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Mars Global Surveyor Mars Orbiter Camera

Not Vegetation! Defrosting Sand Dunes in Late Southern Winter

MGS MOC Release No. MOC2-286, 12 June 2001

As winter gives way to spring in the martian southern hemisphere,
the Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) is
observing the retreat of the south polar frost cap as sunlight falls
upon it for the first time in several months. One of the most
aesthetically-pleasing aspects of the spring defrosting process
is the pattern that is created on the martian sand dune fields.

Dunes are usually among the first surfaces to begin showing signs of
change in late winter when temperatures are just beginning to creep
above -125° C (-193° F; 148 K). The pattern of spots
on the dunes in the above MOC picture was observed on June 8,
2001. The location of the dune field near 62°S, 155°W, is
shown in the color context view, which was acquired at the same

Dark spots and streaks on defrosting sand dunes were first observed by MOC in the
northern hemisphere in 1998. Similar dark-spotted dunes
in the southern hemisphere were described at a
NASA/Mars Global Surveyor Space Science Update media briefing in 1999. Despite the “sensation” one gets
when looking at pictures of spotted, defrosting martian
dunes (i.e., the sensation that these images show
some form of life, like vegetation, growing on Mars) these features
are a normal, common manifestation of the springtime defrosting
process on Mars. The ices involved–because of the low temperatures
at these locations–are probably both frozen water and carbon
dioxide, though it is unclear as to whether one type of ice
dominates over the other in controlling the appearance and
coalescence of the dark spots. It is known from the first martian
year of MOC operations that by summer all of the frost–and thus all
of the spots–on the dunes will be gone.

North is up and sunlight illuminates the scene from
the upper left in both pictures. The color context view covers
an area approximately 115 km (72 miles) across; the high resolution
image covers 3 km by 22 km (1.9 by 13.6 mi).

Images Credit: NASA/JPL/Malin Space Science Systems