- The large diversity of organic molecules detected on Mars is a hint that life once existed there.
- Today, the planet is rather inhospitable. Finding life will be difficult.
- To find it, we should search the salt-rich highlands, caves, lava tubes, and other places deep blow the surface.
Both of the NASA rovers currently exploring Mars — Curiosity and Perseverance — have been turning up more and more evidence of organic compounds at the planet’s surface. A recent paper by Maёva Millan of Georgetown University and colleagues details many types of organics previously unreported at the Gale Crater site, including nitrogen-, oxygen-,, and chlorine-containing molecules, as well as polycyclic aromatic hydrocarbons. And just last month, NASA reported Perseverance’s discovery of organic compounds in Jezero Crater, an ancient lake environment that may have hosted life a long time ago.
The mere presence of organic compounds isn’t evidence for life since there are many ways to produce them abiotically. But the discovery of so many organics, of so many different types, is quite astonishing. Compare this with the state of scientific thinking about Mars following the Viking mission of the 1970s. Neither Viking lander was thought to have found organics, which led Gerald Soffen, the mission’s project scientist, to declare: “No bodies, no life.” In fact, Viking’s gas chromatograph mass spectrometer had detected chlorinated organic compounds, but these were explained away as being due to contamination.
How do we know if it’s biological?
We can only wonder what Soffen, who died before the current era of Mars exploration began, would make of this new treasure trove of organic compounds found by the rovers. Could they be the remnants of microbes that lived billions of years ago? Jacob Heinz from the Technical University Berlin and I have looked at one type of sulfur organic, called thiophenes, discovered by Curiosity. Although we were unable to answer conclusively whether they were of biological origin, we did suggest a test for that: If a light carbon isotope signature is found in sediments or rocks associated with the compounds, they’re likely to be biological.
Why are isotope ratios significant? Carbon-13 has one neutron more than the more common carbon-12 isotope, so it’s heavier. Living organisms, being “lazy,” prefer to use the lighter variety because it takes slightly less energy to process. So, a sedimentary environment rich in carbon-12 would usually be interpreted on Earth as evidence for life.
That’s true of stromatolites (microbial mats dating from Earth’s ancient past), which can be found in the rock record because the microbial communities cemented sedimentary grains together. Stromatolites also may have existed several billion years ago on Mars when the planet’s surface was habitable, with plenty of water available. Intriguingly, an enriched carbon-12 signature recently has been found in mudstones at Gale Crater on Mars, which might indicate microbial activity.
To find life on Mars, follow the salt
But Mars is a very different planet today, and environmental conditions on the surface are extremely harsh. If any organisms are still around, they likely would have retreated to protected niches. But where, exactly? One likely place to look is the salt deposits in the Southern Highlands of Mars. In the driest areas on Earth, microbes live in salt rocks, using the hygroscopic properties of salts to extract water directly from the atmosphere. A similar lifestyle may also be feasible in the salt-rich highlands on Mars. Salt-loving microbes also could be found near the so-called Recurrent Slope Lineae (RSL), dark streaks appearing in images that are thought to be briny water discharging onto the Martian surface.
Another refuge for Martian life might be caves, particularly deep lava tube caves or ice caves that are natural windows to the subsurface. Here Martian organisms, if they exist, might find conditions warm and wet enough to thrive. Many caves are known to exist on Mars. In fact, Martian caves are usually larger than those on Earth, due to the lower gravity. Some of these, like ones seen in the Hebrus Valles, have been suggested as good locations for a future human station on Mars.
Hydrothermally active regions also would be likely places to look for life on Mars, as they would have warmer temperatures, water, and plenty of useful chemical com pounds, including organic molecules. Although no such regions have been identified yet, they should exist. Mars has experienced recent volcanic activity, and many of the planet’s observed geological features are consistent with hydrothermal environments.
Drill, baby, drill
If we want to find Martian life today, we may need to go deep. The planet’s deep subsurface should have temperatures and pressures in the right range, while offering protection from the harsh radiation on the surface. Nutrients might be scarce, though. And reaching these environments is a tougher proposition, requiring drills far beyond the current state of the art. Other locales might be better to search first.
NASA is actively discussing these search strategies, in workshops like the one held in 2019 called Mars Extant Life: What´s Next?. The scientists at that meeting agreed that it’s time to take the next step and send a life detection mission to some of these environments where we might expect life to still be present. What are we waiting for?