Got to face the alien face
I am a big fan of the History Channel’s series The Universe, to the point that I purchased the first two seasons on DVD… and I very rarely purchase DVDs of anything. The series is full of good, strong science related with very good CGI. But I have a few complaints with the episode Alien Faces, which I watched last night for the first time.
The Alien Faces episode was The Universe‘s take on astrobiology. The narrative envisaged several extrasolar planets that support life, with each planet being distinctly different in mass, solar input, amount of available water on the surface, and other such factors. The planetary science was pretty good. All the planets depicted in Alien Faces are fictional, that is none of them correspond with any currently-known extrasolar worlds… though clearly a couple of them were inspired by known planets.
For example, one world in Alien Faces was a close analog of Gliese 581c: a tidally-locked world orbiting close to a red dwarf star. When Gliese 581c was discovered it was heralded as possibly orbiting in the habitable zone (at a stellar orbital distance where liquid water can exist on its surface) of its red dwarf star. I was pretty excited about that discovery, and wrote up a short narrative wherein I imagined conditions there – with a completely speculative depiction of native life – and I published it in the Geochemical News – the society newsletter of the Geochemical Society. This was before I started blogging. Okay, plug finished. Anyway, Gliese 581c probably isn’t in its star’s HZ after all, but other red dwarfs might have analogous worlds, and thinking about what such planets would be like is a useful exercise in extrapolation.
Alien Faces also features a low-gravity world with ample water, a massive arid world with a ring system, a Europa analog with a subsurface ocean below an icy crust, and an airless planet populated by machine life. The selection of hypothetical worlds is an interesting mix, and serves to demonstrate a range of evolutionary habitats where the fundamental constraints of gravity, availability of water and solar energy differ significantly. There are valid questions about what kinds of life would develop on a high-gravity world, for instance, or in the tightly-constrained environment of a tidally-locked world where one side is perpetually blazing hot and the other side is eternally freezing cold. The problem for me is not the selection of theoretical planets, here, but the assumption of more or less terrestrial-style life on each one (barring the machine-life example).
Many of the biospheres are dominated by complex animals or animal-like creatures having four limbs and bilateral symmetry. Some creatures are pretty interesting, such as giant sliding tripods found on a high-gravity world, or floating gasbag animals on a low-gravity/heavy-atmosphere world… but the latter possibility was depicted much more creatively in Wayne Barlowe’s book Expedition. The artist Nemo Ramjet also developed an interesting bestiary of alien animals as part of his Snaiad project. Science fiction novels abound with a plethora of amazing and bizarre – but scientifically plausible – aliens, and the creatures of Alien Faces seem fairly pedestrian by comparison.
In Alien Faces all the life forms on all the worlds – again, except for the machine world – apparently breathe oxygen. Even assuming for argument’s sake that alien life would be based on the same basic biochemistry as ours – DNA, RNA, the same or similar amino acid bases, energy transduction using ATP – there are plenty of possibilities for radically diverse biospheres. For the first two billion years on Earth our atmosphere was devoid of molecular oxygen. Recent work suggests that our planet’s biosphere prior to the evolution of oxygenic photosynthesis was dominated by methanogenic microbial life, yielding a methane-rich atmosphere. Could complex animal and plant life evolve using a methane-CO2 metabolic couple, instead of our existing dominant oxygen-CO2 model? What would that life be like? In a methanic reducing atmosphere would plants be able to evolve, say, fullerene-based structural polymers instead of cellulose? And how tall could they grow, then?
Many microbes on Earth can exhale hydrogen sulfide – rotten egg gas – which is used by other microbes who in turn exhale sulfur or sulfate. Could an entire biosphere of metazoans and plants be supported that way? In that scenario you’d end up with alien land plants using atmospheric CO2 and H2S to grow tall, breathing out yellow sulfur dust that drifts down like pollen… where it would be sucked up by animals that breathe solid sulfur dust instead of oxygen gas, exhaling H2S to complete the plant-animal cycle. What color is the sky in an N2/H2S/CO2 atmosphere? Sulfur redox chemistry yields less energy per mole than oxygen does… so would sulfur-breathing animals have to be slow-moving and small? Perhaps the only kinds of animal life possible on such a world would be sessile sulfur-feeders living at the feet of towering sulfidic plants.
Alien Faces does many things right. For example, scientists interviewed for the program discuss how plant life might differ on worlds orbiting dim red stars, where photosynthetic pigments might evolve to absorb every meager visible and infrared photon… making plants black. At the risk of being utterly shameless I should say that I speculated in exactly this direction in my Gliese 581c story a few years ago. Alien Faces also makes good points about how a planet’s environmental conditions will always dictate the terms under which life evolves, with radically different conditions producing radically different biological adaptations. It’s not a bad episode. I only wish they’d pushed a bit beyond the convention of oxygen chauvanism.