observations of scallop formation and development in the southern hemisphere support a solar-insolation model proposed by previous researchers (e.g. [Morgenstern, A., Hauber, E., Reiss, D., van Gasselt, S., Grosse, G., Schirrmeister, L., 2007. J. Geophys. Res. 112, CiteID E06010; Lefort, A., Russell, P.S., Thomas, N., McEwen, A.S., Dundas, C.M., Kirk, R.L., 2009a. J. Geophys. Res. 114, E04005; Lefort, A., Russell, P.S., Thomas, N., 2009b. Icarus, in press]). Observations made using HiRISE images suggest that scalloped depressions most likely form from small cracks in the mantle, which become larger and deeper through sublimation of interstitial ice from within the mantle. Sublimation is likely enhanced on equator-facing slopes because of increased solar insolation, which accounts for the asymmetric slope profile and hemispherical orientation and is demonstrated by THEMIS-IR images. We suggest that sublimation lag deposits can possibly be removed by dust devils or strong slope winds related to the Hellas Basin, offering an explanation as to why scalloped terrain is so abundant only in this area of the southern hemisphere. Daytime maximum summer temperatures suggest that sublimation in the study area of Malea Planum is possible under current conditions if the sublimation lag is removed. While it cannot be ruled out that scalloped terrain in Malea Planum is presently evolving, we attribute the extensive distribution to geologically recent obliquity excursions when conditions were more conducive to mesoscale modification of the ice-rich mantle.     

Constraints on the origin and evolution of the layered mound in Gale Crater, Mars using Mars Reconnaissance Orbiter data

Gale Crater contains a 5.2 km-high central mound of layered material that is largely sedimentary in origin and has been considered as a potential landing site for both the MER (Mars Exploration Rover) and MSL (Mars Science Laboratory) missions. We have analyzed recent data from Mars Reconnaissance Orbiter to help unravel the complex geologic history evidenced by these layered deposits and other landforms in the crater. Results from imaging data from the High Resolution Imaging Science Experiment (HiRISE) and Context Camera (CTX) confirm geomorphic evidence for fluvial activity and may indicate an early lacustrine phase. Analysis of spectral data from the CRISM (Compact Reconnaissance Imaging Spectrometer for Mars) instrument shows clay-bearing units interstratified with sulfate-bearing strata in the lower member of the layered mound, again indicative of aqueous activity. The formation age of the layered mound, derived from crater counts and superposition relationships, is ∼3.6-3.8 Ga and straddles the Noachian-Hesperian time-stratigraphic boundary. Thus Gale provides a unique opportunity to investigate global environmental change on Mars during a period of transition from an environment that favored phyllosilicate deposition to a later one that was dominated by sulfate formation.     

Distribution and time-variation of spire streaks at Pavonis Mons on Mars

We documented the distribution and the time-variation of the specific dark wind streaks at Pavonis Mons. We focused on the streaks we named “Spire Streaks”, which are overlapping spindle shaped dark streaks at the upper boundary of the coalesced dark streaks on Tharsis volcanoes. We investigated both visible and infrared images obtained by Viking orbiter camera, Mars Orbiter Camera (MOC), THEMIS, CTX and HiRISE of the spire streaks at Pavonis Mons. We also used topographic data obtained by Mars Orbiter Laser Altimeter (MOLA) to see the relationship between the topography and the distribution of the spire streaks. The spire streaks at Pavonis Mons provide us high-resolution information about the direction of the nighttime slope wind, and could be indirect clues for the time-variation of the nighttime environment. We conclude that the spire streaks are erosional features. However, some features of the spire streaks reported in this paper are outside the scope of previous modeling for erosional process, and we need a new category of model for the formation.     

Ice-rich terrain in Gusev Crater, Mars?

The morphology of materials on the floor of Gusev Crater (14° S, 175° W), Mars, imply a history of volcanism and subsequent removal of an ice-rich deposit. Fluid lava flows observed in the western portion of Gusev Crater paradoxically terminate in a steep, thick (<60 m) flow front adjacent to hummocky terrain. The hummocky terrain is morphologically similar to deglaciated terrain on Earth, generated when glacial debris are left behind after the glacier has retreated. We propose the following scenario for the floor of Gusev Crater. First, ice-rich material was deposited adjacent to Thira Crater. Second, fluid lavas were emplaced and ponded against the ice-rich deposits. At some later time, the ice within the deposit sublimated, leaving hummocky terrain. Current age estimates for the Gusev flows are Hesperian, suggesting that the ice removal occurred in the upper Hesperian or more recently. If this hypothesis is correct, quench features (glassy rinds, columnar jointing) should be observed at the lava flow margin; the hummocky deposit should be poorly sorted, angular debris.     

Cratering on Mars with almost no atmosphere or volatiles: Pangboche crater

Pangboche crater (17.2°N, 226.7°E; 10.4 km dia.) lies close to the summit of Olympus Mons volcano, Mars, at an elevation of ~20.9 km above the datum. Given a scale height of 11.1 km for the atmosphere, this relatively large fresh crater most likely formed at an atmospheric pressure <1 mbar in essentially volatile‐free young lava flows. Detailed analysis of Pangboche crater from High Resolution Imaging Science Experiment (HiRISE) and Context Camera (CTX) images reveals that volatile‐related features (e.g., fluidized ejecta layers and pitted floor material) are absent. In contrast, abundant impact melt occurs on the floor, inner walls, and rim of the crater, and there is an extensive field of secondary craters that extend up to approximately 45 km from the rim crest. All of these attributes argue that it was the absence of volatiles in the target rocks at the time of crater formation, rather than the thin atmosphere, which had a controlling influence on crater morphology. Digital elevation data derived from the CTX images reveal that Pangboche crater has a depth of about 954 m (depth/diameter = approximately 0.092) and that uplifted target rocks comprise about 58% of the relief of the 180 m‐high north rim. As the target material comprised a sequence of layered lava flows, Pangboche crater may well represent the best crater on Mars for direct comparison with craters formed on the Moon (permitting variations in gravitational effects to be investigated) or on Mercury (allowing the role of the atmosphere to be studied).