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1.
The seasonal evolution of the H2O snow in the Martian polar caps and the dynamics of water vapor in the Martian atmosphere are studied. It is concluded that the variations of the H2O mass in the polar caps of Mars are determined by the soil thermal regime in the polar regions of the planet. The atmosphere affects water condensation and evaporation in the polar caps mainly by transferring water between the polar caps. The stability of the system implies the presence of a source of water vapor that compensates for the removal of water from the atmosphere due to permanent vapor condensation in the polar residual caps. The evaporation of the water ice that is present in the surface soil layers in the polar regions of the planet is considered as such a source. The annual growth of the water-ice mass in the residual polar caps is estimated. The latitudinal pattern of the seasonal distribution of water vapor in the atmosphere is obtained for the stable regime.Translated from Astronomicheskii Vestnik, Vol. 38, No. 6, 2004, pp. 497–503.Original Russian Text Copyright © 2004 by Aleshin.  相似文献   

2.
《Icarus》1986,67(1):19-36
A quantitative model of the state, distribution, and migration of water in the shallow Martian regolith is presented. Reported results are confined to the region of the planet greater than 40° lat. The calculations take into account (1) expected thermal variations at all depths, latitudes, and times resulting from seasonal and astronomically induced insolation variations; (2) variations in atmospheric PH2O and PCO2 resulting from polar insolation variations and regolith adsorptive equilibria; (3) feedback effects related to latent heat and albedo variations resulting from condensation of atmospheric constituents; (4) two possible regolith mineralogies; (5) variable total H2O content of the regolith; (6) kinetics of H2O transport through the Martian atmosphere and regolith; and (7) equilibrium phase partitioning of H2O between the condensed, adsorbed, and vapor phases. Results suggest that the adsorptive capacity of the regolith is important in controlling the state and distribution of high-latitude H2O; unweathered mafic silicates favor the development of shallow ground ice at all temperate and polar latitudes, while heavily weathered clay-like regolith materials leads to a deeper ground ice interface and far more extensive quantities of adsorbed H2O. The capacity of the high-latitude regolith for storage of H2O and the total mass of H2O exchanged between the atmosphere, polar cap, and subsurface over an obliquity cycle is found to be relatively independent of mineralogy. The maximum exchanged volume is found to be 3.0 × 104 km3 of ice per cycle. Implications for the history of the polar caps and the origin of the layered terrain are discussed. Results also suggest that seasonal thermal waves act to force adsorbed H2O into the solid phase over a wide variety of latitude/obliquity conditions. Seasonal phase cycling of regolith H2O is most common at high latitudes and obliquities. Such phase behavior is highly dependent on regolith mineralogy. In a highly weathered regolith, adsorbed H2O is annually forced into the solid phase at all latitudes ≥40° at obliquities greater than approximately 25°. Seasonal adsorption-freezing cycles which are predicted here may produce geomorphologic signatures not unlike those produced by terrestrial freeze-thaw cycles.  相似文献   

3.
C.B. Leovy 《Icarus》1973,18(1):120-125
A model for exchange of water from the atmosphere to condensing CO2 caps is developed. The rate of water condensation in the caps is assumed to be proportional to the meridional heat flux. It follows that the amount of water condensed in the caps varies inversely with the amount of CO2 condensed. The seasonal phase of the release of water from the caps is not consistent with observed variations in the abundance of atmospheric water. Seasonal variations of atmospheric water abundance are most consistent with vapor exchange between the atmosphere and permafrost in the subtropics. Although water condensation in semipermanent caps is normally very slow, it may take place at a much faster rate at unusually high atmospheric temperatures, such as those produced by absorption of solar radiation by airborne dust.  相似文献   

4.
In this paper we attempt to answer the question, how formation of a small-scale trench in the martian regolith affects local distribution of the subsurface ice. We are especially interested in the consequences of digging a trench to search for buried ice, as has been done during the Phoenix Mars Lander mission. However, the results may be also applicable for natural troughs, or cracks. We present results of simulations of diurnal exchange of water between the regolith and the atmosphere. Our model includes the heat and vapor migration in the regolith surrounding the trench, as well as formation of diurnal frost. We take into account scattering of light in the atmosphere and on the trench facets, as well as changes of atmospheric humidity on diurnal and seasonal time scales. Our calculations show, that the measurements of ice content in a sample obtained within one, or two days from the beginning of digging should not be affected. However, on somewhat longer time scale at the south facing site of the trench the regolith can be significantly depleted from ice. This effect should be taken into account if the excavation and taking samples from different depths will be performed in stages separated in time by a month, or more.  相似文献   

5.
Exchange of CO2 and H2O between the Mars regolith and the atmosphere-cap system plays an important role in governing the evolution of the martian atmosphere and the martian climate. Most of the exchangeable CO2 (perhaps one or two orders of magnitude more than the atmospheric inventory) is currently adsorbed on the deep regolith, and can be “cryopumped” to a large quasipermanent CO2 cap (not now present) during lowest Mars obliquity (θ). During the obliquity driven regolith-cap CO2 exchange cycle, the atmospheric pressure varies harmonically between ~0.1 mb (lowest Θ) and ? 20 mb (highest Θ). The regolith buffer plays only a small or negligible role in the seasonal CO2 pressure variations caused by atmosphere-cap exchange because adsorption greatly inhibits diffusion of the seasonal “pressure wave” into the regolith. In contrast, thermally driven H2O seasonal exchange between the atmosphere and regolith appears to be in large part responsible for observed seasonal variations in the small atmospheric H2O inventory. Long term exchange of H2O may be dominated by transfer between the polar caps and ice in the regolith. Available and potential tests of regolith-atmospheric-cap volatile exchange models using ground-based and spacecraft-based techniques are discussed.  相似文献   

6.
We examine the response of Martian climate to changes in solar energy deposition caused by variations of the Martian orbit and obliquity. We systematically investigate the seasonal cycles of carbon dioxide, water, and dust to provide a complete picture of the climate for various orbital configurations. We find that at low obliquity (15°) the atmospheric pressure will fall below 1 mbar; dust storms will cease; thick permanent CO2 caps will form; the regolith will release CO2; and H2O polar ice sheets will develop as the permafrost boundaries move poleward. At high obliquity (35°) the annual average polar temperature will increase by about 10°K, slightly desorbing the polar regolith and causing the atmospheric pressure to increase by not more than 10 to 20 mbar. Summer polar ground temperatures as high as 273°K will occur. Water ice caps will be unstable and may disappear as the equilibrium permafrost boundary moves equatorward. However, at high eccentricity, polar ice sheets will be favored at one pole over the other. At high obliquity dust storms may occur during summers in both hemispheres, independent of the eccentricity cycle. Eccentricity and longitude of perihelion are most significant at modest obliquity (25°). At high eccentricity and when the longitude of perihelion is close to the location of solstice hemispherical asymmetry in dust-storm generation and in polar ice extent and albedo will occur.The systematic examination of the relation of climate and planetary orbit provides a new theory for the formation of the polar laminae. The terraced structure of the polar laminae originates when eccentricity and/or obliquity variations begin to drive water ice off the dusty permanent H2O polar caps. Then a thin (meters) layer of consolidated dust forms on top of a dirty, slightly thicker (tens of meters) ice sheet and the composite is preserved as a layer of laminae composed predominately of water ice. Because of insolation variation on slopes, a series of poleward- and equatorward-facing scarps are formed where the edges of the laminae are exposed. Independently of orbital variations, these scarps propagate poleward both by erosion of the equatorward slopes and by deposition on the poleward slopes. Scarp propagation resurfaces and recycles the laminae forming the distinctive spiral bands of terraces observed and provides a supply of water to form new permanent ice caps. The polar laminae boundary marks the furthest eqautorward extension of the permanent H2O caps as the orbit varies. The polar debris boundary marks the furthest equatorward extension of the annual CO2 caps as the orbit varies.The Martian regolith is now a significant geochemical sink for carbon dioxide. CO2 has been irreversibly removed from the atmosphere by carbonate formation. CO2 has also benn removed by regolith adsorption. Polar temperature increases caused by orbital variations are not great enough  相似文献   

7.
Tetsuya Tokano 《Icarus》2003,164(1):50-78
In an effort to test and to understand the global hydrogen distribution in the shallow subsurface of Mars retrieved by the Mars Odyssey gamma-ray spectrometer, the present state and movement of water are investigated by a coupled global subsurface-atmosphere water cycle model. It was found that the observed global subsurface hydrogen distribution is largely consistent with the modeled global water cycle, so a large fraction of hydrogen is likely to exist as water, at low and mid latitudes in the form of adsorbed water. Under the present climate the water content in the shallow subsurface becomes higher in the northern hemisphere than in the southern hemisphere as a result of global water cycle, regardless of the initial water distribution in the soil or adsorptive capacity. The higher annual maximum soil temperature in the south, stronger net northward transport of atmospheric water vapor, and the emission of vapor from the northern residual polar cap in northern summer contribute to this hemispheric asymmetry. The generally higher adsorptive capacity of clay minerals in the northern plains may further increase this bias. The longitudinal inhomogeneity is caused by several factors, such as thermal inertia, adsorptive capacity, and atmospheric surface pressure. The water abundance is locally high in low thermal inertia regions (e.g., Arabia Terra) and at deep places where the surface pressure is high (e.g., Hellas); it is low in soil with a low adsorptive capacity (e.g., Tharsis) and high thermal inertia regions (e.g., Solis Planum). Most of the soil humidity near the surface at low and mid latitudes may originate from the atmosphere. The model implies that the upper soil layer should be largely ice-free because otherwise an excessive sublimation and vapor emission into the atmosphere in warm seasons would violate the observational constraints. Moreover, the more uniform latitudinal variation of the observed hydrogen abundance near the surface compared to that of deeper layers is indicative of the presence of adsorbed water instead of ground ice because the adsorbed water content does not as steeply depend on latitude as the ground ice stability. Concerning the regolith mineralogy, montmorillonite can much better account for the observed water cycle than palagonite. While the presence of permanent ground ice appears likely in the polar region below a thin layer, large seasonal cycle of phase change between pore ice and adsorbed water may be possible. Regolith adsorption/desorption is neither negligible nor crucial for the seasonal atmospheric water cycle, but the surface-atmosphere coupling is a major prerequisite for the long-term evolution of subsurface water distribution.  相似文献   

8.
We present here the annual behavior of atmospheric water vapor on Mars, as observed by the OMEGA spectrometer on board Mars Express during its first martian year. We consider all the different features of the cycle of water vapor: temporal evolution, both at a seasonal and at a diurnal scale; longitudinal distribution; and the vertical profile, through the variations in the saturation height. We put our results into the context of the current knowledge on the water cycle through a systematic comparison with the already published datasets. The seasonal behavior is in very good agreement with past and simultaneous retrievals both qualitatively and quantitatively, within the uncertainties. The average water vapor abundance during the year is ∼10 pr. μm, with an imbalance between northern and southern hemisphere, in favor of the first. The maximum of activity, up to 60 pr. μm, occurs at high northern latitudes during local summer and shows the dominance of the northern polar cap within the driving processes of the water cycle. A corresponding maximum at southern polar latitudes during the local summer is present, but less structured and intense. It reaches ∼25 pr. μm at its peak. Global circulation has some influence in shaping the water cycle, but it is less prominent than the results from previous instruments suggest. No significant correlation between water vapor column density and local hour is detected. We can constrain the amount of water vapor exchanged between the surface and the atmosphere to few pr. μm. This is consistent with recent results by OMEGA and PFS-LW. The action of the regolith layer on the global water cycle seems to be minor, but it cannot be precisely constrained. The distribution of water vapor on the planet, after removing the topography, shows the already known two-maxima system, over Tharsis and Arabia Terra. However, the Arabia Terra increase is quite fragmented compared with previous observations. A deep zone of minimum separates the two regions. The saturation height of water vapor is mainly governed by the variations of insolation during the year. It is confined within 5-15 km from the surface at aphelion, while in the perihelion season it stretches up to 55 km of altitude.  相似文献   

9.
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 < 220 K, 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.  相似文献   

10.
An isothermal reservoir of carbon dioxide in gaseous contact with the Martian atmosphere would reduce the amplitude and advance the phase of global atmospheric pressure fluctuations caused by seasonal growth and decline of polar CO2 frost caps. Adsorbed carbon dioxide in the upper ～10 m of Martian regolith is sufficient to buffer the present atmosphere on a seasonal basis. Available observations and related polar cap models do not confirm or refute the operation of such a mechanism. Implications for the amplitude and phase of seasonal pressure fluctuations are subject to direct test by the upcoming Viking mission to Mars.  相似文献   

11.
《Icarus》1986,67(1):1-18
A thermal/diffusive model of H2O kinetics and equilibrium was developed to investigate the long-term evolution and depth distribution of subsurface ice on Mars. The model quantitatively takes into account (1) obliquity variations; (2) eccentricity variations; (3) long-term changes in the solar luminosity; (4) variations in the argument of subsolar meridian (in planetocentric equatorial coordinates); (5) albedo changes at higher latitudes due to seasonal phase changes of CO2 and the varying extent of CO2 ice cover; (6) planetary internal heat flow; (7) temperature variations in the regolith as a function of depth, time, and latitude due to the above factors; (8) atmospheric pressure variations over a 104-year time scale; (9) the effects of factors (1) through (5) on seasonal polar cap temperatures; and (10) Knudsen and molecular diffusion of H2O through the regolith. The migration of H2O into or out of the regolith is determined by two boundary conditions, the H2O vapor pressure at the subsurface ice boundary and the annual average H2O concentration at the base of the atmosphere. These are controlled respectively by the annual average regolith temperature at the given depth and seasonal temperatures at the polar cap. Starting from an arbitrary initial uniform depth distribution of subsurface ice, H2O fluxes into or out of the regolith are calculated for 100 selected obliquity cycles, each representing a different epoch in Mars' history. The H2O fluxes are translated into ice thicknesses and extrapolated over time to give the subsurface ice depth as a function of latitude and time. The results show that obliquity variations influence annual average regolith temperatures in varying degrees, depending on latitude, with the greatest effect at the poles and almost no effect at 40° lat. Insolation changes at the pole, due to obliquity, argument of subsolar meridian, and eccentricity variations can produce enormous atmospheric H2O concentration variations of ≈6 orders of magnitude over an obliquity cycle. Superimposed on these cyclic variations is a slow, monotonic change due to the increasing solar luminosity. Albedo changes at the polar cap due to seasonal phase changes of CO2 and the varying thickness of the CO2 ice cover are critically important in determining annual average atmospheric H2O concentrations. Despite the strongly oscillating character of the boundary conditions, only small amounts of H2O are exchanged between the regolith and the atmosphere per obliquity cycle (<10 g/cm2). The net result of H2O migration is that the regolith below 30–40° lat is depleted of subsurface ice, while the regolith above 30–40° lat contains permanent ice due to the depth of penetration of the annual thermal wave. This result is supported by recent morphological studies. The rate of migration of H2O is strongly dependent on average pore/capillary radius for which we have assumed values of 1 and 10 μm. We estimate that the H2O ice removed from the regolith would produce a permanent ice cap with a volume between 2 × 106 and 6 × 106 km3. This generally agrees with estimates deduced from deflationary features at lower latitudes, depositional features at higher latitudes, and the mass of the polar caps.  相似文献   

12.
Philip B. James 《Icarus》1985,64(2):249-264
The Martian CO2 cycle, which includes the seasonal condensation and subsequent sublimation of up to 30% of the planet's atmosphere, produces meridional winds due to the consequent mass flux of CO2. These winds currently display strong seasonal and hemispheric asymmetries due to the large asymmetries in the distribution of insolation on Mars. It is proposed that asymmetric meridional advection of water vapor on the planet due to these CO2 condensation winds is capable of explaining the observed dessication of Mars' south polar region at the current time. A simple model for water vapor transport is used to verify this hypothesis and to speculate on the effects of changes in orbital parameters on the seasonal water cycle.  相似文献   

13.
The seasonal variation of neutron emissions from Mars in different spectral intervals measured by the HEND neutron detector for the entire Martian year are analyzed. Based on these data, the spatial variations of the neutron emissions from the planet are globally mapped as a function of season, and the dynamics of seasonal variation of neutron fluxes with different energies is analyzed in detail. No differences were found between seasonal regimes of neutron fluxes in different energy ranges in the southern hemisphere of Mars, while the regime of fast neutrons (with higher energies) during the northern winter strongly differs from that during the southern winter. In winter (L s = 270°–330°), the fast neutron fluxes are noticeably reduced in the northern hemisphere (along with the consecutive thickening of the seasonal cap of solid carbon dioxide). This provides evidence of a temporary increase in the water content in the effective layer of neutron generation. According to the obtained estimates, the observed reduction of the flux of fast neutrons in the effective layer corresponds to an increase in the water abundance of up to 5% in the seasonal polar cap (70°–90°N), about 3% at mid-latitudes, and from 1.5 to 2% at low latitudes. The freezing out of atmospheric water at the planetary surface (at middle and high latitudes) and the hydration of salt minerals composing the Martian soil are considered as the main processes responsible for the temporary increase in the water content in the soil and upper layer of the seasonal polar cap. The meridional atmospheric transport of water vapor from the summer southern to the winter northern hemisphere within the Hadley circulation cell is a basic process that delivers water to the subsurface soil layer and ensures the observed scale of the seasonal increase in water abundance. In the summer northern hemisphere, the similar Hadley circulation cell transports mainly dry air masses to the winter southern hemisphere. The point is that the water vapor becomes saturated at lower heights during aphelion, and the bulk of the atmospheric water mass is captured in the near-equatorial cloudy belt and, thus, is only weakly transferred to the southern hemisphere. This phenomenon, known as the Clancy effect, was suggested by Clancy et al. (1996) as a basic mechanism for the explanation of the interhemispheric asymmetry of water storage in permanent polar caps. The asymmetry of seasonal meridional circulation of the Martian atmosphere seems to be another factor determining the asymmetry of the seasonal water redistribution in the “atmosphere-regolith-seasonal polar caps” system, found in the peculiarities of the seasonal regime of the neutron emission of Mars.  相似文献   

14.
The near-infrared reflectance spectra of the martian surface present strong absorption features attributed to hydration water present in the regolith. In order to characterize the relationships between this water and atmospheric vapor and decipher the physical state of water molecules in martian regolith analogs, we designed and built an experimental setup to measure near-IR reflectance spectra under martian atmospheric conditions. Six samples were studied that cover part of the diversity of Mars surface mineralogy: a hydrated ferric oxide (ferrihydrite), two igneous samples (volcanic tuff, and dunite sand), and three potential water rich soil materials (Mg-sulfate, smectite powder and a palagonitic soil, the JSC Mars-1 regolith stimulant). Sorption and desorption isotherms were measured at 243 K for water vapor pressure varying from 10−5 to ∼0.3 mbar (relative humidity: 10−4 to 75%). These measurements reveal a large diversity of behavior among the sample suite in terms of absolute amount of water adsorbed, shape of the isotherm and hysteresis between the adsorption and desorption branches. Simultaneous in situ spectroscopic observations permit a detailed analysis of the spectral signature of adsorbed water and also point to clear differences between the samples. Ferric (oxy)hydroxides like ferrihydrite or other phases present in palagonitic soils are very strong water adsorbent and may play an important role in the current martian water cycle by allowing large exchange of water between dust-covered regions and atmosphere at diurnal and seasonal scales.  相似文献   

15.
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   

16.
Geoffrey A. Briggs 《Icarus》1974,23(2):167-191
A model of the behavior of the Martian polar caps is described which incorporates the heating effects of the atmosphere, as well as insolation and conduction. This model is used to try to match the observed regression curves of the polar caps, and it predicts that all the seasonally condensed CO2 will be lost by around the summer solstice. The implication is that the residual caps are composed of water ice which, it is found by further modeling, should be stable during the Martian summers. However, it is also argued that this model may be too simplistic, and that the effects of wind in redistributing the seasonal condensate may lead to sufficient thickness of CO2 in the central polar region to allow the year-long existence of CO2 without significantly changing the retreat characteristics of the cap, and it is, therefore, concluded that at the present, the nature of the residual caps cannot be reliably determined.  相似文献   

17.
The surface temperature of the Martian polar caps is about 148 K (frost point temperature of CO2 at a surface pressure of about 6 hPa), with the “desert” (frost-free) areas adjacent to the polar caps having much greater surface temperatures. The existence of this steep meridional gradient of temperature between the polar caps and the adjacent “desert” areas may produce in the atmosphere a baroclinic instability which generates an atmospheric circulation system similar in some aspects to the terrestrial sea breeze. We have called this circulation system the Martian polar cap breeze. In this paper, the phenomenology of the Martian polar cap breeze is developed on the basis of the indirect observational evidence. Along with friction and the Coriolis force, other factors influence the polar cap breeze: the prevailing wind, topography, irregularity of the polar cap-edge, and stability of the atmosphere. These factors are studied in a qualitative form, as well as the seasonal variations. In addition, the large-scale polar cap wind is presented as a different Martian atmospheric circulation system.  相似文献   

18.
The mechanisms that can induce short term variations of methane in the Martian atmosphere, and thus explain the observations currently available, are yet to be discovered. Seasonal exchange with the regolith, caused by reversible adsorption, is expected to induce both spatial and time variabilities without the need for additional sources and sinks, thus avoiding difficulties raised by other scenarios. However, a comprehensive view of the role of reversible exchanges with the subsurface was still lacking. We have investigated the efficiency of such a process by implementing a coupled subsurface–atmosphere transport module in a Global Climate Model, taking into account both the thermodynamics and the kinetics of the adsorption process. It is based on recent experimental data on the adsorption of methane. We show that even with an optimistic set of parameters, and although the regolith can potentially take up a large fraction of the atmospheric reservoir, the seasonal variability induced by an exchange with the subsurface is very limited. If a local plume is detected, however, the apparent decay rate of methane in the atmosphere can be affected by the regolith uptake. This study could be extended to any trace gas reacting with the regolith, to help interpret future in situ or orbital measurements.  相似文献   

19.
David Andrew Fisher 《Icarus》2005,179(2):387-397
This paper describes a “simple standard” model of water transport through regolith that includes diffusive migration and phase changes driven by damped seasonal temperature waves. A hitherto unused first-order process is then added, that can produce ice densities much greater than those allowed by the initial dry porosity. Voids are produced in cooling icy regolith when tensile stresses exceed the cracking threshold . These stresses build up through an interaction of thermal contraction and elastic-plastic response. When the cracks open up after tensile failure there is purely thermal void enhancement and subsequent reduction as the regolith warms again. When the cracks are open the porosity is increased and they partially fill with ice crystals. Thus the void reduction on warming cannot go back to the original zero point and the bulk density of ice is increased with each temperature cycle. The cracking and thermal adjustment happen at scales of meters to millimeters. The large cracks can occur in pure ice and/or homogeneous icy material and the smaller cracks are produced by rock cobbles, pebbles, and grains having a much smaller coefficient of thermal expansion than ice. Thus a hierarchy of cracks and voids forms each temperature cycle and augments the ice content. The process can take the upper few meters of a pore-saturated icy soil from 28% by mass ice content to 70% in 10 Ma. This mechanism and the seasonal temperature cycle can plausibly produce massive ice deposits in the upper few meters of Mars' high-latitude regolith by diffusion and also keep the massive-ice regolith effectively porous to water vapor transport. The obliquity cycle can produce tensile stresses nearing 2 MPa down to depth so even deeper cracking could be happening.  相似文献   

20.
The Mars Orbiter Camera onboard the Mars Global Surveyor has obtained several images of polygonal features in the southern polar region. In images taken during the end of the southern spring, when the surrounding surface is free of the seasonal frost, CO2 ice still appears to be present within the polygonal troughs. In Earth's polar regions, polygons such as these are indicative of water ice in the ground below. We analyzed the seasonal evolution of the thermal state and the CO2 content of these features. Our 2-D model includes condensation and sublimation of the CO2 ice, a self consistent treatment of the variations of the thermal properties of the regolith, and the seasonal variations of the local atmospheric pressure which we take from the results of a general circulation model. We find that the residence time of seasonal CO2 ice in troughs depends not only on atmospheric opacity and albedo of the CO2 ice, but also and most significantly on the distribution of water ice in the regolith. Optical properties of the atmosphere and surface CO2 ice can be independently obtained from observations. To date this is not true about the distribution of water ice below the surface. Our analysis quantifies the dependence of the seasonal cycle of the CO2 ice within the troughs on the assumed distribution of the water ice below the surface. We show that presence of water ice in the ground at a depth smaller than the depth of the troughs reduces winter condensation rate of CO2 ice. This is due to higher heat flux conducted from the water ice rich regolith toward the facets of the troughs.  相似文献   

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