Jupiter's impressive 2009 scar, which was first spotted by amateur astronomer Anthony Wesley, was likely created by an asteroid the size of the Titanic cruise liner, say scientists who studied the aftermath of the collision.

The results of the investigation, which are published in two new papers in the journal Icarus, conclude that the impact must have been caused by an asteroid, and not a comet, which was the case for the well-known comet Shoemaker Levy-9 impact into Jupiter in 1994.

A dark black marking was observed visually on the giant planet on 19 July 2009 by Anthony Wesley, who alerted professional astronomers around the world. Follow-up observations from NASA's Infrared Telescope Facility in Mauna Kea, the Gemini North and South telescopes, ESO's Very Large Telescope and even the Hubble Space Telescope in the week after, enabled astronomers to observe the warm temperatures and chemical reactions caused by the impact.

Scientists report that the impact warmed the lower stratosphere by 3-4 kelvin at an altitude of 42 kilometres above the cloud tops. Although the temperature increase may not seem that great, spread over such a large area it is significant. The rocky object likely carved out a channel in the clouds as it plunged through the atmosphere, exploding at depth to release the energy equivalent of five gigatons of TNT. Debris launched back up along the channel above the cloud tops, dredging up ammonia gases from depth and depositing them in the stratosphere.

"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," says Leigh Fletcher of Oxford University. "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."

As Fletcher says, the signature of the gases and the detection of silica, combined with the dark scar, implies that the impactor was much denser than the Shoemaker Levy-9 comet, and thus more likely a rocky asteroid. The depth at which the impactor broke up would not have been possible from a weakly consolidated comet. Taking all the pieces of evidence into account, scientists conclude that the asteroid had a density of approximately 2.5gcm^-3 and was between 200 and 500 metres wide.

Previously, scientists had assumed comets were the primary culprits for impacting into the giant planet, but this observation shows that there are still asteroids on very chaotic orbits in this region of the Solar System, too.

"We weren't expecting to find that an asteroid was the likely culprit in this impact, but we've now learned Jupiter is getting hit by a diversity of objects," says Paul Chodas of NASA's Near-Earth Object Program Office. "Asteroid impacts on Jupiter were thought to be quite rare compared to impacts from the so-called 'Jupiter-family comets,' but now it seems there may be a significant population of asteroids in this category."

Asteroids of a few hundred metres in diameter are predicted to hit Earth about once every 100,000 years, so scientists will now be keen to determine the frequency at Jupiter.