Jupiter’s second greatest hit

Ever wonder what happened to that bruise in Jupiter’s atmosphere that appeared last summer? An amateur astronomer first spotted the strange dark scar in Jupiter’s south polar region on July 19. The scar indicates that some projectile, either a comet or an asteroid, had hit the giant planet (SN: 8/29/2009, p. 8) this summer.

It was only the second time in recorded history that a large body has been known to strike Jupiter. In 1994, chunks of Comet Shoemaker-Levy 9 plowed into a different part of the planet’s atmosphere and were famously imaged by the Hubble Space Telescope.

Curiously, the new bruise is evolving more slowly than those that had been generated by the 1994 encounter, noted planetary scientist Heidi Hammel, of the Space Science Institute in Boulder, while in Fajardo, Puerto Rico, for the annual meeting of the American Astronomical Society’s Division for Planetary Sciences. Three months after the 1994 event, evidence that the Jovian atmosphere had been hit had all but vanished. But not so for the punch that the planet received this past summer. The difference, said Hammel, is that the projectile that struck this summer hit an area where the Jovian winds are much quieter. As a result, the faint bruise — initially about 4,500 kilometers in diameter Fajardo, Puerto Rico is smearing out more slowly and remains visible with powerful telescopes, such as the Keck Observatory atop Hawaii’s Mauna Kea, researchers reported October 5.

The faint scar has now wrapped about a quarter of the way around the planet.

On October 7, Bryan Butler of the National Radio Astronomy Observatory in Socorro, N.M., reported on radio-wavelength observations of Jupiter recorded during the first month after this scar was first seen. If the projectile had contained lots of dust, as comets generally do, it would have changed the energies of electrons in Jupiter’s upper atmosphere. This would have altered the radio emissions from the electrons, but observations with the Very Large Array network of radio telescopes in Socorro showed no such effect, Butler said. That makes it more likely the object that struck Jupiter was an asteroid, he concluded.

Stay tuned, says Hammel, for spectacular images she and her colleagues have taken of the impacted site using the new Wide Field Camera 3 on the Hubble Space Telescope.