I always appreciate a good mystery, so I thought that I would write this post about three things that may turn out to have similar but extraordinary things hidden beneath the surface.
Although they are quite distant from one another, Antarctica, Europa (the 4th largest moon of Jupiter), and Titan (the largest moon of Saturn and the 2nd largest in our solar system) have attracted a fair amount of attention over the fact that they have, or may have, large lakes beneath their icy surfaces. The presence of the nearly 16,000 square kilometre Lake Vostok, sitting around 4,000 metres beneath the surface of the Antarctic ice-sheet, was confirmed in 1993. Whether Europa and Titan have sub-surface lakes or oceans is less certain, but over the last 15 years remote-sensing data has been collected by space probes that strongly supports this hypothesis.
Perhaps the most compelling aspect of this topic is biology. In the case of Antarctica, scientists are eager to sample from and describe an environment that has potentially been sealed from direct contact with Earth's atmosphere for over 15 million years and that could contain undiscovered microorganisms. For Europa and Titan excitement is focused on the possibility of discovering the first example of extra-terrestrial life. To top things off, we can expect much of this discussion to move quickly in the near-future: although not without controversy, Russian scientists may gain samples of Lake Vostok water by the end of this year, and the European Space Agency is planning to launch a space probe in 2022 that will gather data about Europa's chemistry and gain Radar profiles of its subsurface to test the ocean hypothesis.
I think that it's worth briefly discussing how these subsurface lakes and oceans come to be, starting with Antarctica. The presence of lakes beneath an ice-sheet that has the coldest surface environment on Earth (including a world record low of -89 Celsius recorded at Vostok station in 1983) may seem counterintuitive, but their existence was first hypothesized over a century ago by Russian scientist Peter Kropotkin. Kropotkin wrote that the considerable thickness of the Antarctic ice-sheet (over 4 kilometres in some areas) is sufficient to generate pressures at depth that are capable of decreasing the melting point of water by a few degrees1. When combined with knowledge of geothermal heat emanating from the continent's rocky basal layer, we have a picture that is simple and yet complete enough to understand the formation of subglacial lakes. The location and dimensions of Lake Vostok, and other sub-glacial lakes, were confirmed in the early 1990s following studies of Radar echo-sounding profiles of Antarctica, with the liquid surface of these lakes easily discernible because of the strong signal reflectivity of water. While there is uncertainty, recent research appears to favor the hypothesis that Lake Vostok formed beneath the Antarctic ice-sheet 15 million or more years ago, rather than before the formation of the ice-sheet as an open-air lake2.
I mentioned that Lake Vostok has potentially been sealed from the atmosphere for 15 million or more years, but I don't mean to give you the impression that the original lake water has remained throughout this span of time. The Antarctic ice-sheet is slowly but constantly flowing, and water from the lake is continually frozen onto the bottom of the moving ice-sheet, carried away by the sheet's flow, and then replaced with new glacial melt-water. It is estimated that this conveyor-belt-like cycle takes approximately 13,000 years to remove an equivalent to the lake's entire volume2. I mention this cycle because it helps in answering a key practical question for research at this site: how can we get a sample of Lake Vostok's water? There is concern that drilling directly into the lake could damage or contaminate its potentially fragile ecosystem, and scientists were initially unsure that they could gain a sample without taking this risk. However, it was eventually realized that it is not necessary to directly sample Vostok's water to sample some of its biota: by drilling close to the lake, but not actually penetrating its water, they were able to obtain ice-cores formed from frozen lake water that had microorganisms encased within it. The results showed familiar species of small rod-shaped bacteria, but there is still the expectation that unfamiliar microorganisms may await discovery in the lake's deeper waters or on its floor2.
While the great distances to Europa and Titan place constraints on our ability to collect information about them, space probes have gathered some amazing information in recent years. For starters, Europa looks like this (its diameter is about 3,000 kilometres):
If you've ever seen a picture of our Moon, of Mercury, or Mars, you might appreciate that Europa does not have nearly as many craters on its surface as do these other bodies. Europa is thought to be as old as these other objects, so the simple explanation is that it has been "resurfaced", causing the removal of many of the older impact craters. The most probable explanation for this resurfacing appears to be related to the great tidal forces exerted on Europa by Jupiter. Physicists predict that tidal forces generate enough heat through friction to melt ice in Europa's interior and create a global ocean layer somewhere between 10 and 30 km beneath its surface3. Occasionally, because of meteorite impacts or tectonic activity, this water may spill onto the surface and erase craters. The red colouration in the above image is thought to represent the residue of salts (such as ammonia) from such spilled water. You may also notice that the reddest areas surround the large cracks that cross the moon, and these cracks may be the source from which water spilled onto its surface3.
Saturn's moon Titan gained a fair amount of publicity in 2004 when the Cassini-Huygens space probe entered its orbit, with the Cassini satellite using radar to penetrate its visually opaque atmosphere and capturing images of hydrocarbon lakes on its frigid (-180 Celsius) surface. The Huygens module landed on its surface some 6 months later and took this picture:
The discovery of hydrocarbon lakes came as a surprise to many, but mega-props go to Carl Sagan (among other scientists) for predicting this as early as the late 1970s. A recent analysis of Titan's gravity field also indicates that the moon has a subsurface ocean, which would most likely be formed by a combination of radiogenic heating from its core and a very low freezing point due to a high concentration of salts4. Here's a cross-sectional graphic showing how Titan's interior is thought to be layered:
I realize that I'm breaking typical blog etiquette by writing at length, so I'll wrap things up. While it is out of my field, I do not think that it is possible to assess the probability of life occurring in the subsurface environments of Europa and Titan, but it seems that most scientists writing on the issue place the probability above 0. I could naysay based on the fact that both of these environments have more extreme physical conditions than Lake Vostok, and that life may have evolved under more clement conditions on Earth only to eventually adapt to the more hardy conditions experienced beneath an ice sheet, but I don't think that this would change much. Furthermore, many scientists are particularly hopeful about prospects for life on Titan because it contains an abundance of complex molecules that may have been part of the "primordial soup" from whence life began on Earth. Don't expect to wait too long to hear more exciting news about any of these places.
2. Siegert, M.J. (2005). Reviewing the origin of subglacial lake Vostok and its sensitivity to ice sheet changes. Progress in Physical Geography 29, 2, 156-170.
3. Stevenson, D. (2000). Europa's ocean - the case strengthens. Science 289, 1305-1307.