The Science of Discworld Revised Edition

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 announced signs of life on Mars. A meteorite dug up in the Antarctic with the code number ALH84001 had been knocked off Mars 15 million years ago by a collision with an asteroid, and plunged to Earth 13,000 years ago. When it was sliced open and the interior examined at high magnification we found three possible signs of life. These were markings like tiny fossil bacteria, crystals containing iron like those made by certain bacteria, and organic molecules resembling some found in fossil bacteria on Earth. It all pointed to: Martian bacteria! Not surprisingly, this claim led to a big argument, and the upshot is that all three discoveries are almost certainly
not
evidence for life at all. The fossil ‘bacteria’ are much too small and most of them are steps on crystal surfaces that have caused funny shapes to form in the metal coatings used in electron microscopy; the iron-bearing crystals can be explained without invoking bacteria at all; and the organic molecules could have got there without the aid of Martian life.
    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 all 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