The craft would then use a visible-light camera and an infrared spectrometer to chronicle the explosion and study the resulting plume of debris before Martian gravity pulls the material back down to the surface.
The mission, scheduled for a 2011 launch and an arrival at Mars in late 2012, is led by Arizona State University in Tempe and NASA's Jet Propulsion Laboratory (JPL) based at the California Institute of Technology.
THOR's explosive technique was pioneered by NASA's Deep Impact mission, which successfully slammed a half-ton impactor into Comet Tempel 1 on July 4, 2005. (Read a news article about initial results from the Deep Impact mission.)
JPL's David Spencer, the mission manager for Deep Impact, has signed on for the THOR mission as well. He says his task should be a bit easier this time around.
"With such a large target region on Mars, delivering THOR's impactor will be less challenging than the Deep Impact comet encounter," Spencer said.
Unexplored Mid Latitudes
THOR's collision would occur in a visually intriguing but unexplored area of Mars—the planet's middle latitudes between 30° and 60° in either hemisphere.
Images of these regions suggest tantalizing evidence of dust-covered layers of snow or ice.
The suspected ice-rich layers could have been deposited during the past 50,000 to 1 million years, when the Martian climate changed due to variations in the planet's orbit.
Some Martian climate-change theories suggest that when the planet's tilt periodically changes, increased sunshine vaporizes water locked in polar ice. The water is then deposited in the middle latitudes as falling snow and ice.
"The hypothesis that we're trying to test is that the mid-latitudes are very ice rich," Christensen said. "This unexplored region of Mars may provide chemical and mineral clues to tell us about habitable areas on the planet."
"Right now all we have to go on are pictures of this stuff," he said. "Are these mantles of material ice rich? If so, did they form during recent climate oscillations?"
Geological and chemical evidence suggests that much of the planet once boasted abundant water, though no one is sure if it also harbored life.
Instruments reveal that Mars likely has significant ice near its poles. In 2007, NASA will send a non-roving lander named Phoenix to Mars's high northern latitudes, where it will scrape away a thin layer of soil and search for water ice.
But THOR is shooting for Mars's middle latitudes, where ice could support climate-change theories and present some intriguing possibilities.
"If there is a lot of ice at these latitudes, the temperature could get warm enough that the ice can actually melt," Christensen said. "We'll be looking for the origin of these young gullies that we've seen on the Martian surface in the mid-latitudes.
"It's one thing to find ice, that's great," he continued. "But ice that can melt and form liquid water—that gets a lot more exciting in terms of potential habitat."
"Ultimate Prize"
Finding water is the THOR mission's main goal, but other building blocks of life could potentially be present beneath the planet's surface. Organic materials and gasses, if found, could be unearthed by the blast.
Some instruments have detected methane in the planet's atmosphere, but such data are not universally accepted. Other evidence of organic materials on Mars has thus far been very thin on the ground.
"We'll be looking for organics in this plume, but there's certainly nothing to say that it's likely that we'll find them," Christensen said.
"That would be sort of the ultimate prize, but the nature of exploration is that you never know."
In any event, the dramatic collision won't be a one-shot scientific wonder. THOR's massive impact crater will remain as an important research site, silently awaiting future Mars visitors.
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