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WHEN the news broke about what seem to be traces of water on the surface of
Mars, I was at a conference in Valencia. It was a good opportunity to think
about water. Baking hot in summer, this part of Spain takes the stuff very
seriously. Its water court has been meeting outside the cathedral every week for
centuries to deal with irrigation disputes. And by chance, the city’s
beautifully cool botanical garden was the setting for the second of the American
Geophysical Union’s meetings on the Gaia hypothesis, which suggests that the
creatures of Earth have been regulating their water supplies for considerably
longer.

Mars and Gaia go back a long way. Jim Lovelock’s idea that planets with life
can regulate their environment on a global scale grew out of a comparison he
made in the 1960s between the atmospheres of Earth and Mars. His Gaian view sees
life either taking planets over completely—in which case it will pump
reactive gases into the atmosphere, pushing it far from chemical equilibrium, as
it has on Earth—or dying off because it has failed to achieve such global
influence. To Lovelock, the Martian atmosphere—all but completely
inert—shows that the planet is dead. In this, Gaia runs counter to the
general astrobiological presumption that in any place where there’s water and a
supply of nutrients, life will somehow cling on—if it was there in the
first place.

The argument that there was once life on Mars seems quite a good one. There’s
no known factor that graced the early Earth and not the early Mars. And even if
Mars did lack a certain something, life need not have originated there in order
to have flourished. After all, meteorites passed from Earth to Mars by the
billion in the early years of the Solar System. What’s more, water seems to have
flowed more freely on early Mars, and people making models of the atmosphere’s
physics and chemistry find it hard to get the planet warm enough to explain how.
One way out of this problem may be to add biology in the form of a methane
greenhouse produced by bacteria.

Life lurking in the softly seeping springs of the Martian highlands would be
one of the defining discoveries of the 21st century. It would also be a body
blow to the Gaian notion that life is necessarily a planet-wide phenomenon; the
rest of Mars looks as dead as ever. But the more general idea that liveable
environments find ways to maintain themselves might find new support. Models of
water on present-day Mars place it far deeper in the crust than what is being
suggested now. No one expected any within a few hundred metres of the
surface—or has any idea how it got it there.

If physics and chemistry can’t do the job unaided, maybe biology has a role
here too. If there’s life around these putative sort-of springs, maybe it has a
way of maintaining them through such mechanisms as biofilms, antifreezes, gas
emissions or Gaia-knows-what.

Rather like the Valencians, Mars may not regulate itself globally, but merely
locally.