Mars Global Surveyor
Mars Orbiter Camera

One of the most profound discoveries that would not have been possible if the Mars Global Surveyor (MGS) mission had not been extended beyond its 1 Mars year (687 Earth days) Primary Mission is that of dramatic changes that take place in the south polar residual ice cap each martian year. To make this discovery, the MGS Mars Orbiter Camera (MOC) had to be employed during a second Mars year to repeat images of sites on the south polar cap that had been imaged during the Primary Mission.

The initial discovery was made in 2001, when the MOC team repeated images of portions of the south polar cap that had already been imaged in 1999. The goal of these images was to obtain stereo views, which would allow investigators to see the topography of the cap in three dimensions (3-D) and to measure the thickness of the polar ice layers.

It was not possible to produce the desired 3-D views. To the team's surprise, the landforms of the south polar cap had changed.

The south polar residual cap -- that is, the portion of the ice cap that remains bright and retains ice throughout the southern summer season -- was seen in 1997 and 1999 MOC images to have a complex terrain of broad, relatively flat mesas, small buttes, and many pits and troughs. Pits are generally circular and in some areas visually resemble a stack of thin slices of Swiss cheese. Very early in the MGS mission, the MOC team speculated that these landforms must be carved into frozen CO₂, because they look so unfamiliar and because Viking orbiter infrared measurements indicated that the south polar cap is cold enough consist of frozen carbon dioxide, even in summer.

The observations made by MOC in 2001, during the first part of the MGS Extended Mission, showed that the scarps and pit walls of the south polar cap had retreated at an average rate of about 3 meters (~10 feet) since 1999. In other words, 3 meters per Mars year (and, of course, most of that retreat takes place during the summer). In some places on the cap, the scarps will retreat less than 3 meters a Mars year, and in others it can retreat as much as 8 meters per martian year.

Of the two materials that one is likely to encounter in a frozen state on Mars -- water and carbon dioxide -- it is CO₂ that is volatile enough to permit scarp retreat rates like those observed by MOC.

Over time, south polar pits merge to become plains, mesas turn into buttes, and buttes vanish forever. Since 2001, two additional Mars years have elapsed. A scientific benefit of having a long MGS Extended Mission has been the opportunity to use MOC to document how the polar cap is changing each year.

Four images are shown here. Each is located near 86.3°S, 49.4°W, and each shows the same portion of the south polar residual cap as it appeared in 1999, 2001, 2003, and 2005. The animated GIF image shows a "movie" recording the changes that have occurred during the past four martian years. The landscape of the south polar cap is changing very rapidly.

Each year that MGS has been in orbit, the landforms of the south polar residual cap have gotten smaller, and the CO₂ removed from the cap has not been re-deposited. The implication is that Mars presently has a warm (and warming?) climate, with new carbon dioxide going into the atmosphere every year. The other implication is that, at some time in the not-too-distant past, the planet had a colder climate, so that the layers of CO₂ could be deposited in the first place. If one takes the rate of scarp retreat and projects it backwards to fill in all of the pits and troughs with the carbon dioxide that has been removed from them, one finds that the colder climate might only have occurred a few centuries to a few tens of thousands of years ago. This kind of time scale is not unlike that of the climate changes that have been recorded on Earth, including the Ice Ages and the smaller fluctuations that have occurred since the last Ice Age (e.g., the "Little Ice Age" of the mid-14th through mid-19th centuries).

After the discovery that the pits were enlarging and that we were not seeing CO₂ deposition, it was suggested that interannual variations might be large enough to permit such deposition on a short timescale. However, two Mars years of additional observations show no large magnitude annual differences. Variations that would permit CO₂ deposition may require decades. And to see such variations may require many more Mars years of observations by orbiting spacecraft.

Reference Material:

• The discovery of south polar residual cap scarp retreat, and the implication that this requires the materials to be largely composed of carbon dioxide ice, was first detailed in 2001 by M. C. Malin and colleagues at Malin Space Science Systems:
  Malin, M. C., M. A. Caplinger, and S. D. Davis (2001), Observational evidence for an active surface reservoir of solid carbon dioxide on Mars, Science, 294, 2146-2148, doi:10.1126/science.1066416 -- (Click HERE for a no-cost link to the paper)
  
  
• Previous animated GIFs showing south polar scarp retreat:
  1. The Changing South Polar Cap of Mars: 1999-2005, 13 July 2005
  2. MOC Observes Changes in the South Polar Cap: Evidence for Recent Climate Change on Mars, 6 December 2001