P22A-01 10:20h
Mars Northern Lowlands: Wind-eroded Versus Sediment-filled, a Comparison of Hypotheses
The northern lowlands are remarkably smooth at scales from several hundred meters to hundreds of km, are locally covered by obvious layers of sediment that are up to a few tens of meters thick, and lie beyond and below the terminuses of many large outflow channels. Primarily for these reasons, the hypothesis that these lowland areas correspond to the sediment-filled beds of ancient bodies of standing water has been widely accepted (the prevailing hypothesis). On the other hand, atmospheric general circulation model (GCM) simulations incorporating dust lifting and transport consistently indicate that the northern plains are dominated by wind erosion in the present climate regime, and that they have been dominated by wind erosion in most past climate regimes as well. This is a robust result of the simulations that follows from the occurrence of strong mid-latitude storm systems together with the relatively low wind speed threshold for saltation resulting from higher pressure at lower elevation. This model result is supported by orbital and surface observations of frequent storms in this region and widespread seasonally variable surface wind streaks. It is difficult to reconcile this evidence for a robust erosional regime with the prevailing hypothesis of an ancient surface of fine sediments deposited beneath standing water. We develop an alternate hypothesis that the northern plains are erosional rather than depositional. This hypothesis is consistent with widespread areas of high surface thermal inertia, frequent occurrences of rock outcrops and boulders visible in orbital and surface high resolution images, frequent occurrences of multiple rims around craters whose diameters are in the few hundred meter to several kilometer range, absence of dune forms, and relatively infrequent occurrence of small craters. All of these features of the northern plains will be documented in this paper. Very significantly, the alternate hypthosis provides a natural explanation for the properties of the population of very large craters (diameter greater than 100 km) that is otherwise very difficult to understand. Through a side-by-side comparison of the prevailing hypothesis and the alternate hypothesis, we will argue that the alternate hypothesis provides a more satisfactory explanation of all the observations than the prevailing hypothesis. However, it remains to be explained why a much more intense erosional event would have occurred in the northern than in the southern hemisphere, and why dust storms are so frequently observed in the northern plains despite the absence of obvious features associated with saltation.
P22A-02 10:35h
Evidence for Long-term Wind Modification of the Martian Surface
General Circulation Models predict that wind erosion and deposition are persistent in certain areas on Mars regardless of long-term changes in orbital dynamics. When integrated over billion-year time scales, seemingly trivial rates of erosion or deposition $\sim$micron/yr can result in substantial surface alteration of order $\sim$km depth. This is important for interpreting Martian surface evolution. Acting on regolith over billions of years, wind could massively scour or bury landscapes, confusing the timing of events and the interpretation of features thought to be altered by interaction with volatiles. There are several indicators of whether surfaces are undergoing erosion or burial including wind streaks, the presence of bedforms and dunes, and the preservation state of small (20-2000 m) craters. The state of small craters is particularly diagnostic and their absence in a Mars Orbiter Camera narrow angle (NA) image suggests a surface age $ < $9 m.y. either due to complete erosion or burial. We find that erosional and burial states of surface features inferred primarily from imaging data correlate with thermal inertia values and GCM-predicted regions of persistent deposition (e.g., Amazonis, Arabia) and erosion (e.g., Isidis, Acidalia). Small craters are also often absent between 60-80 deg latitude in both hemispheres, perhaps correlating with high wintertime wind stress. Similarly 70-95% of NA images on the Hellas floor lack small craters perhaps due to pressure-enhanced wind stress. Overall, results support the hypothesis that long-term geomorphologic action of wind has substantially altered the surface of Mars.
P22A-03 10:50h
Three Decades of Martian Surface Changes
The surface of Mars has changed dramatically during the three decades spanned by spacecraft exploration. Comparisons of Mars Global Surveyor images with Viking and Mariner 9 pictures suggest that more than one third of Mars' surface area has brightened or darkened by at least 10%. Such albedo changes could produce significant effects on solar heating and the global circulation of winds across the planet. All of the major changes took place in areas of moderate to high thermal inertia and rock abundance, consistent with burial of rocky surfaces by thin dust layers deposited during dust storms and subsequent exposure of the rocky surfaces by aeolian erosion. Several distinct mechanisms contribute to aeolian erosion on Mars. Persistent winds dominate erosion at low latitudes, producing diffuse albedo boundaries and linear wind streaks generally oriented in the direction of southern summer winds. Dust devils darken the mid- to high- latitudes from 45 to 70 degrees during the summer seasons, forming irregular albedo patterns consisting of dark linear tracks. Dust storms produce regional albedo variations with distinct but irregular margins. Dark sand dunes in southern high latitudes appear to be associated with regional darkening that displays diffuse albedo boundaries. No surface changes were observed to repeat regularly on an annual basis, but many of the changes took place in areas that alternate episodically between high and low albedo states as thin mantles of dust are deposited and later stripped off. Hence, the face of Mars remains recognizable after a century of telescopic observations, in spite of the enormous extent of alteration that has taken place during the intervals between spacecraft missions.
http://astrogeology.usgs.gov/Projects/MarsChanges
P22A-04 11:05h
Surface Dust Redistribution on Mars as Observed by the Viking and Mars Global Surveyor Orbiters
The global redistribution of dust by the atmosphere is geologically and climatologically important. Dust deposition and removal at the surface represents ongoing sedimentary geology: a vestige of aeolian processes responsible for the concentration of vast dustsheets and potentially for ancient layered units at various locations on Mars. The varying amount of dust on the surface has also long been hypothesized as a factor in determining whether regional or global dust storms occur in a given year. Indeed, the atmosphere has a very short, sub-seasonal time-scale (or memory) and as such, any inter-annual variability in the climate system that is not simply ascribable to stochastic processes, must involve changing conditions on the surface. An excellent, multi-year dataset is provided by the combined Viking and Mars Global Surveyor (MGS) orbiter albedo and thermal observations, from the Infrared Thermal Mapper (IRTM) and Thermal Emission Spectrometer (TES), and from the MGS Mars Orbiter Camera Wide Angle imager (MOC-WA). This dataset allows investigation into the degree to which surface dust deposits on Mars really change: over decadal time scales, over the course of the annual cycle, and as a result of global and regional dust storms. The MGS mapping orbit data set extends over almost 3 Martian years at the time of writing, while the Viking data set provides a much less complete sampling of three northern summers/autumns and one southern summer/autumn. These data sets include three global dust storms (two for Viking and one for MGS) and smaller regional storms (one in the first TES mapping year and two in the third). We have examined the Viking IRTM, MGS TES, and MGS MOCWA data sets to determine what types of changes in dust coverage have occurred. Viking-to-MGS changes in albedo are highlighted by the drastic modification of a large, low albedo (low dust) feature to the east of Utopia Planitia. Year-to-year changes within the Viking and MGS records are dominanted by the effects of global dust storms. The 2001 storm observed by MGS is the best documented. We found a number of regions that changed significantly after the 2001 global dust storm. Areas with noticeable changes include the brightening of Syrtis Major, Hellas Planitia, and the region east of Hellas Planitia, and the darkening of Tharsis. Solis Planum, a region known to have participated as a secondary source for the 2001 storm became darker following the storm, while an area directly to the east of Solis became brighter. The majority of these changes are visible in both TES maps and MOC wide-angle images. These changes have been slowly relaxing back towards pre-storm conditions since the end of the storm. Similar albedo changes in these same regions were found associated with the global storms observed by the Viking IRTM. This suggests that the very limited number of dust storms observed by spacecraft have tended to produce the same kinds of changes in surface dust coverage. The origin of the long-term changes in Utopia are not understood.
P22A-05 11:20h
Does the present-day wind regime explain the location and geomorphology of dunes in the southern highlands of Mars?
Dozens of dunefields are scattered throughout the southern highlands of Mars, mostly (but not entirely) located in the floors of impact craters. MOC Narrow Angle images reveal that these dunes are not basic barchan or transverse in form, but rather that they are a combination of barchans, reversing, linear, and star dunes, indicating that they were formed in a multi-directional wind regime. In Noachis Terra, west of Hellas Planitia, almost all dunefields show three dominant slipface orientations, indicating formative winds from the SW, SE, and NE. Different dunefields show these three winds in different proportions, suggesting that in different areas, different winds have dominated. To determine how these different winds interact in the present day wind regime, we ran the Mars MM5 over Noachis Terra. The model runs include twelve 10-day runs spaced throughout the martian year, with a grid size of 20 km and a timestep of 10 seconds (physical parameters were saved once every hour). Winds from the SW blow during winter afternoons, caused by geostrophic forcing. Because of coverage from the seasonal polar cap, these winds are more effective in blowing sand at latitudes poleward of $50\deg$ S, roughly consistent with the observed spatial pattern in dune morphology. Winds from the SE blow during the early to mid afternoon in the spring, caused by slope winds blowing up and over the rim of the Hellas basin. These winds should be more common closer to Hellas Planitia, but they appear in dunes throughout Noachis Terra. Winds from the NE blow in the evening during summer, and they are katabatic flows that accelerate into topographic lows, following the diurnal tide. These winds are strongest equatorward of $50\deg$ S, which is fairly consistent with observed dune morphology. Discrepancies between the model and observed dune slipfaces can be explain by shifting physical parameters (i.e., dust loading or obliquity).
P22A-06 11:35h
Wind Erosion Regimes and the Evolution of the Surface of Mars Studied with the NASA Ames Mars General Circulation Model
A billion year integration of Mars orbital parameters and the NASA Ames Mars General Circulation Model are combined to investigate the long-term erosional history of the surface of Mars. In agreement with findings of Robert Haberle et al., we find that the distribution of potential surface erosion by wind is robust with respect to orbital parameter variations. Potential erosion is strongest: (1) in storm tracks following the edges of the seasonal polar caps, (2) in regions of low surface elevation, (3) in regions of strong cross-equatorial solstice flows at moderate to high obliquity. It follows that maximum long-term erosion rates occur throughout most of the northern plains, in Acidalia and portions of Amazonis and Utopia, and in the Hellas basin. We also investigate the sensitivity of wind erosion to changes in global mean surface pressure and find, as expected, very high sensitivity. For example, if global mean surface pressure were to increase from the current 6 mb to 40 mb, model potential erosion rates increase by more than one order of magnitude. In this regime, potential erosion rates are sufficiently high that several km of easily eroded fine regolith could be removed in a time span of 100 million years. Possible observational consequences of these results will be discussed.
P22A-07 11:50h
Dark Streaks on Mars : Systematic Mapping of Geometric, Surface Properties and Rate of Formation From HRSC, THEMIS and MOC images.
Dark slope streaks are among the only processes which are active at the present time on the Martian surface. While their mechanism of formation and triggering is still debated, they may involve liquid water. The HRSC experiment (Mars Express) provides new key data for the study of the mechanism of formation of these objects. The resolution and covering of HRSC images permit a systematic mapping of the geometric properties of these objects (width, length and orientation) which was not possible with MOC or THEMIS-VIS data. We present a systematic mapping of about 800 dark streaks in a region north of Olympus Mons. The mapped area extends from $28^o$N to 38$^o$N from 220$^o$E to 224$^o$E. From these data, it is possible to investigate geographical variations of the density of dark streaks. Few dark streaks occur north of $33^oN$. The density of dark streaks increases rapidly from $32^oN$ and values up to 0.05 dark streaks per $km^2$ are observed. The ubiquitous boundary observed around $33^o$N indicates a possible correlation between the formation of dark streak and surface temperature, a correlation which has been previously reported at the planetary scale [1] giving supports to the role of liquid water in the process of formation. The number of dark streaks varies also with longitude. These variations can be related to surface roughness properties (at the kilometer scale) and surface thermal properties. The orientations of dark streaks have been systematically mapped from the direction of the flow along the slope. The dark streaks at the highest latitude are preferentially oriented on south facing slopes. This result implies a dependence to the sun exposition. At lower latitude, the orientation of dark streaks becomes progressively more randomly distributed, with the exception of an area on the slope of the aureole of Olympus Mons, where a population of dark streaks is emplaced preferentially on west facing slopes. This preferential orientation can not be explained by any particular organization of the topography or slopes. We propose that winds could be involved in the formation of dark streaks by the accumulation of dust particles at hill crests. The comparison of HRSC, THEMIS-VIS and MOC images allow the detection of new dark streaks. These observations are promising for the estimation of possibly geographic variations of rates of formation which will have implications concerning the transport and deposition of dust on Mars. [1] Schorghofer, et. al. Slope streaks on mars: Correlations with surface properties and the potential role of water. Geoph.l Res. Let., 29(23):41-1,41-4.