Scientists have announced the discovery of the largest organic molecules ever found on Mars.
These molecules could have come from the breakdown of fatty acids that existed 3.7 billion years ago before being preserved in sediments laid down by an ancient lake on the Red Planet. While molecules don’t exactly prove the existence of past life on Mars, scientists say that they show that such a discovery might indeed be possible.
“Our study proves that, even today, by analyzing Mars samples we could detect chemical signatures of past life, if it ever existed on Mars,” said Caroline Freissinet, an astrochemist from the Laboratoire Atmosphères et Observations Spatiales in Guyancourt, France, in a statement.
The molecules are known as alkanes, hydrocarbons that combine chains of carbon atoms with hydrogen atoms attached to them. They were found by NASA’s Curiosity rover when it drilled into a sample of mudstone from a rock nicknamed ‘Cumberland’, in the Yellowknife Bay region of Mars’ Gale crater, all the way back in 2013.
Curiosity landed in the 96-mile (154-kilometer) Gale crater in 2012. The crater was once flooded by an ancient lake, making it a promising location to search for signs of past habitability, and Yellowknife Bay lies on what was once the lake floor. The Cumberland rock is made from clay-rich sedimentary material laid down by this lake.
“There is evidence that liquid water existed in Gale crater for millions of years and probably much longer, which means there was enough time for life-forming chemistry to happen in these crater-lake environments on Mars,” said Daniel Glavin of NASA’s Goddard Space Flight Center in the statement announcing this discovery.
The organic molecules were discovered by Curiosity’s Sample Analysis on Mars (SAM) suite of instruments. Using this suite, the rover drills a rock sample and scoops it into the SAM, where it is baked in an oven to 1,800 degrees Fahrenheit (1,000 degrees Celsius). This releases gases from the sample, which are then separated and studied by specialized sensors (in this case, a gas chromatograph and a mass spectrometer) that can identify the elements vital for life — carbon, nitrogen, oxygen, phosphorus and sulfur. A separate laser spectrometer analyzes the gases, searching for signs of water and smaller organic molecules such as methane.
Freissinet and Glavin previously co-led earlier discoveries of organic molecules in the Cumberland sample, including methane as well as chlorine or sulfur-bearing organics, but until this study, the largest organic molecules found on Mars had been only six carbon atoms long.
So Freissinet and Glavin modified the SAM procedure to search for larger organic molecules. In particular they and their team were looking for amino acids. They didn’t find any, but they did find alkanes larger than any found on Mars thus far. These include decane (10 carbon atoms and 22 hydrogen atoms), undecane (11 carbon atoms and 24 hydrogen atoms) and dodecane (12 carbon atoms and 26 hydrogen atoms). Although dodecane is the largest alkane ever found on Mars, it is still dwarfed in comparison to the largest alkanes on Earth, which can feature over 150 carbon atoms.
It’s possible that the large Martian alkanes have a geochemical origin — nothing to do with life — but Freissinet and Glavin’s team performed laboratory experiments showing that they could have come from the breakdown of fatty acids entombed in the clay-rich sedimentary materials over 3.7 billion years.
Life incorporates fatty acids into cell membranes and uses them to regulate various cell and organ processes. However, fatty acids can also be produced by geochemistry, so inferring their existence on Mars 3.7 billion years ago is not necessarily proof of life. However, abiotic fatty acids do tend to be smaller in size than 12 carbon atoms. The timing is also curious — 3.7 billion years ago is also when life is thought to have first started getting a grip on Earth. Could life on both our worlds have developed synchronously?
For now that remains speculation, but while the presence of long-chain alkanes is not a smoking gun for life on Mars, they could be a big clue.
However, Curiosity rover’s SAM instruments are unable to detect organic molecules larger than dodecane, meaning that the future of Mars-life research could lie on Earth.
“We are ready to take the next big step and bring Mars samples home to our labs to settle the debate about life on Mars,” said Glavin, who is NASA’s senior scientist for sample return. However, this would depend upon how soon a mission can be launched to retrieve the samples cached by Curiosity’s sibling rover, Perseverance. NASA has recently run into difficulties designing and funding a retrieval mission and has solicited help from private companies.
The findings were published on March 24 in Proceedings of the National Academy of Sciences.
