Jupiter's icy moon Europa is often pegged as place we're most likely to find extraterrestrial life in our solar system. But if it wants to keep that title, Europa has its work cut out for it. That's because Enceladus—Saturn's tiny, eruptive satellite—not only harbors liquid water beneath its frozen surface, but the moon's ocean is starting to seem very Earth-like.
In a study that appears in pre-print this week on arXiv, researchers give us our first detailed glimpse into the composition of Enceladus's subsurface ocean. At pH 11 or 12, the ice moon's waters are about as basic as bleach—nothing we humans would want to drink. But certain types of life might thrive here. In fact, the same chemical reactions that make this ocean so alkali also produce hydrogen gas, a fuel source that probably powered ancient life in Earth's subsurface aquifers.
"The pH is compatible with life as we know it; life on Earth may have begun under similar conditions," planetary scientist Christopher Glein and colleagues wrote in a statement.
Its diminutiveness notwithstanding—at 310 miles in diameter, the dazzling white moon could sit comfortably between San Francisco and LA—Enceladus is, in some ways, not so different from our blue marble. It's got a hot, rocky core that's probably rich in silica and iron; the same elements that one would use to cook up an Earth-like planet. Here and on Enceladus, some of that core heat seeps into the deep ocean via geothermal vents, creating potential hotspots for life.
But broad outlines aside, everything we know about Enceladus comes from images of the moon's surface, which tell the tale of an almost unimaginably alien place. Towering plumes of water vapor and ice erupt like geysers from the moon's south pole and rocket into space at an astonishing 800 miles per hour. Particles from these icy eruptions encircle the moon in a shell of dust that feeds Saturn's massive E-ring.
Ice volcanoes and ring rain are fascinating, but nothing about them sounds particularly livable.
It's the warmer, liquid waters miles beneath the shiny surface that we'd really like to get a glimpse of. In their paper, Glein and his co-authors use observational data collected by the Cassini spacecraft to construct a chemical model of Enceladus's subterranean ocean. Like Earth's oceans, these waters are rich in sodium chloride, but they also appear to be loaded with sodium carbonate—a substance used in laundry detergent and other cleaning agents—giving them their strongly alkali character.
The researchers suspect Enceladus's peculiar ocean chemistry is caused by serpentinization, a process wherein water reacts with and transforms with iron-rich rocks. Significantly, serpentinization also leads to the production of hydrogen gas, a fuel source that supports microbes living deep in Earth's crust today, and probably helped life get its start billions of years ago, before the rise of oxygen.
Similar conditions could prevail on Europa, which also houses a geothermally heated, subsurface ocean. But intel on Europa's ocean is harder to come by.
"At Enceladus, there is a plume that is providing free samples of its ocean for chemical analysis," Glein told me in an email. "So, it is a lot easier to study its ocean, and we are getting a lot of detailed information."
Lessons learned from Enceladus may well translate to the ice moon's larger cousin. For instance, Glein suspects serpentinization may be a key player in determining the habitability of either moon's ocean.
"Serpentinization creates conditions that can support life, such as hydrogen as an energy source," Glein said. "The process may make both bodies habitable."
Of course, we won't be able to tell if either moon bears alien life until we collect some ocean water samples and analyze them directly. Missions seeking to do so might be entering their heydey, in light of the Obama administration's recent call to boost NASA, spending to $18.5 billion in the coming fiscal year. This week, NASA greenlit the Europa Clipper, an unmanned spacecraft that'll conduct a detailed reconnaissance of Europa's surface.
Meanwhile, Glein is among a team of scientists behind a newly proposed Saturn mission: The Enceladus Life Finder. ELF is a solar-powered Saturn orbiter designed to fly through Enceladus's south pole plume, collecting samples of erupted ocean water and analyzing them for biosignatures.
Now, it seems, the race is on to find the first life-harboring moon. Sci-fi lit junkies may be throwing their lot in with Enceladus, seeing as how future humans in Kim Stanley Robinson's 2312 eat "Encedalian biota" for their supposed medicinal value. Fans of the cult film Europa Report may find themselves partial to the larger moon, hoping that just maybe, we'll snap a photo of a bioluminescent space octopus.
Hey, one can dream.