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(CNN) -- Long before David Bowie crooned, "Is there life on Mars?" scientists have wondered the same - are we alone in our solar system? Now striking pictures suggesting the presence of liquid water on the Martian surface have scientists believing we are getting closer to finding out.
Images taken by the orbiting NASA's Mars Global Surveyor spacecraft enabled scientists to detect changes in the walls of two craters in the southern hemisphere of Mars, apparently caused by the downhill flow of water in the past few years.
The research team looked at two photographs of the same crater, one taken in August 1999, and the other in September 2005, which showed a gully that had not previously been there. Scientists have previously established the existence of water on Mars in the form of ice at the poles and water vapor, and pointed to geological features that appear to have been carved by water ages ago.
However there has long been a quest for "smoking gun" evidence for liquid water currently on the planet.
"Basically, this is the 'squirting gun' for water on Mars," Kenneth Edgett of Malin Space Science Systems in San Diego, a scientist involved in the research told reporters.
The scientists, whose research appears in the journal Science also found 20 newly formed craters left by impacts from space debris. They said water seemed to have flowed down two gullies in the past few years, even though liquid water cannot remain long on the planet's frigid, nearly airless surface because it would rapidly freeze or evaporate.
That seemed to support the notion that underground liquid water may reside close enough to the surface in some places that it can seep out periodically.
Although the images did not directly show water, they showed bright deposits running several hundred meters, seemingly left by material carried downhill inside the crater by running water, occasionally snaking around obstacles and leaving finger-shaped marks diverting from the main flow.
"It could be acidic water, it could be briny water, it could be water carrying all kinds of sediment, it could be slushy, but H2O is involved," Edgett said. Edgett said each apparent flow was caused by an amount equal to "five to 10 swimming pools of water."
Michael Meyer, lead scientist for NASA's Mars Exploration Program, said the observations provided the strongest evidence to date that water still flowed occasionally on the surface of Mars.
"The big questions are: how does this happen, and does it point to a habitat for life?" Meyer said. Edgett and his team conceded the images were only circumstantial evidence, not proof.
In the name of scientific detachment, they cited a possible alternative explanation that the features were caused by the movement of dry dust down a slope.
However, as Michael Meyer suggests, the discovery does raise many questions, including whether the source and abundance of water could serve as a resource in future missions to explore Mars.
Some groups are already preparing for a life on the Red Planet.
The Mars Society is a privately funded organization dedicated to promoting human exploration of the Red Planet, and the recent find will give the society's president Dr Robert Zubrin and his team more hope for a manned mission.
The society currently runs simulated missions in the Arctic, practicing what they think day-to-day life will be like for Martian explorers. The next four-month mission in the northern climes of Canada will see a team of scientists donning mock spacesuits and undertaking field and scientific experiments in complete isolation.
"We're better prepared for a journey to Mars now than we were when we sent men to the moon in the 1960s," Zubrin told CNN. "There have been 15 successful probes to the Red Planet - in fact we know more about it than the American pioneers knew about the western frontier."
That still might not be enough for a mission anytime soon, and a return to the moon seems like a more likely destination.
Last week President Bush gave his full backing to NASA's project to create a lunar base, and many believe that the moon could be a jumping off point for Martian exploration.
As for searching for native Martian life, for now the hunt will continue from afar. Some scientists suspect Mars once sheltered primitive, bacteria-like organisms and previous missions found evidence Mars at one time boasted ample quantities of water.
But whether life was supported is still up for debate.
A Martian meteorite thought to be 4.5 billion years old discovered in the Antarctic in 1984 caused huge controversy in the scientific community. Some claimed it contained fossil like features and evidence of ancient microscopic life on the Red Planet, others dismissed the claims, saying it had been contaminated since it had been on earth.
Caught between a rock and far off place, the quest for life on Mars will continue.
Reuters contributed to this report.
Signs of life - a gully possibly caused by water flowing over the Martian surface.
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