Space / Mars

"HiRISE is built to look at shapes in surface features, so it collects all the light and uses it for spatial resolution," says Scott Murchie, the project scientist for CRISM. "We have to collect light over a bigger patch so we can slice it up into many different colors." The color data will tip off scientists to the presence of clays, sulfates, and other minerals that may have precipitated out of ancient lakes. It will also yield the most detailed global information yet on geologic layering, one key to figuring out how Mars evolved over billions of years.

MRO's eyes will examine not only the surface but also above and below it. The Shallow Radar experiment will peer 30 feet or more below the Martian surface to detect buried water ice; another instrument, an infrared radiometer, will monitor dust storms and other atmospheric disturbances. The craft itself will act as an instrument to measure the behavior of carbon dioxide, which is present on Mars both as a gas and as "snow." The trick is to detect minute fluctuations in the planet's gravity field that occur as the mass of carbon dioxide shifts between the atmosphere and the surface. Engineers on Earth will continuously track a radio beam emitted by MRO and monitor its Doppler shift—the elongation or compression of radio waves caused by slight movements of the probe. By factoring out all known causes of the probe's motion (everything from its flight path around Mars to the movement of the crust on which the tracking station on Earth rests), engineers can isolate the perturbations caused by the cycling of Martian snow or smog.

"We can measure a one-part-in-a-billion change in the gravity field," says Maria Zuber, head of MIT's Department of Earth, Atmospheric, and Planetary Sciences and leader of the gravity-mapping team. "That's enough to tell us the difference between summer and winter." Zuber and her colleagues first experimented with gravity-field measurements on the Mars Global Surveyor and are now ready to piece together a detailed atmosphere survey. "When we said we'd do this, nobody believed us. But we did it anyway."

When MRO's mission is complete in November 2008, it will have sent back between 30 and 50 terabits of data—10 times the output of any previous orbiter and twice as much information as in all the words in the Library of Congress. By then NASA will be readying a new rover, the Mars Science Laboratory, to pursue the next round of puzzles. "Every time we've looked at Mars with the ability to resolve things in finer detail, we've seen new phenomena that changed our ideas about how the planet is today and how it has changed," Zurek says. With luck, MRO will discover a few good sites to pitch camp.