Martian South Polar Pits in Layer of Frozen Carbon Dioxide
One of the most profound benefits of being able to continue
photographing Mars in the Mars Global Surveyor (MGS) Extended Mission
is the opportunity to go back and re-image a site that was seen in the
previous martian year. New MGS Mars Orbiter Camera (MOC) images have
provided a startling observation: The residual martian south polar cap is
changing. The fact that it is changing suggests that Mars may have major,
global climate changes that are occurring on the same time scales as
Earth's most recent climate shifts, including the last Ice Age.
MOC images of the south polar cap taken in 1999 were compared with
images of the same locations taken in 2001, and it was discovered that
pits had enlarged, mesas had shrunk, and small buttes had vanished. In
all, the scarps that enclose the pits and bound the mesas and buttes
retreated about 3 meters (3.3 yards) in 1 martian year (687 Earth days).
This rapid retreat of polar scarps can only occur if the ice is frozen carbon
dioxide (also known as "dry ice"). Retreat of scarps made of
water ice is much slower and would not have been measurable from one
martian year to the next.
The portion of the martian south polar cap that persists through
summer is called the residual polar cap. The two sets of four pictures
shown here are from four places on the residual south polar cap. The
pictures from 1999 were taken in October of that year, the corresponding
pictures from 2001 were acquired in August, approximately 1 Mars year
after the 1999 images were obtained. In each case, the pictures are
illuminated by sunlight from the upper left, and each shows an area
about 250 meters (273 yards) across. The polar cap is layered, and the
layers have eroded to form pits, troughs, mesas, and buttes. The pits
form as sunlight warms frozen carbon dioxide during southern spring
and summer, and the ice sublimes away. There is so much carbon
dioxide that it does not all go away in one summer---in fact, it may
take hundreds to thousands of years to disappear.
These new observations indicate that the south polar residual cap is
not permanent. It is disappearing, a little bit more each southern spring
and summer season. At the present rate, a layer 3 m thick can be
completely eroded away in a few tens of martian years. Since each
layer is equivalent to about 1% of the mass of the present atmosphere
(which is 95% carbon dioxide), if sufficient carbon dioxide is buried in the
south polar cap, the mass of the atmosphere could double in a few
hundred to a thousand Mars years. That could lead to profound changes
in the environment. For example, it would change how much and where
wind erosion would occur, and where and for how long liquid water could
survive at or near the surface.
It also means that Mars may have been very different in the recent
past (perhaps only a few thousands of years ago). On today's Mars, the ice
is eroding, but in the past that material had to have been deposited. The
martian climate was probably colder, and there was more carbon dioxide
in the atmosphere. For some reason, large amounts of carbon dioxide froze
at the south pole---one might say that there was a "Martian Ice
Age"---and this freezing occurred on a time scale similar to that of
the most recent Ice Age on Earth.
Mars is changing, and it is changing on a time scale that we can
measure and observe. If all of the carbon dioxide that is being released
into the atmosphere from the south polar cap is not freezing out
somewhere else, and if it is not being adsorbed into the martian soil, then
it must be causing the atmospheric pressure to increase. If this is so, and
if one were to assume that the entire known volume of the polar cap is
made of carbon dioxide that sublimes at the same rate we see today,
then it could increase the martian atmospheric pressure by as much as
10 times, to about 1/10th the density of Earth's atmosphere, in just the
next few thousand years. Although this atmosphere would not be
breathable, carbon dioxide is a "greenhouse gas" that would
cause the global temperature to increase considerably and make it easier
for liquid water to persist elsewhere on the planet. Perhaps, just perhaps,
a thickening martian atmosphere would eventually make it easier for
people to live on Mars.
This new MGS MOC discovery is described in a paper published
December 7, 2001, in the journal, Science. Read a more
detailed discussion
of these results (but less detailed than the Science article).
Images Credit: NASA/JPL/Malin Space Science Systems
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