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Mars Global Surveyor
Mars Orbiter Camera

8 Years at Mars #4: Four Mars Years of South Polar Cap Scarp Retreat

MGS MOC Release No. MOC2-1223, 20 September 2005

  MOC2-1223a: "Movie" --  MOC2-1223b: 1999 view -- MOC2-1223c: 2001 view --  MOC2-1223d: 2003 view -- MOC2-1223e: 2005 view
Animated GIF showing changes in south polar scarp positions, as they evolved from 1999 to 2001 to 2003 to 2005. View of the south polar scarps in 1999 View of the south polar scarps in 2001 View of the south polar scarps in 2003 View of the south polar scarps in 2005 NASA/JPL/Malin Space Science Systems

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.

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Malin Space Science Systems and the California Institute of Technology built the MOC using spare hardware from the Mars Observer mission. MSSS operates the camera from its facilities in San Diego, California. The Jet Propulsion Laboratory's Mars Surveyor Operations Project operates the Mars Global Surveyor spacecraft with its industrial partner, Lockheed Martin Astronautics, from facilities in Pasadena, California and Denver, Colorado.