The search for life in our solar system just became a bit more interesting.
While scientists have not found life on Enceladus, Saturn's moon, they have found the next best thing: food that could sustain potential life. The food source is in the form of molecular hydrogen, which could feed microbes as it does here on Earth.
"We're very excited to find what looks to be a food source for potential microbes, which is a pretty important step to saying this is a habitable ocean," Hunter Waite, lead author of a paper published in the journal Science, told CBC News. Waite is also on the Cassini Ion and Neutral Mass Spectrometer team lead.
Scientists detected the molecular hydrogen in plumes of water vapour erupting from beneath the moon's icy surface, a critical ingredient in a process known as methanogenesis. On Earth, this provides food and energy to microbes deep in the oceans.
And the only plausible source, they concluded, are hydrothermal reactions between hot rocks and the water in the moon's ocean.
Enceladus is a small, icy moon with a vast ocean beneath its surface. In 2005, Cassini discovered that water vapour was spewing out into space. Subsequent flybys took the spacecraft right through the plumes, where it detected organic material, carbon dioxide and carbon monoxide. And data suggests that there is a large liquid ocean with hydrothermal vents, locations where life thrives on Earth.
While it may seem unlikely that a moon almost 1.5 billion kilometres from the sun can retain a liquid ocean, the orbit of Enceladus around Saturn is more elliptical than most other moons in our solar system. Saturn's massive gravity causes the squeezing and thus heating of the moon.
"I think conditions for habitability are kind of nailed down," Waite said. However, he added that further measurements of both phosphorus and sulfur are needed. But everything else is there: water, organics, an energy source and more.
In order to determine whether the molecular hydrogen is being created by microbial life, further missions need to fly through the plumes, Waite said. Cassini is about to begin its "Grand Finale" tour, where it will plunge into Saturn in September, ending a 13-year mission that garnered some substantial scientific data and a better understanding of the complex system.
Scientists also revealed that in 2016 the Hubble Space Telescope imaged what are believed to be plumes from Europa, an icy moon of Jupiter. In 2014, Hubble spotted a similar display from that location, believed to also be water vapour.
The plume stretches as high as 100 kilometres above the surface of the moon.
The plumes of Europa are located in a warm region on the moon first identified by the Galileo spacecraft which orbited Jupiter in the 1990s. Data from that mission was also used in this current study.
The plumes of Europa don't occur as regularly as those on Enceladus, the scientists noted. Though the observations do not provide "unequivocal" proof that they are water plumes, Europa researcher Bill Sparks from the Space Telescope Science Institute said he is fairly confident that they are similar to those seen on Enceladus.
Expanding search for life
This discovery also could help astronomers find life around exoplanets orbiting other star systems. At the moment, astronomers are searching for gases resembling that found on Earth on these far off worlds.
"If we find a new exotic form of life on an icy moon, like Enceladus or Europa, we might find that they are associated to completely different markers that aren't associated to ours and sort of broaden our horizons and what kind of environment could constitute as being habitable," Queen's University astronomer Nathalie Ouellette said.
Waite said another question could be raised if we discover that Enceladus doesn't support life with so many elements present.
"If everything's right and it's not there, what's going on? Maybe we don't understand [life] too well."