What it would take to discover life on Saturn

Image credit: NASA/Cassini.

The mystery of whether microbial alien life could inhabit Enceladus, one of Saturn’s 83 moons, could be solved by an orbiting space probe, according to a new study led by University of Arizona researchers. In an article published in The Journal of Planetary Science, the researchers trace how a hypothetical space mission could provide definitive answers. When Enceladus was first inspected in 1980 by NASA’s Voyager 1 spacecraft, it looked like a small, not-too-exciting “snowball” in the sky. Later, between 2005 and 2017, NASA’s Cassini probe toured the Saturn system, studying Saturn’s complex rings and moons in unprecedented detail. Scientists were stunned when Cassini discovered that Enceladus’ thick ice shell conceals a vast warm ocean of salty water that releases methane, a gas that typically originates from microbial life on Earth.

Methane, along with other organic molecules that build the building blocks of life, were detected when Cassini flew through gigantic columns of water gushing from Enceladus’s surface. As the small moon orbits the ringed gas giant, Saturn’s immense gravitational field squeezes and pulls on it, heating its interior due to friction. As a result, spectacular plumes of water shoot from cracks and crevices on Enceladus’s icy surface into space. Last year, a team of scientists from UArizona and Université Paris Sciences et Lettres in Paris calculated that if life could have arisen on Enceladus, there is a high probability that its presence explains why the moon is spewing methane. “To find out if that’s the case, we need to go back to Enceladus and look,” said Régis Ferrière, lead author of the new paper and an associate professor in UArizona’s Department of Ecology and Evolutionary Biology.

In their latest paper, Ferrière and colleagues report that while the hypothesized total mass of living microbes in Enceladus’ ocean would be small, all it would take would be one visit from an orbiting spacecraft to know for sure whether similar microbes to Earth populate the ocean of Enceladus. under his shell. “Clearly sending a robot crawling through ice cracks and plunging deep into the seafloor would not be easy,” Ferrière said, explaining that more realistic missions have been designed that would use improved instruments to sample the plumes as Cassini did, or even land on the surface of the moon. “By simulating the data that a more prepared and advanced orbiting spacecraft would collect from the plumes alone, our team has now shown that this approach would be sufficient to confidently determine whether or not there is life within Enceladus’s ocean without having to probe. the depths of the moon,” he said. “This is an exciting prospect.” Located about 800 million miles from Earth, Enceladus completes one orbit around Saturn every 33 hours. While the moon isn’t even as wide as the state of Arizona, it stands out visually for its surface; Like a frozen pond that glistens in the sun, the moon reflects light like no other object in the solar system. Along the moon’s south pole, at least 100 gigantic columns of water gush through cracks in the icy landscape, like lava from a violent volcano.

Scientists believe that the water vapor and ice particles ejected by these geyser-like features contribute to forming one of Saturn’s iconic rings. This ejected mixture, which brings gases and other particles from the depths of Enceladus’ ocean, was sampled by the Cassini spacecraft. The excess methane that Cassini detected in the plumes conjures up images of extraordinary ecosystems found in the lightless depths of Earth’s oceans: hydrothermal vents. Here, at the edges of two adjacent tectonic plates, hot magma beneath the seafloor heats ocean water into porous bedrock, creating “white smokers,” vents that spew scorchingly hot, mineral-saturated seawater. Without access to sunlight, the organisms depend on the energy stored in the chemical compounds released by white smokers to make a living.

“On our planet, hydrothermal vents teem with life, large and small, despite the darkness and insane pressure,” Ferrière said. “The simplest living creatures that exist are microbes called methanogens that feed themselves even in the absence of sunlight.” Methanogens convert dihydrogen and carbon dioxide for energy, releasing methane as a byproduct. Ferrière’s research group modeled their calculations on the hypothesis that Enceladus has methanogens that inhabit oceanic hydrothermal vents similar to those found on Earth. In this way, the researchers calculated what the total mass of methanogens in Enceladus would be, as well as the probability that its cells and other organic molecules could be expelled through the columns. “We were surprised to find that the hypothesized abundance of cells would only equal the biomass of a single whale in the global ocean of Enceladus,” the paper’s first author, Antonin Affholder, a postdoctoral research associate at UArizona who was at Paris Sciences & Lettres, told the Associated Press. do this research. “The Enceladus biosphere may be very sparse. And yet our models indicate that it would be productive enough to feed the columns with enough organic molecules or cells to be detected by instruments aboard a future spacecraft.”

Enceladus has recently attracted attention as a place that will one day be further reviewed and examined. One proposal, the “Enceladus Orbilander,” designed by the Johns Hopkins Applied Physics Laboratory, envisions a mission that would collect vast amounts of data about Enceladus by landing on and orbiting this celestial body beginning in the 2050s. “Our research shows that if a biosphere is present in the ocean of Enceladus, signs of its existence could be detected in the plume material without the need for landing or drilling,” Affholder said, “but such a mission would require an orbiter to fly at through her.” the plume several times to collect a lot of oceanic material.” The document includes recommendations on the minimum amount of material that should be collected from plumes to confidently search for microbial cells and certain organic molecules. Observable cells would show direct evidence of life. “The chance that real cells could be found might be slim,” Affholder said, “because they would have to survive the outgassing process that carries them through the plumes from the depths of the ocean to the vacuum of space, quite a journey for a tiny cell. ” Instead, the authors suggest that the organic molecules detected, such as particular amino acids, would serve as indirect evidence for or against a life-abundant environment.