When a mystery object smacked into Jupiter without warning in July 2009, an event whose aftermath was first spotted by an amateur astronomer in Australia, observers across the globe scrambled to get a look at the planet to figure out just what had happened.

Astronomers working on other campaigns at world-class observatories, including one of the 10-meter Keck telescopes in Hawaii, swung into action, training their borrowed instruments on Jupiter to photograph the after-effects of the collision. Some of the most striking images, so to speak, came from the Hubble Space Telescope, which had only recently been upgraded by visiting astronauts and was still undergoing checkout of some of its new components.

Heidi Hammel, a senior research scientist at the Space Science Institute in Boulder, Colo., was part of the team commandeering Hubble to view the aftermath, as she had been 15 years earlier for the collision into Jupiter of the comet Shoemaker–Levy 9. She and her colleagues published the results of their new observations, and the ways in which the 2009 impact differed from that of Shoemaker–Levy 9 in 1994, in the June 1 issue of the Astrophysical Journal Letters.

Assuming that similar atmospheric processes were in play for Shoemaker–Levy 9 and the 2009 event, Hammel says, the dark bruise (near the bottom of the photo above) observed by Hubble after the more recent impact was essentially a debris field stirred up by a roughly 500-meter body. "A plume of material was blasted out of the atmosphere that then collapsed down onto the tops of the clouds," she says. "I think of all that black stuff as Jovian soot." Watching how that bruise evolved and faded over time, Hammel's group has found that the surprise impactor was more than likely an asteroid, not a comet.

When Shoemaker–Levy 9 struck in 1994, the comet punched into Jupiter in a series of fragments arriving over the course of a week, but the 2009 impactor seems to have held together under the intense gravitational strain from the massive planet, as only a single bruise was observed. "That suggests that it had some strength to it," Hammel says, which seems a better fit with a solid asteroid than a cometary rubble pile.

In addition, the Shoemaker–Levy 9 impact sites were surrounded by fuzzy halos visible in the ultraviolet, attributed to the debris clouds surrounding the nuclei of the comet fragments. But the impact scar of 2009 did not have such a halo. Finally, Hubble's ultraviolet imagery showed that the 2009 bruise faded much more quickly than the marks left by the 1994 comet strike. "That suggests that maybe there weren't as many small particles," Hammel says. "It's really sort of controversial; we've been debating this amongst ourselves."

Another group, which also reported its findings in the June 1 issue of the Astrophysical Journal Letters, ran simulations of the impactor's orbit, tracing it backward in time to try to uncover its provenance. But the team, which was led by Agustín Sánchez-Lavega of the University of the Basque Country in Bilbao, Spain, and also included Anthony Wesley, the Australian amateur astronomer who discovered the impact, found a roughly equal probability for the mystery object originating from the asteroid belt or a cometary reservoir.

Not having seen the object prior to impact, the two groups were forced to do a bit of detective work to infer the most likely scenario for what happened. "When you take every one of these bits and pieces together, you start to get a story," Hammel says. "But we're trying to be very clear that we don't have a smoking gun to prove that it was definitely an asteroid and not a comet."

Photo credit: NASA, ESA, H. Hammel (Space Science Institute, Boulder, Colo.), and the Jupiter Impact Team