Evidence suggesting hydrothermal activity is taking place on Enceladus has been discovered, providing the "tantalizing possibility" that Saturn's icy moon could support alien life.
Scientists have found microscopic grains of rock near Saturn that suggest hydrothermal activity is taking place on Enceladus – where seawater infiltrates and reacts with a rocky crust and emerges as a heated and mineral-laden solution.
Published in the journal Nature, scientists from the University of Colorado Boulder working on the Cassini mission spent four years analysing the grains from the spacecraft, conducting laboratory experiments and computer simulations.
Active geology on Enceladus was first found by Cassini in 2005 following the discovery of an icy spray emerging from its south polar region. They then found a towering plume of water ice and vapour, salts and organic materials coming from the moon's surface.
In 2014, scientists said the gravitational measurements suggest Enceladus has a six-mile-deep ocean beneath its ice shell, which is between 19 to 25 miles thick.
Cassini's Cosmic Dust Analyser instrument has detected miniscule rock particles rich in silicon since 2004. By eliminating sources, researchers came to the conclusion that they must be grains of silica, which is found on Earth in sand and the mineral quartz. Their size, however, provided a clue as what geological process formed them.
On Earth, the most common way of forming silica grains of such a tiny size is through hydrothermal activity - specifically, when slightly alkaline water with modest salinity is super-saturated with silica and undergoes a big drop in temperature.
Scientists determined the Cassini grains were most likely from when hot water containing dissolved minerals from the moon's rocky interior travelled upwards and came into contact with cooler water. Temperatures required to produce the grains would need to be at least 90C.
"It's very exciting that we can use these tiny grains of rock, spewed into space by geysers, to tell us about conditions on - and beneath - the ocean floor of an icy moon," said lead author Sean Hsu.
Study co-author Frank Postberg said: "We methodically searched for alternate explanations for the nano-silica grains, but every new result pointed to a single, most likely origin."
Laboratory tests verified the conditions under which silica grains formed at the same size as those detected by Cassini, prompting researchers to conclude these conditions exist at the sea floor of Enceladus.
The findings add to the possibility that Enceladus could contain environments suitable for hosting living organisms.