Gullies,
channels and other features on the martian surface have long suggested that
water flowed across red planet long ago. But a growing number of observations
show frozen water is there now.
The findings
of NASA's Phoenix
Mars Lander, announced throughout the mission, are now detailed in a set of
four papers in the July 3 issue of the journal Science. They are the
first to describe the northernmost reaches of the red planet and shed light on
the history of water on Mars, as well as the planet's potential habitability.
Among the
most significant finding from Phoenix, which landed on the red planet May 25,
2008: a layer of rock-hard water ice just a few inches beneath the surface of
the dusty arctic plains.
Phoenix unearthed other surprises: The
Martian dirt at Phoenix's landing site proved to be unexpectedly clumpy and
contained some compounds that set it apart from the sites where the lander's
rover cousins have been investigating.
Water
confirmed
Phoenix was sent to the Vastitas Borealis
plains of Mars in part to confirm observations from NASA's Mars Odyssey orbiter
(still circling the red planet) that showed signatures of water ice beneath the
surface of the region.
Using its
robotic arm, Phoenix dug into the Martian surface to see if it could reach the
ice below.
In one
trench, dubbed "Dodo-Goldilocks," the lander exposed what mission
scientists described as "bright material" about 4 to 5 centimeters
below the surface (a similar patch was seen below the lander itself, likely
exposed by the spacecraft's landing thrusters). Over the next two months, the
team watched as Phoenix's cameras showed that the material was sublimating
away, which would be expected of water ice exposed to the Martian atmosphere.
The
tendency of soil samples scooped up by the lander to clump together made it
difficult to get the samples into Phoenix's onboard instruments, but after
several attempts, a sample was coaxed in and the lander's detectors confirmed
that there was indeed water ice hiding under the regolith. The confirmation was
originally
announced on July 31.
Interestingly,
the ice seems to occur at different depths under the surface depending on the
terrain. The plains where Phoenix landed feature polygonal mounds surrounded by
troughs that result from the seasonal expansion and contraction of the ice
underneath the surface, which creates cracks and crevices.
The ice
below the center of the mounds was fairly shallow, "but in the troughs in
between, we went down as much as eight inches and never did find the ice
underneath," said the mission's principal investigator Peter Smith, of the
University of Arizona.
Just how
the ice got there is still not known, though researchers have a few theories.
"It
could have been the remnant of a larger polar ice cap that shrank; could have
been a frozen ocean; could have been a snowfall frozen into the ground,"
Smith said. "The most likely theory is that water vapor from the
atmosphere slowly diffused into the surface and froze at the level where the
temperature matches the frost point."
Surprising
soil
Just as
interesting as the water ice itself are the chemical signs in the soil that
liquid water interacted with the soil in the past.
Phoenix's instrument detected a range of
compounds and elements in the soil, but two were of particular interest because
they imply reaction of the dirt with water.
Calcium
carbonate was detected by Phoenix's Thermal and Evolved-Gas Analyzer (TEGA),
which heated up samples and then analyzed the gases coming off them.
Conditions
on early Mars were expected to be ideal for the formation of carbonates, but
few have been detected by orbiters or surface missions to the red planet. The
calcium carbonate, which makes up only about 3 to 5 percent of the soil by
weight at Phoenix's site, likely formed in the past when carbon dioxide in the
atmosphere interacted with films of water on soil particles.
The
carbonate also acts as a buffer that gives the soil an alkaline pH (about 7.7),
a marked contrast from the acidic soils seen in other Martian locations. An
alkaline soil environment is also interesting because it is similar to that of
many habitable environments, for example, Earth's oceans.
The other
chemical standout was a compound
called perchlorate, a highly oxidizing substance.
"Perchlorate
was not predicted at this landing site and nobody had it on their list of
likely chemicals," Smith said. "There was a very high concentration
of it, higher than the salts we might have expected like sodium chlorate (table
salts)."
Perchlorate
is an interesting find because it has "a strong affinity for water,"
Smith said, adding: "On Earth, microbes use it as a chemical energy
source."
These
findings will help inform scientists' understanding of Mars' past and provide
evidence of a place where any potential life could have thrived.
"Who
knows? Evolution is a powerful force. If life ever started on Mars, there are
niches where it could still survive," Smith said.
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