Mars may be changing

Changes in the polar caps seen over two years

By BBC News Online science editor Dr David Whitehouse

Long-term changes, possibly related to a global climatic shift, have been detected on the surface of Mars.

High-resolution images of Mars' south pole show dramatic erosion in its year-round frosty upper layers.

Astronomers have also identified a reservoir of carbon dioxide that, if released, could alter the planet completely.

The observations "herald a new era in the study of Mars," says David Paige of the University of California, US.

Global change

Pictures of pits and other features on Mars' south pole, captured by the orbiting Mars Global Surveyor (MGS), show changes in ice cover over just two years.

Analysis of the data also indicates that gas exchange between the atmosphere and the frozen ices at the poles could have significant effects on the planet's long-term climatic stability.

The climate is really dynamic and changing with time

Michael Malin, Malin Space Science Systems

"Not all the carbon dioxide that can be in the atmosphere is in the atmosphere today," says Michael Malin of Malin Space Science Systems. "This means that the climate is really dynamic and changing with time."

The size of the reservoir, still unknown, could also influence the presence of liquid water on Mars' surface.

If the reservoir is large, for example, the atmospheric pressure change brought about by its release could create conditions suitable for "liquid water to persist at or near the surface for as long as the pressure is elevated," according to Dr Malin.

Two-year survey

In 1999, MGS made its first observations of irregular to circular-shaped pits, intervening ridges, and isolated mounds carved into layers of ice at the southern pole. To look for changes in these structures over time, Malin and colleagues looked at the same region again in 2001.

Comparisons between the two sets of images suggest that many of these pits are growing and that many small features have disappeared altogether. Between 25 to 50 % of the features measured have been eroded by one to three metres (three to ten feet) during the course of the past Martian year.

Mars Global Surveyor

The amount of erosion seen in these structures suggests that the frost layers are composed of moderately dense solid carbon dioxide, rather than water ice, say the researchers writing in the journal Science.

Remarkably, based on the observed erosion rates, the pits could have been created in a Martian decade and may erode away completely within one to two decades.

"We know that the pits we see at the surface today are not very old, and that they will not last very long. These layers of carbon dioxide are very ephemeral on a geological timescale," says Dr Malin.

Over the course of a Martian year, Mars exchanges up to a third of the carbon dioxide in its atmosphere with its surface. It deposits layers of dry ice "snow" at the northern and southern polar regions during each hemisphere's autumn and winter.

Weather forecast

With the help of another instrument on board MGS scientists have measured elevation changes on the Martian surface that correlate with this seasonal cycle of snow accumulation and evaporation.

These layers of carbon dioxide are very ephemeral on a geological timescale

Michael Malin, Malin Space Science Systems

"We have measured when and where the carbon dioxide resides during the winter over the entire surface of Mars, which will allow more accurate models of Martian weather to be developed," says Nasa's Maria Zuber.

The MGS measured the elevation of the Martian landscape by beaming a pulse of laser light down to the surface and calculating the amount of time it took for the pulse to bounce back to the MGS.

The biggest elevation changes - representing differences in snow depth across the seasons - were observed close to the poles. However, the bulk of snow accumulation and evaporation appears to take place at lower latitudes, below 75 degrees in the north and 73 degrees in the south.

Dr Zuber and colleagues also measured the tiny variations in Mars' gravity field that occur as a result of the seasonal redistribution of snow between the poles.

Using this data along with the elevation observations, the researchers calculated the density of the carbon dioxide snow, determining that it is much denser than snow on Earth.