Written in the rust-red dust, sand, and rock of Mars is the history of water - a compound vital to organic life.
A key tool for unlocking that history has started to trace ever-tighter orbits around the red planet. NASA's 2001 Mars Odyssey orbiter, a solar-powered digital geologist launched last April, is preparing to take up a 2-1/2-year search for clues to the story of water - and, perhaps, of life - on Mars.
In the process, the craft, which arrived at Mars late Tuesday, is breathing new life into US efforts to explore a planet that has captivated the human imagination for millenniums. Mars Odyssey's successful 285-million-mile journey stands in stark contrast to a pair of high-profile Mars missions in 1999 that reached the planet only to be destroyed.
"How sweet it is!" said Daniel Goldin, outgoing administrator of the National Aeronautics and Space Administration, shortly after Mars Odyssey signaled its safe arrival.
His feeling was widely shared. When the orbiter's motor fired to put it into orbit, it hurtled around the backside of Mars, entering a period of radio silence that mission manager David Spencer called "the longest 20 minutes of my life." When Mars Odyssey emerged, cheers and applause erupted in the control center.
Although the craft is now orbiting Mars, it won't begin in earnest the science part of the $297 million mission until February. It will take nearly that long for flight controllers to adjust the orbit's shape so the craft can fulfill its objectives of mapping the chemical composition of the surface and "sniffing" for possible deposits of water ice just below the surface in some regions.
The chemical makeup of rocks and minerals is expected to yield clues about the planet's past climate and how that influenced on Mars' water resources.
Researchers divide roughly into two camps on these topics. One holds that early in its history, the planet was warmer, wetter, and had an atmosphere thick enough to support the presence of vast amounts of surface water. The other camp posits that the planet has always been cold, and that any water would have been locked up as subsurface ice. Periodically, however, that ice could melt and gush to the surface.
Indeed, the Mars Global Surveyor, currently orbiting the planet, has beamed back images of what appear to be relatively new gullies, washes, and debris flows. Some planetary geologists say these images are strikingly similar to aerial shots taken of US Southwestern desert slopes after flash floods and could have been formed after the sudden warming of water ice. Others hold that the formations could have been carved by the rapid release of fluidlike mixtures of soil and carbon-dioxide gas, which also can exist as subsurface ice.
Based on the wide range of temperatures found to support elementary life forms on Earth, many astrobiologists say they suspect that either of the conditions on Mars could have allowed primitive life forms to emerge.
Mars Odyssey carries a thermal imager designed to look for hot springs beneath the surface, if they exist. Such spots would be prime targets for future landers or rovers looking for mineral evidence of early, primitive life forms.
Once it finishes gathering data, the orbiter is designed to serve as a radio-relay station for lander and rover missions planned for later this decade.
Currently, the craft's elliptical orbit takes it within 62 miles of the surface of Mars and as far away as 2,000 miles. Over the next 77 days, controllers will take advantage of atmosphere drag to slow the spacecraft. The orbit should change shape and shrink until it settles into a circular orbit 249 miles above Mars.
The technique, known as aerobraking, presents its own challenge, notes Roger Saunders, project scientist for 2001 Mars Odyssey. "Having gotten this far, we don't want to make any errors," he says.
One factor that controllers will be watching is the Mars' weather. The planet currently is in the grip of a global dust storm that began in late June.
While Mars Odyssey is in little danger of passing through the storm, mission managers say, the sun is heating the upper layers of dust, boosting the upper atmosphere's temperature by some 80 degrees F. As the atmosphere heats, it expands farther out into space and so could present more drag to the orbiter than originally envisioned.