It was a thrill to learn that on Sunday, Russian scientists managed to poke a drill tip through miles of Antarctic ice into Lake Vostok. Samples of water from this extreme environment promise to provide one of biology’s severest tests of life on Earth. Scientists are talking up the possibility that this experiment, the first of several in progress in Antarctica, could tell us more about possible life on the icy satellite of Jupiter named Europa. Is that a stretch?
We’re asking different questions here. At Vostok, we want to know if life has survived; at Europa we want to know if life could have arisen. In that context I think that Vostok and Europa are worlds apart; their similarities are superficial. Let’s look at the two places in a bit more detail.
Lake Vostok is a large tectonic basin, rather like Lake Tahoe, that happened to be overrun some 15 million years ago by the growing Antarctic ice cap. It has been sealed in profound darkness and freezing cold ever since, with the ice flowing slowly over it. Here’s a diagram of the situation.
The lake is kept unfrozen because of a trickle of heat from the Earth’s crust beneath plus the effect of great pressure in depressing the freezing point. Ice melts at the upstream end and lake water freezes at the downstream end, so on the geological time scale there’s an exchange of water, and the water itself must be charged with air carried in by the ice. But the amount of minerals and nutrients entering the lake this way must be astronomically small. Somewhat larger amounts may come from the rock and sediment of the lake’s floor, but the picture is still disheartening.
And yet we have found life everywhere on Earth, from temperatures above the boiling point to below freezing. Microbes are recovered from within the ice cap itself. I believe that the microbes originally sealed into Lake Vostok survive today, because that’s the way to bet on this planet. However, from everything we know, life could never have arisen in such a place. The raw ingredients and energy required are absent.
Is that true for Europa? It’s colder on its warmest day than anywhere on Earth, true. But Europa should have much more of the assets for life than Vostok.
Europa is an old world that formed along with the rest of the planets. Like Earth, Europa separated into a dense interior and a light shell, only with a greater share of water. Its rocks, like those of the early Earth, had lots of natural radioactivity that must have generated enough heat to keep part of the overlying water melted throughout its history. (More recently, Jupiter’s four major satellites have fallen into mutually resonant orbits that wring them with changing tidal forces. The innermost moon, Io, is heated to volcanism this way, and Europa and Ganymede are heated to lesser extents.) The heat must have expressed itself in hydrothermal vents, too, exactly like Earth’s seafloor “black smoker” vents.
In a word, as far as planetary scientists can tell Europa should have started out with the same setting that is commonly thought to have spawned life on Earth. The first structures that served as cell membranes could have arisen at hydrothermal vents, which would exist on Europa just as they do on Earth: springs of hot, chemically active water on the floor of a big cold sea. The water itself should contain ammonia, sulfates, even hydrocarbons. All of this is straightforward modeling based on what we already know about the solar system.
Planetary modelers are finding that the thick ice shell of Europa should have some interesting activity, too. The eerie striped pattern of Europa’s surface shows that the ice fractures regularly due to tidal forces. When that happens, water would rise and its dissolved gases would come out in bubbles. These “Perrier ocean” eruptions would spray over the surface, where the ice and its organic compounds would bake and polymerize and react in the radiation from Jupiter and the Sun.
Eventually, after approximately a billion years, the entire icy crust would become replaced with ice bearing this baked material. And at that point you would have a nutrient cycle. In sum, it’s quite plausible for life to arise and persist on Europa where it’s quite impossible in Lake Vostok. If we ever get a spacecraft to Europaproposals keep being submittedour experience drilling to Vostok would help us drill through Europa’s crust. But a more elegant proposal is to simply swoop over Europa in low orbit and scoop up bits of dust from its icy surface raised by micrometeorite impacts. Just like on Earth, if life is on Europa its signs should be everywhere.