NASA's Phoenix Mars Lander may have found water ice on the red planet, but it still has a lot of work left to do to answer the question that has been on scientists' minds for decades: Has Mars ever been capable of harboring life?
Phoenix scientists announced yesterday that the mission finally confirmed the presence of subsurface water ice in the north polar regions of Mars — first detected by NASA's Mars Odyssey orbiter in 2002 — about two months after touching down on the Martian surface on May 25.
The lander is now analyzing the ice to see if it was ever a liquid and if it contains organic materials, the building blocks of life.
The ice, collected from below the surface at the lander's site in the Martian arctic, could have acted like a freezer, protecting any organics that may have formed there.
"We have an environment where organics could be preserved," said mission scientist William Boynton of the University of Arizona.
The detection of organics on Mars would not necessarily mean there is life. It would just mean that carbon and other molecules that make up life as we know it were present.
"Organics would be the home run or the grand slam of the mission," said Bruce Jakosky, a geologist at the University of Colorado who is not affiliated with the mission. However, if they don't find organics, "that doesn't mean that there wasn't life on Mars," Jakosky said. Other missions, planned and unplanned, will keep the search alive.
Liquid water
The confirmation of the Odyssey ice observations was a key goal of the $420 million Phoenix mission, but only the first of several steps in characterizing the dirt and ice layer of Mars' Vastitas Borealis region to determine whether it may once have been habitable at some point in the planet's past.
'I see this as a step along the way of Phoenix getting to its major science results," Jakosky said. "By itself, that's not a major result."
One of those steps is determining whether the water ice ever existed in a liquid form, said Phoenix robotic arm co-investigator Ray Arvidson of Washington University in St. Louis — liquid water being a key resource for life as we know it.
Phoenix will look for signs of ancient liquid water by heating up samples of the icy dirt mixture in the tiny ovens of its Thermal and Evolved-Gas Analyzer (TEGA), which can analyze the vapors given off by the heated samples to analyze their composition.
TEGA has already begun heating up the sample that confirmed that the rock-hard layer beneath the surface dirt was indeed water ice — when ice begins to melt, it takes more heat to raise the temperature of the sample. Over the course of the next week, TEGA will gradually heat the sample all the way up to 1,832 degrees Fahrenheit (1,000 degrees Celsius).
As the sample is heated, any hydrated minerals, or those that likely formed in the presence of liquid water, show themselves as they break apart and the signature of their water is detected by the instrument, explained Boynton, a TEGA co-investigator.
Finding hydrated minerals, such as carbonates, sulfates or clays, would indicate that liquid water once permeated the Martian regolith where Phoenix now sits, Arvidson said in a telephone interview last night.
This water would not have flowed as rivers or streams, as it once likely did closer to Mars' equator, where hydrated salts have already been found by NASA orbiters and rovers, but would have percolated through the dirt layer as Mars' orbital motions tipped the northern parts of the planet toward the sun, warming them up, Arvidson added.
The stuff of life
The other big signature Phoenix will look for as the mission continues and the probe's ovens heat up dirt samples will be organic molecules, the building blocks of life.
"Finding organics would really change our way of thinking," Boynton said.
But so far, organic molecules haven't shown up on Mars. When the Viking landers heated up dirt samples in the 1970s, "there were really no organic molecules at all," Boynton said.
But dirt near the Martian equator faces strong oxidizing pressures, which can destroy organics, Boynton added, which is why Phoenix is looking for them farther north.
Organics have proven elusive to Phoenix so far; the first dirt sample analyzed by TEGA, taken from the surface, found no sign of them. But this wasn't much of a surprise to mission scientists.
"We didn't really expect to find them in the surface soils," because the surface is subjected to the same oxidizing pressure as the equatorial regions, Boynton told SPACE.com.
TEGA will keep looking for signs of organics in subsequent samples taken closer to the ice layer, he added.
If Phoenix doesn't find organics, the mission won't be a flop, Jakosky said, because it still gives scientists valuable information about the northern region. "Whatever they find is exciting," he said.
If it does detect organics though, mission scientists will be cautious about interpreting the findings, Boynton said. They would assume first that any organics could be contamination brought with the spacecraft from Earth. Mission scientists will check with a "blank" brought from Earth to determine whether or not any organics discovered are terrestrial stowaways.
Even if the blank shows that the organics came from the Martian soil, they still may not be native, Boynton said, since the same organic-bearing meteorites that strike Earth strike Mars as well. Determining the ultimate origin of any organics would likely take bringing back a sample to Earth, a project that both NASA and the European Space Agency are working toward.
And organics don't on their own equal life. "Just because there are organic compounds, doesn't mean that that's life," said Kim Seelos, a postdoctoral researcher at Johns Hopkins University.
Future missions
Whether or not Phoenix finds signs of organics, it won't be the last time that NASA looks for them on Mars. NASA's Mars Science Laboratory (MSL), set to launch in 2009, will pick up where the 1970s Viking missions left off, exploring the regions closer to the equator for signs of them.
MSL will be better equipped than Viking, and even Phoenix, to detect organics in dirt samples, with more powerful and sensitive equipment, as well as the ability to roam around.
While "life is teeming on Earth," Arvidson said, it may not have been so dense on Mars — if it ever existed — perhaps only forming in small pockets. Since Phoenix, as a lander, stays put by definition, it can only explore the patch of ground in its immediate vicinity.
However, any organics detected by MSL would have formed in a much more ancient period of Mars' history because the landscape near Mars' equator formed billions of years ago. The surface that Phoenix is exploring is much younger, only tens of millions of years old.
For now, Phoenix is the only chance of finding organics on this region of Mars, since no future missions in the works now are planned to return to the frozen northern reaches. To get another mission back there to look the region in greater detail, "it would probably take finding organics," Boynton said.
But "Phoenix isn't the last gasp, or MSL isn't the last gasp" to answering the question of whether life ever existed on Mars, Jakosky said.
The evidence available to scientists now suggests that Mars could have harbored life, it's just a matter of finding a spot that preserves the signs of it, he added. And Phoenix's landing site, or even MSL's, may not be the ideal spot to go and look for those signs, whether by sending another rover or staging a mission to return a sample to Earth.
There are plenty of other environments on Mars where Jakosky would like to look for signs of life. While he can't point to a particular spot, there is evidence that Mars once had features that could have supported life, including ancient lake beds, ancient highlands where evidence suggests that water existed for long periods of time, and hot springs — "places like Yellowstone," as Jakosky describes them.
Some scientists, such as Seelos, doubt that clear evidence of Martian life itself will ever be found because any life would have likely been microbial, which is not easily preserved as fossils. But Jakosky sees this as an "unnecessarily pessimistic" view, because ancient microbes have been preserved in some places on Earth.
But it could be awhile before scientists ever answer the question of life on Earth because they currently understand so little about the history and geology of Mars, Jakosky said.
"We don't know the answer and I don't even know how to put odds on "when and whether they will find the answer," Jakosky said.
His advice is to keep using missions like Phoenix, MSL, and eventually a sample return mission to build our knowledge of Mars, as we have built the knowledge of our own planet's past.