Scientists have known for years that the polar caps on Mars shrink and grow between the Martian summer and winter seasons. But they wondered just how much carbon dioxide 'snow' is deposited each season? Are the frost deposits more like snow or more like ice? What role do the ice caps play in seasonal changes on Mars? Now new research from MOLA, the laser altimeter aboard the Mars Global Surveyor, may provide answers to these questions.

Elevation measurements taken by MOLA in combination with gravity determined by tracking the MGS spacecraft have been used to measure seasonal changes in the thickness of seasonal frost deposits and to provide the first direct measurement of their density.

Over the course of a Martian year, which consists of 687 Earth days, as much as a third of Mars' tenuous carbon dioxide (CO2) atmosphere "freezes out" at the polar caps during the winter in the northern and southern hemispheres. A team consisting of Dr. David E. Smith of NASA's Goddard Space Flight Center (Greenbelt, Md.), and Drs. Maria T. Zuber and Gregory A. Neumann of the Massachusetts Institute of Technology (MIT), used MOLA data to measure the changes in the height of the polar caps every two weeks. They showed that small changes in surface height correlate with the expected times of deposition and evaporation of CO2 from the surface. The group also measured miniscule changes in the gravity field of Mars due to the movement of carbon dioxide, which allowed them to make the first measurement of the density of surface frost deposits.

The MOLA results show that a maximum of 1.5 to 2 meters of frozen material are deposited during the Martian winter. During the summer, the frozen CO2 evaporates and returns to the atmosphere. These two graphs show the maximum amount of 'snow' that accumulated over the northern hemisphere

"Snow on Mars is composed of dry ice and last year it reached a maximum depth of 1.5 to 2 meters near both the north and south poles," explained Smith, Principal Investigator of the laser altimeter investigation and lead author of the study published in the December 7 issue of Science.

"Snow on Mars is denser than it is on Earth, being more ice-like than powdery," noted Zuber, the Deputy Principal Investigator of the altimeter investigation. To isolate the signals, the group measured changes of 10 centimeters in the height of the surface of Mars, and changes of a couple parts per billion in the planet's global mass distribution.

MOLA data was also used to track elusive Martian clouds linked to the frozen deposits. The thin lines in these images are the tracks of the laser bouncing off of the clouds. When the tracks are added together, we can see how the range of the clouds (white overlay) change during the Martian year.

Carbon dioxide is overwhelmingly the most abundant gas in the martian atmosphere and an important greenhouse gas. The studies of the CO2 clouds will allow a better understanding of the present atmosphere, which improves the ability to target future landers, and provides important information for understanding the evolution of the atmosphere over time. "Characterizing the present-day behavior of CO2 is a necessary first step towards understanding the past climate on Mars," noted Neumann.

Links & Resources

For more information, including hi-res images, please see the press release this article was adapted from.

See also Seasonal Variations of Snow Depth on Mars, Science, Vol 294, December 7, 2001.

For more figures and animations, see: http://tharsis.gsfc.nasa.gov/MOLA/snow_paper.html