Since the discovery of Mars' polar caps more than 200 years ago, astronomers have known the planet to hold water. But did Mars ever have a climate capable of nurturing life?
Tomorrow, the National Aeronautics and Space Administration is set to launch the Red Planet's first weather satellite to help answer that question.
The lift-off of the Mars Climate Orbiter marks the start of the second wave in NASA's decade-long assault on Mars, which captivated millions last year when a roving robotic geologist named Sojourner rolled onto Martian soil and into the history books.
Now, researchers are turning their attention to the history of water on Mars.
Understanding that history is critical to solving several Martian riddles, says Carl Pilcher, science director for NASA's solar-system exploration program. Foremost among those, he explains, is whether the planet ever had the potential to harbor life.
Planetary scientists also hope to reconstruct the planet's climate history, opening a window on the ancient Martian environment. Researchers and mission planners want to uncover resources that could be used by future robotic and manned Mars missions.
Tomorrow's launch is the first of two for NASA's $190 million Mars 98 project. The second takes place Jan. 3, carrying the Mars Polar Lander and a pair of experimental probes.
When orbiter finally gets to Mars in September, it will relay data and images from the Mars Polar Lander, arriving in December. The lander will collect and test soil samples for evidence of water, act as a Martian weather station, and measure the height of dust haze and clouds above.
On its way to the Martian surface, the lander will release the basketball-sized probes, which will crash onto the surface and bury themselves as much as three feet into the soil to look for water and take measurements.
The lander also will provide the first multimedia Mars experience. As it takes pictures of the surroundings, a small microphone will capture the whisper of a Martian breeze.
Once the lander's tasks end, 60 to 90 Martian days after touching down, the orbiter will spend at least one Martian year tracking seasonal changes in temperatures, clouds, moisture, and dust in the atmosphere.
"We've known for many years that the Martian surface shows indications that past climates were very different," says Daniel McCleese, referring to the deep channels and flood basins that scar the Martian surface.
Fresh evidence has come from the Mars Global Surveyor, which was launched in 1996. Currently orbiting the Red Planet in preparation for a mapping mission, the Global Surveyor shows the region around the Martian north pole to be the flattest ever seen in the solar system. The terrain varies in height by only a few feet over many miles. To planetary geologists, this suggests the region formed through sedimentation.
More intriguing, McCleese says, is the extensive layering found in region around the south pole. The terrain, which is said to resemble alternating layers of dust and ice in Alaskan glaciers, suggest that the material was deposited in cycles.
On the surface, the layers may be so thin that "you could see 100,000 years of history in the first meter of surface," says Richard Zurek, project scientist for the Mars 98 missions.
The challenge to re-creating climate history is in preserving and reading the soil's history. "We don't know what the annual water and dust accumulation rates are," McCleese says. "A 30-meter deep layer may represent 10,000 years or 1 million years."
Dust measurements taken by the orbiter could shed some light on deposition rates. But the key, he says, is to develop new dating techniques that don't require organic remains and that will be small and energy-efficient enough to fit onto a spacecraft.
The orbiter's main goal is to find a way to reconstruct past climate patterns.
"The seasonal cycles of Mars' atmosphere today transports large amounts of water and carbon-dioxide to the north and south poles during their respective winters," McCleese says. "That exchange of material very likely duplicates what happed over millions of years on Mars."
Whatever scientists discover will unlock secrets of one of the solar systems most mysterious - and captivating - planets. For now, little is know about the Red Planet.
"Above all, this is a mission of exploration," says Bruce Jakowski, a planetary geologist at the University of Colorado at Boulder. "We're going to a place on Mars we've never been before."
THE MARS FILE
* Ancients named Mars after the god of war because of its blazing color: red.
* The red color is caused by iron and oxygen combining to form iron oxide, or rust.
* Half the size of Earth, Mars is a frozen desert with temperatures averaging minus 64 degrees F.
* Two tiny moons, which may be captured asteroids, circle Mars: Phobos (fear) and Deimos (terror).
* Mars has the tallest mountain in the solar system - the extinct volcano Olympus Mons. It towers more than 80,000 feet - three times the height of Mt. Everest.