illustration of perseverance rover on mars

Science

Signs of life on Mars may be deeper than any current rover can dig — study

Ionizing radiation may destroy the signs of life Curiosity and Perseverance seek.

NASA

Early this year, NASA announced that its Perseverance rover had found organic molecules on Mars by shining a laser through samples collected in Jezero Crater, adding to the rover Curiosity's finds in 2018. But Curiosity and Perseverance’s discoveries of organic molecules on the Martian surface could just be the tip of the iceberg.

According to a newly published paper in Science Advances, a team working with the European Space Agency’s EXPOSE-R2 system on the International Space Station have found that the technique Perseverance used might have trouble finding many molecules to detect. After exposing the sorts of organic molecules most easily detected by the technique — known as Raman spectroscopy after Indian physicist C.V. Raman, who discovered it nearly a century ago — to a simulated Mars in Earth orbit, only a few traces of life remained at the surface.

But there might be a gold mine for astrobiologists beneath the probes’ wheels — it’s just that rovers haven’t been able to dig deep enough to get at it.

WHAT’S NEW — Perseverance and other Martian probes use a whole suite of instruments to detect the presence of organic molecules in the regolith. Part of Perseverance’s kit is Scanning Habitable Environments with Raman & Luminescence for Organics & Chemicals — or SHERLOC to its friends. Raman spectroscopy relies on shining a laser on a sample, then observing the way the light is scattered by the vibration of bonds between atoms in a molecule’s system.

This system serves two purposes on probes like Perseverance: looking at the makeup of minerals in the Martian regolith, and seeing if it can spot any organic residue along the way. The team’s experiment tested just how long organic molecules might survive in an environment like Mars’ by taking samples of seven organic molecules they thought would be most likely to be akin to what Martian life would need, mixing them into simulated Martian dirt, sealing them in with a Mars-like atmosphere.

EXPOSE-R2 simulated the stuff of life — then battered it with radiation levels like those on Mars. Roscosmos / ESA

Speaking with Inverse, team leader and astrobiologist Mickael Baqué explains, “if Martian life evolved, they — or some similar structures — would play a significant role in Martian life.”

Then the whole thing was strapped to the outside of the International Space Station for 469 days.

After almost sixteen months, only three of the seven were readily detectable by spectroscopy: chlorophyllin, quercetin, and melanin. The other four — beta carotene, naringenin, cellulose, and chitin (which lines the cells of fungi) — were much harder to detect. Even the most resilient molecules (chlorophyllin and quercetin) were just half as common as they were when they were first exposed.

If biomolecules start to break down quickly from exposure to solar radiation through the thin Martian atmosphere (and lack of magnetic field), this means that the surface might not be a very fruitful place to look for traces of Martian life. But samples that were shielded from ultraviolet radiation showed little change — so the best place to look might just be a few feet beneath Perseverance’s wheels.

WHY IT MATTERS — Although Curiosity and Perseverance have had some success in finding organic molecules on the radiation-blasted Martian surface, this experiment confirms that the best place to look for traces of life is well below the sands. “We could still detect a very few molecules on the surface,” says Bacqué, but “they are much better preserved when they are protected from UV just below the surface.”

That isn’t unexpepected—missions to Mars already do their best to dig beneath the surface. But even Perseverance can only dig a few inches below the surface, where organic molecules are still vulnerable to break down over time.

Signs of Martian life would need to stay intact over geologic timescales that are difficult to grasp, much less to duplicate experimentally. Even experiments like EXPOSE-R2, Bacqué notes, aren’t able to come close to the vast timespans and, well, exposures of potential Martian biomolecules. “If we expose them one year around the Earth, it’s just not the same as three billion years on Mars.”

To try to get some semblance of that exposure, complementary experiments focus on setting organic molecules in high-energy particle accelerators. But even blasting molecules with a particle accelerator only involves a fraction of the hostility of eons of exposure to the sun’s radiation. “In space,” Bacque explains, “there are a lot of things happening that we cannot simulate completely on the ground.”

The Rosalind Franklin rover could dig deep enough to find life on Mars — but won’t launch until the end of the decade. ESA

WHAT’S NEXT — The European Space Agency rover Rosalind Franklin was designed to be able to drill two meters below the Martian surface to search for signs of life sheltered well below the reach of ionizing radiation.

But the rover — part of a joint European-Russian mission that was planned to launch this month, with a landing date of June 10, 2023 — has been put on hold after the ESA voted to suspend and then terminate the mission this July in the wake of the Russian invasion of Ukraine. Rosalind Franklin probably won’t arrive on Mars until at least 2028, since ESA will need to build a new lander and find a different ride to the red planet.

“We would have hoped much sooner to be able to compare different places on Mars,” Bacqué said, “especially reaching these depths that we never have.” But the fact that the next Martian rover is currently in deep storage doesn’t mean that the race to interpret even the faintest signs of life has slowed. In the meantime, Bacqué emphasizes that this doesn’t mean Perseverance is totally out of luck. Other instruments more attuned to detecting organic materials were able to detect trickier molecules, like cellulose from a kombucha scoby, that were exposed to the same 15 months of hard radiation. Ultimately, he says, “it’s a nice tool to have, but it’s not that important.”

Plans to return to the moon and set up a long-term scientific base there would help exobiologists delve deeper into the science of finding life amid even the tiniest traces. “Astrobiolabs in lunar orbit or on the lunar surface,” Bacqué notes, would be “where we can push new techniques and instruments to go further, for Mars and the icy moons and so on.”

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