Surficial Deposits at Gusev Crater Along Spirit Rover Traverses

Abstract

The Mars Exploration Rover Spirit has traversed a fairly flat, rock-strewn terrain whose surface is shaped primarily by impact events, although some of the landscape has been altered by eolian processes. Impacts ejected basaltic rocks that probably were part of locally formed lava flows from at least 10 meters depth. Some rocks have been textured and/or partially buried by windblown sediments less than 2 millimeters in diameter that concentrate within shallow, partially filled, circular impact depressions referred to as hollows. The terrain traversed during the 90-sol (martian solar day) nominal mission shows no evidence for an ancient lake in Gusev crater.Gusev crater is 160 km in diameter, is of Noachian age, and lies at the terminus of the 900-km-long branching Ma'adim Vallis. The crater is partially filled by Hesperian-aged materials (1) and was selected as the landing site for the Mars Exploration Rover Spirit to search for evidence of previous liquid water flow and/or ponding that may be responsible for the crater infilling (1-8).Landing occurred on a generally flat plain (14.5692°S, 175.4729°E) characterized by approximately circular, shallow depressions Ͻ20 m in diameter [hollows (9)] (Fig. 1A) and poorly defined ridges up to hundreds of meters long and a few meters high (Plate 1). The 210-m diameter Bonneville crater (9) is ϳ300 m northeast of the lander, and surface albedo increases from ϳ0.19 to ϳ0.26 toward its rim as a result of increased dust mantling (10). Bonneville and its ejecta deposits comprised the primary exploration targets along the 506-m traverse during the nominal mission reported here.The largest rocks within 20 m of the lander are Ͻ0.5 m in diameter, smaller than the largest rocks at the three previous Mars landing sites (11,12). Rocks Ͼ1 cm cover about 5% of the surface, and the area covered by fragments Ͼ10 cm is ϳ50% of the total rock-covered area (12). The size-frequency distribution of rocks Ͼ1 cm generally follows the exponential model distribution based on the Viking Lander and Mars Pathfinder landing sites for 5% rock abundance (1, 11).Most rocks Ͼ1 cm are angular to subangular (13) and of variable sphericity (13), and almost none display obvious rounding (14). The majority of rocks are intrinsically dark gray in color, but some exhibit variable dust coverings and possibly associated weathering coatings or rinds that impart a light-toned and/or reddish color, especially apparent on the lowermost 5 to 15 cm of their surfaces (10-12, 15, 16) (Plate 9). Bulk-rock compositions are consistent with picritic (olivinerich) basalt (16).Rocks Ͼ15 cm and within 20 m of the lander are mostly around hollows or near drift deposits, whereas they are largely absent within hollows (Fig. 1A). Rocks sitting exposed or perched on the surface are up to 10 times as numerous around the immediate exterior of hollows as elsewhere (Fig. 1A). Fractured and split rocks are also concentrated around the hollows, but lighter toned (redder) rocks are often closer to eolian drifts. Faceted rocks are five to e...

Wind-Related Processes Detected by the Spirit Rover at Gusev Crater, Mars

“…We observed an increase in rock size and frequency around Bonneville crater, reflecting ejecta that was thrown out of the crater during an impact event (25). There is a corresponding increase in drift deposits around Bonneville (Fig.…”

The Spirit Rover's Athena Science Investigation at Gusev Crater, Mars

“…The soils on the Gusev crater plains explored by Spirit are ubiquitous; they cover flat regions as well as depressions that we have termed hollows (see below), and also exist as discrete drifts of probable eolian origin (24)(25)(26). A thin cover of dust-sized particles of likely airborne origin covers much of the soil and rocks observed around Columbia Memorial Station and during traverses to and from the rim of Bonneville crater.…”

Pancam Multispectral Imaging Results from the Spirit Rover at Gusev Crater

“…More distant features include a ridge of low-lying (ϳ100 to 150 m high) hills, called the Columbia Hills (Plate 4), ϳ3 to 4 km east of the lander and isolated knobs and mesas ranging from ϳ8 km to ϳ26 km south to southwest of the lander near the mouth of Ma'adim Vallis. There were more rocks on the Bonneville crater ejecta blanket, with ϳ10% of the area covered by rocks (13,14 ). The local relief around the landing site exhibits meter-scale undulations interspersed with small (Ͻ 200-m diameter), low-relief (Ͻ 15-m depth) impact craters and much smaller (tens of m in diameter) circular hollows.…”