During its more than five-month stint on Mars last year, NASA's Phoenix Mars Lander found evidence that liquid water existed at the spacecraft's landing site, some Phoenix team members say.
Water is key to all forms of life as we know it and the discovery of liquid water would suggest a greater opportunity for biology on the red planet.
The new but controversial conclusion comes from observations of a set of "little globules" attached to struts on the lander's legs that were photographed by Phoenix's robotic arm camera over the course of the mission, as first reported at Spaceflight Now.
These globs were seen to apparently move and grow between snapshots, and 22 members of the Phoenix team, including principal investigator Peter Smith of the University of Arizona in Tucson, think that this behavior combined with other Phoenix findings indicates that these blobs might have been liquid water that was splashed up onto the spacecraft as it landed.
The paper making the case for liquid water will be presented on March 23 at the Lunar and Planetary Science Conference in Houston. But not all of Phoenix's team members agree with the paper's interpretation of the globs.
"It's a bit controversial," Smith told SPACE.com. But "obviously they came from somewhere — they weren't there when we launched," he added.
Phoenix touched down at its landing site in the Martian arctic on May 25, 2008. The stationary lander's mission was to search for signs of potential habitability on the red planet, namely, signs that water ice just below the surface was once liquid.
On July 31, 2008, Phoenix confirmed that the hard material it encountered underneath the clumpy surface dirt was indeed water ice. Analyses of dirt done in the lander's onboard instruments also indicated that this ice was likely once liquid and had interacted with the Martian regolith, or dirt, at a period of time in Mars' history when its climate was warmer.
But finding liquid water on the present surface of Mars, which sees temperatures between -20 and -80 degrees Celsius (-4 and -112 degrees Fahrenheight) even in the summer, is a whole different ball game. It was expected that any water ice exposed to the atmosphere would immediately sublimate, or turn to vapor. The Phoenix team saw signs of this when they exposed underground ice in the trenches dug by the spacecraft.
The new report's main author, Phoenix team member Nilton Renno, proposes that perchlorate salts, discovered in the Martian dirt by Phoenix's wet chemistry lab, were concentrated enough in a patch at the spacecraft's landing site that they could lower the freezing point of the water ice, causing it to melt into a salty brine (this is the same phenomenon that causes sidewalk salt to melt down snow and ice in the winter). Nilton and his co-authors think the brine could have been splashed up onto the lander leg when Phoenix touched down.
Of course, Phoenix didn't get any samples of this material and any investigation of the blobs relies on the images taken by Phoenix and knowledge of how perchlorate behaves.
"There's a matter of belief at some level," Smith said.
Smith cautions that the case isn't solved for sure. As Phoenix descended to the surface, its thrusters created a high-pressure, high-temperature environment and blasted ammonia which could have affected the surface below it. There could also be other constituents in the dirt that could affect its chemistry, he said.
But the case for liquid water is compelling, Smith said. Though he added, "I can't say I agree with every statement in the paper."
Michael Hecht, the lead scientist for the instrument that discovered perchlorate, thinks that while the idea of this splash of liquid brine isn't physically impossible, it is "far less likely than simpler explanations," he told SPACE.com.
The imaging of the globules is low resolution, which Smith also pointed out, and some of the apparent changes seen in them could be attributed to changing shadows, Hecht said.
And while perchlorate is an excellent sponge, sucking up water if the surrounding air is warm and dry enough, the temperatures required of the lander mentioned in the paper are too warm and "you would not get liquid droplets of perchlorate brine," Hecht explained.
A more likely explanation, Hecht contends, is that water vapor released by the ground ice stuck to the legs.
The legs would likely be relatively cold compared to the ground during the day, Hecht explained. When sunlight fell on the patches of ice exposed at the landing site, some of that water would sublimate. As that water vapor traveled up through the air, it might encounter cold patches of dirt stuck on the lander legs from the landing and stick.
And "once there's ice there, [other water vapor molecules will] go to the places where there's ice," eventually forming the blobs seen in the photos, Hecht said.
There are circumstances where perchlorate could create liquid brine on Mars though, Hecht said. During periods when Mars might have just a few degrees warmer, perhclorate rinds could melt water ice. Another paper being presented at the Lunar and Planetary Science Conference posits that perchlorates could seep down beneath Mars' polar ice caps, forming a lubricating sludge that lets the ice caps flow.
But these situations are different than the briny blobs described by Renno.
Hecht acknowledges that he could be wrong and the globs on Phoenix could be liquid brine, but "I just don't think it's the likely explanation," he said. "It's just plain old frost, nothing more."
Hecht thinks that the true nature of these blobs will be hashed out over time by the Phoenix team and by reviewers of Renno's paper. (Hecht and Renno, as well as other Phoenix team members, have corresponded at great length over the topic.)
"It hasn't been in front of the jury yet," Hecht said.