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1.
Arnus Vallis (AV) is a >300-km-long sinuous, rille located on the northeastern flank of the Syrtis Major volcano on Mars. Observational evidence presented here suggests that AV formed as an open lava channel that was at least partly incised into the pre-existing terrain. The lava source area consists of a sub-circular pit at the southwestern end of a 7-km-long straight section of channel. AV trends down slope from this source with an average bottom slope of 0.26% or 0.14°. Width varies from ∼1 km at the source to ∼0.6 km near the distal end, with a mean of 0.76 km. Depth decreases from ∼180 m at the source to ∼25 m near the distal end. The AV terminus is obscured by a large impact crater. We suggest that the material that flowed in AV must have been a relatively high temperature, low viscosity lava dynamically and perhaps compositionally similar to terrestrial komatiite or some lunar basalt lavas. If correct, this finding has implications for the mode of construction of Syrtis Major.  相似文献   

2.
The mineralogical composition of the Martian surface is investigated by a Multiple-Endmember Linear Spectral Unmixing Model (MELSUM) of the Observatoire pour la Minéralogie, l’Eau, les Glaces et l’Activité (OMEGA) imaging spectrometer onboard Mars Express. OMEGA has fully covered the surface of the red planet at medium to low resolution (2–4 km per pixel). Several areas have been imaged at a resolution up to 300 m per pixel. One difficulty in the data processing is to extract the mineralogical composition, since rocks are mixtures of several components. MELSUM is an algorithm that selects the best linear combination of spectra among the families of minerals available in a reference library. The best fit of the observed spectrum on each pixel is calculated by the same unmixing equation used in the classical Spectral Mixture Analysis (SMA). This study shows the importance of the choice of the input library, which contains in our case 24 laboratory spectra (endmembers) of minerals that cover the diversity of the mineral families that may be found on the Martian surface. The analysis is restricted to the 1.0–2.5 μm wavelength range. Grain size variations and atmospheric scattering by aerosols induce changes in overall albedo level and continuum slopes. Synthetic flat and pure slope spectra have therefore been included in the input mineral spectral endmembers library in order to take these effects into account. The selection process for the endmembers is a systematic exploration of whole set of combinations of four components plus the straight line spectra. When negative coefficients occur, the results are discarded. This strategy is successfully tested on the terrestrial Cuprite site (Nevada, USA), for which extensive ground observations exist. It is then applied to different areas on Mars including Syrtis Major, Aram Chaos and Olympia Undae near the North Polar Cap. MELSUM on Syrtis Major reveals a region dominated by mafic minerals, with the oldest crustal regions composed of a mixing between low-calcium pyroxenes (LCPs) (orthopyroxenes (OPx)) and high-calcium pyroxenes (HCPs) (clinopyroxenes (CPx)). The Syrtis volcanic edifice appears depleted in LCP (OPx) and enriched in HCP (CPx), which is consistent with materials produced with a lower partial fusion degree at an age younger to the surrounding crust. Strong olivine signatures are found between the two calderas Nili Patera and Meroe Patera and in Nili Fossae. A strong signature of iron oxides is found within Aram Chaos, with a spatial distribution also consistent with thermal emission spectrometer (TES). Gypsum is unambiguously detected in the northern polar region, in agreement with the study of Langevin et al. [2005. Sulfates in the north polar region of Mars detected by OMEGA/Mars Express. Science 307(5715), 1584–1586]. Our results show that the linear spectral unmixing provides good first order results in a variety of mineralogical contexts, and can therefore confidently be used on a wider scale to analyze the complete archive of OMEGA data.  相似文献   

3.
We present radiometrically calibrated spectrophotometric images of Mars taken at the NASA Infrared Telescope Facility (IRTF) near and during the 1995 and 1999 oppositions. Absolute intensity and radiance factor (rF = I/F) values have been calculated for approximately 95% of the surface over all longitudes between −70° to 90° latitude in the 1.5- to 4.1-μm spectral region at a spectral resolution (Δλ/λ) of 1.5%. Values of radiance factor range from rF = 0.4 to 0.6 at 2.2 μm for the bright regions such as Moab and Arabia to rF = 0.12 to 0.3 at 2.2 μm around the dark regions Syrtis Major and Acidalia Planitia. Variations are seen due to seasonal dust and/or condensate cloud cover and viewing geometry. Our results are generally consistent with the few reported previous radiance factor determinations for Mars. These data are unique among ground-based data in their relatively high spatial resolution (?200 km/pixel at the sub-Earth point) and coverage combined with their spectral resolution and coverage. These radiometrically calibrated observations can be used as input to studies focusing on spectral unmixing of surface and atmospheric components, radiative transfer modeling of disk and limb radiances, and photometric modeling of the martian phase function.  相似文献   

4.
《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   

5.
J.F. Bell III  T.M. Ansty 《Icarus》2007,191(2):581-602
We acquired high spectral and spatial resolution hyperspectral imaging spectrometer observations of Mars from near-UV to near-IR wavelengths (∼300 to 1020 nm) using the STIS instrument on the Hubble Space Telescope during the 1999, 2001, and 2003 oppositions. The data sets have been calibrated to radiance factor (I/F) and map-projected for comparison to each other and to other Mars remote sensing measurements. We searched for and (where detected) mapped a variety of iron-bearing mineral signatures within the data. The strong and smooth increase in I/F from the near-UV to the visible that gives Mars its distinctive reddish color indicates that poorly crystalline ferric oxides dominate the spectral properties of the high albedo regions (as well as many intermediate and low albedo regions), a result consistent with previous remote sensing studies of Mars at these wavelengths. In the near-IR, low albedo regions with a negative spectral slope and/or a distinctive ∼900 nm absorption feature are consistent with, but not unique indicators of, the presence of high-Ca pyroxene or possibly olivine. Mixed ferric-ferrous minerals could also be responsible for the ∼900 nm feature, especially in higher albedo regions with a stronger visible spectral slope. We searched for the presence of several known diagnostic absorption features from the hydrated ferric sulfate mineral jarosite, but did not find any unique evidence for its occurrence at the spatial scale of our observations. We identified a UV contrast reversal in some dark region spectra: at wavelengths shorter than about 340 nm these regions are actually brighter than classical bright regions. This contrast reversal may be indicative of extremely “clean” low albedo surfaces having very little ferric dust contamination. Ratios between the same regions observed during the planet-encircling dust storm of 2001 and during much clearer atmospheric conditions in 2003 provide a good direct estimate of the UV to visible spectral characteristics of airborne dust aerosols. These HST observations can help support the calibration of current and future Mars orbital UV to near-IR spectrometers, and they also provide a dramatic demonstration that even at the highest spatial resolution possible to achieve from the Earth, spectral variations on Mars at these wavelengths are subtle at best.  相似文献   

6.
Several hydrated silicate deposits on Mars are observed within craters and are interpreted as excavated Noachian material. Toro crater (71.8°E, 17.0°N), located on the northern edge of the Syrtis Major Volcanic Plains, shows spectral and morphologic evidence of impact-induced hydrothermal activity. Spectroscopic observations were used to identify extensive hydrated silicate deposits, including prehnite, chlorites, smectites, and opaline material, a suite of phases that frequently results from hydrothermal alteration in terrestrial craters and also expected on Mars from geochemical modeling of hydrothermal environments. When combined with altimetry and high-resolution imaging data, these deposits appear associated predominantly with the central uplift and with portions of the northern part of the crater floor. Detailed geologic mapping of these deposits reveals geomorphic features that are consistent with hydrothermal activity that followed the impact event, including vent-like and conical mound structures, and a complex network of tectonic structures caused by fluid interactions such as fractures and joints. The crater age has been calculated from the cumulative crater size-frequency distributions and is found to be Early Hesperian. The evidence presented here provides support for impact-induced hydrothermal activity in Toro crater, that extends phyllosilicate formation processes beyond the Noachian era.  相似文献   

7.
Scott C. Mest  David A. Crown 《Icarus》2005,175(2):335-359
The geology and stratigraphy of Millochau crater (21.4° S, 275° W), located in the highlands of Tyrrhena Terra, Mars, are documented through geomorphic analyses and geologic mapping. Crater size-frequency distributions and superposition relationships are used to constrain relative ages of geologic units and determine the timing and duration of the geologic processes that modified Millochau rim materials and emplaced deposits on Millochau's floor. Crater size-frequency distributions show a Middle Noachian age for rim materials and Middle Noachian to Early Hesperian ages for most of the interior deposits. Valley networks and gullies incised within Millochau's rim materials and interior wall, respectively, indicate fluvial activity was an important erosional process. Millochau contains an interior plateau, offset northeast of Millochau's center, which rises up to 400 m above the surrounding crater floor and slopes downward to the south and west. Layers exposed along the northern and eastern scarp boundaries of the plateau are tens to hundreds of meters thick and laterally continuous in MOC images. These layers suggest most materials within Millochau were emplaced by sedimentary processes (e.g., fluvial or eolian), with the potential for lacustrine deposition in shallow transient bodies of water and contributions of volcanic airfall. Mass wasting may have also contributed significant quantities of material to Millochau's interior, especially to the deposits surrounding the plateau. Superposition relationships combined with impact crater statistics indicate that most deposition and erosion of Millochau's interior deposits is ancient, which implies that fluvial activity in this part of Tyrrhena Terra is much older than in the eastern Hellas region. Eolian processes mobilized sediment to form complicated patterns of long- and short-wavelength dunes, whose emplacement is controlled by local topography. These deposits are some of the youngest within Millochau (Amazonian) and eolian modification may be ongoing.  相似文献   

8.
New insight into the seasonal, diurnal and spatial distribution of water vapor on Mars has been obtained from analyzing the spectra of the short-wavelength channel (SW) of the Planetary Fourier Spectrometer (PFS) onboard Mars Express. The processed dataset, recorded between January 2004 and April 2005, covers the seasons from LS=331° of Mars Year 26 to LS=196° of the following year. In this period the mean column density around vernal equinox was 8.2 pr. μm. The maximum values during northern summer were about 65 pr. μm, located around 75° N latitude with a longitudinally inhomogeneous distribution. Regarding the atmospheric transport, the majority of polar water vapor remains in the north polar region while only about a quarter is transported southward. Geographically there are two water vapor maxima visible, over Arabia Terra and the Tharsis plateau, that are most likely caused both by atmosphere-ground interaction and by atmospheric circulation. A comparison with other instruments generally shows a good agreement, only the SPICAM results are systematically lower. Compared to the results from the PFS long-wavelength channel the results of this work are slightly higher. A strong discrepancy is visible northward of about 50° N during the northern summer that is possibly explained by a non-uniform vertical H2O mixing. In particular, a confinement of the water to the lower few kilometers yields a much better agreement between the retrieved column densities of the two PFS channels.  相似文献   

9.
Syrtis Major Planum is a volcanic plain dominated by lava flows. High resolution stereo camera (HRSC) images of the northern Syrtis Major region display erosional features such as grooves, teardrop-shaped islands and valleys. These landforms are characteristics of outflow channels seen on Mars, therefore implying that a flood event took place in this region. The flow of 100 km long and a few kilometer wide followed the local slopes in most locations. Maximum flood discharges estimated from images and topography vary from about 0.3×106 to 8×106 m3/s, and therefore are in the range of terrestrial mega-floods in the Scablands or Lake Bonneville. In North Syrtis Major, the relationships with surrounding lava flows and the timing of the flood coeval to Syrtis Major volcanic activity suggest that it could be related to the subsurface water discharge mobilized by the volcanic activity. The proximity of Noachian age basement rocks 20 km away from the flood and below lava flows might have played a role in its formation and water presence.  相似文献   

10.
The Tyrrhena Terra region of Mars is studied with the imaging spectrometers OMEGA (Observatoire pour la Minéralogie, l’Eau, les Glaces et l’Activité) onboard Mars Express and CRISM (Compact Reconnaissance Infrared Spectrometer for Mars) onboard Mars Reconnaissance Orbiter, through the observation of tens of craters that impacted into this part of the martian highlands. The 175 detections of hydrated silicates are reported, mainly associated with ejecta blankets, crater walls and rims, and central up-lifts. Sizes of craters where hydrated silicates are detected are highly variable, diameters range from less than 1 km to 42 km. We report the presence of zeolites and phyllosilicates like prehnite, Mg-chlorite, Mg-rich smectites and mixed-layer chlorites–smectites and chlorite–vermiculite from comparison of hyperspectral infrared observations with laboratory spectra. These minerals are associated with fresh craters post-dating any aqueous activity. They likely represent ancient hydrated terrains excavated by the crater-forming impacts, and hence reveal the composition of the altered Noachian crust, although crater-related hydrothermal activity may have played a minor role for the largest craters (>20 km in diameter). Most detected minerals formed over relatively high temperatures (100–300 °C), likely due to aqueous alteration of the Noachian crust by regional low grade metamorphism from the Noachian thermal gradient and/or by extended hydrothermal systems associated with Noachian volcanism and ancient large impact craters. This is in contrast with some other phyllosilicate-bearing regions like Mawrth Vallis where smectites, kaolinites and hydrated silica were mainly identified, pointing to a predominance of surface/shallow sub-surface alteration; and where excavation by impacts played only a minor role. Smooth plains containing hydrated silicates are observed at the boundary between the Noachian altered crust, dissected by fluvial valleys, and the Hesperian unaltered volcanic plains. These plains may correspond to alluvial deposition of eroded material. The highlands of Tyrrhena Terra are therefore particularly well suited for investigating the diversity of hydrated minerals in ancient martian terrains.  相似文献   

11.
A review of the available spectral geometric albedo measurements for Mars was presented earlier for the spectral region 0.3 to 1.1μ. A new observational study has greatly increased the store of data, especially for small Martian regions and for the infrared spectral region 1.0 to 2.5μ. Here we combine the new data with data both from the earlier review and, for the infrared spectral region, from the literature. We present a more complete picture of Martian spectral reflectivity properties than was available. This study should provide a more firm basis upon which models of Martian surface composition can be built. At visible wavelengths the Mars dark area Syrtis Major is red rather than green or grey in color; the bright area Arabia is even redder than Syrtis Major. Absorption bands, which differ between bright and dark areas, appear in the reflection curves. The 1μ absorption feature for dark areas is confirmed and more completely described. A previously unreported absorption band near 0.95μ for bright areas appears along with several absorption features in the infrared. The geometric albedo for Arabia reaches a maximum of about 0.43 at 1μ. The Bond albedo for this same area reaches a maximum of 60%. The bright area Arabia is occasionally three times brighter than the dark area Syrtis Major at red wavelengths. Published infrared reflection data available for Mars are not in complete agreement. Changes in brightness and color of Arabia are discussed which are not in agreement with traditional darkening wave theory.  相似文献   

12.
Atmospheric water vapor abundances in Mars’ north polar region (NPR, from 60° to 90°N) are mapped as function of latitude and longitude for spring and summer seasons, and their spatial, seasonal, and interannual variability is discussed. Water vapor data are from Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES) and the Viking Orbiter (VO) Mars Atmospheric Water Detector (MAWD). The data cover three complete northern spring-summer seasons in 1977-1978, 2000-2001 and 2002-2003, and shorter periods of spring-summer seasons during 1975, 1999 and 2004. Long term interannual variability in the averaged NPR abundances may exist, with Viking MAWD observations showing twice as much water vapor during summer as the MGS TES observations more than 10 martian years (MY) later. While the averaged abundances are very similar in TES observations for the same season in different years, the spatial distributions in the early summer season do vary significantly year over year. Spatial and temporal variabilities increase between Ls ∼ 80-140°, which may be related to vapor sublimation from the North Polar Residual Cap (NPRC), or to changes in circulation. Spatial variability is observed on scales of ∼100 km and temporal variability is observed on scales of <10 sols during summer. During late spring the TES water vapor spatial distribution is seen to correlate with the low topography/low albedo region of northern Acidalia Planitia (270-360°E), and with the dust spatial distribution across the NPR during late spring-early summer. Non-uniform vertical distribution of water vapor, a regolith source or atmospheric circulation ‘pooling’ of water vapor from the NPRC into the topographic depression may be behind the correlation with low topography/low albedo. Sublimation winds carrying water vapor off the NPRC and lifting surface dust in the areas surrounding the NPRC may explain the correlation between the water vapor and dust spatial distributions. Correlation between water vapor and dust in MAWD data are only observed over low topography/low albedo area. Maximum water vapor abundances are observed at Ls = 105-115° and outside of the NPRC at 75-80°N; the TES data, however, do not extend over the NPRC and thus, this conclusion may be biased. Some water vapor appears to be released in plumes or ‘outbursts’ in the MAWD and TES datasets during late spring and early summer. We propose that the sublimation rate of ice varies across the NPRC with varying surface winds, giving rise to the observed ‘outbursts’ at some seasons.  相似文献   

13.
Ground-based observations of the CO2 distribution on Mars were made this past opposition from Cerro Tololo Interamerican Observatory. Almost complete coverage of the Martian surface from 40°N to 60°S was obtained. Agreement with previous Kitt Peak observations is good, and confirmation of a pressure anomaly in the Tharsis region has been obtained. The ridge whose eastern slope is Syrtis Major stops at about 15°S, in agreement with the 1971 radar data. The Noachis-Hellas region south of Syrtis Major appears at about average altitude, indicating that the dust storm of 1971 was already active in that region as early as the end of August.  相似文献   

14.
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   

15.
Recent geomorphic, remote sensing, and atmospheric modeling studies have shown evidence for abundant ground ice deposits in the martian mid-latitudes. Numerous potential water/ice-rich flow features have been identified in craters in these regions, including arcuate ridges, gullies, and small flow lobes. Previous studies (such as in Newton Basin) have shown that arcuate ridges and gullies are mainly found in small craters (∼2-30 km in diameter). These features are located on both pole-facing and equator-facing crater walls, and their orientations have been found to be dependent on latitude. We have conducted surveys of craters >20 km in diameter in two mid-latitude regions, one in the northern hemisphere in Arabia Terra, and one in the southern hemisphere east of Hellas basin. In these regions, prominent lobes, potentially ice-rich, are commonly found on the walls of craters with diameters between ∼20-100 km. Additional water/ice-rich features such as channels, valleys, alcoves, and debris aprons have also been found in association with crater walls. In the eastern Hellas study region, channels were found to be located primarily on pole-facing walls, whereas valleys and alcoves were found primarily on equator-facing walls. In the Arabia Terra study region, these preferences are less distinct. In both study regions, lobate flows, gullies, and arcuate ridges were found to have pole-facing orientation preferences at latitudes below 45° and equator-facing orientation preferences above 45°, similar to preferences previously found for gullies and arcuate ridges in smaller craters. Interrelations between the features suggest they all formed from the mobilization of accumulated ice-rich materials. The dependencies of orientations on latitude suggest a relationship to differences in total solar insolation along the crater walls. Differences in slope of the crater wall, differences in total solar insolation with respect to wall orientation, and variations in topography along the crater rim can explain the variability in morphology of the features studied. The formation and evolution of these landforms may best be explained by multiple cycles of deposition of ice-rich material during periods of high obliquity and subsequent modification and transport of these materials down crater walls.  相似文献   

16.
Centers of tectonic activity in the eastern hemisphere of Mars   总被引：1，自引：0，他引：1  
We compiled a paleotectonic map for the eastern hemisphere of Mars to determine if extensional tectonic features (graben) are radial or compressional tectonic features (wrinkle ridges) are concentric to centers of tectonic activity defined by axisymmetric stress fields. Using a vector analysis technique all latitude and longitude points (1° bins) are tested to see if they lie on great circle extensions of extensional structures (the plane defined by the maximum and intermediate principal stresses) or great circle perpendiculars to compressional structures (the plane defined by the maximum and minimum compressional stresses). Centers of tectonic activity are defined as 5° areas whose concentrations of great circle extensions of tectonic features are statistically significant (e.g., 3σ or 7.4σ for large populations) and therefore are not the result of random noise. Our paleotectonic investigation has identified four statistically significant centers of tectonic activity within the eastern hemisphere: Elysium, Hadriaca/Tyrrhena-Hellas, Isidis-Syrtis, and Arabia Terra. Two of these centers (Hadriaca/Tyrrhena and Isidis-Syrtis) meet the 7.4σ statistical criteria and thus represent primary centers of tectonic activity with axisymmetric stress fields. The remaining two meet the 3σ statistical criteria and thus are defined as secondary centers of tectonic activity. Because the structures that define the centers extend over 80° of the planet the defined centers of tectonic activity are regional in character and related to modified impact basins or volcanic centers (all are more limited in extent than the Tharsis stress system that extends over the entire western hemisphere). The observation that statistically significant centers of tectonic activity are quantifiably and statistically identified argues that the crust and lithosphere of the eastern hemisphere at a regional scale is not dominated by regional inhomogeneities and anisotropies.  相似文献   

17.
Diverse phyllosilicate deposits discovered previously in the Nili Fossae region with near infrared reflectance data are a window into the complex history of aqueous alteration on Mars. In this work, we used thermal infrared data from the Thermal Emission Spectrometer (TES) in combination with near infrared data from the Observatoire pour la Minéralogie, l’Eau, les Glaces, et l’Activité (OMEGA) to better constrain the mineralogy and geologic origin of these deposits. We developed a TES spectral index for identification of clay minerals, which correctly identifies the phyllosilicates in the Nili Fossae area and points to several other interesting deposits in the Syrtis Major region. However, detailed inspection of the TES spectral features of Nili Fossae phyllosilicates shows a feature at low wavenumbers (350-550 cm−1) that is not an exact match to any specific Fe3+-, Al-, or Mg-rich phyllosilicate phase. Instead, the feature is more similar to basaltic glass and may indicate that the phyllosilicates in this region are: (1) rich in Fe2+ (based on similarity to trends seen in laboratory data of clay minerals), (2) poorly crystalline/extremely disordered, and/or (3) present within a matrix of actual basalt glass. This feature is similar to spectral features seen in altered rocks in the Columbia Hills region of Gusev Crater by previous authors. By calibrating measured spectral index values against mathematical spectral mixtures of typical martian dark surfaces and known abundances of alteration minerals, we are able to estimate an enrichment in abundance of alteration minerals in the altered surfaces. Many dark, Noachian deposits in the Nili Fossae area are enriched phyllosilicates by 20-30% (±10-15%) relative to dark, volcanic surfaces in the same region. The distribution and abundance of these phases indicates that alteration in the region was pervasive, but did not completely erase the original mineralogy of what was likely an Fe-rich basalt protolith. As a group, the Nili Fossae phyllosilicate deposits are fundamentally different from those found in the Mawrth Vallis region. Nili Fossae deposits have strong thermal infrared features related to admixed pyroxene, plagioclase, and occasionally olivine, whereas the Mawrth Vallis deposits contain no mafic minerals. Comparison of TES and OMEGA data also illustrates some more general differences between the datasets, including the impact of physical character of the martian surface on detectability of minerals in each spectral range.  相似文献   

18.
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   

19.
Previous observations have noted the change in albedo in a number of North Pole bright outliers and in the distribution of bright ice deposits between Mariner 9, Viking, and Mars Global Surveyor (MGS) data sets. Changes over the summer season as well as between regions at the same season (Ls) in different years have been observed. We used the bolometric albedo and brightness temperature channels of the Thermal Emission Spectrometer (TES) on the MGS spacecraft to monitor north polar residual ice cap variations between Mars years and within the summer season for three northern Martian summers between July 1999 and April 2003. Large-scale brightness variations are observed in four general areas: (1) the patchy outlying frost deposits from 90 to 270°E, 75 to 80°N; (2) the large “tail” below the Chasma Boreale and its associated plateau from 315 to 45°E, 80 to 85°N, that we call the “Boreale Tongue” and in Hyperboreae Undae; (3) the troughed terrain in the region from 0 to 120°E longitude (the lower right on a polar stereographic projection) we have called “Shackleton's Grooves” and (4) the unit mapped as residual ice in Olympia Planitia. We also note two areas which seem to persist as cool and bright throughout the summer and between Mars years. One is at the “source” of Chasma Boreale (∼15°E, 85°N) dubbed “McMurdo”, and the “Cool and Bright Anomaly (CABA)” noted by Kieffer and Titus 2001. TES Mapping of Mars’ north seasonal cap. Icarus 154, 162-180] at ∼330°E, 87°N called here “Vostok”. Overall defrosting occurs early in the summer as the temperatures rise and then after the peak temperatures are reached (Ls∼110) higher elevations and outlier bright deposits cold trap and re-accumulate new frost. Persistent bright areas are associated with either higher elevations or higher background albedos suggesting complex feedback mechanisms including cold-trapping of frost due to albedo and elevation effects, as well as influence of mesoscale atmospheric dynamics.  相似文献   

20.
We present absolute abundances and latitudinal variations of ozone and water in the atmosphere of Mars during its late northern spring (Ls=67.3°) shortly before aphelion. Long-slit maps of the a1Δg state of molecular oxygen (O2) and HDO, an isotopic form of water, were acquired on UT January 21.6 1997 using a high-resolution infrared spectrometer (CSHELL) at the NASA Infrared Telescope Facility. O2(a1Δg) is produced by ozone photolysis, and the ensuing dayglow emission at 1.27 μm is used as a tracer for ozone. Retrieved vertical column densities for ozone above ∼20 km ranged between 1.5 and 2.8 μm-atm at mid- to low latitudes (30°S-60°N) and decreased outside that region. A significant decrease in ozone density is seen near 30°N (close to the subsolar latitude of 23.5°N). The rotational temperatures retrieved from O2(a-X) emissions show a mean of 172±2.5 K, confirming that the sensed ozone lies in the middle atmosphere (∼24 km). The ν1 fundamental band of HDO near 3.67 μm was used as a proxy for H2O. The retrieved vertical column abundance of water varies from 3 precipitable microns (pr-μm) at ∼30°S to 24 pr-μm at ∼60°N. We compare these results with current photochemical models and with measurements obtained by other methods.  相似文献   

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