Since the discovery of the mysteriously warm ‘tiger stripe’ crevasses and the remarkable geyser-like plumes of water vapor erupting from Enceladus’s southern regions, this icy moon has gone from an afterthought to a prime target for astrobiology.
During the years of the Cassini mission’s avatar-like presence in the Saturnian system we’ve seen increasingly convincing evidence that Enceladus is a lot more than just a brightly reflective ball of ice. To summarize all of this in simple terms: the moon has a large internal ocean of liquid, salty water.
That water jets out in visible plumes a few hundred miles high through a system of cracks and (we presume) deep plumbing through an icy crust that is probably anywhere from 10 km to 40 km in average thickness. In fact, so much effluvia jets out from Enceladus that it generates Saturn’s vast E-ring – encircling the giant planet at a roughly 200,000 km orbital radius.
Using instruments never designed for precisely this kind of challenge, Cassini has already demonstrated that the water is roughly as salty as Earth’s oceans and also contains silicate nanoparticles. Together these data are strongly suggestive of water that has been in contact with the hot rocks and minerals of some kind of deep-ocean hydrothermal system. It's precisely the kind of environment that we find along the mid-ocean ridges of Earth’s own waters. And precisely the kind of environment that can support, and possibly even generate, living things.
Now, Waite et al. have published findings in the journal Science that provide evidence of something else in the Enceladus plumes; molecular hydrogen. With some skillful and methodical analysis of Cassini’s mass spectrometer data during its most recent Enceladus flyby (with a closest approach of some 49 kilometers), the researchers make a convincing case that molecular hydrogen is being detected that had its origins deep inside the moon.
Specifically, on Earth, as water reacts with rocks containing reduced iron-bearing minerals in hydrothermal systems it produces molecular hydrogen. That simple molecule is terrific food for microorganisms when it floats around in a soup of oxidants – as it does in these systems. A particularly robust example is the metabolic process we call methanogenesis. Hungry microbes react molecular hydrogen with carbon dioxide to generate methane – and extract the biochemical energy they require. As metabolic processes go, this is a big one, and one that has an ancient genetic lineage here on Earth.
But are conditions actually suitable for methanogenic life deep inside Enceladus? Waite et al. argue that they could easily be. In particular, what we know about the likely alkalinity of the Enceladus ocean (around pH 9-11) together with the newly observed mixing ratio of molecular hydrogen to water (about 0.01 in the plumes), points to an environment that would easily span the chemical sweet spot for methanogenesis to be a favorable life-strategy.
It’s not evidence of life in Enceladus, but it’s awfully tempting to imagine what’s going on in that dark abyss. We have actually already seen methane in the plumes, but we don’t know if any is biologically produced – to determine that will require new instruments and a new mission.
What is truly intriguing is what this implies for our search for life across the solar system. Stepping back for a moment, imagine that we’d come across Enceladus and its ocean before we’d really studied Mars in any detail. If that had been the case I’m not sure that we’d have been so interested in the red planet and it’s cold, arid landscape. Mars's best days may be behind it. Now it's a place of what may have once been.
Of course, Mars is still somewhat easier to explore in engineering terms, and it may well harbor subsurface life even today. But Enceladus? Enceladus is like the ripe fruit on the tree, while we keep grasping at the dried-out cores on the ground.