Science of Discworld

Interferometry uses a widely separated array of small telescopes instead of one big one, and for optical astronomy the array has to be set up in space. This produces an added advantage, because space is big, or, in more Discworldly terms, a place to be big in. The biggest distance between telescopes in the array is called the baseline. Out in space you can create interferometers with gigantic baselines, radio astronomers have already made one that is bigger than the Earth by using one ground-based telescope ante

The biggest dream of all, though, is NASA's Planet Imager, pen­cilled in for 2020. A squadron of spacecraft, each equipped with four optical telescopes, will deploy itself into an interferometer with a baseline of several thousand miles, and start mapping alien plan­ets. The nearest star is just over four light years away; computer simulations show that 50 telescopes with a baseline of just 95 miles (150 km) can produce images of a planet 10 light years away that are good enough to spot continents and even moons the size of ours. With 150 telescopes and the same baseline, you could look at the Earth from 10 light years away and see hurricanes in its atmosphere. Think what could be done with a thousand-mile baseline.

Planets outside our solar system do exist, then, and they probably exist in abundance. That's good news if you're hoping that some­where out there are alien lifeforms. The evidence for those, though, is controversial.

Mars, of course, is the traditional place where we expect to find life in the solar system, partly because of myths about Martian 'canals' which astronomers thought they'd seen in their telescopes but which turned out to be illusions when we sent spacecraft out there to take a close look, partly because conditions on Mars are in some ways similar to those on Earth, though generally nastier, and partly because dozens of science-fiction books have subliminally prepared us for the existence of Martians. Life does show up in nasty places here, finding a foothold in volcanic vents, in deserts, and deep in the Earth's rocks. Nevertheless, we've found no signs of life on Mars.

Yet.

For a while, some scientists thought we had. In 1996 NASA a

However, in 1998 the Mars Global Surveyor did find signs of an ancient ocean on Mars. At some point in the planet's history, huge amounts of water gushed out of the highlands and flowed into the northern lowlands. It was thought that this water just seeped away or evaporated, but it now turns out that the edges of the northern lowlands are ail at much the same height, like shorelines eroded by an ocean. The ocean, if it existed, covered a quarter of Mars's sur­face. If it contained life, there ought to be Martian fossils for us to find, dating from that period.

The current favourite for life in the solar system is a surprise, at least to people who don't read science fiction: Jupiter's satellite Europa. It's a surprise because Europa is exceedingly cold, and cov­ered in thick layers of ice. However, that's not where the life is suspected to live. Europa is held in Jupiter's massive gravitational grasp, and tidal forces warm its interior. This could mean that the deeper layers of the ice have melted to form a vast underground ocean. Until recently this was pure conjecture, but the evidence for liquid water beneath Europa's surface has now become very strong indeed. It includes the surface geology, gravitational measurements, and the discovery that Europa's interior conducts electricity. This finding, made in 1998 by K.K.Khurana and others, came from observations of the worldlet's magnetic field made by the space probe Galileo, The shape of the magnetic field is unusual, and the only reasonable explanation so far is the existence of an under­ground ocean whose dissolved salts make it a weak conductor of electricity. Callisto, another of Jupiter's moons, has a similar mag­netic field, and is now also thought to have an underground ocean. In the same year, T.B.McCord and others observed huge patches of hydrated salts (salts whose molecules contain water) on Europa's surface. This might perhaps be a salty crust deposited by upwelling water from a salty ocean.

There are tentative plans to send out a probe to Europa, land it, and drill down to see what's there. The technical problems are for­midable, the ice layer is at least ten miles (16 km) thick, and the operation would have to be carried out very carefully so as not to disturb or destroy the very thing we're hoping to find: Europan organisms. Less invasively, it would be possible to look for tell-tale molecules of life in Europa's thin atmosphere, and plans are afoot to do this too. Nobody expects to find Europan antelopes, or even fishes, but it would be surprising if Europa's water-based chemistry, apparently an ocean a hundred miles (160 km) deep, has not pro­duced life. Almost certainly there are sub-oceanic 'volcanoes' where very hot sulphurous water is vented through the ocean floor. These provide a marvellous opportunity for complicated chemistry, much like the chemistry that started life on Earth.

The least controversial possibility would be an array of simple bacteria-like chemical systems forming towers around the hot vents, much as Earthly bacteria do in the Baltic sea. More complicated creatures like amoebas and parameciums would be a pleasant sur­prise; anything beyond that, such as multicellular organisms, would be a bonus. Don't expect plants, there's not enough light that far from the sun, even if it could filter down through the layers of ice. Europan life would have to be powered by chemical energy, as it is around Earth's underwater volcanic vents. Don't expect Europan lifeforms to look like the ones round our vents, though: they will have evolved in a different chemical environment.