(See photos of Mars.)
Frozen water is known to exist in the planet's polar regions, and water vapor has been detected in the Martian atmosphere.
But many felt that modern Mars was too frigid to harbor any liquid water today.
"Ten years ago Mars scientists were talking about water [on Mars] billions of years ago," said Philip Christensen, a professor at Arizona State University in Tempe, who is unaffiliated with the research.
"Five years ago they were talking about water [on Mars] millions of years ago. Today, we can honestly talk about liquid water on Mars todaythat revolution in our thinking has changed how we view Mars."
Scientists must now debate just where Martian water could come from. It may originate in subsurface aquifers or melt from ice or snow, but each theory is faced with the same problemthe brutal Martian cold.
If the water is coming from ice or snowmelt, how is it able to melt on a planet with mean surface temperatures of 22 to -124 degrees Fahrenheit (-5 to -87 degrees Celsius)?
If water is flowing from underground, how is it able to remain liquid without freezing?
One theory is that Martian water is highly acidic, which could lower its freezing point to a temperature below those found on the red planet.
But such water, if it did appear, would likely flow only short distances before freezing in the planet's brutal cold.
Life on Mars?
Finding liquid water on Mars would yield clues to more than geological processes. Liquid water has also been a key focus of the search for extraterrestrial life.
Confirming the presence of liquid water on Mars would open the possibility for subterranean environments that might harbor living organisms.
Yet scientists can't yet be 100 percent certain that they've seen what they think they've seen.
"I think the evidence for this recent water is compelling, [but] certain questions still remain," said Arizona State's Christensen at today's press conference.
Future research could definitively solve the mystery. Spectral analysis, which can be done with instruments already on Mars, could provide stronger confirmation.
The future may even promise a visit from a Mars roverbut the surface craft would need luck to be on site at exactly the right time to witness one of the occasional flow events.
Viewing Crater Impacts in "Real Time"
Liquid water was not the only surprising discovery revealed by Mars Global Surveyor, the spacecraft recently deemed lost by NASA after an incredible nine-year run.
(See related photo gallery: "Mars Probe Lost in Space?" [November 27, 2006].)
High-resolution images sent back from the spacecraft allowed scientists to watch the formation of new craters, from 7 feet (2 meters) to 486 feet (148 meters) in diameter, which were caused by celestial objects crashing into the Martian surface.
"We had not anticipated that we could actually see craters forming over time," said Malin of Malin Space Science Systems. "This was a completely serendipitous discovery."
Craters reveal valuable information about the composition of a planet's surface and subsurface. Measuring the number of impacts also allows scientists to estimate a surface's age.
Mars has an impact rate similar to that of the Moon, the scientists reported.
"About 12 craters per year are forming across the surface of Mars," Malin explained.
"If you were to live on the surface for about 20 years you'd live close enough to one of these events to be able to hear it."
The impact data suggest a hazard, albeit a low-frequency one, that future Mars missions may have to take into account.
The findings could also help scientists model the dangers and the likelihood of Earth impacts caused by objects like comets and meteorites.
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