Bushes on Mars ?
Scientists: Defrosting dunes
on Mars look like bushes
MSSS PHOTO RELEASE
"They look like bushes!" That's what
almost everyone says when they see
the dark features found in pictures
taken of sand dunes in the polar
regions as they are beginning to defrost
after a long, cold winter.
It is hard to escape the fact that, at first
glance, these images acquired by the
Mars Global Surveyor (MGS) Mars
Orbiter Camera (MOC) over both
polar regions during the spring and
summer seasons, do indeed resemble
aerial photographs of sand dune fields
on Earth -- complete with vegetation
growing on and around them!
Of course, this is not what the features are, as we describe below
and in related picture captions. Still, don't they look like vegetation
Shown here are two views of the same MGS MOC image. On the
left is the full scene, on the right is an expanded view of a portion
of the scene on the left. The bright, smooth surfaces that are
dotted with occasional, nearly triangular dark spots are sand dunes
covered by winter frost.
The MGS MOC has been used over the past several months
(April-August 1999) to monitor dark spots as they form and
evolve on polar dune surfaces. The dark spots typically appear
first along the lower margins of a dune--similar to the position of
bushes and tufts of grass that occur in and among some sand
dunes on Earth.
Because the martian air pressure is very low--100
than at Sea Level on Earth--ice on Mars
does not melt and become liquid when it warms up. Instead,
ice sublimes--that is, it changes directly from solid to gas, just
as "dry ice" does on Earth. As polar dunes emerge from the
months-long winter night, and first become exposed to sunlight, the
bright winter frost and snow begins to sublime. This process is not
uniform everywhere on a dune, but begins in small spots and then
over several months it spreads until the entire dune is spotted like a
The early stages of the defrosting process--as in the picture shown
here--give the impression that something is "growing" on the
dunes. The sand underneath the frost is dark, just like basalt beach
sand in Hawaii. Once it is exposed to sunlight, the dark sand
probably absorbs sunlight and helps speed the defrosting of each
This picture was taken by MGS MOC on July 21, 1999. The
dunes are located in the south polar region and are expected to be
completely defrosted by November or December 1999. North is
approximately up, and sunlight illuminates the scene from the upper
left. The 500 meter scale bar equals 547 yards; the 300 meter
scale is also 328 yards.
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