Forget the much-vaunted ‘Goldilocks zone’ assumed to be the stellar sweet spot for life as we know it to exist on other planets: a new study suggests that the effect of cosmic rays on extraterrestrial environments could support life without the need for sunlight or geothermal energy, an idea that might open up new possibilities—and places for exobiologists to look—for life on other worlds.
Cosmic rays are a catch-all term used to describe high-energy radiation emitted from the Sun and the other stars in the galaxy, either in the form of charged particles such as protons and electrons, or high-frequency electromagnetic radiation like x-rays and gamma rays. For the most part, these forms of radiation are hazardous due to their ionizing nature, carrying enough energy to either add or remove an electron from atoms they hit, a situation that can cause damage to the DNA of biological cells that can result in the death of the cell, or a mutation that could make the cell cancerous.
As a result, most organisms on Earth have evolved measures to stave off the damage from the small portion of cosmic rays that reach the surface—the majority are blocked by the atmosphere—but a study published through Cambridge University Press suggests that it is conceivable that organisms could use the electrons freed via ionization from either frozen or liquid water that dwells deep beneath the surface of planets or moons as an energy source, eliminating their need for photosynthesis or hydrogen released from volcanic vents for sustenance.
One such organism already known to be nourished in this fashion is a type of bacteria called Desulfotomaculum: similar to the bacteria that can cause the spoilage of canned foods, this bacteria was discovered 2.8 kilometers (1.7 miles) below the surface in South Africa’s Mponeng gold mine. Isolated from sunlight-based energy sources for tens of millions of years in subterranean water, this strain had evolved to consume elemental hydrogen that formed when cosmic rays struck the water molecules the bacteria were found thriving in.
The study evaluated a number of planetary bodies within our Solar System for their suitability for this process, placing a focus on Mars, Enceladus and Europa; their findings were that Saturn’s Enceladus was best suited for organisms subsisting on the byproducts of radiolysis, with an energy density that could support up to 40 nanograms of biomass per square centimeter, meaning that there could be as many as 101,000 individual bacterium in an area the size of a dime. Mars, capable of supporting a little over a quarter of that number, came in second, with Europa trailing with only one-ninth of Enceladus’ capacity.
“This discovery changes the way we think about where life might exist,” according to study lead Dimitra Atri, Group Leader of the Mars Research Group at New York University Abu Dhabi’s Center for Astrophysics and Space Science. “Instead of looking only for warm planets with sunlight, we can now consider places that are cold and dark, as long as they have some water beneath the surface and are exposed to cosmic rays. Life might be able to survive in more places than we ever imagined.”
This notion expands the search area for potential life forms past the habitable zone of a star system—often referred to as the Goldilocks zone—the region around a star where the conditions are neither too hot nor too cold for the proverbial porridge of liquid water to form—even on worlds in our own Solar System thought to be too dark to support life.