A new paper enriches scientists’ understanding of where the rock record
preserved or destroyed evidence of Mars’ past and possible signs of ancient
life.
Today, Mars is a planet of extremes – it’s bitterly cold, has high
radiation, and is bone-dry. But billions of years ago, Mars was home to lake
systems that could have sustained microbial life. As the planet’s climate
changed, one such lake – in Mars’ Gale Crater – slowly dried out. Scientists
have new evidence that supersalty water, or brines, seeped deep through the
cracks, between grains of soil in the parched lake bottom and altered the
clay mineral-rich layers beneath.
The findings published in the July 9 edition of the journal Science and led
by the team in charge of the Chemistry and Mineralogy, or CheMin, instrument
– aboard NASA’s Mars Science Laboratory Curiosity rover – help add to the
understanding of where the rock record preserved or destroyed evidence of
Mars’ past and possible signs of ancient life.
“We used to think that once these layers of clay minerals formed at the
bottom of the lake in Gale Crater, they stayed that way, preserving the
moment in time they formed for billions of years,” said Tom Bristow, CheMin
principal investigator and lead author of the paper at NASA’s Ames Research
Center in California’s Silicon Valley. “But later brines broke down these
clay minerals in some places – essentially resetting the rock record.”
Mars: It Goes on Your Permanent Record
Mars has a treasure trove of incredibly ancient rocks and minerals compared
with Earth. And with Gale Crater’s undisturbed layers of rocks, scientists
knew it would be an excellent site to search for evidence of the planet’s
history, and possibly life.
Using CheMin, scientists compared samples taken from two areas about a
quarter-mile apart from a layer of mudstone deposited billions of years ago
at the bottom of the lake at Gale Crater. Surprisingly, in one area, about
half the clay minerals they expected to find were missing. Instead, they
found mudstones rich with iron oxides – minerals that give Mars its
characteristic rusty red color.
Scientists knew the mudstones sampled were about the same age and started
out the same – loaded with clays – in both areas studied. So why then, as
Curiosity explored the sedimentary clay deposits along Gale Crater, did
patches of clay minerals – and the evidence they preserve – “disappear”?
Clays Hold Clues
Minerals are like a time capsule; they provide a record of what the
environment was like at the time they formed. Clay minerals have water in
their structure and are evidence that the soils and rocks that contain them
came into contact with water at some point.
“Since the minerals we find on Mars also form in some locations on Earth, we
can use what we know about how they form on Earth to tell us about how salty
or acidic the waters on ancient Mars were,” said Liz Rampe, CheMin deputy
principal investigator and co-author at NASA’s Johnson Space Center in
Houston.
Previous work revealed that while Gale Crater’s lakes were present and even
after they dried out, groundwater moved below the surface, dissolving and
transporting chemicals. After they were deposited and buried, some mudstone
pockets experienced different conditions and processes due to interactions
with these waters that changed the mineralogy. This process, known as
“diagenesis,” often complicates or erases the soil’s previous history and
writes a new one.
Diagenesis creates an underground environment that can support microbial
life. In fact, some very unique habitats on Earth – in which microbes thrive
– are known as “deep biospheres.”
“These are excellent places to look for evidence of ancient life and gauge
habitability,” said John Grotzinger, CheMin co-investigator and co-author at
the California Institute of Technology, or Caltech, in Pasadena, California.
“Even though diagenesis may erase the signs of life in the original lake, it
creates the chemical gradients necessary to support subsurface life, so we
are really excited to have discovered this.”
By comparing the details of minerals from both samples, the team concluded
that briny water filtering down through overlying sediment layers was
responsible for the changes. Unlike the relatively freshwater lake present
when the mudstones formed, the salty water is suspected to have come from
later lakes that existed within an overall drier environment. Scientists
believe these results offer further evidence of the impacts of Mars’ climate
change billions of years ago. They also provide more detailed information
that is then used to guide the Curiosity rover’s investigations into the
history of the Red Planet. This information also will be utilized by NASA’s
Mars 2020 Perseverance rover team as they evaluate and select rock samples
for eventual return to Earth.
“We’ve learned something very important: There are some parts of the Martian
rock record that aren’t so good at preserving evidence of the planet’s past
and possible life,” said Ashwin Vasavada, Curiosity project scientist and
co-author at NASA’s Jet Propulsion Laboratory in Southern California. “The
fortunate thing is we find both close together in Gale Crater, and can use
mineralogy to tell which is which.”
Curiosity is in the initial phase of investigating the transition to a
“sulfate-bearing unit,” or rocks thought to have formed while Mars’ climate
dried out.
Reference:
“Brine-driven destruction of clay minerals in Gale crater, Mars” by T. F.
Bristow, J. P. Grotzinger, E. B. Rampe, J. Cuadros, S. J. Chipera, G. W.
Downs, C. M. Fedo, J. Frydenvang, A. C. McAdam, R. V. Morris, C. N.
Achilles, D. F. Blake, N. Castle, P. Craig, D. J. Des Marais, R. T. Downs,
R. M. Hazen, D. W. Ming, S. M. Morrison, M. T. Thorpe, A. H. Treiman, V. Tu,
D. T. Vaniman, A. S. Yen, R. Gellert, P. R. Mahaffy, R. C. Wiens, A. B.
Bryk, K. A. Bennett, V. K. Fox, R. E. Millken, A. A. Fraeman and A. R.
Vasavada, 9 July 2021, Science. DOI:
10.1126/science.abg5449
Tags:
Space & Astrophysics