MGS MOC Release No. MOC2-297, 6 December 2001

Martian South Polar Pits in Layer of Frozen Carbon Dioxide

Compare each image on the left with their counterparts on
the right. Small hills vanished and pit walls
expanded between 1999 and 2001.The pits are formed in frozen
carbon dioxide, and the carbon dioxide
is subliming away a little more each Martian year.
Sunlight illuminates each of the four different scenes
from the upper left.
CLICK HERE for animation
of one of these 4 panels
(1.6 MBytes)

CLICK HERE for animation
of all 4 of these panels
(6.2 MBytes)

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.

Images Credit: NASA/JPL/Malin Space Science Systems