Mars water story spawns kudos and controversy
BY CRAIG COVAULT
Posted: March 24, 2009
HOUSTON -- Phoenix science team findings that their spacecraft discovered liquid water on the surface of Mars sailed through their first scientific peer review unchallenged at the 40th Lunar and Planetary Science Conference in Houston this week.
Peter Smith, principal investigator for the mission at the University of Arizona, said that peer review for the landmark findings are only just beginning. "We have a long way to go" for full acceptance of the discovery, he told Spaceflight Now.
Nilton Renno of the University of Michigan, who lead the water analysis, told a standing-room-only hall filled with 500 people that the Phoenix "evidence for liquid water is overwhelming."
NASA's Mars exploration motto should be updated to "follow the 'liquid' water," he told the audience.
The report on liquid water was signed by 22 science team members but is not supported by some key team members, like Bill Boynton, who headed work with the spacecraft's TEGA Thermal and Evolved Gas Analyzer. He does not believe the evidence is complete.
Smith, who heads the mission and also signed the water finding document, is also the author of another paper that says Phoenix did not see any liquid water.
Reminded that his stance in these two papers places him "with a foot in each bucket, one filled with water and the other not," he noted "that's a good observation."
Renno and his colleagues showed imagery of spheroids apparently splashed onto one landing strut as evidence of the presence of liquid water. They then cited point after point on temperature, pressure, chemistry, and the affect of perchlorate salts as supporting evidence.
The movement and merger of the spheroids is what initially caught Reeno's attention, but he has also conducted substantial ground-based testing, including how the light wavelengths reflected from the spheroids change under different sunlight conditions. Those changes seen on Mars match what was found in the lab on Earth.
The existence of perchlorate salts that lower the freezing point for water at the site is the key to the water, along with a presence of calcium carbonate, which is unlike anything seen on Mars before.
Thermodynamic calculations offer additional evidence that salty liquid water can exist where Phoenix landed and elsewhere on Mars. The calculations also predict a droplet growth rate that is consistent with what was observed. And they show that it is impossible for ice to sublimate from the cold ground just under the strut of the lander's leg and be deposited on a warmer strut, a hypothesis that has been suggested.
The lander's wet chemistry lab found evidence of perchlorates, which likely include magnesium and calcium perchlorate hydrates. These compounds have freezing temperatures of about -90 and -105 Fahrenheit, respectively. The temperature at the landing site ranged from approximately -5 to -140 Fahrenheit, with a median temperature around -75 Fahrenheit. Temperatures at the landing site were mostly warmer than this during the first months of the mission. Scientific literature was also cited, predicting that brines would be found on Mars much like they have been found in Antarctica, where they support exotic life forms.
All of this evidence has given the Mars Phoenix lander's north polar site a higher rating for potential habitability for current and past Martian life that is far greater than that found by either the Spirit and Opportunity rovers, Mars Pathfinder, or the twin Viking landers of the 1970s, says Carol Stoker, who headed habitability assessments for the Phoenix mission.
The specific message the Phoenix data convey is "to search for life, future missions should land at the same location Phoenix did," she said.
The entry, descent and landing planners for any future mission like that would relish the knowledge that the landing area is a vast smooth arctic plain with only a few large rocks and craters.