So far, this alien life series has explored Europa and Mars–both planets we find appealing for their potential to host Earth-style life, the type of life we know to exist. But what about life “not as we know it?”
Saturn’s moon Titan is perhaps the most intriguing world in the solar system. Until very recently, its surface has been hidden from us altogether by its thick, cloudy atmosphere. Only with the recent flyby by the Cassini-Huygens probe did we get a glimpse of what Titan’s surface really looks like, and the results of this mission were astonishing.
Visually, Titan bears a very great resemblance to Earth–it has clouds and raindrops, seas and continents and sand dunes and shorelines that look remarkably like those found at home. It is the only place off of Earth known to host large, stable oceans: it is also the first place naturally existing liquid has been photographed off of Earth. If there’s anywhere extraterrestrial that looks like you’d expect it to host life, it’s here. (Titan-Earth comparison courtesy of NASA. Thank government spending for science.)
Given all that, it may come as a shock to hear that Titan’s average surface temperature is -290 degrees Fahrenheit. If it’s any comfort, that’s somewhat warmer than liquid nitrogen; on Titan, nitrogen is a gas, and composes 98.4% the atmosphere. But water is so perpetually frozen that it can be considered a mineral. And there’s another fun difference: Titan’s atmosphere is so thick relative to its low gravity that a human could literally fly by flapping their arms.
So how does frigid Titan do such a good impression of Earth? Its seas are made up of substances that are gases on Earth–hydrocarbons, like methane, ethane, and even a bit of propane. Its land masses are composed of frozen water and ammonia, which also exist in their liquid states below Titan’s crust much as silica and iron exist in liquid form below Earth’s. So there, in Titan’s mantle, there may be potential for Earthlike life. But with so much liquid methane on its surface, it’s hard not to ask if some decidedly un-Earthly life might live in that.
The suggestion of life in liquid methane is still considered pretty outlandish in the scientific community. If nothing else, it may simply be too cold: temperature effects the speed of biochemical processes, after all, and for all we know even hydrocarbon reactions might proceed at a snail’s pace on Titan.
But other astrobiologists aren’t so sure. After all, talking about the fundamental characteristics of life based on Earth life may be like talking about the fundamental characteristics of animals solely from observing a zebra. And Titan’s atmosphere, like that of Mars, has some tantalizing inconsistencies that some scientists claim as possible evidence of life.
Titan’s gas mixture is different from what we would expect. This should not be at all surprising, given how shocked we were by Venus. However, any differences are always tantalizing–for example, Titan has over 1,000 times more carbon dioxide and gaseous methane than predicted. You may recognize these as waste products of Earth-style life.
In addition, some naturally-produced chemicals are missing. There’s not enough free hydrogen or acetylene. This is especially exciting because these molecules may have the necessary properties to serve the roles of oxygen and glucose in a liquid-methane ecosystem. Their conspicuous absence has led some scientists to suggest that they are being consumed by a never-before-observed form of life native to Titan.
To evaluate the question of life on Titan, let’s look at the basic chemical “tools” that make Earth life possible:
1) A solvent. On Earth, this is water. A solvent is simply the background liquid that everything else floats in; and it’s important, because it creates the proper environment for biochemical reactions to occur. Organic chemicals are unlikely to spontaneously react, for example, if you powder them and mix them together on a dry surface. They’re also unlikely to react if you vaporize them and mix the gases together. Liquid is by far the best environment for metabolism. (Artists’ rendering of the view from Titan’s surface also courtesy of NASA.)
Earth scientists are in the habit of thinking that water is the absolute best solvent to bring about life. It does have some pretty unique properties. Water is, for example, the only liquid that expands instead of contracting when frozen. This reflects the uniqueness of its hydrogen bonds. These same bonds which form the expanded crystal structure of water ice also make it better than any other known liquid for suspending organic molecules in their highly reactive forms. But on Titan, the liquid isn’t water: it’s methane.
How does liquid methane stack up against water as a medium for life? Some scientists think that it’s a terrible medium, because of its low temperatures (which translate to slower chemical reactions) and its complete lack of water-like hydrogen bonds. Others, however, suggest that this very lack of methane reactivity may encourage organic chemicals to react with each other instead of their liquid medium–possibly speeding the formation of large structures like proteins.
2) Hereditary material. All life must have the ability to reproduce itself. The heart of this is a hereditary material, that is, a material that is used to pass down information from parent to offspring. On Earth, nucleic acids (DNA and RNA) serve this purpose due to their convenient complementary binding abilities–you can make a whole copy of a DNA or RNA double-helix by lining up random nucleotides with either of the strand of the original and seeing what sticks. This is the basis for DNA replication on Earth.
What might serve the role of hereditary material on Titan? We don’t quite know what other chemicals might serve as well as our nucleotide bases. However, we do know that our nucleotide bases could very well be created by the interaction of the Sun’s light with Titan’s atmosphere.
In 2010, astrobiology researcher Sarah Horst showed that all five nucleotide bases used by our DNA and RNA (plus some of the amino acids that make up our proteins) could be produced by applying energy to a gas mixture similar to Titan’s atmosphere. Vitally, this did not require the presence of liquid water. All it required was Titan-style gases, and energy. This has led to speculation both that Earth-type life could exist in Titan’s liquid water mantle, and that un-Earthly life could exist on its surface.
3) An energy storage molecule. On Earth, this role is filled by glucose and other sugars. What you essentially need is a molecule that you can form using the energy of sunlight, heat, or something else, and then break down later to release energy. On Earth, organisms use sugars as their first step in energy storage. What might they use on Titan?
Some have suggested that the carbon-nitrogen compound acetylene might do the trick. Like glucose, it contains high-energy bonds, which is created by methane’s interaction with sunlight. And like glucose, it could potentially be broken down by an organism with the proper enzymes to fuel metabolism. This makes the conspicuous absence of acetylene build-ups on Titan’s surface very interesting; scientists had expected to find billions of years worth of acetylene accumulation sitting around, but so far they haven’t.
4) Biochemical legos. This is my pet name for proteins. That is, after all, what amino acids are; they’re building blocks that come in a variety of shapes, sizes, and chemical properties such that they can be stuck together to make virtually anything a cell may need. The instructions for assembling these legos into useful proteins is contained within the hereditary material of Earth cells. This would likely be the case for non-Earth life as well, unless we stumbled across a situation where hereditary material and structural building-blocks were one in the same. (Very early Earth life may have looked like this, using RNA for both purposes.)
Here, there are two possibilities for Titan. To go the more obvious route, the same Sarah Horst experiment that showed that hereditary material could be produced in Titan’s atmosphere also resulted in the production of Earth-style amino acids.
Earth-style amino acids are surprisingly common in the cosmos: they’re found in many atmospheres, and even, according to some hotly-disputed spectroscopy results, in the gas clouds between stars. Scientists are beginning to feel that the basic building blocks of Earth-style life may be virtually everywhere.
But there’s also a known possibility for uniquely Titan structural building blocks. Isaac Asimov, famed science fiction writer and biochemist, suggested that complex hydrocarbon compounds could play the same role there as amino acids on Earth. These hydrocarbons could come in as many shapes and sizes, and arguably be better-suited to interacting with the liquid-methane medium in low temperatures.
5) A membrane. On Earth, we have our lovely phospholipid bilayers. These are highly convenient structures that can form naturally, through inorganic processes. They’re a bit like bubbles, except that in this case it’s a “bubble” of molecules whose nonpolar components want to be shielded from their polar solvent, water. When they form into a sheet and then a closed sphere, they also shield whatever is inside them from the outside, thereby protecting and accelerating the biochemical processes they contain.
In a liquid methane environment, a membrane would look quite different. This is because liquid methane is the exact reverse of liquid water: where liquid water is highly polar, methane is utterly nonpolar. So membrane/solvent polarity may actually be reversed on Titan: cell membranes could be made up of a low-temperature polar liquid, aggregated into a membrane against the polar environment of the methane.
How likely is any of this? The bottom line is that we don’t know. We know next to nothing about the chemical processes taking place on Titan: we know, literally, only what we’ve been able to gather from orbit. Nobody’s ever brought samples back or even taken an on-site chemical measurement at the surface.
It may turn out that Titan, like Venus, is a quite non-living surprise for us. Previously unknown chemical/geological processes could explain the lack of hydrogen and acetylene, and the repletion of methane and carbon dioxide. Or it may turn out that Titan is our first experience with truly alien life, using a biochemistry worlds different from our own. This discovery would utterly transform all we know about life in our Universe.