At the end of last year, microbes were found under thousands of metres of ice in Antarctica. The discovery not only stretched the habitable regions of the Earth to new extremes but also lent hope to the idea that life might eke out a precarious existence on other worlds. Now new research shows where these microbes might come from, and how they might survive the rigours of a life in ice.
The plucky bacteria, encased in ice many thousands of years old, were first reported last December by a team of US scientists
Why are the bacteria in the lowest part of the ice, not closer to the surface, where they might have been deposited on wind-borne dust? The answer is supplied by results reported in
Lake Vostok, all of which is below several kilometres of solid ice, has been mapped out using radar signals, which bounce back from the ice at the top and bottom of the lake to reveal its buried profile. At 670 metres deep and covering 14,000 square kilometres, it is the largest known sub-ice lake.
Siegert's group has analysed radar data from airborne measurements revealing that ice is being lost from the base of the sheet in the north and west of the lake. To the south, on the other hand, the ice over the lake is about 150 metres thicker on average, owing to freezing of the lake water.
This suggests that ice is melting over one part of the lake and being reformed over another. This may induce circulation in the lake water, just as the water in surface lakes circulates because of convection. It may also release rocky debris and other ice-bound impurities into the water, which, the researchers say, could provide nutrients for any organisms living in the lake.
It was precisely because of the presence of the lake that the ice core was drilled. The core was taken from a region where refrozen ice had been accreted from the lake onto the bottom of the ice, and the drilling stopped just 120 metres short of the top of the lake. This meant that it penetrated about 100 metres into the 'accretion' ice, and it was here that bacteria were found -- some still living after being released from the ice core. This suggests that they may grow within the lake itself.
But how, asks physicist P. Buford Price of the University of California in the
Price says that the accretion ice above Lake Vostok provides all three of the ingredients essential to life: water, energy and carbon. Glacier ice, he points out, is laced with a network of water-filled veins between solid ice grains, in which salts accumulate, lowering the freezing point and preventing the veins from icing up. Price estimates that these veins could be several thousandths of a millimetre across in the Vostok accretion ice -- wide enough to accommodate bacterial cells.
The liquid veins also concentrate dissolved acids, including organic acids such as formic and acetic acid (the main component of vinegar). Price argues that chemical reactions involving these acids, which have been detected from the ice-core studies, could provide sufficient energy and carbon to support the number of microbes found in the ice cores.
Lake Vostok is the best terrestrial analogue of Jupiter's moon Europa. Over the past few years, the Galileo spacecraft orbiting Jupiter has sniffed out strong evidence that below the crust of ice covering Europa's surface lurks an ocean of liquid water stretching from pole to pole. This is the only known world in the solar system other than Earth on which a large body of liquid water seem likely to exist (although it is possible that Callisto, another of Jupiter's moons, might also have a subsurface ocean). If life can exist in the ice above Lake Vostok, thousands of metres below frozen Antarctica, who is to say that it might not be found also below the ice fields of Europa?