"Black Rain" on Callisto and Ganymede

Computer simulations show that tiny particles drifting inward from Jupiter's most distant moons — tiny bodies with oddball orbits — could have darkened the surfaces of the two biggest Galilean satellites with deep drifts of dark debris.

At the close of the 19th century, astronomers thought the baker's dozen of known moons around the outer planets were an orderly lot. But that changed with the discovery of Saturn's Phoebe (1898) and Jupiter's Himalia (1904), Elara (1905), and Pasiphae (1908) — all having oddball orbits located nowhere near their planet's equatorial plane. In fact, Phoebe and Pasiphae were even moving retrograde, backward with respect to everything else.

Since then the number of these distant "irregular" moons has mushroomed, and they now dominate the tables of each planet's retinue of known satellites. Most dynamicists believe these were stray objects, probably cometary in nature, captured early in solar-system history by their respective planets.

Meanwhile, the two Voyager flybys in 1979 showed Callisto to be a dark, heavily cratered world that has likely been geologically dead for eons. Curiously, Callisto's reflectivity is only about 19% — even though its surface should be largely water ice. Something has contaminated the ice. Several planetary scientists suggested that carbon-rich debris from the outlying irregular satellites was somehow involved. Others proposed that Callisto's surface ice is slowly sublimating away, leaving behind a veneer of carbon-rich rocky matter.

The case for imported contamination got stronger in the mid-1990s, when the Galileo spacecraft showed that deep drifts of dark material mantled the giant Jovian moon Callisto and, to a lesser extent, neighboring Ganymede.

A few years ago an international team led by William Bottke (Southwest Research Institute) calculated how much bigger the irregular satellites might have been initially and how much they've been ground down over time. The numbers suggest that some 50 million billion tons of small particles, coffee-ground size and smaller, have been winnowed away.

A subtle sunlight-induced effect called Poynting-Robertson drag causes these bits to slowly spiral toward Jupiter. But before getting swallowed up, many of them run into Callisto first, especially the bigger ones. A lesser amount ends up running into Ganymede, and some even coat the surface of Europa.

As Bottke and his team report in a forthcoming issue of Icarus, this "black rain" should have blanketed the oldest terrains on Callisto to depths of up to 450 feet (140 m), a close match to what's seen in Galileo's detailed images.

Ganymede should have gotten a veneer about a fifth as thick, perhaps 100 feet (30 m) of the stuff. "These depths are actually seen on the oldest terrains, which are as densely cratered as Callisto," Bottke points out. Elsewhere the coating is thinner, but those regions shows signs of past geologic activity. Perhaps, he speculates, the dark material there was diluted or partly removed.

Another issue is how the material is distributed globally. "Our calculations suggest we would expect to see a big preference for dark debris on the leading faces of these worlds compared to the trailing faces," Bottke says, dozens of times deeper on the leading hemispheres, in fact.

But the actual distribution is more uniform. "Assuming our model is right, we suspect the easiest answer is non-synchronous rotation for all of these worlds" early in solar-system history, when most of the debris was drifting inward.

Europa should have a dark layer at least 35 feet (10 m) thick, but it's not there — at least not topside. The simplest explanation is most of the "black rain" has gotten churned into the interior by the active geology that continues to reshaped this moon's surface. Conceivably, the researchers speculate, some of the dark stuff could have reached Europa's subsurface ocean, infusing it with organic compounds such as amino acids.

As Bottke's team notes, similar infall was and is occurring around Saturn (particularly from Phoebe), and both Uranus and Neptune should have once possessed extensive sets of irregular satellites. The researchers plan to follow the "dirt trails" in these systems in future work.