Comet rain took life's ingredients to Jupiter's moons

Dust made from pulverised comets may have seeded Jupiter's moons with the raw ingredients for life. That includes Europa, which is thought to harbour a liquid ocean beneath its icy crust.

Jupiter has two kinds of natural satellites: large spherical moons and smaller lumpy bodies that follow elongated orbits. Chemical analysis of the irregular bodies suggests they are made of the same stuff as asteroids and comets. This means they are probably rich in the carbon-containing compounds that are key to life on Earth.

It is thought that a gravitational reshuffling of the planets some 4 billion years ago shook up distant belts of space rocks and sent many of them hurtling towards the sun. Some got caught in Jupiter's orbit and became the irregular satellites. The objects frequently collided as they settled into their new orbits, creating dust as fine as coffee grounds.

Blanketed moons

Models say that Jupiter should have captured about 70 million gigatonnes of rocky material, but less than half that amount remains as irregular moons. "So what happened to all the stuff?" asks William Bottke of the Southwest Research Institute in Boulder, Colorado.

His team ran simulations of the irregular moons' evolution and found that their ground-up material would have fallen towards Jupiter, dragged by gravity and blown by the solar wind. About 40 per cent of it would have hit Jupiter's four largest moons. Most of this landed on Callisto (Icarus, doi.org/kff). The rest hit Ganymede and then Europa.

That's roughly consistent with images from the Galileo spacecraft, which show dark material on Ganymede and Callisto. "Callisto literally looks like it's buried in dark debris," says Bottke, while Ganymede has a lot of similarities but less dark stuff on its surface.

Sinking carbon

But the surface of Europa is relatively clean. Cracks cover the moon's crust, which suggests it has cycled material from deeper inside, so the carbon-rich debris may have been incorporated into the ice and even made it into the ocean, says Bottke. "Would it be important in Europa's ocean? It's hard to say," he says. "But it is kind of interesting to think about."

Bottke's calculations only set a lower limit on the amount of carbon-rich material that could have ended up in Europa's ocean, says Cynthia Phillips of the SETI Institute in Mountain View, California, who studies Europa.

"This could potentially be an even larger source of astrobiologically interesting material for the ocean layer than the authors of this paper estimate," she says.

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