On July 19, 2009, thousands of amateurs were converging on China and thereabouts with just one astronomical thought in mind: seeing a total eclipse of the Sun three days later.
But the cosmos threw us all a curve that day, when Australian astrophotographer Anthony Wesley spied a dark smudge near Jupiter's south pole. Apparently something had slammed into the giant planet for the first time since Comet Shoemaker-Levy 9's celebrated death plunge in 1994. Astronomers at several observatories dropped everything to try to record as much detail of the unexpected impact event as possible before the ashes faded from view.
It's taken a while, but two articles in this month's Icarus detail what infrared observers learned in the days and weeks following the impact. In one report, Leigh Fletcher (Oxford University) and others describe spectra obtained with the Gemini South telescope on July 24th, five days after the blast. Although the impact site looked dark to the eye, it glowed brightly in the infrared, indicating that stratospheric gases at the impact site were still slightly warm (by a few Kelvins).
Fletcher's team concludes that the colliding object delivered at least 200 trillion trillion ergs of energy, the kinetic punch that would have come from a body 230 to 1,600 feet (70 to 500 m) across. Apparently it penetrated deeply enough into the planet's cloud decks to dredge up some 7 million tons of ammonia. But the Gemini spectra also reveal that a soot rich in iron, silicate minerals, and silica lay atop the stratosphere as well — hardly the make-up expected from a comet.
Similar conclusions appear in the second report, which details near- and mid-infrared observations from NASA's Infrared Telescope Facility in Hawaii and the Very Large Telescope in Chile. In fact, because he was already at the IRTF when Wesley sounded the alarm, Glenn Orton (Jet Propulsion Laboratory) was able to scrutinize the warm welt within 18 hours of the impact. Orton and others conclude that the unfortunate interloper was 200 to 500 m across, and the spectra likewise indicate the presence of silicates and silica (SiO2).
The silica is the real smoking gun, so to speak. It wasn't detected during the much more energetic Shoemaker-Levy 9 impacts — and it strongly suggests that 2009's whack was from a small asteroid, not a comet. And that possibility has piques the interest of solar-system dynamicists. They'd assumed that only comets crossed paths with the giant planet — in fact, there's a whole class of these icy bodies known as Jupiter-family comets.
"We weren't expecting to find that an asteroid was the likely culprit in this impact," notes Paul Chodas in a JPL press release. But a little dynamical digging has turned up asteroids with unstable orbits that could make close brushes with Jupiter.
You might recall that the teams were still chewing on their 2009 images and spectra when Jupiter was hit once again, this time not even hard enough to leave a mark, last June 3rd. If this keeps up, the good folks at the IAU's Minor Planet Center will have to come up with some kind of interplanetary scorecard to keep track of what got hit and when.