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.
Editor's Note: The image showing the globules was added at 11:30 a.m. E.T.
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