In April 2017, officials announced that two moons in our solar system, Saturn's Enceladus and Jupiter's Europa, may harbor conditions conducive to supporting life. Both moons contain massive oceans underneath a thick surface layer of ice, with Europa alone estimated to contain two to three times more water in volume than Earth.
On Enceladus, the Cassini spacecraft detected molecular hydrogen within plumes of water vapor venting into space from cracks in the moon's icy surface.
On Europa, the Galileo spacecraft recorded data in 1997 of a short bend in the magnetic field. But it wasn't until 2018 that NASA released new findings that suggest the bend is a water plume. Based on previous observations of Enceladus, NASA researchers already knew that plumes become ionized and leave a blip in the magnetic field. Therefore, this similar blip on Europa is more than likely also a plume.
"This is the closest we've come, so far, to identifying a place with some of the ingredients needed for a habitable environment," said Thomas Zurbuchen, associate administrator for NASA's Science Mission Directorate at Headquarters in Washington, in 2017. "These results demonstrate the interconnected nature of NASA's science missions that are getting us closer to answering whether we are indeed alone or not."
The case for Enceladus
The presence of hydrogen on Enceladus confirms nearly all of the conditions necessary for life to form. According to NASA, life as we know it requires liquid water, a source of energy for metabolism, and chemical ingredients carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur. The only missing pieces that have yet to be verified are phosphorus and sulfur. Because Enceladus' core is thought to be chemically similar to meteorites, which are rich in both elements, it likely has the necessary ingredients to support life.
In findings published in 2017 in the journal Science, the researchers said the only plausible explanation for the presence of hydrogen on Enceladus was hydrothermal reactions of rock on the ocean floor containing minerals and organic materials. This in turn can lead to the production of methane, which alone can support vast underwater colonies of bacteria on Earth even in the absence of light.
"This free energy is really a game-changer for Enceladus," researcher Hunter Waite, lead author of a paper detailing the hydrogen discovery, told Scientific American. "The presence of molecular hydrogen shows there is the chemical potential there to support metabolic systems like methanogenic microbes. This suggests we’ve found a potential food source that would support the habitability of Enceladus’s interior ocean."
Heightened interest in Europa Clipper mission
Both discoveries are bolstering the excitement around NASA's Europa Clipper mission, planned for launch sometime in the 2020s.
The spacecraft will specifically target Europa, conducting 40 to 45 flybys at close range to determine whether its ocean has the necessary ingredients for supporting life. The discovery of potential plumes from its surface provides yet another fantastic opportunity to sample its chemical makeup.
Editor's note: This article has been updated since it was originally published in April 2017.