Gentry Lee (Jet Propulsion Laboratory) discusses the question of extraterrestrial life on a program called Are We Alone, which airs this evening on the Discovery Channel at 2100 EDT (0100 UTC on the 17th). Lee is chief engineer for the Solar System Exploration Directorate at the Jet Propulsion Laboratory, a veteran of Viking and Galileo, and a co-author of Arthur C. Clarke. He was also involved with Carl Sagan on COSMOS, so he knows something about video productions.
Pushing Back Astrobiology
As I’ve noted in the two previous posts, we’re moving into an era of re-examination of the Solar System. It’s one that leads inevitably to a new understanding of the concept of habitable zones, with life now being considered a possibility on places that were once thought off-limits. Europa is unusual enough, but the evidence for that ocean beneath the ice is persuasive. Can we extend the paradigm all the way out to the Kuiper Belt? If so, missions like the Haumea orbiter or probes to other trans-Neptunian objects become imperative.
At the Aosta conference, I had the chance to talk to Joel Poncy after his presentation on the Haumea project, a conversation that led to astrobiology. Poncy referred me to a paper by Steve Desch (Arizona State) and colleagues discussing cryovolcanism on distant objects like Charon, and making some startling statements about the possibility of liquid water 40 AU and more from the Sun.
Liquid Water in Cold Objects
Desch looks at the evolution of Kuiper Belt Objects using a model his team constructed that shows how they might retain enough heat to keep subsurface water in a liquid state. Listen to what these people are saying about Pluto’s large moon Charon:
We predict that Charon contains a rocky core… of radius 330 km, surrounded by a slushy layer about 30 km thick containing a mix of ADH [ammonia dihydrate] and liquid water and ammonia. Above this layer is a layer of solid water ice… from 360 to 470 km, surrounded by an undifferentiated crust of rock, water ice and ADH, about 130 km thick. Only about half of the mass of Charon ever experiences differentiation. Our thermal evolution models suggest that within a few x 108 yr, the subsurface liquid will freeze entirely.
Of course, a few hundred million years is well down the road. The news is that Charon may contain liquid water now. The paper continues:
We conclude that objects with a densities similar to Pluto and Triton, 2.0 g/cm3, as small as 500 km in radius, could retain liquid to the present day. Our time-dependent thermal models of KBOs show that it is possible for Charon and Quaoar and many other small KBOs to retain liquid water to the present day.
Cryovolcanism on Other Worlds
Thus the interesting reflectivity of Haumea, which may well indicate cryovolcanism, a mechanism that should also be at work on objects like Charon as liquid is delivered to the surface. Another world with evidence for water ice is Quaoar. We could nail this process down, Desch notes, if we find evidence of cryovolcanism when New Horizons flies past the Pluto/Charon system in 2015. It’s hard to imagine these coldest objects in the Solar System maintaining sub-surface water, but even the Uranian moons Miranda and Ariel show recent resurfacing, and we all remember Triton.
It’s a long way from possible cryovolcanism to astrobiology, but finding liquid water in the Kuiper Belt would at least open a long-shot window for life in the most unexpected places. The paper is Desch et al., “Cryovolcanism on Charon and Other Kuiper Belt Objects,” 38th Lunar and Planetary Science Conference (2007), available online.