摘要 :Highlights?We perform mesoscale simulations of the water cycle in a region around Gale crater.?Regolith interaction reduces vapour abundances at crater floor by factors of 2-3.?Nighttime subsurface ice amounts are s...
展开Highlights?We perform mesoscale simulations of the water cycle in a region around Gale crater.?Regolith interaction reduces vapour abundances at crater floor by factors of 2-3.?Nighttime subsurface ice amounts are small in all seasons.?Diffused vapour is transported up into the atmosphere at convergence boundaries.?Results at Gale crater are representative of other craters in the mesoscale domain.收起
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We investigate the exchange of water vapour between the regolith and atmosphere of Mars, and how it varies with different orbital parameters, atmospheric dust contents and surface water ice reservoirs. This is achieved through the...
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We investigate the exchange of water vapour between the regolith and atmosphere of Mars, and how it varies with different orbital parameters, atmospheric dust contents and surface water ice reservoirs. This is achieved through the coupling of a global circulation model (GCM) and a regolith diffusion model. GCM simulations are performed for hundreds of Mars years, with additional one-dimensional simulations performed for 50 kyr. At obliquities epsilon = 15 degrees and 30 degrees, the thermal inertia and albedo of the regolith have more control on the subsurface water distribution than changes to the eccentricity or solar longitude of perihelion. At epsilon = 45 degrees, atmospheric water vapour abundances become much larger, allowing stable subsurface ice to form in the tropics and mid-latitudes. The circulation of the atmosphere is important in producing the subsurface water distribution, with increased water content in various locations due to vapour transport by topographically-steered flows and stationary waves. As these circulation patterns are due to topographic features, it is likely the same regions will also experience locally large amounts of subsurface water at different epochs. The dustiness of the atmosphere plays an important role in the distribution of subsurface water, with a dusty atmosphere resulting in a wetter water cycle and increased stability of subsurface ice deposits. Crown Copyright (C) 2016 Published by Elsevier Inc. All rights reserved.
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Reconstructions of the orbital parameters of Mars spanning the last similar to 20 Myr, combined with global circulation models, predict multiple cycles of accumulation and degradation of an ice-rich mantle in the mid-latitudes, dr...
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Reconstructions of the orbital parameters of Mars spanning the last similar to 20 Myr, combined with global circulation models, predict multiple cycles of accumulation and degradation of an ice-rich mantle in the mid-latitudes, driven primarily by insolation at the poles during periods when obliquity was more than ten degrees greater than it is today (i.e., >similar to 35 degrees). While evidence of an ice-rich "latitude dependent mantle" (LDM) consistent with these predictions is abundant, features indicative of cycles of emplacement and degradation of this unit are isolated and rare. In addition, fundamental physical properties of the LDM, such as paleo-thickness maxima, have not been determined. Gullies, which are sinuous channels found on steep slopes in mid- and high-latitudes, interact with the LDM and provide a stratigraphic feature useful for documenting both cyclical emplacement/removal and thickness estimates in past climate regimes. In the southern hemisphere, where gullies are most common, we present extensive evidence of (1) cyclical degradation and removal of gullies in the lower mid-latitudes (30-40 degrees S), and (2) burial and exhumation of inverted gully channels in the transitional latitude band between dissected and preserved LDM (40-50 degrees S), which can only be accounted for if an additional tens of meters of LDM were present at these locations during channel formation. These relationships support a model in which end-to-end gully evolution is controlled by the behavior of the LDM: at lower latitudes, gullies incise an ice-rich substrate and are removed when that ice becomes unstable, and at higher latitudes gullies are buried by successive emplacement of LDM where ice remains stable near the surface. Further, the presence of dormant buried gullies implies that present-day activity within gullies, likely to be controlled by the behavior of CO2 frost, is insufficient to explain the entire gully population, and that conditions conducive to increased gully activity preceded the most recent phase of LDM emplacement. (C) 2015 Elsevier Inc. All rights reserved.
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Morphologic characteristics of ice-rich landforms in the martian mid-latitudes record evidence for significant modification of the landscape in response to spin-axis/orbital parameter-driven shifts in the Late Amazonian climate. T...
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Morphologic characteristics of ice-rich landforms in the martian mid-latitudes record evidence for significant modification of the landscape in response to spin-axis/orbital parameter-driven shifts in the Late Amazonian climate. These landforms are spatially distributed across the mid-latitudes and their co-existing presence has so far not been observed from a single crater to infer how exactly a terrain has been modified while Mars was undergoing major-moderate-minor shifts in its Late Amazonian climate. We have therefore carried out an in-depth investigation of Moreux crater (similar to 135 km, centered at 41.66 degrees N, 44.44 degrees E in the Protonilus Mensae region) for identification of features associated with recent and episodic glacial events and for emphasizing the role played by these glacial events in the modification of the crater. Evidence for extensive modification of the surfaces over crater rim/wall and around central peak by emplacement of multiple scales of ice-rich landforms that represents large history of glacial activities was found. From our results we document phases of major-moderate-minor glacial activities within the crater as: (1) piedmont lobes/lobate debris aprons/linear valley fills (similar to 1 Ga-100 Ma), (2) viscous flow features (similar to 30-0.1 Ma) and (3) gullies/thermal contraction crack polygons (similar to 2.1-0.4 Ma). The form and distribution of the random valleys observed within Moreux suggests their formation by pressure-induced melting and flow occurring beneath an extensive layer of ice. We also suggest that central peak of Moreux probably acted as the locus for accumulation of ice/snow and the diversity of glacial/periglacial features within the crater was possibly controlled by differences in the amount of accumulated ice/snow and the rate at which the terrain responded to the shifts in climate during subsequent periods of obliquity changes. Taken together, these ice-rich deposits within Moreux suggest that sequential modification of the crater surfaces over the rim/wall and around central peak has occurred over the last tens of millions of years of martian history. This new evidence thus adds another well-documented case to rapidly accumulating evidences for widespread glacial activity in the middle latitudes of Mars in recent martian history. (C) 2014 Elsevier Inc. All rights reserved.
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We describe the results of our morphologic, stratigraphic and mineralogic investigations of fluvial landforms, paleolakes and possible shoreline morphologies at the Libya Montes/Isidis Planitia boundary. The landforms are indicati...
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We describe the results of our morphologic, stratigraphic and mineralogic investigations of fluvial landforms, paleolakes and possible shoreline morphologies at the Libya Montes/Isidis Planitia boundary. The landforms are indicative of aqueous activity and standing bodies of water, including lakes, seas and oceans, that are attributed to a complex hydrologic cycle that may have once existed on Mars in the Noachian (>3.7. Ga) and perhaps also in the Hesperian (>3.1. Ga). Our observations of the Libya Montes/Isidis Planitia boundary between 85°/86.5°E and 1.8°/5°N suggest, that (1) the termination of valley networks between roughly -2500 and -2800. m coincide with lake-size ponding in basins within the Libya Montes, (2) an alluvial fan and a possible delta, layered morphologies and associated Al-phyllosilicates identified within bright, polygonally fractured material at the front of the delta deposits are interpreted to be the results of fluvial activity and discharge into a paleolake, (3) the Arabia " shoreline" appears as a series of possible coastal cliffs at about -3600 and -3700. m indicating two distinct still stands and wave-cut action of a paleosea that temporarily filled the Isidis basin the Early Hesperian, and (4) the Deuteronilus " shoreline" appears at -3800. m and is interpreted to be a result of the proposed sublimation residue of a frozen sea that might have filled the Isidis basin, similar to the Vastitas Borealis Formation (VBF) identified in the northern lowlands. We interpret the morphologic-geologic setting and associated mineral assemblages of the Libya Montes/Isidis Planitia boundary as results of fluvial activity, lake-size standing bodies of water and an environmental change over time toward decreasing water availability and a cold and dry climate.
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Inverse maximum gross bedform-normal transport (IMGBNT) analysis has been applied to Context Camera (CTX) images of the largest dune field in Ganges Chasma on Mars. The dune field was selected for its position in a likely complex,...
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Inverse maximum gross bedform-normal transport (IMGBNT) analysis has been applied to Context Camera (CTX) images of the largest dune field in Ganges Chasma on Mars. The dune field was selected for its position in a likely complex, multi-directional wind regime. Results indicate that four main winds are responsible for simultaneous construction of the dune field, including along-chasma winds from the ESE, winds blowing down a nearby re-entrant from the NE, winds blowing down the adjacent chasma wall from the NW, and chasma floor winds from the SW. Each wind represents a transport vector that dominates dune morphology at its respective edge of the dune field, such that the central axis of the dune field reflects the convergence of the three most prominent winds (ESE, NW, and SW). The Mars Regional Atmospheric Modeling System (MRAMS) was run at twelve times throughout the martian year to provide context for the local wind patterns. Potential sand fluxes calculated from MRAMS output show that three major air flows from the ENE-E, NNE-NE, and NNW-N converge near the location of the dune field. These flows likely correspond to the ESE, NE, and NW winds identified from IMGBNT analysis, respectively. MRAMS output shows that the flows with major northerly components are produced by larger-scale Hadley return flow constructively combining with nighttime downslope winds; the flow with a major easterly component is likely produced by equatorial easterly "trade" winds constructively combining with the diurnal tide and/or local topography. Although the model correctly predicts the major elements of the local wind pattern, some aspects are either over- or underrepresented, demonstrating the value of using aeolian morphological analysis to conclusively constrain the major sand-moving winds on Mars. Overlapping High Resolution Imaging Experiment (HiRISE) images of barchanoid dunes at the northernmost edge of the dune field indicate that these dunes are currently migrating southward at ~5 m/Mars year (~2.6 m/Earth year); the direction of migration is consistent with both MRAMS predictions in this location and the NW/NE winds found from IMGBNT analysis. Dune morphology suggests that sand in the northwestern part of the dune field is likely to be derived from the adjacent chasma wall to the north and northwest, and sand in the southeastern part of the dune field was probably transported from farther east along the main chasma floor.
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Atmospheric water vapor abundances in Mars' north polar region (NPR, from 60° to 90°N) are mapped as function of latitude and longitude for spring and summer seasons, and their spatial, seasonal, and interannual variability is d...
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Atmospheric water vapor abundances in Mars' north polar region (NPR, from 60° to 90°N) are mapped as function of latitude and longitude for spring and summer seasons, and their spatial, seasonal, and interannual variability is discussed. Water vapor data are from Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES) and the Viking Orbiter (VO) Mars Atmospheric Water Detector (MAWD). The data cover three complete northern spring-summer seasons in 1977-1978, 2000-2001 and 2002-2003, and shorter periods of spring-summer seasons during 1975, 1999 and 2004. Long term interannual variability in the averaged NPR abundances may exist, with Viking MAWD observations showing twice as much water vapor during summer as the MGS TES observations more than 10 martian years (MY) later. While the averaged abundances are very similar in TES observations for the same season in different years, the spatial distributions in the early summer season do vary significantly year over year. Spatial and temporal variabilities increase between L_s ～ 80-140°, which may be related to vapor sublimation from the North Polar Residual Cap (NPRC), or to changes in circulation. Spatial variability is observed on scales of ～100 km and temporal variability is observed on scales of <10 sols during summer. During late spring the TES water vapor spatial distribution is seen to correlate with the low topography/low albedo region of northern Acidalia Planitia (270-360°E), and with the dust spatial distribution across the NPR during late spring-early summer. Non-uniform vertical distribution of water vapor, a regolith source or atmospheric circulation 'pooling' of water vapor from the NPRC into the topographic depression may be behind the correlation with low topography/low albedo. Sublimation winds carrying water vapor off the NPRC and lifting surface dust in the areas surrounding the NPRC may explain the correlation between the water vapor and dust spatial distributions. Correlation between water vapor and dust in MAWD data are only observed over low topography/low albedo area. Maximum water vapor abundances are observed at L_s = 105-115° and outside of the NPRC at 75-80°N; the TES data, however, do not extend over the NPRC and thus, this conclusion may be biased. Some water vapor appears to be released in plumes or 'outbursts' in the MAWD and TES datasets during late spring and early summer. We propose that the sublimation rate of ice varies across the NPRC with varying surface winds, giving rise to the observed 'outbursts' at some seasons.
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The precise mechanisms by which martian hillside gullies erode and their dependence on the local environment remain subjects of debate. We studied three sharp rimmed craters in Noachis Terra and 37 gully profiles using Context Cam...
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The precise mechanisms by which martian hillside gullies erode and their dependence on the local environment remain subjects of debate. We studied three sharp rimmed craters in Noachis Terra and 37 gully profiles using Context Camera (CTX), Mars Orbiter Laser Altimeter (MOLA) and High Resolution Stereo Camera (HRSC) data. We analysed the gully topographic profiles of seven gullies and nine dry ravines. We measured slope properties using HRSC elevation data and used thermal inertia to infer material types of the gully sites. We compared these with three nearby Noachian age craters possessing crater wall slope angles within the range of previously observed gully formations. In-line with previous findings on individual gullies, we found that the slope angles of gullies in our study area consistently reflect the inherited slope angles of the host escarpment, suggesting that traditional slope-based evidence of fluvial activity in martian gullies needs to be placed in context of its local environment. We also observed a direct relationship between gully morphology and local composition of surface units. Martian gully features, and possibly method of erosion appeared heavily influenced by changes in underlying geology and presence of erodible sediment. Examples included gully shape changing in accordance with type of erodible sediment. We suggest that the degraded rims of gully-free Noachian craters precluded slope angles high enough to trigger creation of precursors to alcoves through mass wasting. Lack of these hollows has probably prevented the accumulation of enough ice-rich sediment for gullies to form in. Our analysis reveals that there is a complex interdependence between slope processes and the local environment, and global martian gully models may not work at the local scale. (C) 2014 Elsevier Inc. All rights reserved.
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We report on new retrievals of water vapor column abundances from the Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES) data. The new retrievals are from the TES nadir data taken above the 'cold' surface areas in the ...
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We report on new retrievals of water vapor column abundances from the Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES) data. The new retrievals are from the TES nadir data taken above the 'cold' surface areas in the North polar region (Tsurf<220K, including seasonal frost and permanent ice cap) during spring and summer seasons, where retrievals were not performed initially. Retrievals are possible (with some modifications to the original algorithm) over cold surfaces overlaid by sufficiently warm atmosphere. The retrieved water vapor column abundances are compared to the column abundances observed by other spacecrafts in the Northern polar region during spring and summer and good agreement is found. We detect an annulus of water vapor growing above the edge of the retreating seasonal cap during spring. The formation of the vapor annulus is consistent with the previously proposed mechanism for water cycling in the polar region, according to which vapor released by frost sublimation during spring re-condenses on the retreating seasonal CO2 cap. The source of the vapor in the vapor annulus, according to this model, is the water frost on the surface of the CO2 at the retreating edge of the cap and the frost on the ground that is exposed by the retreating cap. Small contribution from regolith sources is possible too, but cannot be quantified based on the TES vapor data alone. Water vapor annulus exhibits interannual variability, which we attribute to variations in the atmospheric temperature. We propose that during spring and summer the water ice sublimation is retarded by high relative humidity of the local atmosphere, and that higher atmospheric temperatures lead to higher vapor column abundances by increasing the water holding capacity of the atmosphere. Since the atmospheric temperatures are strongly influenced by the atmospheric dust content, local dust storms may be controlling the release of vapor into the polar atmosphere. Water vapor abundances above the residual polar cap also exhibit noticeable interannual variability. In some years abundances above the cap are lower than the abundances outside of the cap, consistent with previous observations, while in the other years the abundances above the cap are higher or similar to abundances outside of the cap. We speculate that the differences may be due to weaker off-cap transport in the latter case, keeping more vapor closer to the source at the surface of the residual cap. Despite the large observed variability in water vapor column abundances in the Northern polar region during spring and summer, the latitudinal distribution of the vapor mass in the atmosphere is very similar during the summer season. If the variability in vapor abundances is caused by the variability of vapor sources across the residual cap then this would mean that they annually contribute relatively little vapor mass to significantly affect the vapor mass budget. Alternatively this may suggest that the vapor variability is caused by the variability of the polar atmospheric circulation. The new water vapor retrievals should be useful in tuning the Global Circulation Models of the martian water cycle.
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We use a general circulation model (Urata, R.A., Toon, O.B. [2013]. Icarus, submitted for publication) to simulate the martian hydrologic cycle, including the radiative effects of water-ice clouds. We find that the current observe...
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We use a general circulation model (Urata, R.A., Toon, O.B. [2013]. Icarus, submitted for publication) to simulate the martian hydrologic cycle, including the radiative effects of water-ice clouds. We find that the current observed hydrologic cycle can be duplicated by tuning the polar cap albedo. The hydrologic cycle is very sensitive to the size and albedo of the North Polar water-ice cap. The radiative effects of ice clouds on atmospheric temperatures can be significant. Simulations of an ancient climate were performed with a 500mb CO_2 atmosphere and a reduced solar constant. The results show that the climate is highly sensitive to the hydrologic cycle, and can range from cold and dry, to warm and wet depending on initial conditions, cloud particle size, precipitation rates, and cloud cover fraction. A warm climate is obtained by assuming cloud ice particles greater than or equal to 10μm, and by reducing the efficiency of precipitation to maximize the cloud optical thicknesses. The warm, wet climates have precipitation rates that are 10% of the present day Earth. While carbon dioxide plays only a minor role in creating warm temperatures, it is necessary to have more than 250mb of carbon dioxide in order to obtain these high temperatures for the conditions assumed in our simulations due to the need for heat transport to the poles.
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