, which over the past few years has returned thousand of photos of Mars. A pixel in a THEMIS visible-light image will represent 18 meters on the ground, where MGS resolves 1.5 meters per pixel.
But Odyssey will see things MGS has not, because MGS doesn't pick up shadows well. MGS views Mars from a position equal to roughly 2 p.m., whereas Odyssey will see the surface from something more like 4 or 5 p.m. -- a steeper angle from which the Sun's shadows will better illuminate subtle features.
And MGS has mapped only about 3-4 percent of Mars in visible light and will probably never get above roughly 6 percent. Odyssey's goal is to map the whole planet over the next 2 to 3 years. Back in the 1970s, Viking mapped all of Mars, but at roughly 200 to 300 meters per pixel, Christensen said.
Bring on the heat
The more exciting instrument, however, is Odyssey's thermal imager.
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This infrared camera, which senses heat, will improve on the efforts of MGS, sharpening the resolution from 3 kilometers per pixel to a stunning 100 meters. If Mars has any heated football fields, Odyssey will see them. The thermal imager will be most effective at night, when the surface is uniformly cool. During the day, a sun-facing slope is warmer than a shaded area.
But Christensen isn't interest in such daytime views. He'll be searching for inherent differences in the temperature of the Martian soil, which should reveal craters and channels that have the potential to harbor "hot spots," regions of molten rock or warm water that might exist just below the surface.
The thermal imager, which measures heat by sensing infrared radiation, is so sensitive it can spot temperature differences of just 1 degree, even from its perch 250 miles above the surface.
But deciphering the temperature differences will require a human touch. Is it water? Is it lava? Or is it some combination of these -- perhaps an underground pool of water warmed by hot lava that is even deeper below? Scientists will compare visible-light images of surface features with the infrared images in an effort to figure out what the heat source might be. And whatever it is, it will likely come with a lesson of some sort.
"Any evidence of near-surface heating would be extremely interesting," Christensen said.
THEMIS also looks at infrared "spectral bands," described as the ranges of colors given off by various minerals. Studying this data will allow scientists to detect silicates, sulfates, oxides and other minerals on Mars' surface. Some of the minerals Odyssey will look for form only in the presence of water.
"We're going to look at Mars in a totally new way," Christensen said.
Odyssey is expected to begin its mapping mission in early February. Christensen said a flurry of images should then be released to the public -- possibly as many as one a day.
Another revolution, or two
Regardless of what Odyssey does and does not find, Christensen expects the probe to remake what's known about the Red Planet. And he has a good scientific reason why yet another orbiter has been sent to Mars: If Odyssey performs well, it will enhance a perspective that cannot be obtained from the surface.
"Global mapping gives a perspective you just can't get from a single landing site," Christensen said. "Imagine landing in a spot on Earth -- say the Gobi desert, the rain forest, Antarctica -- and trying to get a sense of the planet. Air photos revolutionized geologists thinking about Earth. Satellites did the same. Then Mars Global Surveyor did it for Mars."
And, of course, Odyssey is just one step in the Mars program. There is likely to be at least one more revolution in our understanding of Mars after Odyssey's mission is completed. In 2003, NASA plans to launch a
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