At an April 13 press conference, NASA scientists presented new data from the Cassini mission and Hubble Space Telescope suggesting both Saturn’s moon Enceladus and Jupiter’s moon Europa could be habitable.
During Cassini’s last flyby of Enceladus in October 2015, the probe’s Ion and Neutral Mass Spectrometer (INMS) detected hydrogen gas in plumes of gas and ice upwelling from the surface.
An analysis of the plumes’ content indicated they are composed of 98 percent water vapor, one percent hydrogen, and small amounts of carbon dioxide, ammonia, and methane molecules.
Cassini first observed the plumes coming from hot cracks at Saturn’s south pole in 2005. Designed to study the upper atmosphere of Saturn’s moon Titan, the spectrometer was then also used to study Enceladus.
The hydrogen in the plumes is likely produced by interactions between the small moon’s rocky core and warm water in its subsurface ocean.
Microbial life can exist in the absence of sunlight if it has an alternate source of energy. Through a process known as “methanogenesis,” life forms can obtain energy through combining hydrogen with carbon dioxide dissolved in water.
On Earth, bacteria have been found in hydrothermal vents on ocean floors, locations previously believed inhospitable to life.
There, they make up the base of a food chain that includes shrimp and other marine life.
That discovery led scientists to question whether microbes might exist in similar hydrothermal vents on worlds with underground oceans.
“This is the closest we’ve come to identifying a place with some of the ingredients needed for a habitable environment,” said associate administrator for NASA’s Science Mission Directorate Thomas Zurbuchen.
To be habitable for life as we know it, a world must have water, an energy source, and the necessary chemical ingredients, such as hydrogen, carbon, nitrogen, oxygen, phosphorus, and sulfur.
While Cassini has not spotted phosphorus or sulfur in Enceladus’ ocean, scientists believe both are present because Enceladus’ rocky core is chemically similar to meteorites known to contain both elements.
“Confirmation that the chemical energy for life exists within the ocean of a small moon of Saturn is an important milestone in our search for habitable worlds beyond Earth,” said Cassini project scientist Linda Spilker of NASA’s Jet Propulsion Laboratory.
Cassini’s findings are presented in an article published in the journal Science.
At Europa, the Hubble Space Telescope’s Space Telescope Imaging Spectrograph (STIS), which observes in ultraviolet wavelengths, discovered a likely plume in 2016 at the same site where one was first seen two years earlier.
On both occasions, the plume emanated from an unusually warm region where cracks are visible in Europa’s icy crust.
Initially detected by the Galileo mission during the 1990s, this hot region could be created by water erupting from within the moon.
Scheduled for launch in 2022, NASA’s Europa Clipper will observe this region in ultraviolet wavelengths for comparison with the Hubble images.
“If there are plumes on Europa, as we now strongly suspect, with the Europa Clipper we will be ready for them,” said NASA Director of Planetary Science Jim Green.
Hubble’s Europa findings are published in The Astrophysical Journal Letters.