Even though Mars has met a tragic fate by losing its magnetic field and most
of its atmosphere, the planet remains an important site of interest for
planetologists and exobiologists. The red planet could indeed have sheltered
life in its past, and, if it is the case, could still shelter part of it in
its undergrounds. In this context, NASA's Curiosity rover recently discovered
that traces of ancient Martian life may have been destroyed by biological
processes.
The space agency's Curiosity rover made the surprising discovery while
investigating clay-rich sedimentary rocks around its landing site in Gale
Crater, an ancient lake created when an asteroid hit the Red Planet about 3.6 billion years ago. Clay is a good indicator of signs of life, as
it is typically created when rock minerals erode and break down after
contact with water, a key ingredient in life. It is also an excellent
material for the storage of microbial fossils.
But when Curiosity took two samples of mudstone, a sedimentary rock
containing clay, from patches of the dry lake bed, dated to the same time
and place (3.5 billion years ago and only 400m apart), the researchers
found that one plot contained only half the expected amount of clay
minerals. Instead, this area contained a greater amount of iron oxides, the
compounds that give Mars its red hue.
Signs of a past life potentially destroyed by the brine
The team believe the culprit behind this geological removal process is
brine: super-saline water that has seeped into the mineral-rich clay layers
and destabilized them, flushing them out and cleaning up areas with the both
geological and perhaps even biological. “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.”
The rover completed its analysis by drilling into layers of Martian rock
before using its chemistry and mineralogy instrument, known as the CheMin, to
study the samples. The process of chemical transformation in the sediment is
called diagenesis, and it could have created new life under Mars even as it
erased some of the evidence of old life on its surface. So even though old
traces of life may have been erased in the brine areas, the chemical
conditions caused by the influx of salt water may have allowed more life to
arise in its place.
“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.”
The search for Martian life carried out jointly by Curiosity and Perseverance
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.
Tags:
Space & Astrophysics