At NASA headquarters today, members of the Cassini Team announced that Enceladus contains free hydrogen, an important element that can be metabolized by microscopic organisms to produce energy. This free molecular hydrogen is being produced by chemical reactions happening at Enceladus’ ocean floor, in which minerals react with hot water to produce new minerals and free hydrogen. This makes Enceladus, a moon of Saturn, an increasingly promising candidate for the search for life elsewhere in the solar system.
The Cassini probe assisted with this discovery by flying through plumes that spray water and other compounds from beneath Enceladus’ icy crust and taking measurements of the plumes’ chemical composition. These plumes originate among what Cassini team members call the “Tiger Stripes” that are actually fissures located at Enceladus’ south pole. Cassini had previously detected organic molecules like methane in dives through the plumes, which are being powered by geothermal energy deep under Enceladus’ liquid ocean.
“Cassini’s instruments do not have the ability to detect life itself,” Cassini project scientist Linda Spilker had told Astrobiology Magazine in 2015. However, they are capable of helping scientists calculate the likelihood that a moon like Enceladus might play host to rudimentary life.
This discovery confirms the presence of one of the six elements that biologists list as essential for life: hydrogen, carbon, nitrogen, oxygen, phosphorus and sulfur. Although Cassini has not detected phosphorus or sulfur on Enceladus, scientists suspect that this moon’s rocky core is chemically similar to meteorites that contain these two elements.
Although Hunter Waite, lead author of the Cassini study, said that, “It would be like a candy store for microbes,” the science teams at the press conference warned against jumping to conclusions. Molecular hydrogen levels in Enceladus’ plumes are high enough to imply that a lot of the candy is not being eaten. NASA personnel stressed that, if there is life, it is likely to be similar to the life found around underwater plumes in Earth’s oceans. The ecosystems around plumes are based on microbes that are capable of metabolizing free hydrogen and carbon dioxide to generate energy rather than rely on sunlight. In response to a social media question that was probably meant to be humorous, project scientists said that they would be more inclined to look for similar microbes than to search for giant squids when they look for life on moons in the outer solar system.
A paper on this discovery is set to be published in the journal Science.
Plumes On Europa Detected By Hubble
NASA also took the opportunity to announce that the Hubble Space Telescope has detected a possible plume on Jupiter’s moon, Europa, in 2016. NASA has suspected an active plume in that region of Europa since 2014, when Hubble detected evidence of a plume that could be somewhat similar to the ones in the Tiger Stripes zone of Enceladus. This plume may be further evidence that there is a liquid water ocean under Europa’s frozen surface. The plume activity on Europa coincides with a warmer region that had been detected by the Galileo spacecraft – a discovery that was, incidentally, inspired by the discovery of molecular hydrogen on Enceladus.
“The plumes on Enceladus are associated with hotter regions, so after Hubble imaged this new plume-like feature on Europa, we looked at that location on the Galileo thermal map. We discovered that Europa’s plume candidate is sitting right on the thermal anomaly,” said William Sparks of the Space Telescope Science Institute in Baltimore, Maryland. Sparks led the Hubble plume studies in both 2014 and 2016.
The plumes on Europa were observed using the Hubble Telescope’s Space Telescope Imaging Spectrograph (STIS), which is capable of making observations in ultraviolet light. These observations will help with planning for the Europa Clipper mission, which is set for launch in the 2020s. The Europa Clipper will be capable of not only confirming the presence of the plume on Europa, but also peering under Europa’s ice with specialized radar.
“If there are plumes on Europa, as we now strongly suspect, with the Europa Clipper we will be ready for them,” said Jim Green, Director of Planetary Science at NASA Headquarters.
A paper on the Europa study is set to be published in The Astrophysical Journal Letters.