The planet Mars we know today is a cold, dry, desert world, but suppose the martian climate is changing even now, year to year and decade to decade?

South polar mesas and pits in frozen carbon dioxide. Photo: NASA/JPL/MSSS

New observations by NASA's Mars Global Surveyor spacecraft are expanding our understanding of the martian climate and may indicate the climate is changing significantly even today. This suggests even larger climate changes have occurred during the planet's recent history and may again in its future. The observations were made during a full martian year, 687 Earth days.

If this is so, Mars might someday become warmer and wetter, as some scientists suggest it was during its early history. Papers detailing these observations are published in the Dec. 7, 2001, issue of Science magazine.

"If the environment of Mars has really changed by as much and over as short a time-scale as our observation implies, there should be attributes of Mars reflecting these changes that may be measurable by landers," said Dr. Michael Malin, principal investigator for Global Surveyor's camera system at Malin Space Science Systems, San Diego. "If Mars had a higher atmospheric pressure in the not-too-distant past, it is more likely that water was present as a liquid near the surface."

Liquid water is required to support known forms of life, and the presence of liquid water on Mars would make it more likely life may once have existed there.

"Detecting evidence of climate change and variability on Mars using Mars Global Surveyor data is an important aspect of telling us where to go on the surface this decade," said Dr. James Garvin, lead scientist for Mars exploration, NASA Headquarters, Washington, D.C. "Clearly, the polar regions are a good place where we would like to look for hydrothermal vents to see if they exist on Mars."

Images from Global Surveyor's camera system show that pits -- often referred to as the "Swiss cheese" terrain -- at the southern polar ice cap of Mars have dramatically increased in diameter, indicating the material has evaporated rapidly compared to last year.

"The amount of change is much larger than any previous change we've seen on Mars, and it is much larger than can be explained by the evaporation of water ice. We have calculated the only material that could have changed this much is carbon dioxide ice, what we know as dry ice," said Malin. "This means the Mars environment we see today may not be what it was a few hundred years ago, and may not be what will exist a few hundred years in the future."

Evidence for recent climate change on Mars as seen in these images from 1999 and 2001. Photos: NASA/JPL/MSSS

A separate observation is providing more detail about the behavior of carbon dioxide in the martian atmosphere. Carbon dioxide is a "greenhouse gas" believed to warm climates when its atmospheric concentration increases. The spacecraft's laser altimeter and radio tracking system have made precise measurements of the amount and density of carbon dioxide snow in both polar regions. This information gives scientists the first global measurement of the seasonal exchange of carbon dioxide between the atmosphere and surface.

Due to the tilt of the planet, Mars has seasons just like Earth. Scientists have long known the most important seasonal change on Mars is the autumn and winter "freezing out" of carbon dioxide from the atmosphere in the form of dry-ice frost and snow. The evaporation of the surface frost in spring and summer returns carbon dioxide to the atmosphere. Over the course of a martian year, as much as a quarter of the atmosphere freezes out, but until now scientists didn't know precisely where and how much dry-ice frost and snow would pile up on the surface.

"We have measured how deep the dry-ice snow got on Mars over the course of a year. We have also measured the corresponding tiny change in the gravity field due to carbon dioxide being transported from one pole to the other with the seasons," said Dr. Maria Zuber, deputy principal investigator of the laser altimeter, at the Massachusetts Institute of Technology, Cambridge, and NASA's Goddard Space Flight Center, Greenbelt, Md.

"Snow on Mars is denser than snow on Earth and is really more like ice than snow. Understanding the present carbon dioxide cycle is an essential step towards understanding past martian climates," Zuber said.

JPL manages the Mars Global Surveyor mission for NASA's Office of Space Science, Washington, D.C. JPL is a division of the California Institute of Technology, Pasadena.