Aeolian processes at the Mars Exploration Rover Meridiani Planum landing site

Abstract

The martian surface is a natural laboratory for testing our understanding of the physics of aeolian (wind-related) processes in an environment different from that of Earth. Martian surface markings and atmospheric opacity are time-variable, indicating that fine particles at the surface are mobilized regularly by wind'"'. Regolith (unconsolidated surface material) at the Mars Exploration Rover Opportunity's landing site has been affected greatly by wind, which has created and reoriented bedforms, sorted grains, and eroded bedrock. Aeolian features here preserve a unique record of changing wind direction and wind strength. Here we present an in situ examination of a martian bright wind streak, which provides evidence consistent with a previously proposed formational model'''^ for such features. We also show that a widely used criterion for distinguishing between aeolian saltation-and suspension-dominated grain behaviour is different on Mars, and that estimated wind friction speeds between 2 and 3ms~', most recently from the northwest, are associated with recent global dust storms, providing ground truth for climate model predictions.Pre-landing orbiter observations of Opportunity's landing site showed bright and dark streaks tapering away from craters on the Meridiani plains. From observations of similar features distributed across many locations on Mars, streak orientations indicate formative wind directions'""*. High-resolution images within the 117 km X 18 km landing ellipse obtained over several years show bright streak directions that indicate winds from the northwest and southeast'. What has not been recognized previously, however, is that this apparent bimodality of wind direction has a significant time dependence. Images obtained before the major 2001 dust storm are more likely to show bright streaks oriented in the opposite direction from images obtained after the storm waned. Rare overlapping image pairs provide two examples of an individual streak changing orientation after the intervening 2001 dust storm (Fig.

Transverse Aeolian Ridges (TARs) on Mars

“…Some cohesion exists in the soil, based on observations of trench walls. Inter-ripple areas are dominated by 50-125 μm grains of basaltic sand and additional concretion fragments and intact concretions several millimeters in diameter (Sullivan et al, 2005). As the rover continued south, the bedforms became larger (as shown in Fig.…”

“…14e and f), and the observed fragments of concretions armoring the ripples became smaller (Sullivan et al, 2007). In addition to these large ripples, Sullivan et al (2005) noted another population of smaller (∼10 cm wavelength) ripples that formed almost entirely within basaltic sands trapped in local topographic hollows such as craters, pits, and troughs. These small ripples have orientations perpendicular to the current wind direction, inferred from observations of transient wind streaks from Eagle crater.…”

“…The bedforms on the plains are more akin to terrestrial granule ripples than dunes (e.g., Sullivan et al, 2004) because of their absence of slipfaces, low slope angles, and particle size distribution. In particular, wheel trenches reveal that the crests of these ripples are dominated by a monolayer of 1-2 mm fragments of spheroidal concretions (Sullivan et al, 2005) thought to be mainly composed of hematite , but the interior of the ripples, and the regolith in general, is composed of a poorly sorted mix of ∼ 100 μm sand particles, dust particles and a few larger clasts (Sullivan et al, 2007). Some cohesion exists in the soil, based on observations of trench walls.…”

風成砂丘から見る火星およびタイタンの大気循環システム

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“…MGS-MOC HiRISE e.g., Fenton et al, 2005;Hayward et al, 2007;Greeley et al, 2008;Hobbs et al, 2010;Silvestro et al, 2010;Chojnacki et al, 2011GCM Hayward et al 2007 Mars Greeley and Thompson, 2003;Fenton et al, 2005;Hayward et al, 2007;Taniguchi & Endo, 2007;Greeley et al, 2008;Hobbs et al, 2010;Silvestro et al, 2010;Chojnacki et al, 2011 DCA SF e.g., Thomas et al, 1984;Greeley et al, 1993;Greeley and Thompson, 2003;Sullivan et al, 2005 elongate ellipsoidal fan-shape 3 Greeley et al, 1993 Mariner 9 …”