Some of the places on Mars most likely to find traces of ancient life may also be those least likely to preserve them.

That’s the conclusion of a new study that simulates the effects of cosmic rays hitting the surface of Mars on important building blocks of life called “lipids.” In short, the exposed material appeared to break down very quickly under the bombardment of radiation from space; In fact, when salt is mixed into the sediment, it breaks down even faster; This is probably the case in many areas that we consider to be obsolete. places of residence Marcy.

“We prefer salt-rich environments, but these may be the most harmful environments in terms of radiation,” astrobiologist Anais Roussel of Georgetown University told Space.com.

Erase evidence of a past life

“This is a big limitation we have in astrobiology, and the more we know, the better,” says Roussel.

Research by Roussel and colleagues suggests that this is cause for concern; especially on Mars, places that probably remained habitable when the planet became cooler and drier about 4 billion years ago.

The researchers focused specifically on hopanes and steranes, which are fossil forms of chemicals called hopanols and sterols. Hopanols are important parts of bacterial cell membranes, while sterols are part of eukaryotic cell membranes (organisms whose cells contain nuclei; such as humans). Here on Earth, these two lipids represent some of the most persistent chemical signatures of life; They can survive in rocks or soil for billions of years given the right conditions. Additionally, living cells are the only known source of these chemicals; So if they do emerge, it would likely be clear evidence of life with chemistry similar to life on Earth.

But here on Earth, most rocks and soils aren’t constantly bombarded by cosmic rays, thanks to the shielding of our atmosphere and magnetic field. Not so on Mars. It lost these shields about 4 billion years ago. When Roussel and his colleagues bombarded lipid samples with gamma rays to simulate cosmic ray exposure to Mars, about half of the lipids in his sample dissolved into an unrecognizable mixture of smaller molecules, the equivalent of about 3 million years of exposure. On the surface of Mars.

For context, some rocks in Gale Crater, home of the Curiosity rover, have been exposed to cosmic rays on the Martian surface for about 80 million years.

“Three million years is a very, very short time to lose such good diagnostic biosignatures,” says Roussel.

The team’s lipid samples degraded about twice as fast as another important chemical that previous research had tested in similar experiments: amino acids that form proteins, literally the building blocks of life. Roussel suggests this may be because lipids are much larger molecules and their shape is very different from amino acids, meaning they have more surface area to be exposed to incoming radiation.

And again, radiation exposure isn’t much of a problem on Earth, but it could be serious on Mars.

“When we go to Mars, we really have to keep all these parameters in mind and try to avoid defining one perfect location, one single biosignature, one perfect target,” Roussel says.

Many of the places that astrobiologists see as the most likely evidence of ancient Martian life are very salty.

As the Martian atmosphere thinned and its surface cooled, fresh water either froze in the cold or boiled away under low air pressure (depending on location). Because a lower temperature is required for salt water to freeze, salty streams and lakes will be among the last remaining liquids; Salt also makes it a little harder for water to boil, so when the air pressure drops the salt content should have prevented the water from being lost into the steam club.

Salt does more harm than good when it comes to preserving the chemical signatures of the creatures that once lived in these salt pools.

“At this point, we don’t know what specifically could create something in the structure of the salt that would further degrade the organics,” says Roussel. This is a question that scientists are still working on. Radiation can cause chloride or sodium salts of chemicals to react with organic molecules (such as lipids) and break them into smaller pieces. On the other hand, if there is even a microscopic trace of water among the salts, this can create chemicals called oxidants that break down organic molecules very quickly.

Hopes are endless, even if the springs are salty

While the findings may seem discouraging, Roussel says they make him more optimistic than ever about the possibility of life on Mars.

“Perhaps if we haven’t found anything definitive yet, it doesn’t mean there was never life on Mars, it just means we’re looking in the wrong place or need to dig deeper.”

In 2029, the European Space Agency’s Rosalind Franklin rover will have the chance to do just that. NASA’s Curiosity and Perseverance rovers can only drill about 5 centimeters (about 2 inches) into the ground; this is not deep enough to reach rocks or sediments protected from cosmic radiation. But the Rosalind Franklin drilling will reach about 2 meters (78 inches), which is enough to avoid most, but not all, radiation exposure.

“My dream would be to see a mission that goes to a cave on Mars or a lava tube on Mars, because one of those caves might not have been touched by radiation at all,” Roussel says. “It would be extremely difficult from an engineering standpoint, but I think if you can escape it gives hope.” The study was published Nov. 13 in the journal Astrobiology.