Moon of Saturn could be habitable
Once considered an inert chunk of ice, Enceladus revealed to have fuel for life
Scientists have discovered hydrogen in the geysers erupting from Enceladus, a sign that the moon of Saturn could be habitable.
Since NASA’s Cassini spacecraft first showed that Enceladus is home to liquid water — a necessary condition for life as we know it — researchers have wanted to know what environments might exist in its oceans.
By swooping through kilometreshigh watery plumes ejected from cracks in the moon’s ice-crusted surface, Cassini has detected an abundance of hydrogen, evidence of chemical reactions between warm ocean water and a rocky core that could produce a source of energy for specialized life forms.
The discovery, announced Thursday at a NASA press conference and in the journal Science, does not prove that Enceladus is actually inhabited by some type of life. For that, space agencies will need to design new missions.
“You can have a place that’s habitable (where) the lights are on and nobody’s home. But certainly it makes it a more attractive place to go and look,” said John Moores, a planetary scientist at York University who was not involved with the research.
Enceladus was once presumed to be an inert chunk of ice. But when Cassini completed its first close flyby of the Saturn satellite in 2005, researchers were shocked to discover that the south pole of the moon was laced with warm fractures and covered with clouds of water vapour.
Piecing together evidence collected over more than a decade, scientists have figured out that Enceladus has a rocky core surrounded by a salty, liquid sea. The sea is capped with a thick ice crust that is geologically active. Geysers of water with traces of methane, ammonia, carbon dioxide and other molecules are ejected from deep fissures in the icy crust.
Because Enceladus proved to be so interesting, mission scientists sent Cassini back for 20 more visits than initially planned. During its last and deepest dive through Enceladus’ plumes in October 2015, Cassini came within 49 kilometres of the moon’s surface, looking for evidence of hydrogen.
Most life on Earth relies on energy produced by sun-fuelled photosynthesis. But deep in the ocean, some microbes derive energy from a totally different source: a chemical pro- cess between warm water and rock that produces molecular hydrogen. Some microbes can exploit this process for fuel, getting a jolt of energy by combining hydrogen with carbon dioxide dissolved in the water.
Around hydrothermic vents on the sea floor, vast ecosystems sustained by this chemical fuel thrive. While there is still disagreement about when and where life began on our planet, some of the oldest evidence comes from fossilized microorganisms discovered in rocks that were once the site of ancient oceanic hydrothermal vents.
In the seas of a distant, ice-covered moon, photosynthesis is not likely to occur. But based on prior observations of the traces in Enceladus’ plumes, Cassini’s mission scientists wanted to know whether there was evidence of hydrothermal activity in the form of hydrogen.
In its final deep dives, Cassini recorded an abundance of hydrogen. Other processes might explain this detection, but the likeliest explanation, the authors of the Science paper describing Cassini’s findings concluded, is hydrothermal activity. The detection shows there is fuel, but not whether anything is consuming that fuel.
“We have made the first calorie count in an alien ocean,” said Christopher Glein, a Cassini team associate and research scientist at Southwest Research Institute in San Antonio, Texas. “This is a key step toward understanding the habitability of Enceladus.”
Cassini first launched in 1997 and reached Saturn after seven years of space travel. In September, it will plunge into the planet and end its journey.
Cassini was not designed to detect life in Enceladus’ plumes, and NASA scientists said Thursday that new missions are needed to answer questions about potential life in our solar system’s “ocean worlds.”
In another paper published Thursday in the Astrophysical Journal Letters, researchers announced that Hubble telescope observations indicated that Europa, a moon of Jupiter, may also have a kilometres-high plume. Like Enceladus, Europa is covered by an ocean with an icy shell.
NASA is developing a mission to Europa known as Clipper that is still in the preliminary design phase, but that would launch in the 2020s and reach Europa after a journey of “several years.”