THE cracks in Europa’s icy crust are where life is most likely to be found on
the Jovian moon, the conference heard last week.
This means it might be possible to find life without having to drill down to
the ocean far beneath the ice. It also means that any terrestrial stowaways on a
space probe could easily contaminate Europa.
The cracks would provide an ideal home for life, says Richard Greenberg of
the University of Arizona in Tucson. Tides caused by Jupiter’s strong
gravitational pull make warm water flow up and down the cracks, creating a rich
chemistry along the fissures. “That makes Europa extremely habitable,” says
Greenberg.
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There is growing evidence that Europa has an ocean of liquid water beneath
its icy crust. But for life to survive there, it would also need energy.
Greenberg’s analysis of data from the Galileo spacecraft suggests the ice is at
most 6 kilometres thick, but even if it’s thinner in places it would still block
the sunlight.
However, the ice is riddled with cracks caused by tidal forces, he says.
Europa circles Jupiter every 3.5 days. These periodic tidal stresses form a
network of fissures in the ice, which can open and close by about a metre. When
a crack opens, water will rise 90 per cent of the way to the surface, Greenberg
says.
At the top of the fissures, ice both freezes and evaporates into the near
vacuum. When the walls close again, they squeeze slush to the top, where it
freezes. The cyclic flow of water up and down the cracks continually mixes the
liquid.
The connection to the surface is vital. Here, solar radiation can split
molecules, producing chemicals that could supply energy for life. What’s more,
enough sunlight might reach the upper few metres to drive photosynthesis.
“You could have organisms a metre down, with roots hanging onto the cracks,
unfolding their little leaves,” says Greenberg. “If there were life in the
cracks, the whole crack would be filled, it’s such a hospitable setting.” Some
organisms might stay in one place, while others could ride up and down with
water, he suggests.
Unlike our Moon, which always shows the same face to the Earth, Europa
rotates full circle once every 20,000 to 200,000 years relative to Jupiter. This
means cracks slowly shift into different positions. As old cracks seal up after
thousands of years, organisms would be forced to migrate to new cracks or evolve
ways to survive when frozen. “It’s a great set-up for advancement,” Greenberg
says.
