A bizarre microbe found deep in a gold mine in South Africa could provide a model for how life might survive in seemingly uninhabitable environments through the cosmos. Known as Desulforudis audaxviator, the rod-shaped bacterium thrives 2.8 kilometers underground in a habitat devoid of the things that power the vast majority of life on Earth—light, oxygen, and carbon. Instead, this “gold mine bug” gets energy from radioactive uranium in the depths of the mine. Now, scientists predict that life elsewhere in the universe might also feed off of radiation, especially radiation raining down from space.
“It really grabbed my attention because it’s completely powered by radioactive substances,” says Dimitra Atri, an astrobiologist and computational physicist who works for the Blue Marble Space Institute of Science in Seattle, Washington. “Who’s to say life on other worlds doesn’t do the same thing?”
Most life on Earth's surface takes in the energy it needs through one of two processes. Plants, some bacteria, and certain other organisms collect energy from sunlight through a process called photosynthesis. In it, they use the energy from light to convert water and carbon dioxide into more complex and energetic molecules called hydrocarbons, thus storing the energy so that it can be recovered later by breaking down the molecules through a process called oxidation. Alternatively, animals and other organisms simply feed off of plants, one another, etc., to steal the energy already stored in living things.
D. audaxviator takes a third path: It draws its energy from the radioactivity of uranium in the rock in the mine. The radiation from decaying uranium nuclei breaks apart sulfur and water molecules in the stone, producing molecular fragments such as sulfate and hydrogen peroxide that are excited with internal energy. The microbe then takes in these molecules, siphons off their energy, and spits them back out. Most of the energy produced from this process powers the bacterium’s reproduction and internal processes, but a portion of it also goes to repairing damage from the radiation.
Atri thinks an extraterrestrial life form could easily make use of a similar system. The radiation might not come from radioactive materials on the planet itself, but rather from galactic cosmic rays (GCRs)—high-energy particles that careen through the universe after being flung out of a supernova. They’re everywhere, even on Earth, but our planet’s magnetic field and atmosphere shields us from most GCRs.
The surfaces of other planets like Mars are much more susceptible to cosmic rays because of their thin atmospheres and, in the case of Mars, its lack of a magnetic field. Atri argues GCRs could reach the Red Planet’s surface with enough energy left to power a tiny organism. This could also be the case on any world with a negligible atmosphere: Pluto, Earth’s moon, Jupiter’s moon Europa, Saturn’s moon Enceladus, and, theoretically, countless more outside our solar system. He does note, though, that because GCRs don’t deliver nearly as much energy as the sun, GCR-powered life would be very small, and simple, just like D. audaxviator.
To figure out how this might work, Atri ran simulations using existing data about GCRs to see how much energy they’d provide on some of these other worlds. The numbers were clear: The small, steady shower of cosmic rays would supply enough energy to power a simple organism on all of the planets he simulated except Earth, Atri reports this week in the Journal of the Royal Society Interface. “It can’t be ruled out that life like this could exist,” he says.
Atri thinks Mars is the best candidate to host GCR-powered life. The planet’s composition is rocky like Earth’s with plenty of minerals, and it might even have some water tucked away. Both would offer excellent mediums to be broken down by cosmic rays and gobbled up by a life form. The most essential part of the equation, though, is the thin atmosphere. “It’s funny,” Atri says, “because when we look for planets that contain life currently, we look for a very thick atmosphere. With these life forms, we’re looking for the opposite.”
Duncan Forgan, an astrobiologist at the University of St Andrews in the United Kingdom who was not involved with the work, agrees that Mars might be harboring D. audaxviator-like life because its stable temperatures and physical makeup are similar to that of the South African gold mine. He does worry that on other planets that don’t receive light energy from a sun but still get bombarded with GCRs—such as free floating rogue planets not tied to any solar system—temperatures would dip too low and freeze life in its tracks. He also cautions that too many cosmic rays could wipe life out altogether: “Life forms like this want a steady flux of energy from cosmic rays, but not so much that it’s damaging,” he says. “They might not be able to cope with a huge bout of radiation that pops in."
In the future, Atri wants to bring the gold mine bug into the lab and see how it responds to cosmic radiation levels equivalent to those on Mars, Europa, and others. That data would give him more clues to whether this kind of organism could survive beyond Earth. “Desulforudis audaxviator is proof that life can thrive using almost any energy source available,” he says. “I always think of Jeff Goldblum in Jurassic Park—life finds a way.”
*Correction, 3 January, 2:45 p.m.: This article has been modified to reflect the fact that not all life on Earth gets its energy from either photosynthesis or by eating other life forms.