While Mars can claim some unique features - the largest volcano and the deepest canyon in the solar system - its rocky, dusty, cold landscape has yet to yield signs of the ultimate prize: life.
Three simple words - follow the water - have become the mantra of astrobiologists studying the Red Planet because the presence of water is believed to be a prerequisite for life, either past or present.
|Erosion channel on Mars. The detailed structure of the channel's walls and floor looks much like erosion patterns on Earth.
But as scientists look for evidence of water on Mars, they are faced with an underlying dilemma: Will they know life when they see it?
"Scientists' approach to finding life is very earth-centric," said Kenneth Nealson, holder of the USC Wrigley Chair in Environmental Sciences. "Based on what we know about life on Earth, we set the limits for where we might look on other planets."
In a paper published in the journal Astrobiology, Nealson - and Bruce Jakosky of the University of Colorado - speculated that a microbe that exists in the coldest temperatures on Earth might provide clues about how a similar organism could survive beneath the Martian polar ice caps.
The microbe in question was discovered by Corien Bakermans, a postdoctoral student in Nealson's lab, and remains the only one of its kind. It was isolated from a cryopeg - a small, salty, liquid lake found under the Siberian permafrost. Scientists have known for nearly a century that microorganisms can survive in the Siberian permafrost . How they do this, however, is not fully understood.
The bacteria, named Psychrobacter cryopegella, can grow at -10 Celsius and can stay alive and even keep metabolizing at an astonishing -20 Celsius. While it isn't known yet if the Psychrobacter can replicate itself at that extreme temperature, it maintains the minimal metabolism needed to repair and maintain its cell structures.
"This organism can exist at colder temperatures than any previously discovered," said Nealson, a professor of Earth Sciences and Biological Sciences, USC.
|Concept artwork shows the Active Thermal Probe (Mars Cryobot) melting downg through the northern ice cap on Mars.
Credit: NASA JPL
"We know it's possible here, so certainly it's possible somewhere else. This bacteria expands the limits of life, so if you can find places on Mars that are minus 20 degrees centigrade, you should take a look."
Nealson and Jakosky looked to the Martian polar regions for a habitat similar to the one in which Cryopegella survives.
While temperatures at Mars' equatorial and mid-latitudes regularly rise above -20 Celsius, it is unlikely that there is liquid water there because of its potential to be absorbed into the atmosphere, Nealson said.
But, liquid water could be found under the frozen polar caps, he added.
Climate changes on Mars, as with all of the nine planets that orbit the sun, are tied to its obliquity, or tilt of its axis with respect to its orbital plane.
Nealson and his colleagues proposed that as the Red Planet tilted - exposing more of itself to the sun at various times in its history - temperatures at the polar ice caps were warmed to minus -20 Celsius or higher.
"If the ice at the polar caps warmed to liquid water, organisms like cryopegella could have awakened and repaired any damage that might have occurred to their various cellular components," Nealson said.
"Then, as the obliquity changed a few million years later and the planet got colder and colder, these organisms would have been the last survivors."
|Sandstone algae found in Antarctic rocks.
Credit: British Antarctic Society-BAS
But, he added, "I would never say, 'Go and look for this bacteria.' I would say, 'This is a habitat that we should look at on Mars because on Earth, similar habitats have life.'"
Microbial species are now known to occupy almost the entire range of pH from 1.4 (extremely acid) to rear 13 (extremely alkaline). Life also thrives in extreme temperatures, with some species showing growth up to 114 degrees C (thermal springs at Vulcano, Italy and deep sea vents) and other species surviving down to -15 degrees C (brine films in Siberian permafrost). Life also occupies an equally broad range of salinity, ranging from fresh water up to sodium chloride saturation (about 300 percent), where salt precipitates.
A Mars lander scheduled for 2009 may well have a drill capable of obtaining permafrost samples-if a lander can reach the parts of Mars suspected of having permafrost, not an easy feat. If such instruments work, they would save the considerable trouble of transporting samples of Martian permafrost back to Earth.
The paper's other contributors were USC's Corien Bakerman and the University of Colorado's Ruth Ley and Michael Mellon.