On July 19, 2009, 15 years to the day after the famous comet Shoemaker-Levy 9 peppered Jupiter with huge chunks of ice, the gas giant was hit once again by a cosmic bullet.
Australian amateur astronomer Anthony Wesley was the first to notice a scar in the planet's atmosphere after the fact, leading us to believe that Jupiter had been hit by another comet.
However, in this cosmic game of whodunit, jumping to the conclusion that "another" comet was to blame for the 2009 impact turns out to have been a red herring. Just because we know a comet slammed into the planet in 1994, it doesn't necessarily mean the 2009 event used the same weapon.
After some detective work, two papers recently published in the journal Icarus have concluded that Jupiter was actually shot by an asteroid, not a comet.
What's more, the asteroid that did the damage was probably about the size of the Titanic.
But how do astronomer detectives know if an impact is caused by a comet or asteroid? In this cosmic crime scene, it's all about heat and chemistry. And when an asteroid takes a nosedive into a gas giant's atmosphere, it leaves different thermal and chemical fingerprints to a comet impact.
Using infrared data from ground-based telescopes that observed Jupiter's scar for the days after Wesley's discovery, an international team of scientists were able to piece together all the evidence. But the fact that the data suggest an asteroid was to blame is a huge surprise.
"Both the fact that the impact itself happened at all and the implication that it may well have been an asteroid rather than a comet shows us that the outer solar system is a complex, violent and dynamic place, and that many surprises may be out there waiting for us," said Glenn Orton, an astronomer at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "There is still a lot to sort out in the outer solar system."
Leigh Fletcher, a researcher at Oxford University, summarized why all the evidence is pointing at an asteroid impactor:
"Comparisons between the 2009 images and the Shoemaker-Levy 9 results are beginning to show intriguing differences between the kinds of objects that hit Jupiter. The dark debris, the heated atmosphere and upwelling of ammonia were similar for this impact and Shoemaker-Levy, but the debris plume in this case didn't reach such high altitudes, didn't heat the high stratosphere and contained signatures for hydrocarbons, silicates and silicas that weren't seen before. The presence of hydrocarbons, and the absence of carbon monoxide, provide strong evidence for a water-depleted impactor in 2009."
Until now, it was thought that comets were the only objects that still slammed into Jupiter. The majority of asteroids near Jupiter's orbit should have been sucked into the gas giant's enormous gravitational well by now — it's not known as the "solar system's vacuum cleaner" for nothing — so to witness an asteroid impact is either extremely lucky or our understanding of the outer solar system underestimates how many asteroids are really out there.