In a paper published March 3 in the journal Science, Arizona State University geologists Josh Bandfield, Vicky Hamilton, and Philip Christensen report the first results of a major survey being performed by the Mars Global Surveyor's Thermal Emission Spectrometer (TES) of the planet's mineral composition.

TES measures the thermal infrared energy (heat) emitted from Mars and records the unique infrared spectra emitted by specific minerals. Scientists then use this data to map the mineral composition of the planet's surface.

The instrument captures spectra covering 3 km. patches of the Martian surface. With the mapping now about 75% complete, the instrument "has taken about 100 million spectra in about a year," said Christensen, and the planet should be completely mapped within the next four months.

"From those spectra we have made a detailed map showing mineral abundance. These are the first mineral maps ever made of Mars," Christensen noted. "As of now, we have looked at the basic composition of the Martian surface -- it's an overall view.

"What we've found is that the surface materials are primarily volcanic. People have suspected that this was the case, but we've finally been able to clearly demonstrate it."

There is volcanic rock though, and there is volcanic rock, Christensen points out, and the differences in the rocks' compositions can have very large implications about planetary history.

"What's particularly interesting is the surfaces in the southern hemisphere -- which tend to be very ancient, heavily cratered surfaces dating back to near when Mars was first formed -- are primarily basaltic in composition," said Christensen.

"On earth, most of the lavas that are erupted are primarily basaltic. These are the result of slow, non-explosive lava flooding."

"The northern hemisphere of Mars is much younger -- it may only be a billion years or less in age. Those rocks are still volcanic, but have a different composition -- it's a rock called andesite. On earth, these tend to erupt in a very different way in places like the Andes Mountains or Mount St. Helens."

"To a geologist, this change in materials says that the interior of Mars has changed. We're still debating what changed and why it occurred, but it's fascinating that the change did occur." Christensen believes that the change is likely to be due to a change in water content deeper in the planet's interior, but it could also be caused by the Martian crust getting thicker.

"Now it's the geochemists turn to get excited. We're providing them with data to work with and they get to design models to try to explain why this happened."

Christensen and his team are likely to have some exciting science of their own to contemplate in the months to come.

"This map is a baseline. Now we'll start focusing on the places that are unique and different," he said.

"It's like if you sent a survey out to map the west and you discover the Rocky Mountains and the Sierra Nevadas and the great deserts -- you do an inventory of what's out there, and then you start focusing on places like Yellowstone, places that are unique and different from every place else.

"On Mars, some of those places are going to be really interesting," he said.