In the depths of an Alpine mountain range, a researcher is searching for life that might look just like that on other planets.
The Bedretto tunnel is a disused access way to a railway tunnel underneath the imposing Saint-Gotthard mountain range in the Swiss Alps. Deep within the towering mass of granite, geobiologist Cara Magnabosco collects samples of water that hasn't seen the light of day for millions of years. In those samples, Magnabosco searches for ancient microorganisms that are quite different from those found on Earth's surface.
Unlike most of today's life, these microorganisms don't need oxygen to survive, which makes scientists believe that they might look quite like those that first emerged on our planet over 3.5 billion years ago when Earth's atmosphere had little oxygen. But it's not just the glimpse into the earliest days of life on Earth that Magabosco is seeking. She believes that those inhabitants of the wet mountain darkness could teach her about life on other bodies in the solar system, like Mars or the ice-covered moons of Saturn and Jupiter, and show her how to find it.
Signatures of life
"Essentially, we want to understand what are the products of a planet when there is no life, like water-rock reactions, and compare that to the signatures where life is present," Magnabosco, who is a professor of geobiology at the Swiss Federal Institute of Technology in Zürich (ETH Zurich) told Space.com in an interview. "We can study that underground. We can go to the point where there is no life and look at the products and then look at how these products change when there is life."
The changes may be barely noticeable. For example, scientists know that living organisms produce methane. But so do many geological processes. But the methane that is a byproduct of life may look different than the purely geological gas. It may contain different isotopes, forms of the same chemical element with a different number of neutrons in their nucleus. Scientists reason that learning to distinguish between those isotopic differences on Earth will help them develop tools and techniques to do the same elsewhere in the solar system.
It's not just methane that can give out a planet's or a moon's secret. There is a whole range of chemical elements that scientists are interested in, including carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur, Magnabosco said.
"Methane has been extremely well studied but, for example, sulfate is also really big for microbiology in these extreme environments," Magnabosco said. "But also things like nitrates and ammonia, those are all things that life on Earth uses all the time, they are the key elements of the life cycle."
The water in which Magnabosco searches for the microbes may at first glance appear just like the water running from the tap or raining from the sky. But sensitive scientific instruments reveal that the liquid is, in fact, very different. Trapped miles below the planet's surface by geological faults and fractures, the ancient water is saltier and has less oxygen dissolved in it than waters far above. On the other hand, scientists can detect more hydrogen and traces of methane in these subterranean samples.
The water that drips from the walls of the Bedretto tunnel is no more than 300 million years old, but elsewhere in the world such as in Canada and South Africa, deeper deposits have been found that are up to a billion years old. Microbes trapped in such waters have evolved without contact with the planet's surface for more than a quarter of the time for which life has existed on Earth.
"What we see in this water as it gets older is that the cell numbers are going down. The microbial population sizes are decreasing quite a bit," Magnabosco said. "In this old water, the amount of [living] cells per milliliter can be tens of thousands times lower than what we see in the ocean."
Since conditions on other bodies in the solar system are unlikely to make it easy for any lifeforms to survive, geobiologists hope that learning how life operates on the edge of survivability under Earth's crust will tell them where and how to look for its traces elsewhere.
"We want to understand how the organisms got there, how they evolved and how they continue to survive," Magnabosco said. "They have been isolated from the surface for a long time and haven't had any input from photosynthesis or oxygen."
The common ancestor
The researchers have found that those creatures dwelling in the depths are not completely different from the everyday daylight-savoring microbes, however. They are made of similar types of proteins, and their DNA is so similar to their aboveground counterparts, in fact, that scientists are quite certain the underground creatures must be distant cousins of the surface microbes.
"The next big question for us is to find where the common ancestor is from," Magnabosco said. "Was the original ancestor something underground that went to the surface or something on the surface that went underground?"
Whatever the answer to that question is, Magnabosco thinks that the underground residents are much more similar to those distant ancestors than the much busier life forms on the planet's surface. While the world underground has barely changed for billions of years, allowing the microbes to relax into a predictable existence, conditions on the surface have shifted many times, forcing organisms to adapt and evolve.
"It's definitely a habitat that has been existing for billions of years," Magnabosco said. "The conditions and the reactions that are taking place there and driving the organisms living there are relatively consistent over really long time scales, which is a lot different than the surface of our planet where we have seen huge changes in concentrations of oxygen over the billions of years but also changes in ocean chemistry and the nutrient supply on the surface."
Microbial life may indeed be waiting to be discovered elsewhere in the solar system. In the meantime, it turns out we have also had some very alien neighbors right under our noses this whole time.