A NASA team has found that organic salts are likely present on Mars. Like shards of ancient pottery, these salts are the chemical remnants of organic compounds, such as those previously detected by NASA’s Curiosity rover. Organic compounds and salts on Mars could have formed by geologic processes or be remnants of ancient microbial life.
Besides adding more evidence to the idea that there once was organic matter
on Mars, directly detecting organic salts would also support modern-day Martian
habitability, given that on Earth, some organisms can use organic salts, such
as oxalates and acetates, for energy.
“If we determine that there are organic salts concentrated anywhere on
Mars, we’ll want to investigate those regions further, and ideally drill deeper
below the surface where organic matter could be better preserved,” said James M. T. Lewis, an organic geochemist
who led the research, published
on March 30 in the Journal of Geophysical Research: Planets. Lewis is based at
NASA’s Goddard Space Flight Center in Greenbelt, Maryland.
Lewis’s lab experiments and analysis of data from the Sample Analysis at
Mars (SAM), a portable chemistry lab inside Curiosity’s belly, indirectly point
to the presence of organic salts. But directly identifying them on Mars is hard
to do with instruments like SAM, which heats Martian soil and rocks to release gases
that reveal the composition of these samples. The challenge is that heating
organic salts produces only simple gases that could be released by other
ingredients in Martian soil.
What do you do if you
have a sample from another planet, and you want to find out if it contains a
certain molecule...maybe even one that will reveal whether the planet can
sustain life? When scientists face a situation like this, they use an amazing
tool: the mass spectrometer. It separates out materials, allowing scientists to
look very closely at a sample and see what's inside.
Credits: NASA/Goddard Space Flight Center Download video here
However, Lewis and his team propose that another Curiosity instrument that
uses a different technique to peer at Martian soil, the Chemistry and
Mineralogy instrument, or CheMin for short, could detect
certain organic salts if they are present in sufficient amounts. So far, CheMin
has not detected organic salts.
Finding organic molecules, or their organic salt remnants, is essential in
NASA’s search for life on other worlds. But this is a challenging task on the
surface of Mars, where billions of years of radiation have erased or broken
apart organic matter. Like an archeologist digging up pieces of pottery,
Curiosity collects Martian soil and rocks, which may contain tiny chunks of
organic compounds, and then SAM and other instruments identify their chemical structure.
Using data that Curiosity beams down to Earth, scientists like Lewis and
his team try to piece together these broken organic pieces. Their goal is to
infer what type of larger molecules they may once have belonged to and what
those molecules could reveal about the ancient environment and potential
biology on Mars.
“We’re trying to unravel billions of years of organic chemistry,” Lewis
said, “and in that organic record there could be the ultimate prize: evidence
that life once existed on the Red Planet.”
While some experts have predicted for decades that ancient organic
compounds are preserved on Mars, it took experiments by Curiosity’s SAM to
confirm this. For example, in 2018, NASA Goddard astrobiologist Jennifer
L. Eigenbrode led an international team of
Curiosity mission scientists who reported
the detection of myriad molecules containing an
essential element of life as we know it: carbon. Scientists identify
most carbon-containing molecules as “organic.
Research scientist Dr.
Jennifer Eigenbrode discusses the discovery of ancient organic molecules on
Mars.
Credits: NASA's Goddard
Space Flight Center/Dan Gallagher
Download this video in HD formats
from NASA Goddard's Scientific Visualization Studio
“The fact that there’s organic matter preserved in 3-billion-year-old
rocks, and we found it at the surface, is a very promising sign that we might
be able to tap more information from better preserved samples below the
surface,” Eigenbrode said. She worked with Lewis on this new study.
Analyzing Organic Salts
in the Lab
Decades ago, scientists predicted that organic compounds on Mars could be breaking down into salts. These salts, they
argued, would be more likely to persist on the Martian surface than big,
complex molecules, such as the ones that are associated with the functioning of
living things.
If there were organic salts present in Martian samples, Lewis and his team
wanted to find out how getting heated in the SAM oven could affect what types
of gases they would release. SAM works by heating samples to upwards of
1,800 degrees Fahrenheit (1,000 degrees Celsius). The heat breaks apart
molecules, releasing some of them as gases. Different molecules release
different gases at specific temperatures; thus, by looking at which
temperatures release which gases, scientists can infer what the sample is made
of.
“When heating Martian samples, there are many interactions that can happen
between minerals and organic matter that could make it more difficult to draw
conclusions from our experiments, so the work we’re doing is trying to pick
apart those interactions so that scientists doing analyses on Mars can use this
information,” Lewis said.
Lewis analyzed a range of organic salts mixed with an inert silica powder
to replicate a Martian rock. He also investigated the impact of adding
perchlorates to the silica mixtures. Perchlorates are salts containing chlorine
and oxygen, and they are common on Mars. Scientists have long worried that they
could interfere with experiments seeking signs of organic matter.
This is the first photo
ever taken on the surface of Mars. It was taken by NASA’s Viking 1 spacecraft
just minutes after it landed on the Red Planet on July 20, 1976.
Credits: Credits: NASA/JPL More information here.
Indeed, researchers found that perchlorates did interfere with their
experiments, and they pinpointed how. But they also found that the results they
collected from perchlorate-containing samples better matched SAM data than when
perchlorates were absent, bolstering the likelihood that organic salts are
present on Mars.
Additionally, Lewis and his team reported that organic salts could be
detected by Curiosity’s instrument CheMin. To determine the composition of a
sample, CheMin shoots X-rays at it and measures the angle at which the X-rays
are diffracted toward the detector.
Curiosity’s SAM and CheMin teams will continue to search for signals of
organic salts as the rover moves into a new region on Mount Sharp in Gale
Crater.
Soon, scientists will also have an opportunity to study better-preserved
soil below the Martian surface. The European Space Agency’s forthcoming ExoMars
rover, which is equipped to drill down to 6.5 feet, or 2 meters, will carry a
Goddard instrument that will analyze the chemistry of these deeper Martian
layers. NASA’s Perseverance rover doesn’t have an instrument that can detect
organic salts, but the rover is collecting samples for future return to Earth,
where scientists can use sophisticated lab machines to look for organic
compounds.
Banner image: This look back at a dune that NASA's Curiosity Mars rover
drove across was taken by the rover's Mast Camera (Mastcam) on Feb. 9, 2014, or
the 538th Martian day, or sol, of Curiosity's mission. For scale, the distance
between the parallel wheel tracks is about 9 feet (2.7 meters). The dune is
about 3 feet (1 meter) tall in the middle of its span across an opening called
"Dingo Gap." This view is looking eastward. Credits:
NASA/JPL-Caltech/MSSS. More
information here.
May 20, 2021 By Lonnie Shekhtman
NASA’s Goddard Space Flight Center, Greenbelt, Md.
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