The chances that life exists on Jupiter鈥檚 moon Europa look better than ever. Researchers think the moon鈥檚 icy crust may be just a few kilometres thick 鈥� perhaps thin enough to crack open under tidal stresses and allow life in the oceans below to flourish by absorbing the Sun鈥檚 energy.
Astrobiologists fear that if the crust is too thick, any possible life would be limited to organisms that use chemical energy from hydrothermal vents, a very restricted niche.
Now a team from the University of Arizona鈥檚 Lunar and Planetary Laboratory, led by Richard Greenberg, have been studying data from NASA鈥檚 Galileo probe, which is in orbit around Jupiter. They have built up a strong case over the past few years that Europa鈥檚 ice is less than 10 kilometres thick. And they outlined their most recent results in two talks at the Division for Planetary Sciences meeting in Birmingham, Alabama, US, as well as in Reviews of Geophysics (vol 40, p 1).
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Looking at the 10 per cent of Europa鈥檚 surface that Galileo has imaged at high resolution, the Arizona group found that roughly half of it consists of 鈥渢ectonic鈥� terrain, dominated by long, straight ridges and fault lines. The other half appears to be blocks of ice randomly frozen in place, as if they had tumbled together while the surrounding medium was liquid. The researchers say both types of landscape show that cracks or molten regions in the ice have repeatedly exposed the ocean below to the surface.
Convergence zone
Another sign that the ice must be thin is a new analysis of cracking patterns that reveals a telltale feature of Europa鈥檚 tectonic cycle. Researchers have clearly identified zones where the plates are moving apart, but what happens where the plates converge?
On Earth, colliding plates either subduct 鈥� one plate slipping below the other 鈥� or wrinkle up to form mountain ranges. Greenberg鈥檚 group has now identified convergence zones on Europa, but nothing as dramatic as on Earth. 鈥淭he reason you don鈥檛 get anything like the Himalayas or subduction,鈥� says Greenberg, 鈥渋s because there鈥檚 just not much solid stuff there.鈥�
The group鈥檚 calculations suggest that daily tides would push warmer water up into the cracks, keeping them relatively stable. And that would give any organisms access to sunlight and organic material from impacting comets, providing a habitat where life could thrive (see graphic).
The researchers imagine photosynthetic organisms might anchor themselves to the sides of cracks, to reach sunlight, while jellyfish-like creatures might ride up and down with the current.
Tidal stress
Large cracks could last for a few thousand years, the researchers say, but as Europa rotates relative to Jupiter, tidal stress points in the crust would change, and cracks would periodically freeze shut. Organisms would either need to hibernate in the ice until the crack reopened or migrate elsewhere.
But not everyone is convinced that Europa鈥檚 ice is so fragile. Robert Pappalardo of the University of Colorado in Boulder says Greenberg has done some brilliant work in demonstrating the existence of a liquid ocean, but does not agree that the ice is thin. According to Pappalardo, Greenberg鈥檚 results can be explained by flows of warmer ice like those found in glaciers on Earth.
Galileo last photographed Europa in 1999, but the ongoing disagreement means it is urgent that we return and settle the question, says astrobiologist Bruce Jakosky of the University of Colorado. 鈥淎fter the Earth, the two most habitable places in the Solar System are Mars and Europa. We have a vigorous Mars programme, but we don鈥檛 have a vigorous Europa programme right now.鈥�
Hopefully that could soon change: the US Congress passed a resolution last week instructing NASA to spend $40 million on just such a mission to Europa.