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1.
The 174 km diameter Terby impact crater (28.0°S-74.1°E) located on the northern rim of the Hellas basin displays anomalous inner morphology, including a flat floor and light-toned layered deposits. An analysis of these deposits was performed using multiple datasets from Mars Global Surveyor, Mars Odyssey, Mars Express and Mars Reconnaissance Orbiter missions, with visible images for interpretation, near-infrared data for mineralogical mapping, and topography for geometry. The geometry of layered deposits was consistent with that of sediments that settled mainly in a sub-aqueous environment, during the Noachian period as determined by crater counts. To the north, the thickest sediments displayed sequences for fan deltas, as identified by 100 m to 1 km long clinoforms, as defined by horizontal beds passing to foreset beds dipping by 6-10° toward the center of the Terby crater. The identification of distinct sub-aqueous fan sequences, separated by unconformities and local wedges, showed the accumulation of sediments from prograding/onlapping depositional sequences, due to lake level and sediment supply variations. The mineralogy of several layers with hydrated minerals, including Fe/Mg phyllosilicates, supports this type of sedimentary environment. The volume of fan sediments was estimated as >5000 km3 (a large amount considering classical martian fan deltas such as Eberswalde (6 km3)) and requires sustained liquid water activity. Such a large sedimentary deposition in Terby crater is characteristic of the Noachian/Phyllosian period during which the environment favored the formation of phyllosilicates. The latter were detected by spectral data in the layered deposits of Terby crater in three distinct layer sequences. During the Hesperian period, the sediments experienced strong erosion, possibly enhanced by more acidic conditions, forming the current morphology with three mesas and closed depressions. Small fluvial valleys and alluvial fans formed subsequently, attesting to late fluvial processes dated as late Early to early Late Hesperian. After this late fluvial episode, the Terby impact crater was submitted to aeolian processes and permanent cold conditions with viscous flow features. Therefore, the Terby crater displays, in a single location, geologic features that characterize the three main periods of time on Mars, with the presence of one of the thickest sub-aqueous fan deposits reported on Mars. The filling of Terby impact crater is thus one potential “reference geologic cross-section” for Mars stratigraphy.  相似文献   

2.
There is now widespread agreement that the surface of Mars underwent some degree of fluvial modification, but there is not yet full understanding of its surface hydrological cycle and the nature of standing bodies of water, rivers, and precipitation that affected its surface. In this paper we explore Erythraea Fossa (31.5 W, 27.3 S), a graben adjacent to Holden crater, which exhibits strong evidence that it once housed a chain of three lakes, had overland water flow, and was subject to precipitation. The inlet valley, outlet valley, and fan morphologies in the paleolakes are used to qualitatively discern the hydrologic history of the paleolakes; based on topography constraints, the three basins combined once held 56 km3 of water. Depositional features within the basins that change with drainage area and nearby valleys that start near drainage divides indicate that the paleolakes may have been fed by precipitation driven runoff. This suggests the presence of an atmosphere, at least locally, that was capable of supporting a hydrological cycle.  相似文献   

3.
P.C. Thomas  P.B. James  R. Haberle 《Icarus》2009,203(2):352-798
The residual south polar cap (RSPC) of Mars includes a group of different depositional units of CO2 ice undergoing a variety of erosional processes. Complete summer coverage of the RSPC by ∼6-m/pixel data of the Context Imager (CTX) on Mars Reconnaissance Orbiter (MRO) has allowed mapping and inventory of the units in the RSPC. Unit maps and estimated thicknesses indicate the total volume of the RSPC is currently <380 km3, and represents less than 3% of the total mass of the current Mars atmosphere. Scarp retreat rates in the CO2 ice derived from comparison of High Resolution Imaging Science Experiment (HiRISE) data with earlier images are comparable to those obtained for periods up to 3 Mars years earlier. These rates, combined with sizes of depressions suggest that the oldest materials were deposited more than 125 Mars years ago. Most current erosion is by backwasting of scarps 1-12 m in height. This backwasting is initiated by a series of scarp-parallel fractures. In the older, thicker unit these fractures form about every Mars year; in thinner, younger materials they form less frequently. Some areas of the older, thicker unit are lost by downwasting rather than by the scarp retreat. A surprising finding from the HiRISE data is the scarcity of visible layering of RSPC materials, a result quite distinct from previous interpretations of layers in lower resolution images. Layers ∼0.1 m thick are exposed on the upper surfaces of some areas, but their timescale of deposition is not known. Late summer albedo changes mapped by the CTX images indicate local recycling of ice, although the amounts may be morphologically insignificant. Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) data show that the primary material of all the different forms of the RSPC is CO2 ice with only small admixtures of water ice and dust.  相似文献   

4.
Uzboi Vallis (centered at ∼28°S, 323°E) is ∼400 km long and comprises the southernmost segment of the northward-draining Uzboi-Ladon-Morava (ULM) meso-scale outflow system that emerges from Argyre basin. Bond and Holden craters blocked the valley to the south and north, respectively, forming a Late Noachian-to-Hesperian paleolake basin that exceeded 4000 km3. Limited CRISM data suggest lake deposits in Uzboi and underlying basin floor incorporate relatively more Mg-clays and more Fe-clays, respectively. The short-lived lake overflowed and breached Holden crater’s rim at an elevation of −350 m and rapidly drained into the crater. Fan deltas in Holden extend 25 km from the breach and incorporate meter-sized blocks, and longitudinal grooves along the Uzboi basin floor are hundreds of meters long and average 60 m wide, suggesting high-discharge drainage of the lake. Precipitation-derived runoff rather than regional groundwater or overflow from Argyre dominated contributions to the Uzboi lake, although the failure of most tributaries to respond to a lowering of base level indicates their incision largely ended when the lake drained. The Uzboi lake may have coincided with alluvial and/or lacustrine activity in Holden, Eberswalde, and other craters in southern Margaritifer Terra, where fluvial/lacustrine activity may have required widespread, synoptic precipitation (rain or snow), perhaps associated with an ephemeral, global hydrologic system during the Late Noachian into the Hesperian on Mars.  相似文献   

5.
HiRISE images together with other recent orbital data from Mars define new characteristics of enigmatic Hesperian-aged deposits in Sirenum Fossae that are mostly 100-200 m thick, drape kilometers of relief, and often display generally low relief surfaces. New characteristics of the deposits, previously mapped as the “Electris deposits,” include local detection of meter-scale beds that show truncating relationships, a generally light-toned nature, and a variably blocky, weakly indurated appearance. Boulders shed by erosion of the deposits are readily broken down and contribute little to talus. Thermal inertia values for the deposits are ∼200 J m−2 K−1 s−1/2 and they may incorporate hydrated minerals derived from weathering of basalt. The deposits do not contain anomalous amounts of water or water ice. Deflation may dominate degradation of the deposits over time and points to an inventory of fine-grained sediment. Together with constraints imposed by the regional setting on formation processes, these newly resolved characteristics are most consistent with an eolian origin as a loess-like deposit comprised of redistributed and somewhat altered volcanic ash. Constituent sediments may be derived from airfall ash deposits in the Tharsis region. An origin directly related to airfall ash or similar volcanic materials is less probable and emplacement by alluvial/fluvial, impact, lacustrine, or relict polar processes is even less likely.  相似文献   

6.
I. Romeo  D.L. Turcotte 《Icarus》2009,203(1):13-19
The areas of volcanic units on Venus have been measured on the 1:5000000 geological maps published by NASA/USGS. These data were used to obtain a frequency-area distribution. The cumulative frequency-area distribution of 1544 specific occurrence of units cover six orders of magnitude from the largest unit (30 × 106 km2) to the smallest (20 km2). The probability distribution function has been calculated. The medium and large volcanic units correlate well with a power-law (fractal) relation for the dependence of frequency on area with a slope of −1.83. There are fewer small units than the expected values provided by the power-law relation. Our measurements cover 21.02% of the planetary surface, 3.59% of the study area was found to be tessera terrain and is excluded from this study of volcanism. The measurements were restricted to areas where geological maps have been published. The analysis was performed on two independent areas of the planet, with a complete coverage of published maps. In both areas the largest volcanic unit covers a significant portion of the surface (58.75% and 63.64%, respectively). For the total measured volcanic units (excluding tessera), these two largest units (that could correspond to the same unit or not) cover the 61.18% and they are stratigraphically superimposed on older volcanic units which cover 3.37% of the area. The remaining area (35.45%) is occupied by younger volcanic units stratigraphically superimposed on the large volcanic unit(s). These results are based on the independent mapping of a large number of geologists with different ideas about the geodynamical evolution of Venus and different criteria for geological mapping. Despite this fact, the presence of these very large units is incompatible with the equilibrium resurfacing models, because their generation at different ages would destroy the crater randomness. Our frequency-area distribution of the mapped volcanic units supports a catastrophic resurfacing due to the emplacement of the largest unit(s) followed by a decay of volcanism. Our data for the frequency-area distribution of volcanic units provide new support for catastrophic resurfacing models. It is difficult to make our observations compatible with equilibrium, steady-state resurfacing models.  相似文献   

7.
HiRISE images of the lower member of the Medusae Fossae Formation (MFF) were used to identify characteristics of two specific landforms that are well expressed in this particular geologic unit; yardangs and sinuous ridges. Yardangs are wind-eroded ridges that are usually confined to arid environments where the bedrock materials can be easily eroded by windblown sand. Yardangs are common in the lower member of MFF, where many individual yardangs show evidence of a caprock unit overlying a more friable unit, most consistent with an ignimbrite origin for these MFF deposits. Heights of the yardangs in the lower member materials are generally less than a few tens of meters, in contrast to yardangs in the thicker middle member MFF materials to the east of the study area. The yardangs may form in materials comprised of discrete depositional units, and there is good evidence that at least a dozen such depositional events contributed to the emplacement of the lower member of MFF. The lower member yardang heights indicate aeolian erosion has removed at least 19,000 km3 of lower member MFF materials. Sinuous ridges are elongate, positive-relief landforms that have been attributed to a variety of possible fluvial flow processes on Mars. Sinuous ridges are very common within exposures of the lower member of MFF. Multiple ridge types are present, but all forms seen at HiRISE scale are most consistent with some form of aqueous channel flow rather than other possible origins. The results from this initial examination of HiRISE images indicate the potential utility of comparing yardangs and sinuous ridges in the lower member to other members of MFF, although it remains to be determined if sinuous ridges are abundant in the younger MFF members.  相似文献   

8.
Ismenius Cavus, Mars: A deep paleolake with phyllosilicate deposits   总被引：1，自引：0，他引：1  
Ismenius Cavus is a basin where several fluvial valleys converge. Three depositional fan deltas are observed at the valleys outlets at similar elevations. These fans suggest long-term fluvial activity accompanied by a lake inside the basin. The elevational difference between the delta plains and the deepest part of the basin floor implies that this lake was 600 m deep. Iron-magnesium phyllosilicates, which are mapped from near-infrared spectral data, are associated with layered sediments >300 m thick at the base of one of the fans. Stratigraphic relationships with the surrounding plateau show that the valleys are hesperian in age (3.0-3.7 ga), thus dating the lake activity to this period. The coexistence of a deep lake and phyllosilicates demonstrates that persistent bodies of liquid water were present during the hesperian period.  相似文献   

9.
Laboratory simulations using the Arizona State University Vortex Generator (ASUVG) were run to simulate sediment flux in dust devils in terrestrial ambient and Mars-analog conditions. The objective of this study was to measure vortex sediment flux in the laboratory to yield estimations of natural dust devils on Earth and Mars, where all parameters may not be measured. These tests used particles ranging from 2 to 2000 μm in diameter and 1300 to 4800 kg m−3 in density, and the results were compared with data from natural dust devils on Earth and Mars. Typically, the cores of dust devils (regardless of planetary environment) have a pressure decrease of ∼0.1-1.5% of ambient atmospheric pressure, which enhances the lifting of particles from the surface. Core pressure decreases in our experiments ranged from ∼0.01% to 5.00% of ambient pressure (10 mbar Mars cases and 1000 mbar for Earth cases) corresponding to a few tenths of a millibar for Mars cases and a few millibars for Earth cases. Sediment flux experiments were run at vortex tangential wind velocities of 1-45 m s−1, which typically correspond to ∼30-70% above vortex threshold values for the test particle sizes and densities. Sediment flux was determined by time-averaged measurements of mass loss for a given vortex size. Sediment fluxes of ∼10−6-100 kg m−2 s−1 were obtained, similar to estimates and measurements for fluxes in dust devils on Earth and Mars. Sediment flux is closely related to the vortex intensity, which depends on the strength of the pressure decrease in the core (ΔP). This study found vortex size is less important for lifting materials because many different diameters can have the same ΔP. This finding is critical in scaling the laboratory results to natural dust devils that can be several orders of magnitude larger than the laboratory counterparts.  相似文献   

10.
The Planetary Fourier Spectrometer (PFS) experiment on board the Mars Express mission has two channels covering the 1.2-5 μm (SWC) and the 5-50 μm (LWC). The Long Wavelength Channel (LWC) measures the thermal emission spectrum of Mars between 200 and 2000 cm−1 with a spectral resolution of 1.4 cm−1, in absence of apodisation. We present here the calibration of this channel and its performance. The instrument calibration has been performed on ground, before launch, in space during Near Earth Verification (NEV) measurements, and at Mars. Special attention has been given to the problem of microvibrations on board the spacecraft.In order to obtain correct results, the source-instrument-detector interaction is studied very accurately. The instrument variations during a pericentre pass impose a complex procedure for the LW channel calibration, but fortunately the procedure adopted seems to work well. Samples of the calibrated data are given (as single spectrum and as an average over a few spectra) to show the performance of the experiment and its scientific potentialities.  相似文献   

11.
Laura Kerber  James W. Head 《Icarus》2010,206(2):669-684
The Medusae Fossae Formation (MFF), covering about 2.1 × 106 km2 (with an estimated volume of 1.4 × 106 km3) and straddling the equatorial region of Mars east of Tharsis, has historically been mapped and dated as Amazonian in age. Analysis of the MFF using a range of new observations from recent mission data at multiple resolutions reveals evidence that the formation is older than previously hypothesized, with parts of the MFF having formed in the Hesperian and parts having been reworked and reformed throughout the Amazonian, up to the present. Ancient outcroppings of the MFF, edged with jagged yardangs, became a “mold” for embaying Hesperian-aged lavas. The erosion of the MFF left solidified lava “casts” in the embaying lava unit. This lava edge morphology permits the identification of ancient contacts between the MFF and Hesperian-aged lava terrain. Additionally, the flanking fan of the Hesperian-aged Apollinaris Patera volcano embays the formation at its foot, indicating that parts of the MFF were formed in the Hesperian. Erosion has erased and inverted many of the superposed craters in the region, showing that very young Amazonian ages derived from impact crater size-frequency distributions are resurfacing ages, and not emplacement ages. We find abundant evidence that the formation is extremely mobile and continuously reworked. We conclude that a significant part of the MFF may have originally been emplaced in the Hesperian. These observations place new constraints on the mode of origin of the MFF.  相似文献   

12.
Fifteen organic and three inorganic compounds were tested for methane (CH4) evolution under simulated martian conditions of 6.9 mbar; UVC (200-280 nm) flux of 4 W m−2; 20 °C; simulated optical depth of 0.1; and a Mars gas composition of CO2 (95.3%), N2 (2.7%), Ar (1.7%), O2 (0.13%), and water vapor (0.03%). All three inorganic compounds (i.e., NaCl, CaCO3, graphite) failed to evolve methane at the minimum detection level 0.5 ppm, or above. In contrast, all organic compounds evolved methane when exposed to UV irradiation under simulated martian conditions. The polycyclic aromatic hydrocarbon, pyrene, released the most methane per unit of time at 0.175 nmol CH4 g−1 h−1, and a spectral reflectance target material used for the MER rovers and Phoenix lander released the least methane at 0.00065 nmol CH4 cm−2 h−1. Methane was also released from UV-killed bacterial endospores of Bacillus subtilis. Although all organic compounds evolved methane when irradiated with UV photons under martian conditions, the concentrations of residual organics, biogenic signature molecules, and dead microbial cells should be relatively low on the exterior surfaces of the MSL rover, and, thus, not significant sources of methane contamination. In contrast, kapton tape was found to evolve methane at the rate of 0.00165 nmol CH4 cm−2 h−1 (16.5 nmol m−2 h−1) under the UV and martian conditions tested. Although the evolution of methane from kapton tape was found to decline over time, the large amount of kapton tape used on the MSL rover (lower bound estimated at 3 m2) is likely to create a significant source of terrestrial methane contamination during the early part of the mission.  相似文献   

13.
We have studied a terraced fan deposit with unique characteristics located within a trough of Coprates Catena. The fan has an average length of 6.8 km, and is approximately 44 km2 in area and 18 km3 in volume. The fan's broad contributing valley is approximately 35 km long and it noticeably increases in depth about 12.8 km before it intersects the trough, where a rounded knickpoint marks the transition between flat-floored upstream and V-shaped downstream cross-sections. A 14-km-long channel with no apparent source enters the contributing valley from the south. A much smaller sinuous channel has incised along a smaller V-shaped valley in the uppermost eastern portion of the fan deposit. We explored several possible origins for the terraced fan, including mass wasting, volcanic flow, alluvial fan, and delta. We propose that water sourced from volcanic melting of ice eroded and transported material along the contributing valley. This material was then deposited as a delta in a lake within the trough. The concentric terraces are most likely the result of shoreline or ice cover erosion during drops in lake level. A light-toned layered deposit to the east of the fan deposit along the floor of the trough may represent a sedimentary unit formed during the terminal stages of the lake. Although other terraced fans have been identified on Mars, the Coprates Catena fan is unique because it has many more terraces and its surface was incised by a channel and associated valley. The identification of several other valleys to the east suggests that volcanic melting of volatiles during the Hesperian Period created favorable conditions for water flow along the plains in this region.  相似文献   

14.
The evaluation of the planetary Fourier spectrometer performance at Mars is presented by comparing an average spectrum with the ISO spectrum published by Lellouch et al. [2000. Planet. Space Sci. 48, 1393.]. First, the average conditions of Mars atmosphere are compared, then the mixing ratios of the major gases are evaluated. Major and minor bands of CO2 are compared, from the point of view of features characteristics and bands depth. The spectral resolution is also compared using several solar lines. The result indicates that PFS radiance is valid to better than 1% in the wavenumber range 1800-4200 cm−1 for the average spectrum considered (1680 measurements). The PFS monochromatic transfer function generates an overshooting on the left-hand side of strong narrow lines (solar or atmospheric). The spectral resolution of PFS is of the order of 1.3 cm−1 or better. A large number of narrow features to be identified are discovered.  相似文献   

15.
A variety of sedimentary deposits is observed in Xanthe Terra, Mars, including Gilbert-type deltas, fan deltas dominated by resedimentation processes, and alluvial fans. Sediments were provided through deeply incised valleys, which were probably incised by both runoff and groundwater sapping. Mass balances based on High-Resolution Stereo Camera (HRSC) digital terrain models show that up to ～30% of the material that was eroded in the valleys is present as deltas or alluvial fan deposits. Stratigraphic relationships and crater counts indicate an age of ～4.0 to ～3.8 Ga for the fluvial activity. Hydrologic modeling indicates that the deposits were probably formed in geologically very short time scales. Our results point to episodes of a warmer and wetter climate on early Mars, followed by a long period of significantly reduced erosion rates.  相似文献   

16.
Most valley networks have been identified primarily in the heavily cratered uplands which are Noachian in age (>3.5 Gyr). A striking exception to this general observation is Warrego Valles located on the southeastern part of the Tharsis bulge. Recent data obtained by the Mars Orbiter Laser Altimeter, the Thermal Emission Imaging System (THEMIS) spectrometer and the Mars Orbiter Camera give new insight into the formation of valley networks and the early Mars climate. We focus our study on the southern Thaumasia region especially on Warrego Valles and determine the organisation of valleys in relation to regional topography and structural geology. Warrego Valles is the most mature valley network that incised the southern side of Thaumasia highlands. It developed in a rectangular-shaped, concave-up drainage basin. Four times more valleys are identified in THEMIS infrared images than in Viking images. Valleys exist on both sides of the main tributary contrary to what was visible in Viking images. Their distribution is highly controlled by topographic slope, e.g. there is a parallel pattern on the sides and dendritic pattern on the central part of Warrego Valles. We quantitatively analyse valley morphology and morphometry to determine the processes responsible for valley network formation. Warrego Valles displays morphometric properties similar to those of a terrestrial fluvial valley network. This valley network is characterised by seven Strahler's orders, a bifurcation ratio of 3, a length ratio of 1.7, a drainage density of 0.53 km−1 and a ruggedness number of 3.3. The hypsometric curve and integral (0.46) indicate that Warrego Valles reached the mature Davis’ stage. Valleys have undergone external degradation since their incision, which masks their main morphological characteristics. Our study supports the assertion that valley networks formed by fluvial processes controlled by an atmospheric water cycle. Further, they seem to develop by successive stages of erosion that occurred during Noachian through the late Hesperian.  相似文献   

17.
The Planetary Fourier Spectrometer (PFS) experiment on board the Mars Express mission has two channels covering the 1.2-5.5 μm short wavelength channel (SWC) and the 5.5-45 μm (LWC). The SWC measures part of the thermal emission spectrum and the solar reflected spectrum of Mars between 1700 and 8200 cm−1 with a spectral resolution of 1.3 cm−1, in absence of apodisation. We present here the calibration of this channel and its performance. The instrument calibration has been performed on ground, before launch, in space during near earth verification (NEV) measurements, and at Mars. Special attention has been given to the problem of microvibrations on board the spacecraft.In order to obtain correct results, the source-instrument-detector interaction for the thermal part is studied very accurately. The instrument shows a nonlinear behaviour with source intensity. The SNR increases with amplification, hence high gain factors are usually used. The detector is, in space, cooled by a passive radiator, and works around 210-215 K. The calibration source (an internal lamp) shows variations during a pericentre pass and therefore impose a complex procedure for the SW channel calibration. Mechanical microvibrations strongly affect part of the spectrum. We discuss the validity of the present calibration, and indicate possible future developments. Samples of the calibrated data are given to show the performance of the experiment and its scientific potentialities.  相似文献   

18.
Volcanism has been a major process during most of the geologic history of Mars. Based on data collected from terrestrial basaltic eruptions, we assume that the volatile content of martian lavas was typically ∼0.5 wt.% water, ∼0.7 wt.% carbon dioxide, ∼0.14 wt.% sulfur dioxide, and contained several other important volatile constituents. From the geologic record of volcanism on Mars we find that during the late Noachian and through the Amazonian volcanic degassing contributed ∼0.8 bar to the martian atmosphere. Because most of the outgassing consisted of greenhouse gases (i.e., CO2 and SO2) warmer surface temperatures resulting from volcanic eruptions may have been possible. Our estimates suggest that ∼1.1 × 1021 g (∼8 ± 1 m m−2) of juvenile water were released by volcanism; slightly more than half the amount contained in the north polar cap and atmosphere. Estimates for released CO2 (1.6 × 1021 g) suggests that a large reservoir of carbon dioxide is adsorbed in the martian regolith or alternatively ∼300 cm cm−2 of carbonates may have formed, although these materials would not occur readily in the presence of excess SO2. Up to ∼120 cm cm−2 (2.2 × 1020 g) of acid rain (H2SO4) may have precipitated onto the martian surface as the result of SO2 degassing. The hydrogen flux resulting from volcanic outgassing may help explain the martian atmospheric D/H ratio. The amount of outgassed nitrogen (∼1.3 mbar) may also be capable of explaining the martian atmospheric 15N/14N ratio. Minor gas constituents (HF, HCl, and H2S) could have formed hydroxyl salts on the surface resulting in the physical weathering of geologic materials. The amount of hydrogen fluoride emitted (1.82 × 1018 g) could be capable of dissolving a global layer of quartz sand ∼5 mm thick, possibly explaining why this mineral has not been positively identified in spectral observations. The estimates of volcanic outgassing presented here will be useful in understanding how the martian atmosphere evolved over time.  相似文献   

19.
We conducted a systematic, global survey using Thermal Emission Imaging System Infrared (THEMIS IR) coverage (∼100 m/pixel) to search for large alluvial fans in impact craters on Mars. Our survey has focused on large fans (apron areas greater than ∼40 km2, usually located in craters greater than 20 km in diameter) due to the resolution of the THEMIS images and Mars Orbiter Laser Altimeter (MOLA) coverage. We find that the host craters are found to have a distinctive diameter range from 30-150 km. The fans generally cluster in three geographic areas—southern Margaritifer Terra, southwestern Terra Sabaea, and southwestern Tyrrhena Terra, however several outliers do exist. The alluvial fans do not form in a particular orientation along the crater rim nor are they associated with the location of current high rim topography. Fan area magnitude and variability increase with crater diameter while fan concavity magnitude and variability increase with decreasing crater diameter. Smaller fan aprons in general have higher, more variable concavity. The source of the water forming these fans is uncertain given the challenges of accommodating the global distribution pattern and formation patterns within the craters.  相似文献   

20.
With 2 years of tracking data collection from the MRO spacecraft, there is noticeable improvement in the high frequency portion of the spherical harmonic Mars gravity field. The new JPL Mars gravity fields, MRO110B and MRO110B2, show resolution near degree 90. Additional years of MGS and Mars Odyssey tracking data result in improvement for the seasonal gravity changes which compares well to global circulation models and Odyssey neutron data and Mars rotation and precession (). Once atmospheric dust is accounted for in the spacecraft solar pressure model, solutions for Mars solar tide are consistent between data sets and show slightly larger values (k2 = 0.164 ± 0.009, after correction for atmospheric tide) compared to previous results, further constraining core models. An additional 4 years of Mars range data improves the Mars ephemeris, determines 21 asteroid masses and bounds solar mass loss (dGMSun/dt < 1.6 × 10−13 GMSun year−1).  相似文献   

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