Topical science time! This recent news article caught my eye the other day. Russian scientists have succeeded in drilling through the 3.7 km thick layer of ice covering Lake Vostok in Antartica and hope to be able to extract samples from the lake later this year. Lake Vostok has been completely sealed off from the surface for about twenty million years. This makes it a very interesting place to look for life forms, and what they’re doing in Lake Vostok right now is not a million miles away from the way we’ll likely find the first alien life forms.
The environment inside Lake Vostok is, to put it mildly, just a little bit exotic, and not the first place you’d expect to find bacteria. The ice sheet covering it is so thick that we didn’t even know for sure that there was water down there until aircraft- and space-based radar picked it up in the early 90s. Because it’s got a 3.7 km thick ice layer sitting on top of it, that water is under a tremendous amount of pressure – enough so that it remains liquid1 despite having an average temperature of -3oC. This high pressure also results in that water being supersaturated with high concentrations of oxygen and nitrogen than you’d find in an equivalent surface lake, much of which has been forced into “cages” of water molecules called clathrates. There is no light at the bottom of Lake Vostok, but that’s okay because there’s no light at the bottom of the ocean either and we find plenty of stuff living – in fact, thriving – down there, most of which tends to cluster around geothermal vents on the ocean floor – and we suspect the same might be true of Lake Vostok.
Not that you particularly need a geothermal vent to support life these days. Hopefully you’ll have heard of these guys – the extremophiles. Over the last few decades we’ve kept finding them in locations which were thought to be completely inhospitable to life – too hot, too cold, too acid, too alkali, too saline, even too radioactive. In all of these places extremophiles thrive, and some of them even need those harsh conditions in order to survive; for example the halophile loves salt and can’t grow without at least a reasonably high concentration of it present in its surrounding environment.
This has completely revolutionised our view of the conditions required for life to exist, and it’s entirely possible that at the bottom of Lake Vostok there will be a colony of extremophiles that has been evolving entirely on its own for the last 15-20 million years, and which has adapted to exist in the freezing, oxygen-rich environment down there. If they do exist, then those extremophiles will be completely unlike anything we’ve so far observed; bear in mind that twenty million years ago the ancestors of the human race looked something like this and then consider what those twenty million years of divergent evolution might do to bacteria. If they’re down there, and the Russians do find them, then it’ll be as close as we can get to finding alien life without actually going to another planet to do it.
What does this mean for the chances of alien life existing? Well, it improves them considerably, at least in terms of micro-organisms — you might not be able to have a conversation with one but you can at least be comforted that somewhere out there in the Milky Way is the alien equivalent of a tardigrade. However, it also improves the odds of us, personally (or at least a probe controlled by us) going out into space and actually digging some of these things up. You see, Lake Vostok isn’t the only place in the Solar System where there’s a large quantity of water trapped under several kilometres of ice. Several of the moons of the gas giants also qualify; notably Europa, Ganymede2 and Callisto.
Europa is the really interesting one. The surface of Europa is entirely composed of ice, making it by far the smoothest body in the Solar System, but we think that ice surface is only 10-30 km thick. Beneath that is a layer of conducting fluid – overwhelmingly likely to be saltwater – that was detected by the Galileo probe via its magnetic moment. This water is kept relatively warm by tidal heating3 provided by Jupiter, and it’s possible, although nobody knows exactly how possible, that there might be microbial life down there living in environments similar to the ones we find extremophiles inhabiting on Earth – in cold, dark environments like the bottom of Lake Vostok, or next to deep sea thermal vents that spit out heat from the moon’s interior (it does have radioactive decay heating, although this is deemed insufficient to create the subsurface oceans). Missions to visit Europa to try and find some of this life have been mooted for years, although I don’t believe any of the proposals have satisfactorily tackled the question of how exactly you drill through ten kilometres of ice on another planet with a robot probe when it’s taken the team at Lake Vostok decades to bore through four kilometres with the benefit of having humans on hand to operate the equipment. Regardless, it’s likely that it’ll be tried sooner or later depending on what future technology does or does not render possible, and so if you’re unlucky enough to be alive seventy or eight years from now you might end up having the news of the first discovery of alien life piped into your brain through your neuro-cortical machine interface.
The other possibilities are somewhat less interesting. Ganymede has an ice-silicate lithosphere over 200 km thick covering its ocean, so it’s doubtful we’ll ever be able to tunnel down there. Callisto too has a surface layer up to 150 km thick. Enceladus on the other hand is kind of promising; if you’ve watched that EXPLETIVE DELETED Brian Cox on his EXPLETIVE DELETED EXPLETIVE DELETED series Wonders of the Solar System, you’ll be aware that Enceladus has a surface which, like Europa, is composed entirely of ice, but that Enceladus is unique in that the Cassini probe observed water vapour outgassing from the surface in 2008. This points to the presence of liquid water in some form or other, the most likely source of which is yet another subsurface ocean – and since it’s outgassing directly from the surface, it points to this ocean being somewhat more accessible than that of Europa (after all, it needs some way to get there from the interior). At the very least it should be possible to collect material that has come directly from the ocean and determine its composition, and thus its likelihood of being able to support life. While Enceladus isn’t necessarily a better bet for finding life than Europa, it’s potentially far more useful for telling us what the odds of that bet actually are.
The first step, though, is to look at these subsurface water environments on Earth. The Lake Vostok team has been painstakingly drilling through the ice for years while constantly at the mercy of the Antarctic elements (Lake Vostok is the coldest place on Earth). They used a thermal sensor to detect when they were approaching a source of free water and stopped the drill, the general idea being that the decrease in pressure would allow the ice separating the borehole and the lake to melt and then refreeze; they can then retrieve this portion of ice to examine water that has come directly from the lake. Meanwhile they’ve examined the last piece of ice extracted from the borehole – which contains ice thought to have frozen onto the bottom of the ice sheet hundreds of thousands of years ago – and they’ve managed to turn up evidence of extremophile microbes, so the odds are good that there’s a colony of something down there. The team has plans to go even further by sending a robot down in late 2013 to retrieve samples from the sediment making up the lake bed. I suggest you pay attention to the results, if only to make sure that this hasn’t happened.
1. If you think back to the geophysics post you’ll recall that the normal rules of physics don’t really apply when you crush something underneath hundreds of millions of tons of pressure. Rock cannot melt despite being at a high temperature because melting would require it to have some room into which it can expand – and because it’s compressed so much by the high pressure, this is the one thing it doesn’t have. Because water is unusual in that it actually expands in volume upon freezing into ice, the same rule applies to Lake Vostok.
2. CORRECTION: I stated in the geophysics post that Earth is the only body in the Solar System known to have a liquid iron core. While this is broadly correct, it is also suspected that Ganymede, Venus and Mercury might have them as well; Ganymede is very uncertain, Venus is likely based on its similar size to Earth (with the absence of plate tectonics being explained by there being no water on the surface of Venus to soften up the crust enough to allow plate formation/subduction) and Mercury has a strong magnetic field relative to its size which also points to the existence of a partially liquid core. Tectonic behaviour is observed on many of the icy moons of the gas giants, but their tectonics all involve chunks of ice shifting around and not rock. Earth is still a unique case in terms of the strong magnetosphere and plate tectonics.
3. Ever seen a picture of Io? I don’t particularly recommend it as a holiday destination because it’s absolutely covered in volcanoes. In terms of raw geological activity it puts the Earth in the shade, but unlike the Earth this is entirely down to outside forces: tidal heating caused by a constant tug of war on Io between Jupiter and the other Galilean moons which constantly squeezes and stresses it like… well, like somebody constantly squeezing and stressing a stress ball. Io has to relive this internal pressure somehow, and it does it by forming volcanoes which constantly spew out poisonous sulphur gas, which has stained the entire planetary surface a sickly yellow colour.