NASA’s Curiosity rover, currently exploring Gale crater on Mars, is providing new details about how the ancient Martian climate went from potentially suitable for life – with evidence for widespread liquid water on the surface – to a surface that is inhospitable to terrestrial life as we know it.
This is an artist's concept of an early Mars with
liquid water (blue areas) on its surface. Ancient regions on Mars bear signs of
abundant water - such as features resembling valleys and deltas, and minerals
that only form in the presence of liquid water. Scientists think that billions
of years ago, the atmosphere of Mars was much denser and warm enough to form
rivers, lakes, and perhaps even oceans of water. As the planet cooled and lost
its global magnetic field, the solar wind and solar storms eroded away to space
a significant amount of the planet’s atmosphere, turning Mars into the cold,
arid desert we see today.
NASA/MAVEN/The Lunar and Planetary Institute
Although the surface of Mars is
frigid and hostile to life today, NASA’s robotic explorers at Mars are searching for clues as to whether it could
have supported life in the distant past. Researchers used instruments on board
Curiosity to measure the isotopic composition of carbon-rich minerals
(carbonates) found in Gale crater and discovered new insights into how the Red
Planet’s ancient climate transformed.
“The isotope values of these
carbonates point toward extreme amounts of evaporation, suggesting that these
carbonates likely formed in a climate that could only support transient liquid
water,” said David Burtt of NASA’s Goddard Space Flight Center in Greenbelt,
Maryland, and lead author of a paper describing this research published October
7 in the Proceedings of the National Academy of Sciences. “Our samples are not
consistent with an ancient environment with life (biosphere) on the surface of
Mars, although this does not rule out the possibility of an underground
biosphere or a surface biosphere that began and ended before these carbonates
formed.”
Isotopes are versions of an element with different masses. As water
evaporated, light versions of carbon and oxygen were more likely to escape into
the atmosphere, while the heavy versions were left behind more often,
accumulating into higher abundances and, in this case, eventually being
incorporated into the carbonate rocks. Scientists are interested in carbonates
because of their proven ability to act as climate records. These minerals can
retain signatures of the environments in which they formed, including the
temperature and acidity of the water, and the composition of the water and the
atmosphere.
The paper proposes two formation
mechanisms for carbonates found at Gale. In the first scenario, carbonates are
formed through a series of wet-dry cycles within Gale crater. In the second,
carbonates are formed in very salty water under cold, ice-forming (cryogenic)
conditions in Gale crater.
“These formation mechanisms
represent two different climate regimes that may present different habitability
scenarios,” said Jennifer Stern of NASA Goddard, a co-author of the paper.
“Wet-dry cycling would indicate alternation between more-habitable and less-habitable
environments, while cryogenic temperatures in the mid-latitudes of Mars would
indicate a less-habitable environment where most water is locked up in ice and
not available for chemistry or biology, and what is there is extremely salty
and unpleasant for life.”
These climate scenarios for ancient
Mars have been proposed before, based on the presence of certain minerals,
global-scale modeling, and the identification of rock formations. This result
is the first to add isotopic evidence from rock samples in support of the
scenarios.
The heavy isotope values in the
Martian carbonates are significantly higher than what’s seen on Earth for
carbonate minerals and are the heaviest carbon and oxygen isotope values
recorded for any Mars materials. In fact, according to the team, both the wet-dry
and the cold-salty climates are required to form carbonates that are so
enriched in heavy carbon and oxygen.
“The fact that these carbon and
oxygen isotope values are higher than anything else measured on Earth or Mars
points towards a process (or processes) being taken to an extreme,” said Burtt.
“While evaporation can cause significant oxygen isotope changes on Earth, the
changes measured in this study were two to three times larger. This means two
things: 1) there was an extreme degree of evaporation driving these isotope
values to be so heavy, and 2) these heavier values were preserved so any
processes that would create lighter isotope values must have been significantly
smaller in magnitude.”
This discovery was made using
the Sample Analysis at Mars (SAM) and Tunable Laser Spectrometer (TLS)
instruments aboard the Curiosity rover. SAM heats samples up to nearly 1,652
degrees Fahrenheit (almost 900°C) and then the TLS is used to analyze the gases
that are produced during that heating phase.
Funding for this work came from
NASA’s Mars Exploration Program through the Mars Science Laboratory project.
Curiosity was built by NASA’s Jet Propulsion Laboratory (JPL), which is managed
by Caltech in Pasadena, California. JPL leads the mission on behalf of NASA’s
Science Mission Directorate in Washington. NASA Goddard built the SAM
instrument, which is a miniaturized scientific laboratory that includes three
different instruments for analyzing chemistry, including the TLS, plus
mechanisms for handling and processing samples.
By William Steigerwald, NASA’s Goddard Space Flight Center, Greenbelt, Maryland
Source: NASA: New Insights into How Mars Became Uninhabitable - NASA Science
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