NASA's Curiosity Mars rover took this
selfie at a location nicknamed Mary Anning, after a 19th-century English
paleontologist. Curiosity snagged three samples of drilled rock at this site on
its way out of the Glen Torridon region, which scientists believe was a site
where ancient conditions would have been favorable to supporting life, if it
ever was present. Credit: NASA/JPL-Caltech/MSSS
A
new analysis of chemical signatures measured by NASA's Curiosity Rover gives a
peek at Mars's past to a time some 3.7 billion years ago, when it was warmer
and wetter.
Through measurements of isotopic ratios
of oxygen, a team of collaborators, including researchers from Caltech's campus
and NASA's Jet Propulsion Laboratory (JPL) have discovered that the lake which
once existed in Mars's Gale Crater was undergoing significant evaporation
earlier than the mineralogy and geochemistry of the lake bed sediments would
suggest.
The process of evaporation, while
commonplace to us on Earth, gives important clues to the ancient Martian
climate. The presence of evaporation signatures in the isotopic compositions of
water extracted from clay minerals in
the Martian rocks indicates that the Martian atmosphere was warm but also dry,
promoting evaporation of standing water.
"'Warm' is relative," says Amy
Hofmann, Ph.D., a visiting associate at Caltech and research scientist with
JPL, which Caltech manages for NASA. "We're talking a little above
freezing, but it was warm enough to potentially support the kinds of prebiotic
chemistries that astrobiologists are interested in.
"This was a dynamic time in Mars's
history. The planet was in the midst of a global climate transition, but we
know from the rocks at Gale that Mars's surface was still experiencing chemical
weathering, and the lake waters had a roughly circumneutral pH and were not
particularly salty. So, add to that mix the simple organic compounds previously
discovered in these same rocks, and you've got yourself a compellingly
habitable local environment."
Hofmann is the lead author on a paper
describing the study, which appears in the journal Proceedings
of the National Academy of Sciences.
The study focuses on oxygen isotopes rather than more commonly studied hydrogen
isotopes. The project is the first to find strong enrichments of oxygen-18 in
an ancient Martian water reservoir. Oxygen-18 is a relatively rare form of
oxygen that is heavier than its typical counterpart, oxygen-16, due to having
two more neutrons. When water evaporates, the H2O molecules containing a lighter oxygen atom tend to
be the first to go, leaving behind liquid water containing a higher concentration of heavy
oxygen.
The team studied samples collected by
the Curiosity rover between 2012 and 2021 from the Gale Crater region of Mars.
This deep depression on Mars shows signs of once having contained a large lake.
The rover sampled clay minerals, which are known to more accurately retain the
oxygen and hydrogen isotopic signatures imparted from the time they were
formed.
Though the oxygen isotope ratios in
Mars's atmosphere look quite similar to the ratios on Earth, water extracted
from the clay minerals showed strong enrichments of heavier oxygen. This
discovery indicates the evaporation was indeed occurring in Gale Crater at the
time when those sediments were deposited.
"This discovery of the Curiosity
rover team is an important step forward in our long struggle to understand how
water shaped the surface of Mars in ways that remind us of Earth yet are so
different in their details and their outcomes," says co-author John Eiler,
the Robert P. Sharp Professor of Geology and Geochemistry and the Ted and
Ginger Jenkins Leadership Chair of the Division of Geological and Planetary
Sciences.
"Most important to me is the new understanding we have gained of ways the drier atmosphere and wildly changing hydrosphere on Mars controlled the life cycles of its lakes—arguably our best targets for discovering evidence of life or its chemical precursors beyond Earth."
Source: Postcards from ancient Mars: Isotopes illuminate early Martian climate
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