342 Encyclopedia of the Solar System
FIGURE 16 False color mosaic of star sand dunes in the bottom
of Endurance crater. Dark bluish surface is basalt with a surface
lag of hematite spherules. Lighter sides of dunes are likely dust
that has settled from the atmosphere. Note the light-toned
outcrop in the foreground.
enough to be identified from orbit. Sand dunes have been
identified at theMars Pathfinderlanding site where a small
barchan dune was discovered in a trough by the rover and
at Meridiani Planum where star dunes were found at the
bottom of Endurance crater (Fig. 16).
Ripples are eolian bedforms formed by saltation-induced
creep of granules, which are millimeter-sized particles.
They typically have a coarse fraction of granules at the crest
and poorly sorted interiors indicating a lag of coarser grains
after the sand-sized particles have been removed (Fig. 17).
Ripples have been found at theMars Pathfinder, Gusev,
and Meridiani sites. Drifts of eolian material have also been
identified at many of the landing sites behind rocks as wind
tails and other configurations. Finally the reddish dust on
Mars is only several micrometers in diameter and is car-
ried in suspension in the atmosphere giving rise to the om-
nipresent reddish color. Although it takes high winds to en-
train dust-sized particles in the atmosphere, once it is in the
atmosphere it takes a long time to settle out. Dust has been
identified on the surface at all the landing sites (in addition
to being in the atmosphere), giving everything a reddish
color, and has fallen on the solar panels decreasing solar
power. Dust devils, or wind vortices, have been observed at
theMars Pathfinderand Gusev sites and appear to be an
important mechanism for lifting dust into the atmosphere.
4.6 Craters
Impact craters are ubiquitous on Mars, so it is no surprise
that craters have been imaged at most of the landing sites.
AtViking 1(Fig. 6) and theMars Pathfinderlanding sites,
the uplifted rims of craters have been imaged from the
side. At Gusev (Fig. 10) and Meridiani (Fig. 11), the rovers
have investigated a number of craters of various sizes during
their traverses, including the interiors of some. Because im-
pact craters resemble nuclear explosion craters and because
many fresh craters have been characterized on the Moon,
much is known about the physics of impact cratering and
FIGURE 17 Large ripple called Serpent that was studied by
Spiriton the cratered plains. (A) A Hazard Camera image
showing the rover front wheels and the tracks produced by a
wheel wiggle maneuver to section the drift. (B) Color image of
the dusty (reddish) surface and darker more poorly sorted
interior. (C) MI image of the brighter (dust cover) granule-rich
surface (millimeter-sized particles) and poorly sorted, but
generally finer grained, basaltic sand interior. The dusty,
granule-rich surface indicates the eolian feature is an inactive
(dust cover) ripple formed by the saltation induced creep of
granules, which are left as a lag.