Vaduz, an unusual fresh crater on Mars: Evidence for impact into a recent ice-rich mantle
2011; American Geophysical Union; Volume: 38; Issue: 7 Linguagem: Inglês
10.1029/2010gl046605
ISSN1944-8007
AutoresE. I. Schaefer, J. W. Head, S. J. Kadish,
Tópico(s)Space Science and Extraterrestrial Life
ResumoGeophysical Research LettersVolume 38, Issue 7 PlanetsFree Access Vaduz, an unusual fresh crater on Mars: Evidence for impact into a recent ice-rich mantle Ethan I. Schaefer, Ethan I. Schaefer eis@rams.colostate.edu Department of Geological Sciences, Brown University, Providence, Rhode Island, USA Department of Geosciences, Colorado State University, Fort Collins, Colorado, USASearch for more papers by this authorJames W. Head, James W. Head Department of Geological Sciences, Brown University, Providence, Rhode Island, USASearch for more papers by this authorSeth J. Kadish, Seth J. Kadish Department of Geological Sciences, Brown University, Providence, Rhode Island, USASearch for more papers by this author Ethan I. Schaefer, Ethan I. Schaefer eis@rams.colostate.edu Department of Geological Sciences, Brown University, Providence, Rhode Island, USA Department of Geosciences, Colorado State University, Fort Collins, Colorado, USASearch for more papers by this authorJames W. Head, James W. Head Department of Geological Sciences, Brown University, Providence, Rhode Island, USASearch for more papers by this authorSeth J. Kadish, Seth J. Kadish Department of Geological Sciences, Brown University, Providence, Rhode Island, USASearch for more papers by this author First published: 09 April 2011 https://doi.org/10.1029/2010GL046605Citations: 13AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Abstract [1] A fresh, 1.85 km diameter impact crater in the midlatitudes of Mars (38°N) named Vaduz exhibits distinctive crater-related geological subunits (facies) extending up to ∼15 radii from the rim crest and perched >10 m above the adjacent plains. Knobby terrain fringing and underlying the facies is interpreted as degraded thermal contraction crack polygons, consistent with an ice-rich mantle buried by ejecta. The almost complete regional disappearance of this ice-rich unit, and consequent lowering of regional topography by over ten meters, is interpreted to mean that the ice-rich mantle was formed by climate-related deposition of snow, ice, and dust during recent periods of high obliquity. Transition to lower obliquity, and the attendant poleward retreat of the mantling unit, is the most likely cause of the regional loss of the ice-rich layer. The depth of this crater, relative to the thickness of the ice-rich unit, indicates that the impact event excavated bedrock and that Vaduz can be classified as an excess ejecta crater. Excavated silicate regolith and fragmented target debris appear to have been an important factor in the armoring mechanism. 1. Introduction [2] Since ∼5 Ma, high obliquity (spin axis inclination) excursions have produced multiple subtle ice ages, with stable ground ice [Mellon and Jakosky, 1995] and ice-rich deposits [Head et al., 2003] extending to ∼30°. In the most recent history, decrease in the amplitude of obliquity has caused surface and near-surface ice stability to migrate poleward to the high midlatitudes and poles [Mellon and Jakosky, 1995; Head et al., 2003]. [3] At issue is whether the migration and emplacement of volatiles during recent ice ages has taken place solely by vapor diffusion and ice deposition into preexisting regolith pore space [Mellon and Jakosky, 1995] or whether there was also surface deposition of ice, snow, and dust to create mantling layers. For example, several workers have described evidence for a recent ice-rich, latitude-dependent mantle, meters to tens of meters thick, currently preserved under a sublimation lag [Mustard et al., 2001; Kreslavsky and Head, 2002; Head et al., 2003; Schon et al., 2009]. [4] Impact craters on Mars show a wide variety of morphologies [Barlow et al., 2000], and some appear to have excavated into ice-rich units. Two such types are pedestal craters (Pd) and excess ejecta craters (EEC); both are elevated above their surroundings, a configuration generally attributed to local preservation of an underlying ice-rich unit that has been regionally removed [e.g., Barlow, 2006; Black and Stewart, 2008; Kadish et al., 2009]. Pedestal craters (Pd) generally occur in the center of a circular plateau (pedestal) that is typically tens to several hundreds of meters high; the margins of the pedestal extend well beyond the typical radial extent of the continuous ejecta deposit of the central crater [Kadish et al., 2009]. The observed depth of a pedestal crater is typically less than the height of the pedestal [Kadish et al., 2009]. Excess ejecta craters (EEC) are characterized by a crater that excavates through the ice-rich substrate into the subjacent regolith and bedrock and deposits ejecta on the surroundings; the ejecta becomes perched after regional removal of the ice-rich substrate [Black and Stewart, 2008]. The armoring mechanism involved in Pd and EEC is debated [see Kadish et al., 2009]. [5] In this analysis, we document a very fresh midlatitude impact crater (Figure 1), Vaduz, characterized by distinctive and extensive crater-related geological subunits (facies) that are perched over ten meters above adjacent plains on a substrate with a knobby surface texture. Together, these deposits provide insight into the nature, distribution, and timing of ice-rich mantles and the mechanisms for armoring and preserving these mantles during recent climatic history. Figure 1Open in figure viewerPowerPoint Vaduz, a very fresh, 1.85 km diameter impact crater centered at ∼38.2°N, 15.8°E. Inset shows regional context. White box indicates common extent of Figures 1a and 1b. (a) Portion of CTX image P15_007057_2179. A portion of Mamers Valles is seen at the lower right corner. (b) Geomorphic map, with map units labeled. Dashed circle indicates 10 radii radius from the rim crest. Box indicates extent of Figure 2a. 2. Crater Characterization [6] Located at ∼38.2°N, 15.8°E, just west of Mamers Valles near the dichotomy boundary, Vaduz is a 1.85 km diameter crater characterized by a number of distinctive facies (Figure 1) and is extremely fresh, with very few superposed impact craters; there is also no evidence of related secondary craters out to a search distance of ∼40 R (radii from the rim crest). The crater has minimal infilling, and the rim crest is sharp. [7] Six geomorphic units are associated with Vaduz and overlie the background Hesperian ridged plains (Hr) [Skinner et al., 2006] (Figure 1). From the center outward, these are crater interior (CI), crater rim (CR), lobate facies (LF), smooth facies (SF), radial facies (RF), and knobby terrain (KT). We use “crater facies” to denote the lobate, smooth, and radial facies collectively. [8] The outer margins of the lobate and smooth facies have lobateness values (Γ = (outer perimeter) / [4π (area)]1/2) of 1.24 and 2.61, respectively. This differs from pedestal craters, which are generally very circular, having an average Γ of 1.10 [Kadish et al., 2009]. The radial facies (RF; Figures 1 and 2a) is composed of radial, curvilinear ridges that parallel, cross, or truncate each other. Prominent, elongate projections of RF extend outward semicontinuously up to ∼15 R. RF has a Γ of 4.78. Figure 2Open in figure viewerPowerPoint Comparison of the knobby terrain of Vaduz with degraded polygonal terrain elsewhere on Mars. Note the strong morphologic similarity. (a) Knobby terrain knobs, generally with summit pits, outcrop between an enclave (lower left) and a tongue (upper right) of radial facies distinctly exhibiting radial, curvilinear ridges; the crater center is to the northwest. Portion of CTX image P15_007057_2179. (b) Degraded thermal contraction crack polygons in Noachis Terra at ∼43.6°S, 0.0°. Portion of High Resolution Imaging Science Experiment image PSP_003818_1360. [9] Knobby terrain (KT; Figures 1 and 2a) commonly fringes RF and, in places, extends in tongues well beyond it. KT is composed of polygonal to rounded knobs that range from equidimensional to elongate in planform and are both sparsely and densely distributed (Figure 2a). Knobs appear relatively low (compared to adjacent RF) and flat-topped, and are frequently centrally pitted. They are usually ∼25–60 m wide, and may be smaller, but features <∼25 m are poorly resolved in CTX data. Knobs have a moderate albedo and are always in a relatively lower-albedo background that is generally darker than adjacent Hr. [10] The surface of the radial facies (RF) generally lies ∼10–25 m above adjacent Hr in MOLA profile and HRSC DTM data (Figure 3a). Where the knobby terrain (KT) fringes the radial facies (RF; Figures 1, 2a, and 3), it is always lower than RF and slopes away from it. Figure 3Open in figure viewerPowerPoint Diagrammatic cross-sections illustrating the relationship of geologic units (Figure 1) and topography. (a) Cross-section of Vaduz and surrounding terrain showing current crater morphology and inferred stratigraphy. Proposed sequence of formation is illustrated in Figures 3b–3d. (b) During a recent high obliquity period, atmospheric precipitation of snow and codeposition of ice and dust emplaced a layered excess ice mantle over ten meters thick. (c) Thermal contraction crack polygons formed and matured on this mantle. (d) An impact emplaced the crater facies and possibly secondary craters, but any secondary craters would have been too shallow to excavate below the mantle. Later, a return to lower obliquity prompted removal of the mantle by sublimation, except where it was armored by proximal crater facies, forming the observed 10–25 m radial facies scarp and erasing secondary craters (Figure 3a). Not to scale. 3. Analysis and Interpretation [11] The rim crest sharpness, minimal crater infilling, crater facies crispness, lack of mantling material, and dearth of superposed craters all suggest that Vaduz is very young; superposed impact crater size-frequency distribution plots suggest an age of a few million years. The extreme ∼15 R extent of the projections in the radial facies is highly unusual, but both the gross morphology and crater-scaled extents of the crater facies are similar to those of larger “quasi-multiple-layer ejecta craters” found at higher latitudes [Barlow and Boyce, 2008]. Notably, Barlow and Boyce [2008] interpreted the large crater-scaled extents and high lobateness values of these craters to suggest a high volatile content for the target material; this conclusion may also apply to Vaduz. The presence of knobby terrain adjacent to, and lower than, the crater facies strongly suggests that the knobby terrain underlies the crater facies. MOLA profile and HRSC DTM data indicate that the crater facies and knobby substrate form a deposit ∼10–25 m thick that overlies the Hesperian ridged plains. [12] Is this knobby substrate impact-related or is it a preimpact relict? To answer this question, we used a method very similar to that used by Black and Stewart [2008] to identify craters with apparently excess ejecta. They compared the volume of material above the preimpact surface, including both ejected and uplifted material, to the volume of the crater cavity below that surface: VAbove/VCavity. To approximate the preimpact surface, they identified background terrain points near the crater and used the HMars program to interpolate a digital elevation model across the ejecta deposit and crater cavity using Delaunay triangulation. They found a mean VAbove/VCavity of 0.99 ± 0.41 for 572 fresh craters and identified ten craters with ratios ≥2.5 as excess ejecta craters (EEC). [13] We similarly identified background terrain points, interpolated a preimpact surface using Delaunay triangulation, and calculated VAbove/VCavity, although we used ArcGIS instead of HMars. We restricted the area of integration to the outermost contiguous extent of the crater facies but excluded a pie slice shaped region, originating at the crater center, that encompasses two craters (Figure 1) east of Vaduz. Black and Stewart [2008] also excluded pie slice shaped regions for some craters to avoid unrepresentative results; like them, we assumed axisymmetry to compensate. These calculations were performed on three different elevation datasets: MOLA (profile data for background points, gridded data for volumes), an HRSC DTM, and an HRSC DTM-MOLA hybrid. Given the small size of Vaduz, the HRSC DTM is likely more representative than the lower spatial resolution MOLA data, but MOLA data suggest a significantly deeper crater cavity. To allow for this possibility, we fit a paraboloid, centered at the centroid of the crater interior, to the MOLA profile data in the crater cavity and inserted this paraboloidal cavity into the HRSC DTM to produce an HRSC DTM-MOLA hybrid. [14] We found VAbove/VCavity values of 20.6, 16.4, and 3.2, corresponding to average excess thicknesses of 25.8 m, 16.1 m, and 11.2 m, for the MOLA data, HRSC DTM, and HRSC DTM-MOLA hybrid, respectively. These values clearly indicate that Vaduz is an excess ejecta crater (EEC), and it is the smallest EEC yet identified. Because uplifted material and ejecta bulking cannot explain the high Vabove/Vcavity of EEC [Black and Stewart, 2008], we conclude that the crater facies are emplaced on a relict substrate over ten meters thick that has been regionally removed (Figure 3). [15] The common fringing of radial facies (RF) with knobby terrain (KT) and the position of KT topographically below RF suggest that KT is part of this relict substrate. KT also lacks crater-radial and crater-concentric textures, is unlike common ejecta morphologies, and occurs regionally up to ∼41 km from Vaduz, consistent with a nonejecta origin. Additionally, its polygonal to rounded knobs resemble thermal contraction crack polygons (Figure 2), which occur in Mars-like areas on Earth [e.g., Marchant et al., 2002] and widely on Mars and are associated with icy substrates [Marchant and Head, 2007; Levy et al., 2006, 2010]. Although polygons are usually <25–40 m wide [Mangold, 2005; Levy et al., 2010], they reach widths of 50–300 m in some areas of Mars, suggesting micro- and/or paleoclimates allow for wider polygons [Mangold, 2005]. [16] The central pitting of many knobs suggests that they vary radially in their properties. Indeed, sublimation polygons such as those seen on Earth [Marchant et al., 2002] have ice-rich centers topped with fine-grained sublimation lag but marginal sand-wedge troughs [e.g., Levy et al., 2010]. Summit pits could form when wind deflates the fine-grained center. Sublimation could also be more efficient through the relatively thin polygon summit lag than through the thicker trough sediments [Levy et al., 2009]. Both mechanisms could also work together. [17] Could the presence of a regional ice-rich mantle also explain the lack of secondary craters? Typically, the largest secondary craters have diameters <∼5% of that of the primary crater [Schultz and Singer, 1980], or ∼92 m in this case. Because the depth/diameter ratio for martian secondary craters of that size is ∼0.11 [McEwen et al., 2005], secondaries associated with Vaduz would have been ≤∼10 m deep and thus would have been erased when the underlying unit was removed (Figure 3d). Notably, double-layer ejecta craters also generally lack secondary craters [Boyce and Mouginis-Mark, 2006]. Because these craters are similarly found in the midlatitudes and share some morphologic characteristics with Vaduz, it is possible that proposed explanations for their lack of secondary craters also apply to Vaduz, including entrainment and/or crushing of ejected blocks by a high-velocity outflow or fragmentation of ejected blocks due to water in the target material [Boyce and Mouginis-Mark, 2006]. [18] The young age of the crater requires its relict substrate to have been present regionally a few million years ago. Obliquity excursions sufficient to stabilize ice at the latitude of Vaduz have punctuated the climate of Mars since ∼5 Ma [Mellon and Jakosky, 1995; Laskar et al., 2004]. The ice- and dust-rich latitude-dependent mantle described by Head et al. [2003] also is interpreted to have formed in this regime, specifically during the most recent glacial period of ∼0.4–2.1 Ma. On the basis of these observations, we interpret the relict substrate to be similar in nature to the latitude-dependent mantle deposit. Currently, and for the last several hundred thousand years, the amplitude of obliquity oscillations has decreased, causing near-surface ice stability conditions to retreat to higher latitudes [Mellon and Jakosky, 1995; Head et al., 2003; Laskar et al., 2004]. Under these conditions, unarmored surface ice deposits would sublimate and migrate poleward. [19] The evidence presented here strongly suggests that an ice-rich deposit over ten meters thick covered the region a few million years ago. The significant topography (generally more than ten meters) associated with the remaining knobby facies is evidence that the material forming the substrate was removed from the unarmored surrounding areas; this removal supports the interpretation that the material was not regolith with minor vapor diffusion emplaced pore ice but rather a very ice-rich deposit, as was recently observed to have been excavated by five impacts at higher midlatitudes [Byrne et al., 2009]. The presence of such extensive excess ice and its preservation for millions of years favor atmospheric precipitation of ice/snow and dust over equilibrium vapor diffusion in regolith. [20] What is the nature of the armoring mechanism? The crater cavities of excess ejecta craters (EEC), including Vaduz, are largely below the background surface, indicating that these impacts penetrated through the surficial ice-rich unit to excavate mostly silicate rock [Black and Stewart, 2008]. This ejected material forms their associated facies and seems to insulate the substrate, as described by Black and Stewart [2008]. In contrast, pedestal craters (Pd), formed by impacts that do not typically excavate below the background surface, likely do not penetrate through the ice-rich layer. Kadish et al. [2009] review several proposed Pd armoring mechanisms, including an airblast/thermal pulse process [Wrobel et al., 2006] that could explain the great extent of the pedestal beyond the ejecta and the extreme circularity of pedestals [see Kadish et al., 2009]. Because this model is driven by energy transferred from an impact-induced vapor cloud, it may be that EEC, which excavate mostly silicate material, produce less significant vapor and thus are characterized by ejecta-related armoring, whereas Pd excavate only the ice-rich substrate and produce robust vapor clouds, favoring an airblast/thermal pulse mechanism. Although consistent with our observations, we acknowledge that this dual process model is only one possible interpretation. 4. Conclusions [21] A very fresh, 1.85 km diameter impact crater in the midlatitudes of Mars (38°N) named Vaduz exhibits distinctive crater-related facies extending up to ∼15 radii from the rim crest and perched >10 m above the adjacent plains. Knobby terrain fringing and underlying the crater facies is interpreted as degraded thermal contraction crack polygons, consistent with an ice-rich mantle buried by ejecta. The knobby terrain rapidly degrades, then disappears, away from the margins of the ejecta; this is interpreted to signal the progressive degradation and loss of the ice-rich substrate following the time of impact (estimated to be a few million years ago). The almost complete disappearance of this ice-rich unit, and consequent lowering of regional topography by over ten meters, suggests that the ice-rich unit was formed by climate-related deposition of snow, ice, and dust during recent periods of high obliquity, rather than by vapor diffusion into regolith pore space. The onset of the current lower-amplitude obliquity period, and the attendant poleward retreat of the mantling unit, is the most likely cause of the regional loss of the ice-rich layer. [22] We propose the following tentative relationship between crater morphology and the armoring mechanism that protects near-surface volatile deposits: 1) Pedestal craters, representing impacts solely into icy substrates, create blast waves that sinter surface deposits, forming a protective layer [Wrobel et al., 2006]. 2) Excess ejecta craters, which penetrate through the icy substrate into underlying deposits, excavate debris and bedrock units that cover and armor the icy substrate [Black and Stewart, 2008]. The depth of Vaduz relative to the thickness of the ice-rich unit, coupled with the shape of its crater facies, indicates that in contrast to typical pedestal craters, the impact event excavated bedrock, and that excavated regolith and fragmented target debris are an important factor in the armoring mechanism. [23] The preservation of the crater facies to such great radial ranges, and the well-displayed relationships of those facies with the underlying substrate, suggest that this type of crater can serve as a laboratory for the future analysis of crater-related armoring mechanisms. We leave to future work explanation of the distinctive crater facies associated with Vaduz but believe that such investigations will likely benefit from the previous analysis of quasi-multiple-layer ejecta craters, given their similar morphology. Acknowledgments [24] The Editor would like to thank Nadine G. Barlow and an anonymous reviewer. References Barlow, N. G. (2006), Impact craters in the northern hemisphere of Mars: Layered ejecta and central pit characteristics, Meteorit. Planet. Sci., 41, 1425– 1436, doi:10.1111/j.1945-5100.2006.tb00427.x. Wiley Online LibraryCASADSWeb of Science®Google Scholar Barlow, N. G., and J. M. Boyce (2008), Quasi-MLE craters: an unusual crater morphology at high Martian latitudes, Lunar Planet. Sci., XXXIX, Abstract 1164. Google Scholar Barlow, N., J. Boyce, F. Costard, R. Craddock, J. Garvin, S. Sakimoto, R. Kuzmin, D. Roddy, and L. Soderblom (2000), Standardizing the nomenclature of Martian impact crater ejecta morphologies, J. Geophys. Res., 105, 26,733– 26,738, doi:10.1029/2000JE001258. Wiley Online LibraryADSWeb of Science®Google Scholar Black, B. A., and S. T. Stewart (2008), Excess ejecta craters record episodic ice-rich layers at middle latitudes on Mars, J. Geophys. Res., 113, E02015, doi:10.1029/2007JE002888. Wiley Online LibraryADSWeb of Science®Google Scholar Boyce, J. M., and P. J. Mouginis-Mark (2006), Martian craters viewed by the Thermal Emission Imaging System instrument: Double-layered ejecta craters, J. Geophys. Res., 111, E10005, doi:10.1029/2005JE002638. Wiley Online LibraryADSPubMedWeb of Science®Google Scholar Byrne, S., et al. (2009), Distribution of mid-latitude ground ice on Mars from new impact craters, Science, 325, 1674– 1676, doi:10.1126/science.1175307. CrossrefCASADSPubMedWeb of Science®Google Scholar Head, J. W., J. F. Mustard, M. A. Kreslavsky, R. E. Milliken, and D. R. Marchant (2003), Recent ice ages on Mars, Nature, 426, 797– 802, doi:10.1038/nature02114. CrossrefCASADSPubMedWeb of Science®Google Scholar Kadish, S. J., N. G. Barlow, and J. W. Head (2009), Latitude dependence of Martian pedestal craters: Evidence for a sublimation-driven formation mechanism, J. Geophys. Res., 114, E10001, doi:10.1029/2008JE003318. Wiley Online LibraryADSPubMedWeb of Science®Google Scholar Kreslavsky, M. A., and J. W. Head (2002), Mars: Nature and evolution of young latitude-dependent water-ice-rich mantle, Geophys. Res. Lett., 29(15), 1719, doi:10.1029/2002GL015392. Wiley Online LibraryADSWeb of Science®Google Scholar Laskar, J., A. C. M. Correia, M. Gastineau, F. Joutel, B. Levrard, and P. Robutel (2004), Long term evolution and chaotic diffusion of the insolation quantities of Mars, Icarus, 170, 343– 364, doi:10.1016/j.icarus.2004.04.005. CrossrefADSWeb of Science®Google Scholar Levy, J. S., D. R. Marchant, and J. W. Head (2006), Distribution and origin of patterned ground on Mullins Valley debris-covered glacier, Antarctica: The roles of ice flow and sublimation, Antarct. Sci., 18, 385– 397, doi:10.1017/S0954102006000435. CrossrefWeb of Science®Google Scholar Levy, J. S., J. W. Head, and D. R. Marchant (2009), Concentric crater fill in Utopia Planitia: History and interaction between glacial “brain terrain” and periglacial mantle processes, Icarus, 202, 462– 476, doi:10.1016/j.icarus.2009.02.018. CrossrefCASADSWeb of Science®Google Scholar Levy, J. S., D. R. Marchant, and J. W. Head (2010), Thermal contraction crack polygons on Mars: A synthesis from HiRISE, Phoenix, and terrestrial analog studies, Icarus, 206, 229– 252, doi:10.1016/j.icarus.2009.09.005. CrossrefCASADSWeb of Science®Google Scholar Mangold, N. (2005), High latitude patterned grounds on Mars: Classification, distribution and climatic control, Icarus, 174, 336– 359, doi:10.1016/j.icarus.2004.07.030. CrossrefCASADSWeb of Science®Google Scholar Marchant, D. R., and J. W. Head (2007), Antarctic dry valleys: Microclimate zonation, variable geomorphic processes, and implications for assessing climate change on Mars, Icarus, 192, 187– 222, doi:10.1016/j.icarus.2007.06.018. CrossrefADSWeb of Science®Google Scholar Marchant, D. R., A. R. Lewis, W. M. Phillips, E. J. Moore, R. A. Souchez, G. H. Denton, D. E. Sugden, N. Potter, and G. P. Landis (2002), Formation of patterned ground and sublimation till over Miocene glacier ice in Beacon Valley, southern Victoria Land, Antarctica, Geol. Soc. Am. Bull., 114, 718– 730, doi:10.1130/0016-7606(2002)114 2.0.CO;2. CrossrefADSWeb of Science®Google Scholar McEwen, A. S., B. S. Preblich, E. P. Turtle, N. A. Artemieva, M. P. Golombek, M. Hurst, R. L. Kirk, D. M. Burr, and P. R. Christensen (2005), The rayed crater Zunil and interpretations of small impact craters on Mars, Icarus, 176, 351– 381, doi:10.1016/j.icarus.2005.02.009. CrossrefADSWeb of Science®Google Scholar Mellon, M. T., and B. M. Jakosky (1995), The distribution and behavior of Martian ground ice during past and present epochs, J. Geophys. Res., 100(E6), 11,781– 11,799, doi:10.1029/95JE01027. Wiley Online LibraryADSWeb of Science®Google Scholar Mustard, J. F., C. D. Cooper, and M. K. Rifkin (2001), Evidence for recent climate change on Mars from the identification of youthful near-surface ground ice, Nature, 412, 411– 414, doi:10.1038/35086515. CrossrefCASADSPubMedWeb of Science®Google Scholar Schon, S. C., J. W. Head, and R. E. Milliken (2009), A recent ice age on Mars: Evidence for climate oscillations from regional layering in midlatitude mantling deposits, Geophys. Res. Lett., 36, L15202, doi:10.1029/2009GL038554. Wiley Online LibraryADSWeb of Science®Google Scholar Schultz, P. H., and J. Singer (1980), A comparison of secondary craters on the Moon, Mercury, and Mars, Proc. Lunar Planet. Sci. Conf., 11th, 2243– 2259. ADSGoogle Scholar Skinner, J. A., T. M. Hare, and K. T. Tanaka (2006), Digital renovation of the atlas of Mars 1:15,000,000-scale global geologic series maps, Lunar Planet. Sci., XXXVII, Abstract 2331. Google Scholar Wrobel, K., P. Schultz, and D. Crawford (2006), An atmospheric blast/thermal model for the formation of high-latitude pedestal craters, Meteorit. Planet. Sci., 41, 1539– 1550, doi:10.1111/j.1945-5100.2006.tb00434.x. Wiley Online LibraryCASADSWeb of Science®Google Scholar Citing Literature Volume38, Issue7April 2011 FiguresReferencesRelatedInformation
Referência(s)