Abstract
1 Linear, low-albedo patterns ( termed dark wind streaks) formed
on the floor of Gusev crater between September 2003 and February
2004, as seen on High Resolution Stereo Camera images taken on board
the Mars Express Orbiter. Pancam images from the Mars Exploration
Rover Spirit show that the rover crossed a dark streak during its
traverse to Bonneville crater. Spirit Microscopic Imager data reveal
that sand grains within the dark wind streak are relatively free
of dust, whereas grains outside the streak are mantled with dust.
During the September 2004 solar conjunction, Spirit remained in one
location from sol 240 to sol 260. Comparison of images taken before
and after the conjunction shows that patches of soil beneath the
rover darkened with respect to the adjacent soils, suggesting removal
of relatively bright material. Two MI mosaics taken 18 sols apart
of the surface within 0.5 m of the nearest dark patch show that some
larger ( 12 mm) sand grains moved as far as 0.7 mm. These observations
support the hypothesis that some dark surface patterns result from
the removal and/or repositioning of fine-grained material by winds,
exposing a relatively lower albedo substrate, such as coarse sand
grains. Other variable features on the Gusev floor seen from orbit
faded between September 2003 and February 2004 and are interpreted
to represent settling of dust from the atmosphere, consistent with
the accumulation of dust observed on Spirit. The observation of dark
streaks fading with time, while some dark streaks were newly formed,
is consistent with local wind gusts or the passage of dust devils
that locally sweep dust from the surface or cause a redistribution
of fine grains among larger particles.
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