Timothy J. Parker

COASTAL GEOMORPHOLOGY OF THE MARTIAN NORTHERN PLAINS

The paper considers the question of the formation of the outflow channels and valley networks discovered on the Martian northern plains during the Mariner 9 mission. Parker and Saunders (1987) and Parker et al. (1987, 1989) data are used to describe key features common both in the lower reaches of the outflow channels and within and along the margins of the entire northern plains. It is suggested, that of the geological processes capable of producing similar morphologies on earth, lacustrine or marine deposition and subsequent periglacial modification offer the simplest and most consistent explanation for the suit of features found on Mars.

Transitional morphology in West Deuteronilus Mensae, Mars: Implications for modification of the lowland/upland boundary

Abstract West Deuteronilus Mensae, which lies along the lowland/upland boundary of Mars at 45° latitude, 345° longitude, exhibits both gradational ( L. A. Rossbacher, 1985 ,in Models in Geomorphology (Woldenberg, Ed), pp. 343–372, Allen & Unwin, Boston) and fretted terrain ( R.P. Sharp, 1973 , J. Geophys. Res. 78, 4073–4083) boundary types. To the west, the boundary is gradational and to the east it is defined by fretted terrain. The two types of boundaries do not merge with one another within the west Deuteronilus Mensae region, however. The gradational boundary materials appear to overlap the fretted terrain by about 500 km. Contacts between units associated with the gradational boundary and the fretted terrain boundary can both be recognized in the region of overlap. The fretted terrain can be identified as much as 300 km north of the gradational boundary in this region. Lowland units associated with the gradational boundary embay canyons of the fretted terrain in a topographically conformal fashion. Changes in the fretted terrain across the gradational boundary (from uplands to lowlands) include a reduction of canyon wall slopes and depths, such that the fretted terrain north of the gradational boundary appears mantled but not obscured. There are at least two major classes of processes which might explain the lateral overlap: (1) erosion of stratified upland terrain and (2) deposition of plains materials onto the sloping upland margin and fretted terrain. Erosion of stratified upland terrain does not adequately explain the plainward decrease in crater densities across the gradational boundary nor is it consistent with evidence that lowland plains material appears to onlap the sloping upland margin in several places. Of the possible plains emplacement mechanisms, eolian deposition would not produce the sharp, apparently topographically conformal gradational unit contacts. Volcanics plains emplacement would not preserve the complex geometry of the underlying fretted terrain. Sediment deposition in either a liquid or ice-covered sea could produce the draped appearance of the fretted terrain. Outflow channels along the lowland/upland boundary, particularly those of the circum-Chryse and west Elysium regions, may have flooded the northern lowlands to depths of tens to hundreds of meters. The gradational unit contacts may represent the shorelines of such a sea. The scale of characteristic morphologies along these contacts may require an unfrozen condition of sufficient duration to allow lacustrine-style wave erosion and redistribution of material along the contacts. Our limited understanding of the Martian paleoclimate and H 2 O inventories allows the possibility of clement periods in the past, and other geological evidence (e.g., small valley networks and outflow channels) strongly suggests an extensive role of liquid water.

Dust devil vortices seen by the Mars Pathfinder Camera

Discovery of dust devil vortices in Mars Pathfinder (MPF) images reveals a dust entrainment mechanism at work on Mars. Scattering of visible light by dust in the Martian atmosphere creates a pronounced haze, preventing conventional image processing from displaying dust plumes. Spectral differencing techniques have enhanced five localized dust plumes from the general haze in images acquired near midday, which we determine to be dust devils. Processing of 440 nm images highlights dust devils as distinct occultation features against the horizon. The dust devils are interpreted to be 14–79 m wide, 46–350 m tall, travel at 0.5–4.6 m/s, with dust loading of 7E-5 kg m-3, relative to the general haze of 9E-8 kg m-3, and total particulate transport of 2.2–700 kg. The vortices match predictions from terrestrial analog studies.

Water on early Mars: Possible subaqueous sedimentary deposits covering ancient cratered terrain in western Arabia and Sinus Meridiani

Western Arabia and northern Sinus Meridiani (30°N–10°S; 40°W–40°E) are almost entirely covered by wind-eroded, horizontally-bedded, sedimentary layers that lap against local topographic features. This portion of Mars ancient cratered terrain is relatively low (< 1 km elevation) and flat (sloping westward 0.7 m/km over 3,000 km). The region lacks valley network channels. We propose that at some time in early Martian history, this region was under water. The water hypothesis is strengthened by the presence of (1) a shore-like contact between smooth-surfaced deposits and ancient cratered terrain in central Sinus Meridiani, (2) polygonal structures in northern Sinus Meridiani, and (3) sand (which cannot be from airfall), possibly reworked by wind from the underlying sedimentary units. The September 1997 arrival of Mars Global Surveyors thermal emission spectrometer offers an opportunity to begin to test this hypothesis by searching for carbonate and evaporite minerals among the sediment covering the region.

Drainage evolution in the Margaritifer Sinus region, Mars

[1]xa0Margaritifer Sinus, Mars, records a complex history of water transport, storage, and release extending from the late Noachian into at least the mid-Hesperian. Collection, transport, and discharge of water were accomplished by systems of differing character flanking opposite sides of the Chryse Trough. Drainage on the western side of the trough was via the segmented Uzboi-Ladon-Margaritifer mesoscale outflow system that heads in Argyre Basin and incises and fills as it crosses ancient multiringed impact basins. By contrast, Samara and Parana-Loire Valles, two of the largest and best integrated valley systems on Mars, dominated drainage on the eastern trough flank. Valley morphometry suggests formation was due to precipitation-recharged groundwater sapping. All systems discharged into Margaritifer Basin, located along the Chryse Trough axis, and caused ponding that persisted into the early Hesperian. The Uzboi-Ladon-Margaritifer system dominated discharge that was coincident with widespread geomorphic activity on Mars. As channel and valley formation ended, some water in Margaritifer Basin infiltrated the subsurface. Collapse and release of water began shortly thereafter and persisted into mid-Hesperian times, thereby forming Margaritifer and Iani Chaos and incising Ares Vallis.

Properties of Filamentary Sublimation Residues from Dispersions of Clay in Ice

During vacuum sublimation experiments on simulated Martian polar deposits and cometary dirty ices, a fluffy filamentary sublimate residue material with unique physical properties was produced. The silica-to-silica bonds that we believe join the particles together are the result of conditions that may exist in some Martian polar deposits and on some cometary surfaces. Submicron particles of montmorillonite clay thinly dispersed (1 : 1000 clay/water) and not contacting one another in water ice can form very-low-density structures (density as low as 10 ~ gcm 3) during sublimation of the ice. The lightweight constructs, when viewed in scanning electron microscopy micrographs, are composed of long network chains of the clay particles. The material is sufficiently electrically conductive to drain away the scanning electron microscopy charge. It is also resistant (no change in electronic properties are apparent) to scanning electron microscopy electron-beam heating for hours in vacuo. Infrared spectra and X-ray diffraction patterns of the sublimate residues show little difference from spectra and patterns of the original minerals. Heating in an oven, in air, to 370°C produces little change in the structure of the sublimate residual material. The particle bonding forces are strong and produce a resilient, elastic lightweight material. The material is porous and will allow vapors to diffuse through it, and its thermal conductivity is very low. These properties produce a high-performance vacuum insulation. This material may have applications for insulating ice bodies (solid cryogens) in space. The incoming heat is partially carried away by the out-flowing water vapor. ~ 1986 Academic Press. Inc.

Degradation of Victoria Crater, Mars

[1]xa0The ∼750 m diameter and ∼75 m deep Victoria crater in Meridiani Planum, Mars, is a degraded primary impact structure retaining a ∼5 m raised rim consisting of 1–2 m of uplifted rocks overlain by ∼3 m of ejecta at the rim crest. The rim is 120–220 m wide and is surrounded by a dark annulus reaching an average of 590 m beyond the raised rim. Comparison between observed morphology and that expected for pristine craters 500–750 m across indicates that the original, pristine crater was close to 600 m in diameter. Hence, the crater has been erosionally widened by ∼150 m and infilled by ∼50 m of sediments. Eolian processes are responsible for most crater modification, but lesser mass wasting or gully activity contributions cannot be ruled out. Erosion by prevailing winds is most significant along the exposed rim and upper walls and accounts for ∼50 m widening across a WNW–ESE diameter. The volume of material eroded from the crater walls and rim is ∼20% less than the volume of sediments partially filling the crater, indicating eolian infilling from sources outside the crater over time. The annulus formed when ∼1 m deflation of the ejecta created a lag of more resistant hematite spherules that trapped <10–20 cm of darker, regional basaltic sands. Greater relief along the rim enabled meters of erosion. Comparison between Victoria and regional craters leads to definition of a crater degradation sequence dominated by eolian erosion and infilling over time.

Geology of the Melas Chasma landing site for the Mars Exploration Rover mission

[1]xa0The Melas Chasma landing site was considered a high-priority site for the Mars Exploration Rover (MER) mission because of the opportunity to land on and study potential layered sedimentary deposits. Though no longer considered a candidate site because of safety concerns, the site remains a scientifically interesting area that provides insight into the geologic history of Valles Marineris. Within the landing ellipse are dunes, landslide material, and unusual blocky deposits. The blocky deposits are composed of rounded blocks, some of which have meter-scale layering, and they show evidence of ductile deformation, including bending and distortion of coherent blocks around each other. The morphologic characteristics of the blocks are unique, and they appear to have no terrestrial analogue. However, the gross morphology of these blocky deposits and their superposition on adjacent wallrock is consistent with the blocks having been transported downslope. Given the existence of other large mass failures in the area, we propose the blocky deposits may also have originated from mass movement events. The size of the blocks coupled with the distances they traveled indicates high mobility. The distances the blocks were transported and their rounded, irregular shapes suggest either water in the source material or deposition in a subaqueous environment. The source for the main blocky deposit inside the landing ellipse is considered to be the wallrock to the south, while the other two blocky deposits have source regions along the northern canyon walls. The southern wallrock of Melas Chasma contains numerous valleys not seen elsewhere in Valles Marineris. The identification of valley networks along the southern wallrock suggests that a source of water existed below the surface of the plateau and produced the valleys after intersecting the edge of the exposed canyon walls.

Analysis of MOLA data for the Mars Exploration Rover landing sites

[1]xa0We have used Mars Orbiter Laser Altimeter (MOLA) data to demonstrate that selected landing sites meet the Mars Exploration Rover (MER) landing system topography and slope requirements at hectometer and kilometer scales. To provide a comprehensive analysis, we constrained slopes within each landing ellipse using four approaches: (1) measurements of local slopes at 1.2 km length scales using both an adirectional maximum gradient method and a higher-resolution bidirectional, along-track method, (2) predictions of 100 m slopes using self-affine statistics in conjunction with (3) calculations of both pulse width and slope corrected pulse width to constrain slopes at scales smaller than the MOLA spot size (<180 m), and (4) comparisons of simultaneously acquired MOLA data with Mars Orbiter Camera data to identify the geomorphologic features associated with variations in observed slopes, pulse width, and, for this analysis only, reflectivity. The results of the analysis indicate that the selected landing sites are consistent with the MER topography requirement of being below −1.3 km, as well as having slopes less than 5° at length scales of 100 m and <2° at length scales of 1.2 km.

Surface processes recorded by rocks and soils on Meridiani Planum, Mars: Microscopic Imager observations during Opportunity's first three extended missions

The Microscopic Imager (MI) on the Mars Exploration Rover Opportunity has returned images of Mars with higher resolution than any previous camera system, allowing detailed petrographic and sedimentological studies of the rocks and soils at the Meridiani Planum landing site. Designed to simulate a geologists hand lens, the MI is mounted on Opportunitys instrument arm and can resolve objects 0.1 mm across or larger. This paper provides an overview of MI operations, data calibration, and analysis of MI data returned during the first 900 sols (Mars days) of the Opportunity landed mission. Analyses of Opportunity MI data have helped to resolve major questions about the origin of observed textures and features. These studies support eolian sediment transport, rather than impact surge processes, as the dominant depositional mechanism for Burns formation strata. MI stereo observations of a rock outcrop near the rim of Erebus Crater support the previous interpretation of similar sedimentary structures in Eagle Crater as being formed by surficial flow of liquid water. Well-sorted spherules dominate ripple surfaces on the Meridiani plains, and the size of spherules between ripples decreases by about 1 mm from north to south along Opportunitys traverse between Endurance and Erebus craters.