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1.
The Mars Global Surveyor Mars Orbiter Camera was used to obtain global maps of the martian surface with equatorial resolution of 7.5 km/pixel in two wavelength ranges: blue (400-450 nm) and red (575-625 nm). The maps used were acquired between March 15, 1999 (Ls=110°) and July 31, 2001 (Ls=205°), corresponding to approximately one and a quarter martian years. Using the global maps, cloud area (in km2) has been measured daily for water ice clouds topographically corresponding to Olympus Mons, Ascraeus Mons, Pavonis Mons, Arsia Mons, Alba Patera, the western Valles Marineris canyon system, and for other small surface features in the region. Seasonal trends in cloud activity have been established for the three Tharsis volcanoes, Olympus Mons, and Alba Patera. Olympus, Ascraeus, and Pavonis Mons show cloud activity from about Ls=0°-220° with a peak in cloud area near Ls=100°. One of our most interesting observational results is that Alba Patera shows a double peaked feature in the cloud area with peaks at Ls=60° and 140° and a minimum near Ls=100°. Arsia Mons shows nearly continuous cloud activity. The altitudes of several of these clouds have been determined from the locations of the visual cloud tops, and optical depths were measured for a number of them using the DISORT code of Stamnes et al. (1988, Appl. Opt. 27, 2502-2509). Several aspects of the observations (e.g., cloud heights, effects of increased dust on cloud activity) are similar to simulations in Richardson et al. (2002, J. Geophys. Res. 107, 5064). A search for short period variations in the cloud areas revealed only indirect evidence for the diurnal cloud variability in the afternoon hours; unambiguous evidence for other periodicities was not found.  相似文献   

2.
Using images from the Mars Orbiter Camera, we have identified several linear ridges located 10-60 km north of the volcano Olympus Mons, Mars, at the edge of the Olympus Mons aureole materials. These ridges appear to be made of unconsolidated material by virtue of the many dust avalanche scars seen on their upper slopes. Based upon their morphology (several ridges have crater-like central depressions) and superposition relationships, the ridges appear to have formed very recently and post-date the formation of the youngest lava flows spilling over the northern escarpment of Olympus Mons. Several possible origins for the ridges, including an eolian, periglacial, or depositional origin have been considered, but we favor a ridge origin by a series of small explosive eruptions initiated by the intrusion of a dike into a volatile-rich substrate. To explore this process, we develop a numerical model for dike intrusion into a volatile-rich substrate that yields plausible dike widths between 2.4-3.5 m. The total volume of a single ridge system is ∼65×106 m3, and we calculate that it may have taken only a few minutes to form. Viable solutions only exist when the thicknesses of the ice-rich layer is less than ∼1000-2000 m. This strongly suggests that the ice-rich region is limited in its vertical extent to a value of this order.  相似文献   

3.
We use Viking and new MGS and Odyssey data to characterize the lobate deposits superimposed on aureole deposits along the west and northwest flanks of Olympus Mons, Mars. These features have previously been interpreted variously as landslide, pyroclastic, lava flow or glacial features on the basis of Viking images. The advent of multiple high-resolution image and topography data sets from recent spacecraft missions allow us to revisit these features and assess their origins. On the basis of these new data, we interpret these features as glacial deposits and the remnants of cold-based debris-covered glaciers that underwent multiple episodes of advance and retreat, occasionally interacting with extrusive volcanism from higher on the slopes of Olympus Mons. We subdivide the deposits into fifteen distinctive lobes. Typical lobes begin at a theater-like alcove in the escarpment at the base of Olympus Mons, interpreted to be former ice-accumulation zones, and extend outward as a tongue-shaped or fan-shaped deposit. The surface of a typical lobe contains (moving outward from the basal escarpment): a chaotic facies of disorganized hillocks, interpreted as sublimation till in the accumulation zone; arcuate-ridged facies characterized by regular, subparallel ridges and interpreted as the ridges of surface debris formed by the flow of underlying ice; and marginal ridges interpreted as local terminal moraines. Several lobes also contain a hummocky facies toward their margins that is interpreted as a distinctive type of sublimation till shaped by structural dislocations and preferential loss of ice. Blocky units are found extending from the escarpment onto several lobes; these units are interpreted as evidence of lava-ice interaction and imply that ice was present at a time of eruptive volcanic activity higher on the slopes of Olympus Mons. Other than minor channel-like features in association with lava-ice interactions, we find no evidence for the flow of liquid water in association with these lobate features that might suggest: (1) near-surface groundwater as a source for ice in the alcoves in the lobe source region at the base of the scarp, or (2) basal melting and drainage emanating from the lobes that might indicate wet-based glacial conditions. Instead, the array of features is consistent with cold-based glacial processes. The glacial interpretations outlined here are consistent with recent geological evidence for low-latitude ice-rich features at similar positions on the Tharsis Montes as well as with orbital dynamic and climate models indicating extensive snow and ice accumulation associated with episodes of increased obliquity during the Late Amazonian period of the history of Mars.  相似文献   

4.
Using Mars Global Surveyor Mars Orbiter Camera daily global maps, cloud areas have been measured daily for water ice clouds associated with the topography of the major volcanoes Olympus Mons, Ascraeus Mons, Pavonis Mons, Arsia Mons, Elysium Mons, and Alba Patera. This study expands on that of Benson et al. [Benson, J.L., Bonev, B.P., James, P.B., Shan, K.J., Cantor, B.A., Caplinger, M.A., 2003. Icarus 165, 34-52] by continuing their cloud area measurements of the Tharsis volcanoes, Olympus Mons and Alba Patera for an additional martian year (August 2001-May 2003) and by also including Elysium Mons measurements from March 1999 through May 2003. The seasonal trends in cloud activity established by Benson et al. [Benson, J.L., Bonev, B.P., James, P.B., Shan, K.J., Cantor, B.A., Caplinger, M.A., 2003. Icarus 165, 34-52] for the five volcanoes studied earlier are corroborated here with an additional year of coverage. For volcanoes other than Arsia Mons, interannual variations that could be associated with the large 2001 planet encircling dust storm are minimal. At Arsia Mons, where cloud activity was continuous in the first two years, clouds disappeared totally for ∼85° of LS (LS=188°-275°) due to the dust storm. Elysium Mons cloud activity is similar to that of Olympus Mons, however the peak in cloud area is near LS=130° rather than near LS=100°.  相似文献   

5.
In this paper we report about a small region on the northern scarp of Olympus Mons showing an increase of the 3 μm hydration band in the OMEGA spectra, together with low superficial temperatures. Although water ice clouds can occurs on the flank of big martian volcanoes, radiative transfer modeling indicates that atmospheric water ice alone cannot justify the shape of the observed band. A fit of the 1.9–3 μm absorption features is obtained by hypothesizing that the study region consists of a mixture of dust and water ice covered by an optically thin (τ=0.08 at 3 μm) layer of dust. Thermal modeling also suggests that water ice in this region may be stable during most of the martian year due to the saturation of the atmosphere. If water ice is responsible for the observed spectral behavior, it might consist of a number of ice or snow patches possibly deposited in small depressions.  相似文献   

6.
A total of 205 dust devils were detected in 23 High Resolution Stereo Camera (HRSC) images taken between January 2004 and July 2006 with the ESA Mars Express orbiter, in which average dust devil heights were ∼660 m and average diameters were ∼230 m. For the first time, dust devil velocities were directly measured from orbit, and range from 1 to 59 m/s. The observed dust devil directions of motion are consistent with data derived from a General Circulation Model (GCM). In some respects HRSC dust devil properties agree favorably with data from the NASA Mars Exploration Rover Spirit dust devil analyses. The spatial distribution of the active dust devils detected by HRSC supports the conjecture that the ascending branch of the Hadley circulation is responsible for the increase in dust devil activity, especially observed during southern summer between 50° and 60° S latitude. Combining the dust-lifting rate of 19 kg/km2/sol derived from the Spirit observations with the fewer in number but larger in size dust devils from various other locations observed by HRSC, we suggest that dust devils make a significant contribution to the dust entrainment into the atmosphere and to the martian dust cycle.  相似文献   

7.
Observations of water ice clouds and dust are among the main scientific goals of the Planetary Fourier Spectrometer (PFS), a payload instrument of the European Mars Express mission. We report some results, obtained in three orbits: 37, 41 and 68. The temperature profile, and dust and water ice cloud opacities are retrieved from the thermal infrared (long-wavelength channel of PFS) in a self-consistent way using the same spectrum. Orographic ice clouds are identified above Olympus (orbit 37) and Ascraeus Mons (orbit 68). Both volcanoes were observed near noon at Ls=337° and 342°, respectively. The effective radius of ice particles is preliminary estimated as 1-3 μm, changing along the flanks. The corresponding visual opacity changes in the interval 0.2-0.4 above Olympus and 0.1-0.6 above Ascraeus Mons. In the case of Ascraeus Mons, the ice clouds were observed mainly above the Southern flank of the volcano with maximum opacity near the summit. In the case of Olympus, the clouds were found above both sides of the top. A different type of ice cloud is observed at latitudes above 50°N (orbit 68) in the polar hood: the effective particle radius is estimated to be 4 μm. Below the 1 mb level an inversion in the temperature profiles is found with maximum temperature at around 0.6 mb. Along orbit 68 it appears above Alba Patera, then it increases to the north and decreases above the CO2 polar cap. Beginning from latitude 20°S above Tharsis (orbit 68), the ice clouds and dust contribute equally to the spectral shape. Further on, the ice clouds are found everywhere along orbit 68 up to the Northern polar cap, except the areas between the Northern flank of Ascraeus Mons (below 10 km) and the edge of Alba Patera. Orbit 41 is shifted from the orbit 68 by roughly 180° longitude and passes through Hellas. Ice clouds are not visible in this orbit at latitudes below 80°S. The dust opacity is anticorrelated with the surface altitude. From 70°S to 25°N latitude the vertical dust distribution follows an exponential law with a scale height of 11.5±0.5 km, which corresponds to the gaseous scale height near noon and indicates a well-mixed condition. The 9 μm dust opacity, reduced to zero surface altitude, is found to be 0.25±0.05, which corresponds to a visual opacity of 0.5-0.7 (depending on the particle size).  相似文献   

8.
A geologic analysis of 274 images acquired by the high-resolution MOC camera onboard the Mars Global Surveyor spacecraft within the Arabia Terra low neutron flux anomaly (which is indicative of an anomalously high abundance of hydrogen: up to 16 wt % of the equivalent amount of water) was performed. Correlation between the enhanced abundance of equivalent water with the presence of dust on the surface was found. Since dust plays a key role in condensation of water from the atmosphere, we suppose that the anomalies could result from the retention of atmospheric moisture. To analyze this suggestion, we performed a theoretical modeling that allowed us to map the planetary-scale distributions of several meteorological parameters responsible for the atmospheric moisture condensation. Two antipodal regions coinciding rather well with the Arabia Terra anomaly and the geographically antipodal anomaly southwest of Olympus Mons were found in the maps. This suggests that the anomalies are rather recent than ancient formations. They were probably formed by a sink of moisture from the atmosphere in the areas where present meteorological conditions support this sink. Geological parameters, primarily the presence of dust, only promote this process. We cannot exclude the possibility that the Martian cryosphere, rather than the atmosphere, supplied the studied anomalies with moisture during their formation: the thermodynamic conditions in the anomaly areas could block the moisture flux from the Martian interior in the upper regolith layer. The moisture coming from the atmosphere or from the interior is likely held as chemically bound water entering into the structure of water-bearing minerals (probably, hydrated magnesium sulfates) directly from the vapor; or the moisture precipitates as frost, penetrates into microfissures, and then is bound in minerals. Probably, another geologic factor—the magnesium sulfate abundance—works in the Arabia Terra anomaly.  相似文献   

9.
Karl R. Blasius 《Icarus》1976,29(3):343-361
Mariner 9 images of the four great volcanic shields of the Tharsis region of Mars show many circular craters ranging in diameter from 100mm to 20 km. Previous attempts to date the volcanoes from their apparent impact crater densities yielded a range of results. The principal difficulty is sorting volcanic from impact craters for diameters ?1 km. Many of the observed craters are aligned in prominent linear and concentric patterns suggestive of volcanic origin. In this paper an attempt is made to date areas of shield surface, covered with high resolution images using only scattered small (?1 km) craters of probable impact origin. Craters of apparent volcanic origin are systematically excluded from the dating counts.The common measure of age, deduced for all surfaces studied, is a calculated “crater age” F′ defined as the number of craters equal to or larger than 1 km in diameter per 106km2. The conclusions reached from comparing surface ages and their geological settings are: (1) Lava flow terrain surfaces with ages, F′, from 180 to 490 are seen on the four great volcanoes. Summit surfaces of similar ages, F′ = 360 to 420, occur on the rims of calderas of Arsia Mons, Pavonis Mons, and Olympus Mons. The summit of Ascraeus Mons is possibly younger; F′ is calculated to be 180 for the single area which could be dated. (2) One considerably younger surface, F′ < 110, is seen on the floor of Arsia Mon's summit caldera. (3) Nearly crater free lava flow terrain surfaces seen on Olympus Mons are estimated to be less than half the age of a summit surface. The summit caldera floor is similarly young. (4) The pattern of surface ages on the volcanoes suggests that their eruption patterns are similar to those of Hawaiian basaltic shields. The youngest surfaces seem concentrated on the mid-to-lower flanks and within the summit calderas. (5) The presently imaged sample of shield surface, though incomplete, clearly shows a broad range of ages on three volcanoes—Olympus, Arsia, and Pavonis Mons.Estimated absolute ages of impact dated surfaces are obtained from two previously published estimates of the history of flux of impacting bodies on Mars. The estimated ranges of age for the observed crater populations are 0.5 to 1.2b.y. and 0.07 to 0.2b.y. Areas which are almost certainly younger, less than 0.5 or 0.07b.y., are also seen. The spans of surface age derived for the great shields are minimum estimates of their active lifetimes, apparently very long compared to those of terrestrial volcanoes.  相似文献   

10.
The Thermal Emission Spectrometer aboard the Mars Global Surveyor spacecraft has produced an extensive atmospheric data set, beginning during aerobraking and continuing throughout the extended scientific mapping phase. Temperature profiles for the atmosphere below about 40 km, surface temperatures and total dust and water ice opacities, can be retrieved from infrared spectra in nadir viewing mode. This paper describes assimilation of nadir retrievals from the spacecraft aerobraking period, LS=190°–260°, northern hemisphere autumn to winter, into a Mars general circulation model. The assimilation scheme is able to combine information from temperature and dust optical depth retrievals, making use of a model forecast containing information from the assimilation of earlier observations, to obtain a global, time-dependent analysis. Given sufficient temperature retrievals, the assimilation procedure indicates errors in the a priori dust distribution assumptions even when lacking dust observations; in this case there are relatively cold regions above the poles compared to a model which assumes a horizontally-uniform dust distribution. One major reason for using assimilation techniques is in order to investigate the transient wave behavior on Mars. Whilst the data from the 2-h spacecraft mapping orbit phase is much more suitable for assimilation, even the longer (45–24 h) period aerobraking orbit data contain useful information about the three-dimensional synoptic-scale martian circulation which the assimilation procedure can reconstruct in a consistent way. Assimilations from the period of the Noachis regional dust storm demonstrate that the combined assimilation of temperature and dust retrievals has a beneficial impact on the atmospheric analysis.  相似文献   

11.
The possibility of reconstructing the surface topography from single images with the photometric method in the linear approximation is analyzed. The photometric method or surface topography Reconstruction employs a statistical approach to the problem formulation and is the most mathematically correct. This method allows determination of the most probable surface topography given specific observational data. When only one image is available, the photometric method is superior in comparison with the currently available photoclinometry. The processing of test surface topography with the photometric method shows that, under typical conditions, the error of surface relief reconstruction is of higher than 40% in terms of the standard deviation of the surface height. The surface relief of some Martian areas are reconstructed from HRSC images obtained by the Mars Express spacecraft. It is shown that the image-reconstructed surface topography is in good agreement with the topographic information for the same Martian areas obtained by the MOLA altimeter.  相似文献   

12.
Merton E. Davies 《Icarus》1974,21(3):230-236
Mariner 9 took many pictures of the giant Olympus Mons during its year in orbit around Mars. Control points have been identified on the top of Olympus Mons, on the volcanic shield, and on the surrounding plains, and their locations have been measured on the television pictures. These measurements were used to compute the aerographic coordinates and the planetary radii of the points. The radii at some of the points were derived from radar elevation measurements and from radio occultation measurements. The mountain rises about 21 km above its base.  相似文献   

13.
Analysis of the distribution of the epithermal and fast neutron fluxes from the Martian surface within the ±60° latitude zone measured by the High-Energy Neutron Detector (HEND) from mid-February through mid-June 2002 has revealed regional neutron-flux variations outside the zones of climatic effects, which appear to be attributable to the presence of chemically bound water. With the exception of the epithermal neutron fluxes in Arabia and southwest of Olympus Mons (Medusae Fossae), these variations show no correlation with the geologic structure of the terrain at the level of global geologic maps. The lack of such a correlation probably implies that to the formation depth of the epithermal neutron flux (1–2 m), let alone the fast neutron flux (20–30 cm), much of Mars is covered by a surface material that bears little relation in composition to local bedrocks. Clearly, this is an aeolian cover whose fine-grain component was mixed by dust storms in the geologic time on the scale of large regions. The decrease in the flux of epithermal neutrons in Arabia and southwest of Olympus Mons (Medusae Fossae) appears to be attributable to an enhanced concentration of materials containing chemically bound water (clay minerals, palagonite, hydroxides, and hydrosalts) in the surface layers of these regions.  相似文献   

14.
Building on previous studies of volcanoes around the Hellas basin with new studies of imaging (High-Resolution Stereo Camera (HRSC), Thermal Emission Imaging System (THEMIS), Mars Orbiter Camera (MOC), High-Resolution Imaging Science Experiment (HiRISE), Context Imager (CTX)), multispectral (HRSC, Observatoire pour la Minéralogie, l’Eau, les Glaces et l’Activité (OMEGA)), topographic (Mars Orbiter Laser Altimeter (MOLA)) and gravity data, we define a new Martian volcanic province as the Circum-Hellas Volcanic Province (CHVP). With an area of >2.1 million km2, it contains the six oldest central vent volcanoes on Mars, which formed after the Hellas impact basin, between 4.0 and 3.6 Ga. These volcanoes mark a transition from the flood volcanism that formed Malea Planum ～3.8 Ga, to localized edifice-building eruptions. The CHVP volcanoes have two general morphologies: (1) shield-like edifices (Tyrrhena, Hadriaca, and Amphitrites Paterae), and (2) caldera-like depressions surrounded by ridged plains (Peneus, Malea, and Pityusa Paterae). Positive gravity anomalies are found at Tyrrhena, Hadriaca, and Amphitrites, perhaps indicative of dense magma bodies below the surface. The lack of positive-relief edifices and weak gravity anomalies at Peneus, Malea, and Pityusa suggest a fundamental difference in their formation, styles of eruption, and/or compositions. The northernmost volcanoes, the ～3.7–3.9 Ga Tyrrhena and Hadriaca Paterae, have low slopes, well-channeled flanks, and smooth caldera floors (at tens of meters/pixel scale), indicative of volcanoes formed from poorly consolidated pyroclastic deposits that have been modified by fluvial and aeolian erosion and deposition. The ～3.6 Ga Amphitrites Patera also has a well-channeled flank, but it and the ～3.8 Ga Peneus Patera are dominated by scalloped and pitted terrain, pedestal and ejecta flow craters, and a general ‘softened’ appearance. This morphology is indicative not only of surface materials subjected to periglacial processes involving water ice, but also of a surface composed of easily eroded materials such as ash and dust. The southernmost volcanoes, the ～3.8 Ga Malea and Pityusa Paterae, have no channeled flanks, no scalloped and pitted terrain, and lack the ‘softened’ appearance of their surfaces, but they do contain pedestal and ejecta flow craters and large, smooth, bright plateaus in their central depressions. This morphology is indicative of a surface with not only a high water ice content, but also a more consolidated material that is less susceptible to degradation (relative to the other four volcanoes). We suggest that Malea and Pityusa (and possibly Peneus) Paterae are Martian equivalents to Earth's giant calderas (e.g., Yellowstone, Long Valley) that erupted large volumes of volcanic materials, and that Malea and Pityusa are probably composed of either lava flows or ignimbrites. HRSC and OMEGA spectral data indicate that dark gray to slightly red materials (often represented as blue or black pixels in HRSC color images), found in the patera floors and topographic lows throughout the CHVP, have a basaltic composition. A key issue is whether this dark material represents concentrations of underlying basaltic material eroded by various processes and exposed by aeolian winnowing, or if the material was transported from elsewhere on Mars by regional winds. Understanding the provenance of these dark materials may be the key to understanding the volcanic diversity of the Circum-Hellas Volcanic Province.  相似文献   

15.
Kenneth L. Tanaka 《Icarus》1985,62(2):191-206
Gravity sliding and spreading at low strain rates can account for the general morphology and structure of the aureoles and basal scarp of Olympus Mons. Detachment sliding could have occurred around the volcano if either pore-fluid pressures were exceptionally high (greater than 90%) or the rocks had very low resistance to shear (about 1 × 105 Pa or 1 bar). Because of the vast areal extent and probable shallow depth of the detachment zone, development of ubiquitous, high pore-fluid pressures beneath aureole-forming material was unlikely. However, a zone of sufficiently weak material consisting of about 10% interstitial or interbedded ice could have been present. If so, a simple rheologic model for the aureole deposits can be applied that consists of a thin ductile layer overlain by a thicker brittle layer. According to this model, extensional deformation would have occurred near the shield and compressional deformation in its distal parts. Proximal grabens and distal corrugations on aureole surfaces support this model. A submarine slide at Kitimat Arm, British Columbia, is a valid qualitative analogy for the observed features and inferred emplacement style of the aureole deposits. Ground-ice processes have been considered the cause of many geologic features on Mars; a 3% average concentration of ground ice in the regolith is predicted by theoretical models for the ice budget and cryosphere. Ice may have been deposited in higher concentrations below the aureole-forming material; the source of the ice could have been juvenile water circulated hydrothermally by Olympus Mons volcanism. The basal scarp of Olympus Mons apparently demarcates the transition between the upper, stable part of the shield and its lower part that decoupled and formed the aureole deposits. This transition may reflect a change in the bulk shear strength of the shield, caused either by a radial dependence in the abundance of ice or fluid in the shield materials or by the concentration of intrusive dikes within the volcano. Other Martian volcanoes exhibit virtually no evidence of similar large-scale gravity spreading and basal scarps. Perhaps such evidence, if it existed, has been buried by lava flows, or perhaps the smaller size of other volcanoes did not permit the development of these features.  相似文献   

16.
The Mars Express spacecraft has a highly inclined orbit around Mars and so has been able to observe the south pole of Mars in illuminated conditions at the end of the southern summer (Ls=330). Spectra from the planetary Fourier spectrometer (PFS) short wavelength (SW) channel were recorded over the permanent ice cap to study its composition in terms of CO2 ice and H2O ice. Models are fitted to the observed data, which include a spatial mixture of soil (not covered by ice) and CO2 frost (with a specific grain size and a small amount of included dust and H2O ice). Two different kinds of spectra were observed: those over the permanent polar cap with almost pure CO2 ice, negligible water ice, no soil fraction required, and bright; and those over mixed terrain (at the edge of the cap or near troughs) containing a significant soil spatial fraction, more water ice and smaller CO2 grain size. The amount of water ice given by fits to scaled albedo models is less than 10 ppm by weight. When using multi-stream reflectance models with the appropriate lighting geometry, the water amount must be 2-5 times greater than the albedo fit (less than 50 ppm). At the periphery of the residual polar cap, we found a region almost completely covered by water frost, modeled as a mixture of micron-sized and sub-mm sized grains. Our result using a granular mixture of micron-sized grains of water ice and dust with the CO2 grains is different from the modeling of OMEGA polar cap observations using molecular mixtures.  相似文献   

17.
D. Reiss  M. Zanetti  G. Neukum 《Icarus》2011,215(1):358-369
Active dust devils were observed in Syria Planum in Mars Observer Camera - Wide Angle (MOC-WA) and High Resolution Stereo Camera (HRSC) imagery acquired on the same day with a time delay of ∼26 min. The unique operating technique of the HRSC allowed the measurement of the traverse velocities and directions of motion. Large dust devils observed in the HRSC image could be retraced to their counterparts in the earlier acquired MOC-WA image. Minimum lifetimes of three large (avg. ∼700 m in diameter) dust devils are ∼26 min, as inferred from retracing. For one of these large dust devil (∼820 m in diameter) it was possible to calculate a minimum lifetime of ∼74 min based on the measured horizontal speed and the length of its associated dust devil track. The comparison of our minimum lifetimes with previous published results of minimum and average lifetimes of small (∼19 m in diameter, avg. min. lifetime of ∼2.83 min) and medium (∼185 m in diameter, avg. min. lifetime of ∼13 min) dust devils imply that larger dust devils on Mars are active for much longer periods of time than smaller ones, as it is the case for terrestrial dust devils. Knowledge of martian dust devil lifetimes is an important parameter for the calculation of dust lifting rates. Estimates of the contribution of large dust devils (>300-1000 m in diameter) indicate that they may contribute, at least regionally, to ∼50% of dust entrainment by dust devils into the atmosphere compared to the dust devils <300 m in diameter given that the size-frequency distribution follows a power-law. Although large dust devils occur relatively rarely and the sediment fluxes are probably lower compared to smaller dust devils, their contribution to the background dust opacity by dust devils on Mars could be at least regionally large due to their longer lifetimes and ability of dust lifting into high atmospheric layers.  相似文献   

18.
We present the results of 20 months of observations of Mars by the Russian HEND instrument onboard the NASA 2001 Mars Odyssey spacecraft. We show that there are two extended subpolar regions with a soil water content of several tens of percent in the northern and southern hemispheres of Mars. The southern subpolar region is well described by a two-layer model, according to which a soil with a water content of up to 55% by mass lies under a relatively dry soil with a water mass fraction of 2% and a thickness of 15–20 g/cm2. The distribution of water in Martian regolith northern subpolar region is in good agreement with the homogeneous model and does not require invoking the more complex two-layer soil model. The water-ice content in the subsurface layer of the northern subpolar region reaches 53 % by mass. We show that there are two regions with a relatively high water content near the Martian equator. These are Arabia Terra and the Medusae Fossae formation region southwest of Olympus Mons. In these regions, a lower layer with 9–10% of water by mass may underlie the upper layer of relatively dry material 30 g/cm2 in thickness. The moistest spot near the equator is at about 30° E and 10° N. Its lower-layer soil may contain more than 16% of water by mass.  相似文献   

19.
The Tharsis rise on Mars with a diameter of about 8000 km and an elevation up to 10 km shows extensive volcanism and an extensional fracture system. Other authors explained this structure by (I) an uplift due to mantle processes and by (II) volcanic construction. Gravity models of four profiles are in accordance with a total Airy isostatic compensation of the whole rise with mean crustal thicknesses of 50 km and 100 km. But two regions exhibit significant mass deficits: (i) the area between Olympus Mons and the three large Tharsis volcanoes and (ii) central Tharsis. This can be explained by (1) a heated upper mantle, (2) a chemically modified upper mantle, (3) a crustal thickening, or (4) a combination of these three processes. Crustal thickening is mainly a constructional process, but the mass deficit should contribute to a certain degree of uplift causing the extensional area of Labyrinthus Noctis. Gravity modelling results in a different isostatic state of the three Tharsis volcanoes. Pavonis Mons is not compensated, Ascraeus Mons is highly or totally compensated, and Arsia Mons is medium or not compensated. The large, flat volcanic structure Alba Patera has been explained by a hot spot with an evolution of a mantle diapir.The results have shown that the Tharsis rise is a very complex structure. The central and eastern part of the rise is characterized by extensional features and a mass deficit (Extensional Province). The western part is dominated by many volcanic features and a central elongated mass deficit (Volcanic Province). The northern part consists of Alba Patera. It seems unlikely that the whole rise has been generated by one stationary large axisymmetric plume or hot spot. There could have been one or more active hot spots with an evolution in space and time.Contribution Nr. 421, Institut für Geophysik der Universität Kiel, Germany.  相似文献   

20.
The time evolution of atmospheric dust at high southern latitudes on Mars has been determined using observations of the south seasonal cap acquired in the near infrared (1-2.65 μm) by OMEGA/Mars Express in 2005. Observations at different solar zenith angles and one EPF sequence demonstrate that the reflectance in the 2.64 μm saturated absorption band of the surface CO2 ice is mainly due to the light scattered by aerosols above most places of the seasonal cap. We have mapped the total optical depth of dust aerosols in the near-IR above the south seasonal cap of Mars from mid-spring to early summer with a time resolution ranging from one day to one week and a spatial resolution of a few kilometers. The optical depth above the south perennial cap is determined on a longer time range covering southern spring and summer. A constant set of optical properties of dust aerosols is consistent with OMEGA observations during the analyzed period. Strong variations of the optical depth are observed over small horizontal and temporal scales, corresponding in part to moving dust clouds. The late summer peak in dust opacity observed by Opportunity in 2005 propagated to the south pole contrarily to that observed in mid spring. This may be linked to evidence for dust scavenging by water ice-rich clouds circulating at high southern latitudes at this season.  相似文献   

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