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Did life ever exist on Mars? Humanity is one small step closer to learning the answer to that question thanks to the latest batch of organic molecules identified by NASA’s Curiosity Rover.
In a paper published today in Nature, NASA scientists led by Dr. Amy J. Williams of the University of Florida announced the results of the first “wet chemistry” experiments carried out by Curiosity on Martian soil.
The rover has been exploring Mars since 2012. Almost immediately, it began returning data on organic molecules found on the planet’s surface. However, its Sample Analysis at Mars (SAM) apparatus includes multiple methods for identifying those molecules, and we’re just now starting to get the results from some of them.
These methods tend to require breaking the molecules down and examining the parts. The simplest of these is pyrolysis, which just uses heat. However, the latest experiment used on-board capsules of tetramethylammonium hydroxide — a very strong and toxic base — to decompose the organics and analyze them in more detail.
The team was particularly excited to discover N-heterocycles among the molecules. These are ring-shaped compounds with a nitrogen atom where one of the carbon atoms would normally be. The paper explains that N-heterocycles are “fundamental components of astrobiologically relevant molecules, such as nucleic acids.”
Nucleic acids include DNA and RNA, whose presence would unambiguously indicate that Earth-like life once existed on the Red Planet. N-heterocycles on their own are not quite that kind of smoking gun. However, you could not have DNA or RNA without them, so their presence is a hopeful sign.
These compounds have been buried in the Martian soil for about 3.5 billion years, so any hypothetical biological originator is long gone. However, evidence that any sort of life ever existed on Mars would dramatically increase the likelihood of finding it elsewhere, demonstrating that Earth isn’t a once-in-a-Universe fluke.
The Difference Between Organic and Biological
It’s easy to get excited when hearing about “organic” molecules on other worlds. After all, in everyday English, we use the term to mean “natural,” or more specifically, “coming from living organisms.”
In chemistry, the meaning is quite different. Any molecule containing bonded carbon and hydrogen atoms is “organic.”
Carbon (and related elements like silicon) is special because it has exactly four electrons in its outer shell. Elements with fewer than that tend to want to bond with elements that have more, and vice versa. But carbon, being right in the middle, can bond with almost anything, including itself.
Hydrogen complements carbon by, essentially, “tying up the loose ends” of whatever complex structure it might form. Those structures mostly consist of some combination of long chains and rings of carbon atoms, with the occasional other molecule mixed in. The reason for the multiple meanings of “organic” is that those rings and chains are the building blocks of all life, and most organic molecules on Earth come from organic processes.
Most, but not all.
There are various metamorphic processes that produce simple organic compounds. Another paper, published in 2022, has already presented evidence that one of these, serpentinization, was taking place on Mars around 4 billion years ago. At that time, Mars still had liquid water on its surface, and the same process takes place on Earth at the bottom of our oceans.
The current paper acknowledges that there are, essentially, three ways that the organic molecules identified by Curiosity could have ended up in the Martian clay. It could have originated with life on Mars, or it could have come from serpentinization and similar processes.
The third possibility is that the molecules come from Earth or somewhere else entirely, carried to Mars aboard meteorites. That’s not far-fetched, as we have found chunks of Mars on Earth. These were knocked loose by ancient impacts and happened to find their way here, so the reverse could happen as well. In fact, the 2022 paper that found evidence of serpentinization on Mars did so by analyzing one of those Martian meteorites.
Martian Clay Makes a Great Preservative
The most important thing about this latest research isn’t the specific molecules identified. In fact, Curiosity’s experiment found at least two dozen different molecules, but could only confirm the identity of about half of them.
The important thing is that the abundance of such molecules in the sample shows that the clay of Gale crater — once a Martian seabed — does a pretty good job of keeping them intact.
One problem with organic molecules is that their size and complexity makes them fragile. Even things like sunlight can cause them to break over time. If you’ve ever had a sunburn, you’ve experienced that first-hand.
Take away Earth’s protective ozone layer and multiply that sunburn by 3.5 billion years, and you can see the potential problem. Until we actually found them, it was never a sure thing that any organic molecules would still be intact close to the Martian surface, if they’d existed in the first place.
The fact that a variety of identifiable organic compounds is present means there’s lots of meaningful science yet to do on Mars. Scientists don’t know yet where they came from, but it seems clear enough that there should be enough evidence present to answer that question eventually.
In the meantime, perhaps they can bring back some of that Martian mud and market it as sunscreen.
