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1.
Dynamical transport of gases within the martian regolith controls many climatic processes, and is particularly important in the deposition and/or mobilization of shallow ground ice, as well as exchange of other volatiles between the martian regolith and atmosphere. A variety of theoretical studies have addressed issues related to ground ice dynamics on Mars and in the terrestrial analog environment of the Antarctic Dry Valleys. These theoretical studies have drawn on a limited set of empirical measurements to constrain the structural parameters controlling gas diffusion and flow in soils. Here, we investigate five groups of Mars-analog soils: glass spheres, JSC Mars-1, aeolian dune sand, Antarctic Dry Valley soils, and arctic loess. We present laboratory measurements of the structural properties most relevant to gas transport in these soils: porosity, tortuosity, permeability, bulk and intrinsic densities, grain-size distribution, pore-size distribution and BET surface area. Our results bear directly both on the appropriateness of assumptions made in theoretical studies and on current outstanding issues in the study of shallow ground ice on Mars and in the Dry Valleys. Specifically, we find that (1) measured values of tortuosity are lower than values commonly assumed for Mars by a factor of two to three; (2) diffusive loss of ground ice on Mars can likely proceed up to four times faster than predicted by theoretical studies; (3) soil permeabilities are sufficiently high that flushing of the soil column by bulk flow of atmospheric gases may further speed loss or deposition of shallow ground ice; (4) the pore volume in some Mars-analog soils is sufficiently high to explain high volumetric ice abundances inferred from Mars Odyssey Gamma Ray Spectrometer data as simple pore ice; and (5) measured properties of soils collected in Beacon Valley, Antarctica agree well with assumptions made in theoretical studies and are consistent with rapid loss of ground ice in the current climate. 相似文献
2.
The Antarctic Dry Valleys (ADV) are generally classified as a hyper-arid, cold-polar desert. The region has long been considered an important terrestrial analog for Mars because of its generally cold and dry climate and because it contains a suite of landforms at macro-, meso-, and microscales that closely resemble those occurring on the martian surface. The extreme hyperaridity of both Mars and the ADV has focused attention on the importance of salts and brines on soil development, phase transitions from liquid water to water ice, and ultimately, on process geomorphology and landscape evolution at a range of scales on both planets. The ADV can be subdivided into three microclimate zones: a coastal thaw zone, an inland mixed zone, and a stable upland zone; zones are defined on the basis of summertime measurements of atmospheric temperature, soil moisture, and relative humidity. Subtle variations in these climate parameters result in considerable differences in the distribution and morphology of: (1) macroscale features (e.g., slopes and gullies); (2) mesoscale features (e.g., polygons, including ice-wedge, sand-wedge, and sublimation-type polygons, as well as viscous-flow features, including solifluction lobes, gelifluction lobes, and debris-covered glaciers); and (3) microscale features (e.g., rock-weathering processes/features, including salt weathering, wind erosion, and surface pitting). Equilibrium landforms are those features that formed in balance with environmental conditions within fixed microclimate zones. Some equilibrium landforms, such as sublimation polygons, indicate the presence of extensive near-surface ice; identification of similar landforms on Mars may also provide a basis for detecting the location of shallow ice. Landforms that today appear in disequilibrium with local microclimate conditions in the ADV signify past and/or ongoing shifts in climate zonation; understanding these shifts is assisting in the documentation of the climate record for the ADV. A similar type of landform analysis can be applied to the surface of Mars where analogous microclimates and equilibrium landforms occur (1) in a variety of local environments, (2) in different latitudinal bands, and (3) in units of different ages. Documenting the nature and evolution of the ADV microclimate zones and their associated geomorphic processes is helping to provide a quantitative framework for assessing the evolution of climate on Mars. 相似文献
3.
The discovery of microbiota in the Dry Valleys of Antarctica has encouraged the construction of new models of Martian ecosystems in order to determine if life could have once existed on Mars. The Antarctic cyanobacteria reside just below the surface of sandstone rocks where they are protected from the extreme cold and dry environment. Analogy with the Antarctic Dry Valleys supports speculation that hypothetical micro-organisms existed on Mars in the early history of the planet and could have migrated into suitable rocks as the availability of liquid water decreased. Although evidence for sandstone layers on Mars has not been substantiated, the palaeohydrology of Martian fluvial valleys (MFVs) reveals the evidence of lake bed sediment depositions which have formed consolidated sediments. As the MFVs formation may result from underground drainage processes, the sediment material would be expected to contain debris such as pumice washload, and pumilith of volcanic and meteoritic origin. These materials may have formed consolidated porous terrains similar to the Antarctic sandstone. Therefore, the endolithic model is consistent with the Martian liquid water habitat model of perenially ice-covered lakes. 相似文献
4.
Through a combination of aerobraking (drag deceleration) and ablation, meteoroids which enter planetary atmospheres may be slowed sufficiently to soft-land as meteorites. Results of an earlier study suggest that the current 6 mbar atmosphere of Mars is sufficient to aerobrake significant numbers of small (<10 kg) asteroidal-type meteoroids into survivable, low-velocity (<500 m s−1) impacts with the planet's surface. Since rates of meteorite production depend upon the density of Mars's atmosphere, they must also change as the martian climate changes. However, to date, martian meteorite production has received relatively little attention in the literature Here we expand upon our previous work to study martian meteorite production rates and how they depend upon variations of the martian atmosphere, and to estimate the ranges of mass, velocity and entry-angle that produce meteorites. We find that even the current atmosphere of Mars is sufficient to soft-land significant fractions of incident stony and iron objects, and that these fractions increase dramatically for denser martian atmospheres. Therefore, like impact cratering, meteorite populations may preserve evidence of past martian climates. 相似文献
5.
New impacts in the martian mid-latitudes have exposed near-surface ice. This ice is observed to slowly fade over timescales of months. In the present martian climate, exposed surface ice is unstable during summer months in the mid-latitudes and will sublimate. We model the sublimation of ice at five new impact sites and examine the implications of its persistence. Even with generally conservative assumptions, for most reasonable choices of parameters it is likely that over a millimeter of sublimation occurred in the period during which the ice was observed to fade. The persistence of visible ice through such sublimation suggests that the ice is relatively pure rather than pore-filling. Such ice could be analogous to the nearly pure ice observed by the Phoenix Lander in the “Dodo-Goldilocks” trench and suggests that the high ice contents reported by the Mars Odyssey Gamma Ray Spectrometer at high latitudes extend to the mid-latitudes. Our observations are consistent with a model of the martian ice table in which a layer with high volumetric ice content overlies pore-filling ice, although other structures are possible. 相似文献
6.
We have used Mars Reconnaissance Orbiter data from 2007 and 2009 to compare summer behaviors of the seasonal and residual south polar caps of Mars in those two years. We find that the planet-encircling dust storm that occurred in the first of the two Mars years enhanced the loss of seasonal CO2 deposits relative to the second year but did not have a large effect on the continuing erosion of the pits and mesas within the residual cap materials. This suggests that the increase of bright frost in some regions of the residual cap observed between Mariner 9 and Viking can be accommodated within observed martian weather variability and does not require unknown processes or climate change. 相似文献
7.
Recent geological observations in the northern mid-latitudes of Mars show evidence for past glacial activity during the late Amazonian, similar to the integrated glacial landsystems in the Dry Valleys of Antarctica. The large accumulation of ice (many hundreds of meters) required to create the observed glacial deposits points to significant atmospheric precipitation, snow and ice accumulation, and glacial flow. In order to understand the climate scenario required for these conditions, we used the LMD (Laboratoire de Météorologie Dynamique) Mars GCM (General Circulation Model), which is able to reproduce the present-day water cycle, and to predict past deposition of ice consistent with geological observations in many cases. Prior to this analysis, however, significant mid-latitude glaciation had not been simulated by the model, run under a range of parameters.In this analysis, we studied the response of the GCM to a wider range of orbital configurations and water ice reservoirs, and show that during periods of moderate obliquity (? = 25-35°) and high dust opacity (τdust = 1.5-2.5), broad-scale glaciation in the northern mid-latitudes occurs if water ice deposited on the flanks of the Tharsis volcanoes at higher obliquity is available for sublimation. We find that high dust contents of the atmosphere increase its water vapor holding capacity, thereby moving the saturation region to the northern mid-latitudes. Precipitation events are then controlled by topographic forcing of stationary planetary waves and transient weather systems, producing surface ice distribution and amounts that are consistent with the geological record. Ice accumulation rates of ∼10 mm yr−1 lead to the formation of a 500-1000 m thick regional ice sheet that will produce glacial flow patterns consistent with the geological observations. 相似文献
8.
The residual south polar cap of Mars (RSPC) is distinct from the residual north polar cap both in composition and in morphology. CO2 frost in the RSPC is stabilized by its high albedo during southern spring and summer despite the relatively large insolation during that period. The morphology of the RSPC in summer displays a bewildering variety of depressions that are formed in relatively thin layers of CO2. The increase of the size of these depressions between each of the first three years of Mars Global Surveyor (MGS) observations may possibly signal some sort of climate change on the planet. For example, the erosion of the bright plateaus might reduce the RSPC albedo and affect the energy balance. The Mars Orbiter Cameras (MOC) on MGS observed Mars for four consecutive martian years before contact with the spacecraft was lost in late 2006. During this period coverage of the polar regions was particularly dense because MGS flew over them on every orbit. In this paper we report on the four-year behavior of the morphological features in the RSPC and on the large-scale variability in RSPC albedo over the period. The changes in the size of the surface features in the RSPC due to backwasting that were first observed between Mars years (MY) 24 and 25 and subsequently between MY25 and M26 was observed to continue at the same rate through MY 27. The results indicate that on average thicker layers in the RSPC retreat faster than thinner ones, roughly in proportion to their thickness. We argue that a simple difference in porosity between the A and B layers can explain this difference although other factors could be involved. The large-scale albedo of the RSPC decreases as the depressions are uncovered by sublimation of seasonal CO2. However, any interannual differences in albedo due to the backwasting process are masked by interannual differences in the summer dust opacity in the RSPC region. 相似文献
9.
Alberto G. Fairén 《Icarus》2010,208(1):165-48
Water on Mars has been explained by invoking controversial and mutually exclusive solutions based on warming the atmosphere with greenhouse gases (the “warm and wet” Mars) or on local thermal energy sources acting in a global freezing climate (the “cold and dry” Mars). Both have critical limitations and none has been definitively accepted as a compelling explanation for the presence of liquid water on Mars. Here is considered the hypothesis that cold, saline and acidic liquid solutions have been stable on the sub-zero surface of Mars for relatively extended periods of time, completing a hydrogeological cycle in a water-enriched but cold planet. Computer simulations have been developed to analyze the evaporation processes of a hypothetical martian fluid with a composition resulting from the acid weathering of basalt. This model is based on orbiter- and lander-observed surface mineralogy of Mars, and is consistent with the sequence and time of deposition of the different mineralogical units. The hydrological cycle would have been active only in periods of dense atmosphere, as having a minimum atmospheric pressure is essential for water to flow, and relatively high temperatures (over ∼245 K) are required to trigger evaporation and snowfall; minor episodes of limited liquid water on the surface could have occurred at lower temperatures (over ∼225 K). During times with a thin atmosphere and even lesser temperatures (under ∼225 K), only transient liquid water can potentially exist on most of the martian surface. Assuming that surface temperatures have always been maintained below 273 K, Mars can be considered a “cold and wet” planet for a substantial part of its geological history. 相似文献
10.
Phyllosilicates on Mars are widespread in the ancient crust suggesting the presence of liquid water at the martian surface and therefore warmer conditions during its early history. However, the role of the ancient climate in the alteration process, which produced these phyllosilicates, remains under debate, because similar mineral assemblages can be produced by hydrothermal alteration at depth. This paper focuses on the origin of coincident outcrops of Fe/Mg bearing phyllosilicates and Al-bearing phyllosilicates, which are observed in several regions of Mars. We performed a detailed mineralogical comparison between a section in Nili Fossae, Mars, and a weathering profile located at Murrin Murrin, Western Australia. The Murrin Murrin profile is developed in Archaean serpentinized peridotite massifs over a ∼40 m thick sequence. It has three alteration zones: the serpentine mineral saprolite is found at the bottom, immediately overlain by Fe/Mg-bearing smectites and then Al-bearing phyllosilicates (kaolinite) mixed with iron hydroxides. This example illustrates how Al-dominated minerals can derive from the alteration of initially Al-poor ultramafic rocks by the intense leaching of Mg2+. This mineralogical sequence is very similar to that detected locally in Nili Fossae by orbital spectroscopy. By analogy, we propose that the mineral assemblage detected on Mars is the result of long-term weathering, and thus could be the best evidence of past weathering as a direct result of a climate significantly warmer than at present. 相似文献
11.
Relief inversion has been invoked to explain a number of geomorphic features of the martian surface. Terrestrial relief inversion occurs when former depressions become elevated because their fill is more resistant to erosion than the surrounding terrain. It is a common product of long-term landscape evolution on Earth, especially in relatively stable intra-cratonic settings and flat, or near flat lying successions. The inverted relief will preserve relicts of former land surfaces and is therefore older than the surrounding terrain. Relief inversion can occur by a range of processes, including infill of depressions by intrinsically resistant material, selective secondary cementation via diagenesis and weathering, or surface armouring. We examine a number of possible cases of inverted relief on Mars that appear to have formed by these three processes. We suggest that the most likely cementing agents for surface induration are iron oxides, silica, and sulfates. Possible cementation mechanisms include fluid mixing during regional groundwater flow, cooling of hydrothermal or basinal fluids as they near the surface, and evaporation and sublimation of near surface water. Wind action appears the most common erosive process on Mars capable of the regional landscape lowering necessary for relief inversion to occur, unlike on Earth where both deflation and runoff are important. Preliminary crater densities of selected features show that the tops of the proposed inverted relief have considerably more craters than the surrounding plains, as is predicted by the inversion hypothesis. More accurate dating of inverted surfaces and the adjacent areas may provide a simple way of measuring the degree of erosion over time in at least some areas of Mars. 相似文献
12.
Following the work of Kieffer and Titus (2001, Icarus 154, 162-180), we present results of thermal IR observations of Korolev crater, located at ∼73° latitude in the martian northern polar region. Similar to techniques employed by Titus et al. (2003, Science 299, 1048-1050), we use infrared images from the Thermal Emission Imaging System (THEMIS) aboard Mars Odyssey to identify several regions within the crater basin with distinct thermal properties that correlate with topography. The THEMIS results show these regions exhibit temperature variations, spatially within the crater and throughout the martian year. In addition to the variations identified in the THEMIS observations, Mars Global Surveyor Thermal Emission Spectrometer (TES) observations show differences in albedo and temperature of these regions on both daily and seasonal cycles. Modeling annual temperature variations of the surface, we use TES observations to examine the thermal properties of these regions. This analysis reveals the crater interior deposits are likely thick layers (several meters) of high thermal inertia material (water ice, or extremely ice-rich regolith). Spatial variations of the physical properties of these regions are likely due to topography and possibly variations in the subsurface material itself. The nature of these deposits may help constrain polar processes, as well as provide context for the polar lander mission, Phoenix. 相似文献
13.
Thermal inertia derivation techniques generally assume that surface properties are uniform at horizontal scales below the footprint of the observing instrument and to depths of several decimeters. Consequently, surfaces with horizontal or vertical heterogeneity may yield apparent thermal inertia which varies with time of day and season. To investigate these temporal variations, we processed three Mars years of Mars Global Surveyor Thermal Emission Spectrometer observations and produced global nightside and dayside seasonal maps of apparent thermal inertia. These maps show broad regions with diurnal and seasonal differences up to 200 J m−2 K−1s−1/2 at mid-latitudes (60° S to 60° N) and 600 J m−2 K−1s−1/2 or greater in the polar regions. We compared the seasonal mapping results with modeled apparent thermal inertia and created new maps of surface heterogeneity at 5° resolution, delineating regions that have thermal characteristics consistent with horizontal mixtures or layers of two materials. The thermal behavior of most regions on Mars appears to be dominated by layering, with upper layers of higher thermal inertia (e.g., duricrusts or desert pavements over fines) prevailing in mid-latitudes and upper layers of lower thermal inertia (e.g., dust-covered rock, soils with an ice table at shallow depths) prevailing in polar regions. Less common are regions dominated by horizontal mixtures, such as those containing differing proportions of rocks, sand, dust, and duricrust or surfaces with divergent local slopes. Other regions show thermal behavior that is more complex and not well-represented by two-component surface models. These results have important implications for Mars surface geology, climate modeling, landing-site selection, and other endeavors that employ thermal inertia as a tool for characterizing surface properties. 相似文献
14.
The present-day thermal state of the martian interior is a very important issue for understanding the internal evolution of the planet. Here, in order to obtain an improved upper limit for the heat flow at the north polar region, we use the lower limit of the effective elastic thickness of the lithosphere loaded by the north polar cap, crustal heat-producing elements (HPE) abundances based on martian geochemistry, and a temperature-dependent thermal conductivity for the upper mantle. We also perform similar calculations for the south polar region, although uncertainties in lithospheric flexure make the results less robust. Our results show that the present-day surface and sublithospheric heat flows cannot be higher than 19 and 12 mW m−2, respectively, in the north polar region, and similar values might be representative of the south polar region (although with a somewhat higher surface heat flow due to the radioactive contribution from a thicker crust). These values, if representative of martian averages, do not necessarily imply sub-chondritic HPE bulk abundances for Mars (as previously suggested), since (1) chondritic composition models produce a present-day total heat power equivalent to an average surface heat flow of 14-22 mW m−2 and (2) some convective models obtain similar heat flows for the present time. Regions of low heat flow may even have existed during the last billions of years, in accordance with several surface heat flow estimates of ∼20 mW m−2 or less for terrains loaded during Hesperian or Amazonian times. On the other hand, there are some evidences suggesting the current existence of regions of enhanced heat flow, and therefore average heat flows could be higher than those obtained for the north (and maybe the south) polar region. 相似文献
15.
We describe the morphology and spatial relationships between composite-wedge polygons and Mars-like gullies (consisting of alcoves, channels, and fans) in the hyper-arid Antarctic Dry Valleys (ADV), as a basis for understanding possible origins for martian gullies that also occur in association with polygonally patterned ground. Gullies in the ADV arise in part from the melting of atmospherically-derived, wind-blown snow trapped in polygon troughs. Snowmelt that yields surface flow can occur during peak southern hemisphere summer daytime insolation conditions. Ice-cemented permafrost provides an impermeable substrate over which meltwater flows, but does not significantly contribute to meltwater generation. Relationships between contraction crack polygons and sedimentary fans at the distal ends of gullies show deposition of fan material in polygon troughs, and dissection of fans by expanding polygon troughs. These observations suggest the continuous presence of meters-thick ice-cemented permafrost beneath ADV gullies. We document strong morphological similarities between gullies and polygons on Mars and those observed in the ADV Inland Mixed microclimate zone. On the basis of this morphological comparison, we propose an analogous, top-down melting model for the initiation and evolution of martian gullies that occur on polygonally-patterned, mantled surfaces. 相似文献
16.
Lipids can be present within gypsum as intracrystalline inclusions if they become incorporated within the mineral as is it precipitates. The lipids that comprise these inclusions are protected against alteration or destruction by an external oxidising chemical environment because a protective mineral matrix surrounds them. Sulfate minerals are abundant on the surface of Mars and were present in the samples that were analysed by the Viking landers. The quantities of secondary intracrystalline fossil-lipids that are present in samples of gypsum and gypsum-rich soils from the Haughton Impact Structure, Devon Island, Canadian High Arctic are sufficient to suggest that if a similar concentration of fossil lipids was present in the sulfate-rich samples analysed by the Viking Landers then they could have been detected. Possible reasons why a secondary fossil-lipid signature was not detected include a poor rate of conversion during pyrolysis, exposure of intracrystalline lipids during periods of weathering to oxidative martian diagenesis, a low level of biological productivity or an absence of a source for lipids on the surface of Mars. Polycyclic aromatic hydrocarbons of meteoritic origin, and terpane biomarkers such as hopanes and steranes, are not present in the Haughton gypsum in sufficient quantities to have been readily detected. 相似文献
17.
18.
We present the seasonal and geographical variations of the martian water vapor monitored from the Planetary Fourier Spectrometer Long Wavelength Channel aboard the Mars Express spacecraft. Our dataset covers one martian year (end of Mars Year 26, Mars Year 27), but the seasonal coverage is far from complete. The seasonal and latitudinal behavior of the water vapor is globally consistent with previous datasets, Viking Orbiter Mars Atmospheric Water Detectors (MAWD) and Mars Global Surveyor Thermal Emission Spectrometer (MGS/TES), and with simultaneous results obtained from other Mars Express instruments, OMEGA and SPICAM. However, our absolute water columns are lower and higher by a factor of 1.5 than the values obtained by TES and SPICAM, respectively. In particular, we retrieve a Northern midsummer maximum of 60 pr-μm, lower than the 100-pr-μm observed by TES. The geographical distribution of water exhibits two local maxima at low latitudes, located over Tharsis and Arabia. Global Climate Model (GCM) simulations suggest that these local enhancements are controlled by atmospheric dynamics. During Northern spring, we observe a bulge of water vapor over the seasonal polar cap edge, consistent with the northward transport of water from the retreating seasonal cap to the permanent polar cap. In terms of vertical distribution, we find that the water volume mixing ratio over the large volcanos remains constant with the surface altitude within a factor of two. However, on the whole dataset we find that the water column, normalized to a fixed pressure, is anti-correlated with the surface pressure, indicating a vertical distribution intermediate between control by atmospheric saturation and confinement to a surface layer. This anti-correlation is not reproduced by GCM simulations of the water cycle, which do not include exchange between atmospheric and subsurface water. This situation suggests a possible role for regolith-atmosphere exchange in the martian water cycle. 相似文献
19.
In this work we estimate the minimum persistence time of subsurface ice in water rich sediment layers remaining after sublimation of a martian lake. We simulate sublimation of ice from layers of different granulations and thicknesses. Presented results assume insolation and atmospheric conditions characteristic for the present day southern Elysium, where data from Mars Express have identified surface features possibly indicating the very recent presence of a frozen body of water [Murray et al., 2005. Nature 434, 352-356]. The age of these features is estimated to be several million years. On this time scale, we find that most of the water ice must have sublimated away, however remnant ice at a few percent level cannot be excluded. This amount of water ice is sufficient for chemical cementation of the observed features and explains their relatively pristine appearance, without significant signs of erosion. 相似文献
20.
We report on laboratory experiments in which we allowed an SNC-derived mineral mix to react with pure water under a simulated Mars atmosphere for 7 months. These experiments were performed at one bar and at three different temperatures in order to simulate the subsurface conditions that most likely exist where liquid water and rock interact on Mars today. The dominant cations dissolved in the solutions we produced, which may be characterized as dilute brines, are Ca2+, Mg2+, Al3+, and Na+, while the major anions are dissolved C, F−, SO2−4 and Cl−. Typical solution pH was in the range of 4.2-6.0. Abundance patterns of elements in our synthetic sulfate-chloride brines are distinctly unlike those of terrestrial ocean water or continental waters, however, they are quite similar to those measured in the martian fines at the Mars Pathfinder and Viking 1 and 2 Landing sites. This suggests that salts present in the martian regolith may have formed over time as a result of the interaction of surface or subsurface liquid water with basalts in the presence of a martian atmosphere similar in composition to that of today. If most of the mobile surface layer was formed during the Noachian when erosion rates were much higher than at present, and if this layer is homogeneous in salt composition, the total amount of salt in the martian fines is approximately the same as in the Earth's oceans. The minimum quantity of circulating water necessary to deposit this amount of salt is approximately equivalent to a global layer 625 m deep. 相似文献