July 8, 2011 | 6
If there is anything that is predictable about planetary science it is the unpredictability. One of the best examples of this trait has been the remarkable story of Saturn’s moon Enceladus. This brilliantly white and reflective sphere of water ice could fit easily within the borders of Texas, is about 100,000 times the mass of Mt. Everest, and up until recently was just another of the 62 moons orbiting the great ringed gas-giant. This all changed in 2005 when the Cassini mission spotted extraordinary plumes of what appeared to be a water-rich material jetting out from the moon’s southern plains.
Since that discovery this modest ball of rock and ice has become a focus of attention. Far from being an inert memento to the formative years of the solar system Enceladus is geophysically (cryophysically) active. While its surface temperature is on average a chilly 75 Kelvin (-320 F), there are great fissure-like regions in its ice that can rise quite a bit higher. Indeed, some 16 Gigawatts of thermal energy emerges from the moon’s interior, concentrated in these zones. This coincides with the base of the 500-kilometer high plumes that Cassini has spied.
The immediate interest has been whether or not this active cryo-volcanism (for want of a more specific term) indicates a sub-surface environment containing liquid water. Why should we care? We care because liquid water is so central to life on Earth. It is both a unique and incredibly versatile biological solvent and chemical mediator. It is also a critical ingredient for our planet-wide cycles of geophysics and atmospherics. Taken altogether, liquid water may serve as a potential flag for habitats for life elsewhere in the universe. Furthermore, here on Earth we are finding increasing numbers of environments where life exists and thrives with nary a care for the kind of temperate sun-drenched lifestyle that we ourselves enjoy. Sub-surface life may in fact rule our planet, so the conditions beneath the visible scalp of other worlds are of primary interest as we sniff for signs of organisms elsewhere.
Discovering precisely what’s happening on and in Enceladus is a tremendous challenge. However, clever use of the Cassini probe has allowed us to not only observe the icy plumes but to actually fly through enough of them to detect the ping-ping-ping of individual grains of material and to measure their general chemical content. These sampling raids have confirmed the watery composition of the plumes and indicate the likely presence of simple hydrocarbons and ammonia. Together these data have suggested a few potential scenarios for the origins of the material, including some that really didn’t require any great bodies of liquid water.
Now a new study by Postberg et al., reported in Nature, seems to have nailed the case for liquid, lots of it. Their analysis of plume content and structure reveals a highly salty plume composition close to the moon’s surface. Sodium and potassium salts readily dissolve into water through its contact with rocks – precisely as happens here on Earth. The details of this salt-rich water point towards an origin as evaporation into space directly from exposed liquid. It is as if Cassini flew across the salty spray that fogs your sunglasses while you lie on an ocean beach.
The implication is that deep beneath the moon’s surface there is indeed a significant ocean of water that extends all the way down to a rocky, salt-rich, core. Above this ocean there is a thick layer of ice – perhaps 80 kilometers deep. Cracks in this crust allow for pressurized ocean water to seep upwards to eventually form thinly frozen over reservoirs. A squeeze of gravitational tides or ice-tectonics can temporarily break this outermost layer, allowing the ocean water to jet into space.
While this is unlikely to be the last word, it certainly appears that tiny Enceladus could have all the ingredients within which we might expect to find a subsurface biosphere; liquid water, chemical resources, and possibly radiogenic energy supply from a rocky core. Which raises the next big question, could such a biosphere actually originate and evolve in-situ?
It would be fair to say that we don’t even know if our own terrestrial sub-surface biosphere actually began beneath the Earth’s surface, or migrated there (and perhaps even moved back and forth). Despite the incredible variety and differences in lifestyle of organisms on our homeworld – from photosynthesis to chemo-autotrophism (the use of basic environmental chemistry like oxidation to drive metabolism, a.k.a. the bottom of the food chain) the common basis of life does seem to be just that, common to us all. But what we do not know is the relative importance of different environments during the 4 billion year history of life on this planet. This makes it extremely hard to guess whether life could originate and survive forever trapped within the icy bauble of a place like Enceladus, or whether a place like this might lack some critical ingredient – a temporary optional environment like the Earth’s surface – necessary for the recipe.
All the more reason to go and find out if we can. Detecting the presence or absence of life within an incredible natural test-tube like Enceladus could provide us with unique and fundamental clues to the nature of life here on Earth, beneath our feet. It would be a rather wonderful twist to the story if the final piece of the puzzle of our origins comes not from humid tidal pools on a tropical island, but rather from the icy interior of a frozen moon in the depths of the solar system.
Photo Credit: NASA/JPL/SSI
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