Saturn's moon is hiding a secret.
Years ago, NASA and the ESA's Cassini-Huygens spacecraft swung through the salt-rich plumes spewing from the interior of Saturn's moon, Enceladus. But in the middle of this, the probe detected a collection of compounds that scientists have also linked to hydrothermal vents at the bottom of Earth's oceans. Scientists had thought the amount of methane in the plumes could be the result of known geochemical, or non-biological, processes. Until now.
A team of scientists has said that no known lifeless process could be responsible for the amount of methane observed firing out of the planet's surface, which means it could come from a resident life form, according to a recent study published in the journal Nature Astronomy.
Enceladus could have hydrothermal vents like Earth's
"We wanted to know: Could Earthlike microbes that 'eat' the dihydrogen and produce methane explain the surprisingly large amount of methane detected by Cassini?" said Régis Ferrière, a biologist at the University of Arizona and one of the authors of the study, in a blog post at the university's website. "Searching for such microbes, known as methanogens, at Enceladus' seafloor would require extremely challenging deep-dive missions that are not in sight for several decades." Despite the vast distances between this phenomenon and the scientific instruments needed to further study it, we can still employ mathematics as a means of modeling known variables. So Ferrière and his team looked to processes that produce methane in the same way, here on Earth.
Enceladus is an intriguing place, orbiting Saturn at many times the distance from the sun than the Earth, and is coated in thick armor of solid ice. But swirling beneath it is a colossal ocean that spans the entire moon, and it might move to and fro in currents brimming with the crucial ingredients for life as we know it. Then again, it might also house life that breaks radically from anything we've seen before. If the moon houses life, it is likely due to planetary tidal forces that stretch and compress the core of Enceladus, heating the interior and providing energy for the basic forms of life.
If the core is hot, this would keep the ocean from freezing, in addition to generating hydrothermal vents, which (at least on Earth) are centers of brimming activity, where heat from the warming interior escapes into the abyssal depths of the ocean. On our planet, this warmth sustains an ecosystem, providing the food necessary for chemical reactions — in a process called chemosynthesis, as opposed to photosynthesis (which is how plants gather energy from the sun).
Excess methane could mean life on Enceladus
If these hydrothermal vents are on Enceladus, and scientists think they probably are, then we might find familiar forms of life hiding in the vast oceans of Saturn's moon. "Not only could we evaluate whether Cassini's observations are compatible with an environment habitable for life, but we could also make quantitative predictions about observations to be expected, should methanogenesis actually occur at Enceladus' seafloor," added Ferrière in the blog post. The research team's work was maximally challenging, and analyzed the temperature at the floor of hydrothermal vents, in addition to how this affects a population of microbes would have on their environment. And the team discovered that the abundance of methane present was too high to have come from non-biological sources, alone.
While this is very exciting news, there could still be other, unforeseen geochemical processes at work in the depths of Enceladus' oceans responsible for the excess methane. It could be primordial methane trapped inside the moon from a solar nebula, from the ancient past when the solar system was still forming. There are other possibilities, but we won't know for sure until we (or a robot) go there, and find out.