I got an email the other day from a colleague who teaches science at Soap Lake High School, a rural high school in eastern Washington that, in my opinion, punches above its weight in the sciences. One of his students is preparing a presentation on Europa, and she had some questions for a Europa expert. I’m not a Europa expert, but I interact with people who are, so I suppose I’m only one degree removed from actual knowledge of Europa…
Soap Lake High School is located (predictably) near Soap Lake; a highly alkaline, highly stratified saline lake. Because of the lake’s physical structure and chemistry it is a useful analogue for the Europan ocean. It’s not Lake Vostok, but no analogy is perfect. Europa is a pretty hot topic in astrobiology, and there is plenty of information out there on it, but I figured it wouldn’t hurt to post my answers to her questions here. It also lets me expand on the original answers a little.
Why do scientists think that life could be on Europa?
Scientists think that life needs two things – liquid water and a source of energy. Europa is one of the few places in our solar system (other than Earth) where both of these things are likely abundant. There is a lot of evidence that underneath Europa’s ice shell is a huge ocean, much larger in volume than the oceans on Earth. Because Europa is so close to Jupiter, and because of Jupiter’s massive size, there is a huge gravitational force exerted on Europa. This gravitational force heats Europa much in the way that Earth is heated by radioactive decay in its core, possibly enough to produce low-level volcanism at the sea floor. As on Earth, seafloor volcanism produces springs, black smokers, and other sources of reduced elements, useful for life as an energy source (when paired with oxidized material). On Earth these volcanic sites are hotspots for life.
For energy, life requires both oxidized and reduced material. Oxidized material should be abundant at the surface of Europa’s ice shell, thanks to constant bombardment by radiation. There is lots of evidence that Europa’s ice shell turns over gradually, bringing oxidized material into the ocean and exposing fresh ice to radiation. On Earth sea ice is a hotspot for photosynthetic life because sunlight is abundant – like blacks smokers it is energy rich. A similar situation could exist on Europa at the interface between ice and water, since this environment is optimally located between the oxidized ice and the reduced ocean. If Europa were a battery the seafloor and ice surface would be the negative and positive terminals (respectively). Life at the interface acts as a circuit, using the electrons for biological processes as they flow from the negative to the positive terminal (from seafloor derived material to ice surface derived material).
From Seeking Europa’s Ocean (Pappalarado, 2010). This cut away shows many putative features of the Europan ice shell, including the strong temperature gradient, convective diapirs, and the surface features produced by these internal processes.
How is Soap Lake similar to Europa? How is it different?
There is a lot of “chemoautotrophic” life in Soap Lake. This is life that, rather than using the sun as a primary energy source (in the case of plants by creating reduced carbon and free oxygen), uses reduced and oxidized chemicals – the negative and positive battery terminals discussed before. These chemicals may have a biological source (for example ammonia or nitrate or organic carbon) or a geological source (like iron). If the chemical being used is not organic (does not contain carbon-hydrogen bonds), life is said to live lithoautotrophically – this kind of life can theoretically happen in the absolute absence of sunlight (no photosynthesis going on anywhere in the environment). Finding locations on Earth where this is happening is challenging, because photosynthesis influences almost everything here. On Europa there is essentially no light that can be used in photosynthesis, so we would expect a lot of lithoautotrophy. Without photosynthesis the ecosystem of Soap Lake would look pretty different, but there are good examples of microbial life there that can serve as a model for life on Europa. One example is bacteria that gain energy via sulfur oxidation, where reduced sulfur compounds are oxidized by oxygen.
If there is life of Europa, what do you think it looks like?
I think it would look pretty much like life on Earth – or at least microbial life from the right habitats on Earth. Evolution seems to produce the same solutions to problems over and over again – this idea is called convergent evolution – and the problems life would face on Europa are not so different than what it faces on Earth. It seems likely that evolution would produce the same basic structures and metabolic strategies to deal with these problems.
Do YOU think there’s life on Europa, and what will it mean if there is?
I think Europa is the most likely place in the solar system, other than Earth, for life to exist. The fact that it is so far from Earth (unlike Mars) makes this even more exciting. If we find life on Europa there is almost no chance that it came from Earth, it would have to have had a separate origin. This would tell us a lot about how widespread life is in the universe. Finding life on Mars would be a little different, chances are that it would have something to do with life on Earth (Earth and Mars exchange asteroids and other material all the time, providing a vector for biological contamination).
How soon do you think it will be before we know for sure if there’s life on Europa?
Unfortunately it will be a long time before we know if there is life on Europa – a couple of generations if we maintain an interest in exploring the outer solar system. The most we’ve ever been able to study Europa came with the Galileo spacecraft which was sent to study the entire Jupiter system. It flew past Europa multiple times and from those observations we know a little about the composition of the moon and the likelihood of liquid water. We’ve never had a spacecraft in orbit around Europa however, let alone landed on its surface! Since life is most likely to exist below several miles of hard, thick ice even getting to the surface won’t tell us if life exists. Somehow we need to land on the surface and melt or drill through all that ice into the ocean below (warm ice rising in the shell might also contain life, but that would still involve a lot of drilling). This is technically feasible but would be very very expensive to do. We might find tantalizing evidence of life from a spacecraft in orbit or on the surface, perhaps in the form of chemicals that were most likely produced by life, but this isn’t absolute proof.
