Scientists have found a potential food source for life on a world in our solar system, raising the tantalizing possibility that organisms could thrive in a place besides Earth.
The researchers emphasize they did not find evidence of life itself. What they did find was hydrogen gas in the geyser-like plumes spurting from the surface of Saturn’s moon Enceladus. On Earth, hydrogen from seafloor hot springs, also known as hydrothermal vents, serves as a food stock for microbes and as the base of an elaborate ecosystem.
With the new discovery, nearly every item on the list of supplies essential to life has now been found on Enceladus, including carbon-containing molecules called organic compounds, says Hunter Waite of the Southwestern Research Institute in San Antonio, Texas, co-author of a new study in this week’s Science about the find.
“Water’s there – check. Organics are in the plume – check. Now we have a chemical source of energy for food – check,” Waite says. Two chemicals essential to living organisms — sulfur and phosphorus — have not been confirmed, but all the same, “Enceladus is rising to the top of habitable places that exist in the solar system.”
Though Enceladus looks from a distance like a glimmering ball of ice, research in the past few years established that the tiny moon has a salty ocean sloshing underneath its frozen outer shell. Recent evidence also suggested the water percolates into cracks and fissures in the rocky seafloor.
On Earth, seawater that follows a similar path is an important player in marine life. Heated and filled with minerals, the water wafts from the seabed into the open ocean and nourishes bustling ecosystems. Some researchers even argue that life on Earth arose at hydrothermal vents.
Until now, Enceladus lacked evidence of something to keep life well fed. In October 2015, NASA sent its Cassini spacecraft diving through the geysers jetting from Enceladus. A new measurement technique allowed Cassini to measure the plume’s hydrogen level.
After ruling out other possible sources of the hydrogen in the plume, the researchers did a “calorie count” of Enceladus’s ocean to determine whether the water holds enough hydrogen and other chemicals to keep living things alive, says study co-author Christopher Glein, also of the Southwest Research Institute. The team concluded that as long as the seawater has a certain level of acidity, it is replete with hydrogen and carbon dioxide, providing “a lot of food for microbes,” Waite says.
The vents on Enceladus are similar to those at Lost City, a vent system in the Mid-Atlantic known for its towering limestone pillars. At Lost City and elsewhere, microbes consume hydrogen and carbon dioxide to make energy, which they spend on making essentials such as proteins.
The results from Enceladus are “extraordinary and exciting,” says Kevin Hand of NASA’s Jet Propulsion Laboratory, who was not involved in the discovery. But he also points out that no one knows the age of Enceladus’s ocean, and a young ocean may mean less time for life to evolve.
Nor does anyone know whether Enceladus’s ocean is as acidic as the study proposes, says David Catling of the University of Washington, Seattle. “The reason that hydrogen is detected could be that no one is home to eat it,” he says.
New discoveries at another “water world,” Jupiter’s moon Europa, were also announced Thursday. At a NASA briefing, scientists reported that the Hubble Space Telescope spotted evidence of plumes like those on Enceladus rising from Europa, which again like Enceladus is thought to have a subsurface ocean. If the plumes really do exist, they could provide scientists with samples of Europa’s seawater.
NASA is planning to launch a mission to Europa in the 2020s. But no new mission to Enceladus is on the books, and the Cassini spacecraft now orbiting Saturn will die in September. Scientists are anxious to learn more about both worlds.
“We need to probe them,” NASA’s Jim Green said at Thursday’s briefing, “because they are one of the best locations, we believe, that may harbor life today.”