For more than a century, Mars has been looked on as a possible home for life. In this column I want to examine an alternative world; but first, let's see why Mars is usually the popular candidate.

It begin in the late 19th century when Giovanni Schiaparelli, betrayed by the power of the human eye to "connect the dots," thought he could see a system of "canali" - channels - on the Martian surface. Other astronomers observed the shrinking of the polar caps as the seasons changed. Percival Lowell interpreted Schiaparelli's "canali" as canals, and he saw a planet-wide system of them that annually brought melted water from the poles to irrigate the surface of Mars. So Mars must be the home not only of life, but of intelligence.

What does Mars look like today? No canals, but a cratered, sand- worn surface. No surface water, but signs of past water runoff. Stupendous mountains, twice the size of any on Earth, and a great rift, Valles Marineris, that would easily swallow the Grand Canyon.

This describes the surface appearance, but doesn't rule out life. More relevant are the planet's physical parameters. Mars is about half the size of Earth. This, combined with the fact that the density is a good deal less, gives a surface gravity about 2/5 of what we are used to. The Martian day length is almost the same as ours, and the axial tilt is so close that the changing seasons would follow a similar pattern to ours. The land area of Mars is about the same as the total land area of Earth.

On the negative side, Mars is half as far again from the Sun as we are. That gives less solar heating. On a midsummer day at the Mars equator, the temperature may creep up as high as the freezing point of water. There is little surface water to freeze. The white polar caps of Mars are mostly dry ice - frozen carbon dioxide, which makes the temperature at most 78 degrees below zero. At night the cold is intense, with little or no protective cloak of atmosphere to hold in heat. And of that atmosphere, a negligible amount is oxygen. The air on Mars is mostly carbon dioxide.

To summarize, we have no heat, no water, and almost no air. Why, then, does anyone even consider so inhospitable a world as a home for life? The answer is simple: everywhere else in the solar system seems much worse.

This presumes, however, that we require life to be like that with which we are most familiar. Let's broaden our scope.

There is abundant water on some moons of the outer planets of the solar system, although almost all is in the form of ice. I want to concentrate on one of those moons, a satellite of Jupiter called Europa. This is the smallest of the four "Galilean" satellites of Jupiter, so called because they were first seen and recognized by Galileo in 1610, looking through his home-made telescope. Thirty years ago, Europa would have been ruled out at once as a place where anything could live. Airless and freezing, with a surface temperature almost 200 degrees below zero, it was no place for life.

New discoveries have changed that perception. First, images from the Voyager spacecraft in the 1980s show that the ice of Europa's surface has a fractured pattern, suggesting the presence of a liquid water ocean beneath a thick ice cap.

Second, here on Earth we discovered life forms that are now often called "extremophiles." As the name suggests, they flourish in conditions under which we would previously have said life was impossible. Some forms of life do not need sunlight to perform photosynthesis and build organic compounds from simple molecules. Instead they are chemical-powered, deriving the energy they need from sulfur and hydrogen. Bacteria survive near deep-water sea vents known as "black smokers," places that match early visions of Hell. The hot water that issues from a black smoker is so high in sulfur and other minerals that we would find it poisonous to drink, but certain bacteria thrive on it. Other life forms feed on the bacteria, to create great colonies of tube worms and strange crustaceans.

Different anaerobic forms, known as Archaea, thrive at high temperatures underground. No one knows how far down, though there are suggestions that some form of life may dwell in the heat and dark ten miles and more beneath the surface. At the other extreme, organisms survive in the Antarctic, living and breeding in small lenses of liquid water that form just below the surface when the weak Antarctic sun is able to melt the ice. They return to a dormant condition in the Antarctic winter. Lichens cling to the bare rock of high mountains, well above the snow line, exposed to the sun's fierce ultra-violet radiation from which we normally are shielded by the bulk of the atmosphere.

Life seems able to adapt to almost anything: cold, darkness, pressure, heat, intense radiation. On the basis of what we have seen so far, it would be no great surprise if living creatures were to be found in the icy depths of Europa's ocean - provided that some energy source is available there.

The energy for Europa can't be sunlight, which does not penetrate into the waters of the interior. But there are other sources. The most powerful of these is tidal energy. As the other three Galilean satellites, Io, Ganymede, and Callisto, move past Europa in their orbits, their gravity fields continuously squeeze and pull at the other moon. This creates tides in Europa, just as the Moon in its orbit creates tides on Earth. The Europan tides do not break the ice cap, but they do provide heat to keep the interior ocean liquid. The squeezing in different directions also forces chemicals out from Europa's core of rocky silicates. We know there must be such a core, because Europa's average density is much higher than water.

The depth of Europa's ice cap is an open question. The range seems to be from a kilometer to more than 20 kilometers. A recent article (November, 2001) argues that the higher figure would make life impossible, but this assumes we are depending on photosynthesis. If the life is chemosynthetic, depth of ice is not relevant.

So Europa remains a candidate for indigenous life. It surely won't look much like you or me. But doesn't that just make the prospect more exciting?

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