USGS space scientist Timothy Titus and his colleagues Hugh Kieffer of USGS and Philip Christensen of Arizona State University noted that although it has long been known that water ice should be present in the southern polar region of Mars, until recently little evidence for water ice had been found.

Previously, surface water ice had been documented on the northern polar cap of Mars, but this is the first time exposed water ice has been documented on the southern polar cap of the solar system's fourth planet.

Titus and his colleagues used images from the Mars Odyssey Thermal Emission Imaging System (THEMIS) and temperature data from the Mars Global Surveyor Thermal Emission Spectrometer (TES) to confirm the presence of water ice at the surface of the southern polar cap.

In addition, the Mars Odyssey Gamma Ray Spectrometer (GRS) observations showed that there is quite a bit of water ice buried beneath the surface in the southern polar region, but because GRS could not "identify" exposed water ice at the surface when the ice is only a few miles wide, scientists were not sure if exposed water ice existed at the surface of the southern polar regions of Mars.

Enter THEMIS and TES -- with their high-resolution images and sensitive temperature-monitoring data, THEMIS was "able" to tell the scientists where to look for possible water areas. "When we first saw the images from THEMIS, we noticed that areas that were dark were not all the same temperature, which suggested that the areas were composed of different stuff, perhaps even water ice," Titus said.

The scientists then looked at TES data that overlapped the THEMIS images and found that in one area, called Unit I, the water ice warmed up slowly in the summer after the dry ice covering had sublimated away. (Under martian conditions water ice does not melt, it goes directly from solid to a gaseous state, a process called sublimation.)

The temperature remained under about -90 degrees Fahrenheit, the hottest martian ice gets and about the temperature of the northern summer ice cap on Mars, which is composed of dirty water ice (ice mixed with dirt and dust).

"On the southern polar ice caps, the differences between daytime and nighttime temperatures were small, which also suggested to us that the "stuff" might be water ice," Titus said.

Titus and his colleagues also examined unit S, located adjacent to unit I. It showed a different trend in temperatures than unit I. In unit S, as temperatures warmed early in the Mars summer, the dry ice covering changed from solid ice to gas much earlier than in unit I, and in a matter of a few days or so.

Suddenly, said Titus, daytime temperatures jumped and the nighttime temperature stayed the same, which told us that as the dry ice sublimated, probably what was left behind was a 2-7 mm layer of dust over ice.

"This suggests that the top layer changed from dirty water ice to dry dust," Titus said. "The cool nighttime temperatures are what one would expect from having a layer of water ice underneath the thin layer of dust."

Titus and his colleagues are excited about the implications of these findings. "In some ways, this water ice may just be the 'tip' of the iceberg," Titus said. "The speculation is that there may be a whole mass of water ice underneath the southern polar cap."

Determining the abundance and distribution of surface and near-surface water ice is fundamental for both understanding the water cycle of Mars and for the future exploration of Mars, Titus noted in the paper. Water ice, at or near the surface, is available for surface interactions and exchange with the atmosphere, causing, for example, atmospheric changes such as water vapor and water ice clouds.

In addition, water ice that is in the top few inches of soil will most likely be accessible to future robotic probes and, ultimately, human exploration.

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