Perched on a vast plain above the arctic circle of the Red
Planet, NASA’s Phoenix Mars Lander has found new evidence that liquid water was
once present in the north polar region and interacted with minerals there. Phoenix scientists
reported the findings September 29 during a NASA press briefing.
experiments identified calcium carbonates and clays in soil samples scooped up
by the craft’s robotic arm. On Earth, both minerals are associated with the
presence of liquid water.
Carbonates such as limestones form on Earth when carbon
dioxide from the atmosphere dissolves in liquid water, making carbonic acid.
The acid eats away at rocks, which eventually become carbonate deposits such as
the White Cliffs of Dover.
An interaction between carbon dioxide and liquid water is “just
what we think is going on on the surface of Mars,” says Bill Boynton of the University of Arizona
Boynton is lead scientist for Phoenix’s
suite of eight miniature ovens and one mass spectrometer collectively known as
the Thermal and Evolved-Gas Analyzer, or TEGA.
Spacecraft orbiting the planet have found evidence of carbonates
elsewhere on Mars, Boynton notes, but never before in a smooth region devoid of
channels, which would suggest the flow of water, he adds.
TEGA’s evidence for calcium carbonates comes from one of its
ovens. The robotic arm delivered Martian soil into the ovens, and the high
temperature at which carbon dioxide was finally released from the samples
matches the temperature at which calcium carbonates decompose.
wet chemistry experiment, known as the Microscopy, Electrochemistry and
Conductivity Analyzer, or MECA, first dissolved a sample of the soil in liquid.
It then found that the soil solution had a stable, alkaline pH of 8.3 that could
not be easily altered, as if it were being buffered. At the same time, the concentration
of calcium measured in the wet chemistry analysis is exactly that expected from
a solution, such as Earth’s seawater, in which calcium carbonate stabilizes the
The two independent measurements, from the ovens and wet
experiments, reveal that the detection of calcium carbonate is “unambiguous,”
says Boynton. The amount of calcium carbonate in the soil is substantial,
between 3 and 8 percent, he says.
Both experiments also reveal clays that are likely to be
phyllosilicates, common on Earth. Such clays have sheetlike structures with
water interspersed between the layers. The Phoenix scientists determined this based on
the high temperatures required to release water vapor from the clay soil
samples in a TEGA oven. In addition, an atomic force microscope on the lander
has detected smooth, platelike particles that suggest the presence of the
clays, said Michael Hecht of NASA’s Jet Propulsion Laboratory in Pasadena, Calif.,
the lead scientist for MECA.
It remains a puzzle why water may once have been liquid in
this cold arctic region. One possibility, says Phoenix
principal investigator Peter Smith, also of the University of Arizona,
is that the axis about which Mars rotates has wobbled over millions of years. Many
other researchers have suggested that, in the past, the polar region of Mars
may have tipped over such that it received much more sunlight than today,
becoming warm enough that water might have been liquid.
“We’re approaching that hypothesis,” Smith says, but with
the “understanding that Mars has many surprises for us and we haven’t finished
The presence of liquid water would have created a climate
that was once hospitable for life.
Another finding, previously announced, may relate to the
melting of water-ice, he says. Phoenix
has discovered perchlorate, an oxidizing agent that not only acts as a possible
energy source for bacteria, but is also known to drastically lower the
temperature at which water freezes. The presence of even a sprinkling of
perchlorate could keep water liquid at temperatures as low as 60 to 70 degrees
C, he notes.
With the discovery of calcium carbonate and perchlorate in
the soil samples, there is enough new data “to begin rewriting the book of
Martian geochemistry,” says Hecht.
In the meantime, winter is about to begin in the arctic region,
landed on May 25. Not only is the sun now dipping below the horizon for more
than four hours of the Martian day, but the craft must also expend more energy
to keep its instruments from freezing. Scientists are racing against the clock
to fill TEGA’s remaining four ovens and continue other experiments. The craft
is not expected to survive past November.