What lives under the ice?

In January 2013 a project drilled into Subglacial Lake Whillans, under about 800 metres of West Antarctic ice, using a hot water drill with filtration and ultraviolet treatment designed to keep surface organisms out.

The water held about 130,000 cells per millilitre and more than 3,900 distinct types of bacteria and archaea, one of which made up 13 per cent of the sequences. It was not a trace of life: it was an ecosystem.

The energy comes from chemistry. The abundant groups are relatives of organisms that oxidise reduced iron, sulphur and nitrogen compounds, entirely without sunlight, and those compounds appear to come from minerals weathered in the sediments beneath the lake.

In Antarctica two lakes have been reached this cleanly, Whillans and Mercer Subglacial Lake, both in the west. How much they say about the hundreds of others is not known.

Some of these lakes fill and drain. Satellite measurements of the ice surface rising and falling reveal water moving between them, probably along channels at the bed, over years.

So an active lake is connected, and the organisms in it are not sealed in a single body of water for millions of years. That is not true of every lake, and it is not how the bed was first understood: after the lakes were found, the consensus among glaciologists was that water moved very slowly there.

A 2022 inventory counts 675 subglacial lakes under the Antarctic ice sheet, most of them found in radar surveys flown across the continent, and the count is still growing.

A 2016 review describes a range rather than one kind of lake: at one end, lakes that outburst and refill over years, at the other, lakes hydrologically isolated for millions of years. Water moves between the active ones along channels cut at the bed.

How that water interacts with the ice above it is still in question, which is why this is not only a question about biology. What the water is doing under the ice is part of how quickly the ice leaves.

Some lakes have probably been cut off for millions of years. Others fill and drain over years, passing water to neighbouring lakes and out to the sea. Most lakes show no activity from above, which a 2022 inventory reads as either a closed system or inflow and outflow in balance, and from the surface the two look alike.

The question matters because a community isolated for millions of years would be a different kind of result from one that has been exchanging with the sea.

The same reasoning is applied to the ice-covered oceans of Europa and Enceladus. Astrobiologists study the low-energy subsurface of this planet, subglacial lakes among it, as an analogue for those places.

The comparison is a useful analogy and not evidence. A lake fed by ground-up continental rock beneath a terrestrial ice sheet is not the same environment as the brine ocean thought to lie over the rocky core of a moon of Jupiter or Saturn.