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1.
《Icarus》1986,66(1):2-21
A refined technique is presented for deriving 9-μm extinction opacities of the Mars atmosphere from brightness temperature measurements made by the Viking Infrared Thermal Mapper. Improvements include modeling of the vertical temperature profile, a surface-atmosphere temperature discontinuity, and the effects of surface emissitivity and particle scattering. The routine is applied to the Mars Average Data Set to yield zonal mean opacities for more than one Mars year. The mean 9-μm opacity for one year is 0.5, yet the mode value is only 0.056, due to the very skewed distribution. Time histories of opacity at the Viking Lander 1 and 2 sites are generated and compared to in situ data. Opacity maps are presented for the period Ls 168–270 covering the 1977a dust storm; these show the genesis and spread of the storm with 10° Ls resolution.  相似文献   

2.
Stephen D. Wall 《Icarus》1981,47(2):173-183
A thin light-colored ground covering appeared on the surface of Mars near the Viking 2 lander from Ls = 230° to Ls = 16°, a total of 249 Mars days, during the lander's first winter on the surface. This paper presents a reduction of applicable lander imagery during the period. Imaging sequences, relative surface albedo, spectral reflectance estimates, and limited photometric data are presented and compared with previous laboratory measurements. Photometric data are best fit by an average Minnaert k = 1.1 (blue), k = 1.0 (green), and k = 0.95 (red). Appearance and disappearance rates, spectral reflectance, and photometric data all tend to confirm an earlier proposal that the covering was a combination of H2O and CO2, which fell already condensed onto dust particles brought northward by the season's first major dust storm. Under this assumption, the covering thickness is estimated to be between 0.5 and a few millimeters.  相似文献   

3.
We present observations of a local dust storm performed by the OMEGA and PFS instruments aboard Mars Express. OMEGA observations are used to retrieve the dust single-scattering albedo in the spectral range 0.4-4.0 μm. The single-scattering albedo shows fairly constant values between 0.6 and 2.6 μm, and a sharp decrease at wavelengths shorter than 0.6 μm, in agreement with previous studies. It presents a small absorption feature due to ferric oxide at 0.9 μm, and a strong absorption feature due to hydrated minerals between 2.7 and 3.6 μm. We use a statistical method, the Independent Component Analysis, to determine that the dust spectral signature is decoupled from the surface albedo, proving that the retrieval of the single-scattering albedo is reliable, and we map the dust optical thickness with a conventional radiative transfer model. The effect of the dust storm on the atmospheric thermal structure is measured using PFS observations. We also simulate the thermal impact of the dust storm using a one-dimensional atmospheric model. A comparison of the retrieved and modeled temperature structures suggests that the dust in the storm should be confined to the 1-2 lowest scale heights of the atmosphere. However, the observed OMEGA reflectance in the CO2 absorption bands does not support this suggestion.  相似文献   

4.
Michael D Smith 《Icarus》2004,167(1):148-165
We use infrared spectra returned by the Mars Global Surveyor Thermal Emission Spectrometer (TES) to retrieve atmospheric and surface temperature, dust and water ice aerosol optical depth, and water vapor column abundance. The data presented here span more than two martian years (Mars Year 24, Ls=104°, 1 March 1999 to Mars Year 26, Ls=180°, 4 May 2003). We present an overview of the seasonal (Ls), latitudinal, and longitudinal dependence of atmospheric quantities during this period, as well as an initial assessment of the interannual variability in the current martian climate. We find that the perihelion season (Ls=180°-360°) is relatively warm, dusty, free of water ice clouds, and shows a relatively high degree of interannual variability in dust optical depth and atmospheric temperature. On the other hand, the aphelion season (Ls=0°-180°) is relatively cool, cloudy, free of dust, and shows a low degree of interannual variability. Water vapor abundance shows a moderate amount of interannual variability at all seasons, but the most in the perihelion season. Much of the small amount of interannual variability that is observed in the aphelion season appears to be caused by perihelion-season planet-encircling dust storms. These dust storms increase albedo through deposition of bright dust on the surface causing cooler daytime surface and atmospheric temperatures well after dust optical depth returns to prestorm values.  相似文献   

5.
The initiation and evolution of a planet-encircling dust storm on Mars have been observed in unprecedented detail by the Thermal Emission Spectrometer (TES) on board the Mars Global Surveyor. A local dust storm began to expand quickly on 26 June 2001 (areo-centric longitude, Ls=185°), becoming a planet-encircling dust storm by 11 July 2001. The dust storm had dust optical depth >2 over wide areas and caused atmospheric temperatures over a large vertical range to warm by >40 K from 20°N latitude to the south pole.  相似文献   

6.
Bruce A. Cantor 《Icarus》2007,186(1):60-96
From 15 September 1997 through 21 January 2006, only a single planet-encircling martian dust storm was observed by MGS-MOC. The onset of the storm occurred on 26 June 2001 (Ls=184.7°), earliest recorded to date. It was initiated in the southern mid-to-low latitudes by a series of local dust storm pulses that developed along the seasonal cap edge in Malea and in Hellas basin (Ls=176.2°-184.4°). The initial expansion of the storm, though asymmetric, was very rapid in all directions (3-32 m s−1). The main direction of propagation, however, was to the east, with the storm becoming planet encircling in the southern hemisphere on Ls=192.3°. Several distinct centers of active dust lifting were associated with the storm, with the longest persisting for 86 sols (Syria-Claritas). These regional storms helped generate and sustain a dust cloud (“haze”), which reached an altitude of about 60 km and a peak opacity of τdust∼5.0. By Ls=197.0°, the cloud had encircled the entire planet between 59.0° S and 60.0° N, obscuring all but the largest volcanoes. The decay phase began around Ls∼200.4° with atmospheric dust concentrations returning to nominal seasonal low-levels at Ls∼304.0°. Exponential decay time constants ranged from 30-117 sols. The storm caused substantial regional albedo changes (darkening and brightening) as a result of the redistribution (removal and deposition) of a thin veneer of surface dust at least 0.1-11.1 μm thick. It also caused changes in meteorological phenomena (i.e., dust storms, dust devils, clouds, recession of the polar caps, and possibly surface temperatures) that persisted for just a few weeks to more than a single Mars year. The redistribution of dust by large annual regional storms might help explain the long period (∼30 years) between the largest planet-encircling dust storms events.  相似文献   

7.
The Mars Orbiter Laser Altimeter (MOLA), functioning as a high-resolution radiometer, has observed several appearances of the Martian residual ice caps. We examine these data to quantify both seasonal behavior and interannual differences. The northern residual cap (NRC) was found to be mostly stable with the exception of one, previously identified, region of strong variability. Interannual change in the extent of the NRC appears to be small and reversible on timescales of 1 or 2 years. The NRC has an elaborate seasonal evolution of albedo. Annuli of fine-grained CO2 and water frost, which track the inner and outer edges of the seasonal CO2 cap, cause large temporary brightenings. The NRC albedo is stable from just after solstice to Ls 150°, after which albedo decreases steadily. This late-summer darkening can be explained by shadowing within the rough topography of the NRC, leading to a lower limit on topographic relief of 80 cm. The southern residual cap (SRC) appears stable in extent. As has been previously discovered, its seasonal frost albedo behavior appears to be correlated with insolation. However, residual CO2 appears not to share this characteristic; we use this behavioral difference to infer net deposition of CO2 ice on the SRC during 1 out of 3 years. Uncharacteristically, the SRC abruptly darkens at Ls 320° in 1 Martian year (year beginning April 2002). Circumstantial evidence suggests atmospheric scattering by dust is responsible. The 2001 global dust-storm appears, either, to have had no effect on the polar cap albedos, or, resulted in slightly brighter ice deposits.  相似文献   

8.
A time-sequential set of bolometric albedo maps for Mars has been constructed from Viking Infrared Thermal Mapper data. The maps provide global coverage in longitude for latitudes -60° to +60° at 1° by 1° spatial resolution. Individual maps are constructed under strict geometric constraints for a narrow range of Ls. The set of albedo maps spans a Martian year and includes maps before, during, and after the global dust storms of 1977. Transient brightenings associated with local dust storms or condensate clouds are apparent in some of the maps. During dust-free periods, bolometric albedo maps are generally similar to classical, visual albedo map of Mars. The distribution of bolometric surface albedos is bimodal with typical, clear-sky, Lambert albedos of 0.27 and 0.16 for bright and dark areas, respectively. Atmospheric effects strongly influence apparent surface albedos, especially for dark areas. Neither bright nor dark regions show measurable, long-term variations of bolometric albedos during clear periods.  相似文献   

9.
C.F. Capen 《Icarus》1976,28(2):213-230
This paper is a qualitative and quantitative investigation of recent seasonal and secular albedo feature variations on Mars using the improved red-filter photography of the International Planetary Patrol Program obtained during the 1971 and 1973 apparitons. The boundaries of telescopic albedo features showing significant changes and uncommonly bright areas were measured with an image projection reader and plotted on Mars maps in order to compare changes with Mariner 9 variable feature data obtained in 1972. The Daedalia-Claritas darkening (90–130°W, 25°S) of 1973 was the most significant change observed on Mars since the Laocoontis-Alcyonius darkening of the 1950 decade. The 1973 apparition presented six excellent examples of normal seasonal changes.Photographic photometry of 21 albedo features located in three longitude regions was systematically done on 120 original red-filter negatives, about half representing the Martian southern spring season and the other half the southern summer season. These two sets of photographs differed about 90° in Ls but were nearly equal in viewing geometry. They were identical in image scale, emulsion, color response, and quality. The data, consisting of 3960 density measurements, were analyzed in terms of albedo ratios and their fluctuations. Several types of diagnostic graphs are presented.Seasonal differences of the mean brightness ratios for most of the features were found to be consistent with the contrast changes shown on photographic Mars maps. Seasonal variations of brightnesses with latitude and region were indicated, but not in the strict classical sense of a wave-of-darkening. Most albedo features were more variable during southern spring than they were in summer. Most light albedo features were more active than dark ones during both seasons. There was a general loss of relative contrast in southern summer. No systematic dependence on solar phase angle was detected within the range (38° > i > 16°) spanned by both sets of photographs.  相似文献   

10.
Edwin S. Barker 《Icarus》1976,28(2):247-268
The patrol of Martian water vapor carried out with the echelle-coudé scanner at McDonald Observatory during the 1972–1974 apparition has produced 469 individual photoelectric scans of Doppler-shifted Martian H2O lines. Almost an entire Martian year was covered during the 1972–1974 period (Ls = 118?269° and 301?80°). Three types of coverage have been obtained: (1) regular—the slit placed pole to pole on the central meridian; (2) latitudinal—the slit placed parallel to the Martian equator at various latitudes; (3) diurnal—the slit placed parallel to the terminator at several times during a Martian day measured from local noon.Both the seasonal and diurnal effects seem to be controlled by the insolation and not the local topography with respect to the 6.1 mb surface. A slight negative correlation with elevation was noted which improved during the seasons of greater H2O content. The previous seasonal behavior has been confirmed and amplified. The following are the primary conclusions: (1) The planetwide abundance is low (5?15 μm of ppt H2O) during both equinoctical periods. (2) The maximum abundance of about 40 μm occurs in each hemisphere after solstice at about 40° latitude in that hemisphere. (3) The latitude of the maximum amount in the N-S distribution precedes the latitude of maximum insolation by 10–20° of latitude. (4) During the “drier” seasons (5–20 μm) near the equinoxes on Mars, the atmospheric water vapor changes by a factor of 2–3x over a diurnal cycle with the maximum near local noon. (5) The effects of the 1973 dust storm during the southern summer reduced the amount of water vapor over the southern hemisphere regions to 3–8 μm.  相似文献   

11.
Surface materials exposed throughout the equatorial region of Mars have been classified and mapped on the basis of spectral reflectance properties determined by the Viking II Orbiter vidicon cameras. Frames acquired at each of three wavelengths (0.45 ± 0.03 μm, 0.53 ± 0.05 μm, and 0.59 ± 0.05 μm) during the approach of Viking Orbiter II in Martian summer (Ls = 105°) were mosaicked by computer. The mosaics cover latitudes 30°N to 63°S for 360° of longitude and have resolutions between 10 and 20 km per line pair. Image processing included Mercator transformation and removal of an average Martian photometric function to produce albedo maps at three wavelengths. The classical dark region between the equator and ～30°S in the Martian highlands is composed of two units: (i) and ancient unit consisting of topographic highs (ridges, crater rims, and rugged plateaus riddled with small dendritic channels) which is among the reddest on the planet (0.59/0.45 μm ? 3); and (ii) intermediate age, smooth, intercrater volcanic plains displaying numerous mare ridges which are among the least red on Mars (0.59/0.45 μm ? 2). The relatively young shield volcanoes are, like the oldest unit, dark and very red. Two probable eolian deposits are recognized in the intermediate and high albedo regions. The stratigraphically lower unit is intermediate in both color (0.59/ 0.45 μm ? 2.5) and albedo. The upper unit has the highest albedo, is very red (0.59/0.45 μm ? 3), and is apparently the major constituent of the annual dust storms as its areal extent changes from year to year. The south polar ice cap and condensate clouds dominate the southernmost part of the mosaics.  相似文献   

12.
A distinctive terrain named cryptic region which is characterized by regions of low albedo and low temperature has been identified on the Martian south polar cap. In this zone, many fan- and spider-shaped features of km-scale appeared following the sublimation of the CO2 frost layer. These peculiar features were apparently caused by a wind-blown system of dust-laden jets. During the warming period starting at Ls∼180°, the seasonal ice cap regresses and fans and spiders appear in sequence. These surface features are repeatable events that tend to occupy the same areas from year to year. In this study, we use the Mars Orbiter camera (MOC) narrow-angle images to produce a statistical study of the time distributions of the fans and spiders as functions of Ls and as functions of the topography. The time variations and spatial distributions of these features are further correlated with the CO2 ice coverage measured by the Mars Orbiter laser altimeter (MOLA) instrument. We have documented that most of the fans are found in the early spring with Ls<230° and the fans and spiders coexist at Ls=250°±20°. It is also found that there is a strong dependence on latitude and altitude with fans and spiders most often observed at high latitude (>83°S) and high altitude (>2500 m). Our statistical result also indicates that the occurrence of fans is highly correlated with the thickness of the CO2 frost thus providing support for the venting model.  相似文献   

13.
We have obtained estimates of the threshold wind speed Vgt near the top of the atmospheric boundary layer on Mars and of the rotation angle α between this wind velocity and the direction of the surface stress. this calculation has been accomplished by combining wind tunnel determinations of the friction velocity with semi-empirical theories of the Earth's atmospheric boundary layer. Calculations have been performed for a variety of values of the surface pressure, ground temperature, roughness height, boundary layer height, atmospheric composition atmospheric stability, particle density, particle diameter, and strength of the cohesive force between the particles.The curve of threshold wind speed as a function of particle diameter monotonically decreases with decreasing particle diameter for a cohesionless soil but has the classical U shape for a soil with cohesion. Observational data indicate that the latter condition holds on Mars. Under “favorable” conditions minimum threshold wind speeds between about 50 and 100m/sec are required to cause particle motion. These minimum values lie close to the highest wind speeds predicted by general circulation models. Hence, particle motion should be an infrequent occurence and should be strongly correlated with nearness to small topographic features. The latter prediction is in accord with the correlation found between albedo markings and topographic obstacles such as craters. For equal wind speeds at the midpoint of the boundary layer, particle movement occurs more readily in general at night than during the day, more readily in the winter polar areas than the equatorial areas noon, and more readily for ice particles than for silicate particles.The boundary between saltating and suspendable particles is located at a particle diameter of about 100 μm. This value is close to the diameter at which the Vgt curve has its minimum. Hence, the wind can set directly into motion both saltating and larger-sized suspendable particles, but dust-storm-sized particles usually require impact by a saltating particle for motion to be initiated. Albedo changes occur most often in regions containing a mixture of dust-stoorm-sized particles and saltating particles. The threshold wind speed for surfaces containing large, nonerodible roughness elements can either be larger or smaller than the value for surfaces with only erodible material. The former condition for Vgt holds when the roughness height z0 is less than about 1 cm and may be illustrated by craters that have experienced less erosion than their environs. The latter condition for Vgt may be partly responsible for albedo changes detected on the elevated shield volcano, Pavonis Mons. Values of the angle α generally lie between 10 and 30°. These figures place a modest limitation on the utility of surface albedo streaks as wind direction indicators.  相似文献   

14.
John Caldwell 《Icarus》1977,32(2):190-209
Ultraviolet photometric and spectrophotometric observations of Mars and Saturn obtained by two Earth-orbiting satellites are combined in this report. High-resolution data from the S59 experiment aboard TD1A reveal no definite absorption features in the spectra of either planet. The absence of a prominent absorption in the Mars data near 2150 Å can be reconciled with the preliminary Viking measurement of NO only if that gas is preferentially concentrated at high Martian altitudes. Broadband photometry from OAO-2 shows that atmospheric dust on Mars during the great dust storm of 1971–1972 reduced the ultraviolet geometric albedo by a factor of ?3 at the height of the storm. This atmospheric energy deposition is probably an important mechanism in the storm dynamics. Diurnal variation in the ultraviolet brightness of Mars appears to be marginally detectable during the dust storm. A real brightness variation during a clear season is observed. The combined Saturn data from the two satellites strongly suggest that NH3 does not influence the ultraviolet spectrum of Saturn, but that some other absorber does. A candidate for such an absorber, H2S, is investigated. OAO-2 broadband photometry of Jupiter and of Saturn demonstrate that these planets have very similar albedos from 2100 to 2500 Å. This implies a common ultraviolet absorber on both planets, other than NH3.  相似文献   

15.
Mesospheric clouds have been previously observed on Mars in a variety of datasets. However, because the clouds are optically thin and most missions have performed surface-focussed nadir sounding, geographic and seasonal coverage is sparse. We present new detections of mesospheric clouds using a limb spectra dataset with global coverage acquired by NASA’s Mars Climate Sounder (MCS) aboard Mars Reconnaissance Orbiter. Mesospheric aerosol layers, which can be CO2 ice, water ice or dust clouds, cause high radiances in limb spectra, either by thermal emission or scattering of sunlight. We employ an object recognition and classification algorithm to identify and map aerosol layers in limb spectra acquired between December 2006 and April 2011, covering more than two Mars years. We use data from MCS band A4, to show thermal signatures of day and nightside features, and A6, which is sensitive to short wave IR and visible daytime features only. This large dataset provides several thousand detections of mesospheric clouds, more than an order of magnitude more than in previous studies.Our results show that aerosol layers tend to occur in two distinct regimes. They form in equatorial regions (30°S–30°N) during the aphelion season/northern hemisphere summer (Ls < 150°), which is in agreement with previous published observations of mesospheric clouds. During perihelion/dust storm season (Ls > 150°) a greater number of features are observed and are distributed in two mid-latitude bands, with a southern hemisphere bias. We observe temporal and longitudinal clustering of cloud occurrence, which we suggest is consistent with a formation mechanism dictated by interaction of broad temperature regimes imposed by global circulation and the propagation to the mesosphere of small-scale dynamics such as gravity waves and thermal tides.Using calculated frost point temperatures and a parameterization based on synthetic spectra we find that aphelion clouds are present in generally cooler conditions and are spectrally more consistent with H2O or CO2 ice. A significant fraction has nearby temperature retrievals that are within a few degrees of the CO2 frost point, indicating a CO2 composition for those clouds. Perihelion season clouds are spectrally most similar to H2O ice and dust aerosols, consistent with temperature retrievals near to the clouds that are 30–80 K above the CO2 frost point.  相似文献   

16.
We report high-spectral-resolution (λ/δλ = 800-2300) near-infrared mapping observations of Mars at Ls = 130° (April 1999), which were obtained by drift-scanning the cryogenic long-slit spectrometer at the KPNO 2.2-m telescope across the disk. Data were reformatted into calibrated spectral image cubes (x,y,λ) spanning 2.19 to 4.12 μm, which distinguish atmospheric CO2 features, solar lines, and surface and aerosol features. Maps of relative band depth between 3.0 and 3.5 μm trace water ice clouds and show the diurnal evolution of features in the persistent northern summer aphelion cloud belt, which was mapped contemporaneously but at fixed local time by the Mars Global Surveyor Thermal Emission Spectrometer (MGS/TES). Cloud optical depth, particle sizes, and ice aerosol content were estimated using a two-stream, single-layer scattering model, with Mie coefficients derived from recently published ice optical constants, followed by a linear spectral deconvolution process. A comparison of data and model spectra shows evaporating nighttime clouds in the morning followed by afternoon growth of a prominent orographic cloud feature on the west flank of Elysium Mons. Cloud optical depth at 3.2 μm evolved to 0.28 ± 0.13 and ice aerosol column abundance to 0.9 ± 0.3 pr μm in the afternoon. Column abundances as large as 0.17 pr μm were retrieved in nonorographic clouds within the aphelion cloud band around midday. These clouds exhibit a modest decline in optical depth during the afternoon. Results show that ice particle radii from <2 μm to >4 μm exist in both cloud types. However, large particles dominate the spectra, consistent with recent MGS/TES emission phase function measurements of aphelion cloud aerosol properties.  相似文献   

17.
《Icarus》2003,166(1):1-20
We have analyzed observations of the Acidalia hemisphere of Mars taken by the Hubble Space Telescope's Near-Infrared Camera Multi-Object Spectrograph (HST/NICMOS) during July of 1997 (Ls=152°, northern martian summer). The data consist of images at ∼60 km/pixel resolution, using both narrow- and medium-band filters specifically selected to allow us to study the hydration state of the martian surface. Calibration was performed by comparison to Phobos-2 ISM observations of overlapping regions, and atmospheric gas correction was performed by modeling the atmosphere for each pixel using a line-by-line radiative transfer code coupled with the MOLA altimetry data. Our results indicate the presence of at least three spectrally different large-scale (>1000 km diameter) terrains corresponding to the dark regions of northern Acidalia, the southern hemisphere classical dark terrain, and the classical intermediate terrain adjacent to southern Acidalia. We also identified two other spectrally unique terrains, corresponding to the northern polar ice cap, and to the southern winter polar hood. Comparisons with mineral spectra indicate the possibility of different H2O- or OH-bearing (i.e., hydroxides and/or hydrates) minerals existing both in northern Acidalia and in the nearby intermediate albedo terrain. Hydrated minerals do not appear to be spectrally important components of the southern hemisphere dark terrains imaged by HST in 1997.  相似文献   

18.
Measurements of martian atmospheric water vapor made throughout Ls = 18.0°-146.4° (October 3, 1996-July 12, 1997) show changes in Mars humidity on hourly, daily, and seasonal time scales. Because our observing program during the 1996-1997 Mars apparition did not include concomitant measurement of nearby CO2 bands, high northern latitude data were corrected for dust and aerosol extinction assuming an optical depth of 0.8, consistent with ground-based and HST imaging of northern dust storms. All other measurements with airmass greater than 3.5 were corrected using a total optical depth of 0.5. Three dominant results from this data set are as follows: (1) pre- and post-opposition measurements made with the slit crossing many hours of local time on Mars’ Earth-facing disk show a distinct diurnal pattern with highest abundances around and slightly after noon with low abundances in the late afternoon, (2) measurements of water vapor over the Mars Pathfinder landing site (Carl Sagan Memorial Station) on July 12, 1997, found 21 ppt μm in the spatial sector centered near 19° latitude, 36° longitude while abundances around the site varied from as low as 6 to as high as 28 ppt μm, and (3) water vapor abundance is patchy on hourly and daily time scales but follows the usual seasonal trends.  相似文献   

19.
We used MGS-MOC and MRO-MARCI daily mapping images of the North Polar Region of Mars from 16 August 2005 (Ls = 270°) to 21 May 2009 (Ls = 270°), covering portions of three consecutive martian years (MY 27-MY 29), to observe the seasonal behavior of the polar ice cap and atmospheric phenomena. The rate of cap regression was similar in MY 28 and MY 29, but was advanced by 3.5° of Ls (∼7-8 sols) in MY 29. The spatial and temporal behaviors of dust and condensate clouds were similar in the two years and generally in accord with prior years. Dust storms (>100 km2) were observed in all seasons, with peak activity occurring at Ls = 10-20° from 50°N to 70°N and at Ls = 135-140° from 70°N to 90°N. The most active quadrant was 0-90°W in MY 28, shifting to 180-270°W in MY 29. The majority of regional storms in both years developed in longitudes from 10°W to 60°W. During late summer the larger storms obscure the North Polar Region in a cloud of dust that transitions to north polar hood condensate clouds around autumnal equinox.Changes in the distribution of perennial ice deposits, especially in Olympia Planum, were observed between the 2 years, with the MY 29 ice distribution being the most extensive observed to date. Modeling suggests that the small, bright ice patches on the residual cap are not the result of slope or elevation effects. Rather we suggest that they are the result of local meteorological effects on ice deposition. The annual darkening and brightening of peripheral areas of the residual cap around summer solstice can be explained by the sublimation of a brighter frost layer revealing an underlying darker, ice rich layer that itself either sublimes to reveal brighter material below or acts as a cold trap, attracting condensation of water vapor that brightens the surface. An alternative explanation invokes transport and deposition of dust on the surface from the cap interior, and later removal of that dust. The decrease in cap albedo and accompanying increase in near surface atmospheric stability may be related to the annual minimum of polar storm activity near northern summer solstice.  相似文献   

20.
Geologic evidence of the prior existence of liquid water on Mars suggests surface temperatures Ts were once considerably warmer than at present; and that such a condition may have arisen from a larger atmospheric greenhouse. Here we develop a simple climate model for a CO2/H2O Mars atmosphere including water vapor-longwave opacity feedback in the atmosphere and temperature-albedo feedback at surface icecaps, under the assumption that once the Martian surface pressure was ps ≥ 1 atm CO2. Longwave flux to space is computed as a function of Ts and ps using band-absorption models for the effect of the 15-μm fundamental, and the 10- and 15-μm hot bands, of the CO2 molecule; as well as the pure rotation bands and e continuum of H2O. The derived global radiative balance predicts a global mean surface temperature of 283°K at 1 atm CO2. When the emission model is coupled to a latitudinally resolved energy balance climate model, including the effect of poleward heat transfer by atmospheric baroclinic eddies, the solutions vary, depending on ps. We considered two cases: (1) the present Mars (ps ? 0.007 atm) with pressure-buffering by solid CO2 icecaps, and limited poleward heat flux by the atmosphere; and (2) a hypothetical “hot Mars” (ps ? 1.0 atm), whose much higher CO2 amount augmented by H2O evaporative feedback yields a theoretical Ts distribution with latitude admitting liquid water over 95% of the surface, water icecaps at the poles, and a diminished equator-to-pole temperature gradient relative to the present.  相似文献   

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