The recent news that there may be an underground ocean on Titan tantalizes us in astrobiological terms. It also brings us up against the question of how to define a habitable zone. The standard definition involves the presence of liquid water at the surface, a reasonable requirement when you’re looking for carbon-based life. But it’s also true that exobiology may one day be studying forms of life that are nothing like us, living in environments we would at first dismiss from our list of living places. Just how far does a habitable zone extend?
First, a short defense of the status quo. As we expand our exoplanet hunt and become capable of detecting the signature of life on distant planets, we need a target list to optimize our search time. It’s entirely reasonable to fine-tune that list toward conditions similar to what we find on Earth because the life on our planet is the only kind we’ve been able to study. Detecting its biomarkers in an alien atmosphere is a sensible goal. We know what we’re looking for (within reasonable parameters), and trying to identify such life doesn’t mean that we’re not aware that far more exotic possibilities may occur elsewhere.
So calling a habitable zone a region where liquid water is possible helps us keep our focus as we begin to learn what possibilities the universe presents. But it’s also clear that even here in our own Solar System there may lurk surprises. Enceladus reminds us of that, as does Europa, and now we learn that Titan may be even more interesting from an astrobiological perspective. Thus Ralph Lorenz (Johns Hopkins Applied Physics Laboratory), lead author of the recent Titan paper, commenting on the possible presence of liquid water mixed with ammonia 100 kilometers down:
“The combination of an organic-rich environment and liquid water is very appealing to astrobiologists. Further study of Titan’s rotation will let us understand the watery interior better, and because the spin of the crust and the winds in the atmosphere are linked, we might see seasonal variation in the spin in the next few years.”
Lorenz mentions Titan’s rotation because Cassini’s science team has been using the spacecraft’s Synthetic Aperture Radar to collect data, establishing fifty unique landmarks on the moon’s surface. The intriguing result is that when earlier data are compared to more recent flybys, some of these features seemed to shift as much as thirty kilometers from their prior positions. There was no actual movement at the surface — the apparent ‘shift’ was the result of the fact that Titan was spinning about its axis in a different way than expected. An internal ocean is almost demanded as a way of explaining how the moon could move in such a way. Couple deep water with all those useful chemical compounds that preceded life on Earth and things get interesting.
One day we will discover whether places like Titan or Europa do contain life. Once confirmed, even simple bacteria become reason for us to extend the search for life into even more remote territory, surveying possible biospheres in the Edgeworth/Kuiper belt, for example. Life may prove astoundingly adaptive, even if conditions in the outer Solar System cannot support the complex fauna of Earth.
And that’s really what the notion of habitability comes down to. The question philosophers and scientists have asked for centuries is whether there are other places in the universe where humans could live. The term ‘habitable zone’ is a way of phrasing such a region, but we’ll need to start talking about ‘biozones’ as we adapt our astrobiological thinking to more extreme environments. Humans would be out of their element on Hal Clement’s Mesklin (the world of his novel Mission of Gravity), but bizarre creatures might still evolve there.
And we might as well push this as far as possible: If some form of string theory proves out and we do live within a multiverse containing as many as 10100 universes, we may have to extend our idea of biozone zone to our entire universe. Some theorists suggest we live in one of the few out of the multitude of universes that could sustain life, its parameters — the precise strength of the nuclear force, for example — exquisitely tuned for life. Many of the others could be lifeless, formless, inimical to organization.
The poet John Dryden’s lovely line ‘And music shall untune the sky’ comes to mind, this theory suggesting that we inhabit a universe whose harmonies keep all essential forces in balance. Just hope nothing happens to put us off-key.
If we’re living within a single living bubble amidst a froth of dead universes, then in the ultimate scheme of things life of any kind whatsoever may be an extreme rarity. But before we go quite that far, let’s find out whether we’re not looking at potential biospheres much closer to home. The Titan paper is Sotin and Tobie, “Titan’s Hidden Ocean,” Science Vol. 319. no. 5870 (21 March 2008), pp. 1629-1630.