Comet dust may have seeded Jupiter's moons, including Europa and its liquid ocean beneath an icy crust, with the raw ingredients for life, U.S. researchers say.

Asteroids and comets rich in the carbon-containing compounds that are key to life on Earth have been captured by Jupiter's gravity, becoming orbiting moons that frequently collided as they settled into new orbits billions of years ago and created a fine dust of those compounds, they say.

The question is, where has all that dust gone?

Computer models suggest Jupiter should have captured about 70 million gigatons of rocky material but less than half that amount remains as irregular moons orbiting the planet.

William Bottke of the Southwest Research Institute in Boulder, Colo., said the ground-up material would have fallen toward Jupiter, dragged by gravity and blown by the solar wind and almost half of it would have hit Jupiter's largest moons, including Callisto, Ganymede and Europa.

Images from NASA's Galileo spacecraft have shown dark material on Ganymede and Callisto.

"Callisto literally looks like it's buried in dark debris," Bottke told NewScientist.com, noting the surface of Ganymede looks similar.

In comparison, Europa's surface appears relatively clean but cracks in the moon's icy crust suggest material is being cycled from the surface to deeper inside.

Carbon-rich debris settling on Europa may have been incorporated into the ice and made it into the ocean, Bottke said.

"Would it be important in Europa's ocean? It's hard to say," he said. "But it is kind of interesting to think about."