TENTACLED monsters, pale skinny humanoids, shimmery beings of pure energy... When it comes to the question of what alien life forms might look like, we are free to let our imagination roam. The science-in-waiting of extraterrestrial anatomy has yet to acquire its first piece of data, so nobody knows what features we will behold if and when humans and aliens come face-to-face. Or face to squirmy something.
Despite this lack of hard evidence, a blend of astronomy and earthly biology offers some clues to what is out there. A few bold scientists are even willing to make an educated guess at the nature of aliens that might exist on faraway worlds.
What these extraterrestrials will be like depends on where and how we expect to meet them. Barring the appearance of flying saucers, there are two broad possibilities: either we have a close encounter with our neighbours by visiting the planets and moons next door; or we make an interstellar phone call to creatures inhabiting much more distant planets that circle alien suns. These two options have different implications for the shape of what we find living there.
What extraterrestrials will look like depends on where and how we meet them.
If first contact turns out to be within our solar system, then at least we have some prior knowledge about the available habitats. Several spots might be suitable for Earth-like life based on carbon biochemistry and using water as a solvent. The subsoil of Mars may be warm enough to host microbes akin to Earth's bacteria, for example, and there could be larger beasts swimming in the watery oceans of some outer moons of the solar system - especially Jupiter's moon Europa. There's every chance that a deep aquatic ocean lies beneath Europa's ice, stretching right down to the moon's rock core, where volcanic vents pump out hot, nutrient-rich water.
Astrobiologist Dirk Schulze-Makuch of Washington State University in Pullman calculates that the energy supplied by these vents could feed a large population of microbes, which in turn could support a pyramid of predators. Europa's top predator, the equivalent of our great white shark, would be a fearsome creature with a mass of - wait for it - about 1 gram. "Europa could support a shrimp-sized organism," he says. There would not be enough prey to feed a viable population of predators bigger than that.
Shrimp-sized doesn't have to mean shrimp-shaped, of course. "It is kind of difficult to say anything about how it would look," says Schulze-Makuch. Even on Earth animals have evolved an astonishing diversity of shapes and body plans, but Schulze-Makuch is nevertheless prepared to speculate. "I would make a guess at something worm-like," he says. "That is a pretty successful kind of organism on Earth."
While the hypothetical ice worm of Europa would be swimming about in boring old water, a few astrobiologists are pushing the boat out and pondering the possibility of life that is not water-based. Most places in the solar system are too hot or too cold for liquid water to exist, but there are several other liquids that might host some kind of biochemistry, says Steven Benner of the Foundation for Applied Molecular Evolution in Gainesville, Florida.
The clouds of Venus hold droplets of sulphuric acid, and billions of years ago there may have been pools of the stuff on the planet's surface. Though pretty destructive to bags of water like ourselves, it could be a refreshing draught for beings with the right biochemistry. These acid-dwellers would have to be formed of chemically resistant materials. "Multicellular Venusians living in liquid sulphuric acid could have veins made of glass," Benner suggests, conjuring up visions of delicate, transparent glassware creatures, rolling carefully over the Venusian rocks. But glass is not the only option: more mechanically robust materials would also fit the bill. "There are flexible polymers that are acid-stable, such as Teflon, polyethylene and silicone," Benner points out.
Elsewhere in our solar system, surface lakes and seas exist to this day - though not watery ones. On Saturn's moon Titan, they are formed from a chilled hydrocarbon cocktail of ethane and methane, and Schulze-Makuch speculates about what forms of life they might harbour. "Things might be bigger," is his unexpected conclusion. "Water has a high surface tension, which constrains the volume of single cells. That's why bacteria on Earth are so small." The surface tension in a methane-ethane blend is much lower, so single cells could be enormous, a possibility that Schulze-Makuch has explored in his novel Voids of Eternity. "I have boulder-sized microbes moving over the surface and guzzling up hydrocarbons," he says. "That is science fiction of course, but there may be something in it."
In our eyes, the Titanians might seem pretty laid back. At around 93 kelvin, Titan's seas are very cold and that makes chemical reactions super slow. "Things could be very slow-moving and slow-growing," says Schulze-Makuch. "The lifetime of such an organism may be 10,000 years, or perhaps as much as a million."
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