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 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 "dryice"). 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 polarcap 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 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.