The Mission Team

Profile: Dr. Philip Christensen/THEMIS

Professor Philip Chirstensen, Arizona State University, Tempe
Principal Investigator, Thermal Emission Imaging Spectrometer on Mars Odyssey

Arizona State University's Dr. Philip Christensen, professor of planetary geology, is the scientist responsible for the two-camera thermal emission imaging system called THEMIS on Mars Odyssey. The spectrometer-camera system identifies the mineral content of rocks and other surface materials based upon their unique spectral signatures from the infrared heat they emit, and by from reflected sunlight.

Themis will be Christensen's second instrument operating at Mars; he already has one experiment -- the thermal emission spectrometer -- currently in orbit around Mars on NASA's Mars Global Surveyor. Measuring the infrared signatures of minerals on Mars' surface, it has produced the first global mineral map of Mars, laying out in a very real sense the groundwork for scientists to track down the history of water on Mars.

Now Odyssey's imaging spectrometer will follow up, identifying where ancient, wet environments once existed on Mars. These are the places where there might be signs of life past or present.

Beyond the two instruments now in space, Christensen and his Arizona team are preparing a smaller version of his instrument in duplicate. These will be installed on the two NASA Mars Exploration Rovers scheduled for launch in 2003. He and his team had an earlier imaging spectrometer experiment on the Mars Observer spacecraft lost in 1993. "That failure made me, and all of the students, scientists, programmers, and data processors who worked on Mars Observer, very aware of how complex and risky sending spacecraft to other planets can be," says Christensen. His Arizona State University lab wasoriginally built to operate the spectrometer on Mars Observer and analyze its data. After the loss, the lab remained in use and has given birth to more and more instruments and more and more trained planetary geologists under Christensen's tutelage. "Fortunately, it made all of us even more determined to make the project work and to carry on the exploration of Mars," says Christensen. Appropriately, his lab is located in greater Phoenix.

Relating Rocks to Water in Mars' Past

The search for life on Mars right now focuses on seeking out watery places of the past. Environments of two particular types are being sought: those containing ancient groundwater, or ancient, hot water environments. The organisms that live in these environments on Earth don't need sunlight, oxygen, or to live on the surface.

"The chemistry that goes on there is ideally suited to very simple organisms to get started," Christensen says. "It's a wonderful environment for them. There's a lot of energy around. It's easy living."

These environments can exist several miles beneath the surface, where water comes in contact with hot, volcanic rock. Finding basaltic rocks on Mars' surface with Christensen's Global Surveyor experiment helped target areas for a closer look with Odyssey's imaging system. Those rocks may reveal telltale minerals that form when hot water runs over basalt.

With Global Surveyor data, Christensen found that the surface materials are primarily volcanic. 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. On Earth, most of the erupted lavas that are basaltic, and when weathered by the circulation of hot water, they contain telltale minerals. The team hopes to find those distinctly water related minerals in the basaltic rocks of Mars.

The results from Mars Global Surveyor's thermal emission spectrometer have provided the mineralogical roadmap for geologists and geochemists following the trail to water's past on Mars. "Now we'll start focusing on the places that are unique and different," says Christensen. "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," Christensen promises, "some of those places are going to be really interesting."