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Biophysical Signatures of Extraterrestrial Life

At this year’s meeting of the American Geophysical Union, over 28,000 earth scientists stormed the streets of San Francisco with their puffy jackets, REI backpacks, and flannels; they were decked out to sit inside and talk about the latest discoveries in earth science (and drink over 4,500 cups of coffee). In one room, however, a small group of scientists were talking about science on planets a bit further away.

Reader, they were talking about aliens.

Finding life on other planets is no easy task, and it’s not likely we’ll find tiny little green dudes. Even finding microbes would be difficult. It’s possible that planets in our solar system had life long ago, but changing conditions have rendered them uninhabitable.

We don’t even know how life is created on earth, so how are we supposed to see how it created on other planets?

So, instead of looking for life itself, we need to look for signs. Looking for traces of life on other planets is not as easy as looking for fossils, footprints, or anything like that. The traces that microbes leave behind are exactly that: traces. To the untrained eye, these traces would be indistinguishable from a regular space rock.

Last year, astrobiologists at NASA published a list of 15 features that could indicate the presence of life on another planet, shown below. On the top of the ladder are the most convincing lines of evidence for life on other planets. Consequently, these would be the hardest for astronauts to observe on a mission, or for satellites to detect. To realistically find convincing signs of life, we need to climb further down the ladder.

The rungs on the ladder of life detection, as determined by NASA astrobiologists. (From Neveu et al. 2018)

We need to detect metabolism. SPACE METABOLISM.

Whether its an angiosperm or an alpaca, all living things need to convert matter into energy to survive. On the chemical level, energy production creates two products: one is oxidized, one is reduced. Based on this idea, we should be able to detect the existence of microbial communities on other planets by detection of these products.

So how can we detect them? The answer lies in a surveying technique that’s commonly used on earth, called induced polarization. An induced polarization survey can detect the chargeability of materials. In other words, we can measure how long certain materials can hold their charge over time. This works by placing two electrodes along the surface you’re measuring and sending electrical pulses with different frequencies. When the pulse travels in a porous media with an external electric field, like soil, a rock, or a sponge, it can become polarized.

We’re not sending people to shock the surface of mars anytime soon, but we could send satellites. Spectral Induced Polarization (SIP) can detect the same signals, without inducing a charge. These instruments can look meters below the surface, detecting signatures that even astronauts on the planet’s surface wouldn’t see.

The problem is, we can’t pinpoint what’s causing these signatures, at least not exactly. These signals could be signs of metabolism, or they could be something else entirely. This method can help us find places on the planet that likely have life, but to get the complete picture, we need to talk to geologists. Based on the physical and geological properties of other planets, geologists have developed a strategy to assessing possible microbial sites.

Within our solar system, there are three major celestial bodies that may be, or may have been capable of life. Mars is one of the top candidates, with evidence of ancient water. Scientists think Europa and Enceladus - two icy ocean worlds - are also capable of hosting life.

Because a high volume of microbial species are found in rocks underground, this is a likely place to find extraterrestrial microbes. What this means for current missions is that Mars rovers should be looking for freshly eroded surfaces to gain insight into what’s going on below the surface.

Areas where microbes could be living inside rocks (From Onstott et al. 2019)

Some people can get disappointed when they think of space bacteria, since it sounds like a less exciting life form. But if anything, I think it’s more exciting. It could be an entire microscopic world, probably different than anything we’ve ever seen; one that’s been just under the surface this whole time.

--Lissie Connors

Lissie Connors is a human person living on planet earth, but she would be open to other celestial options. She’s a regular contributor to Physics Buzz, covering anything from snow to mayonnaise.


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