Updated July 21, 2009—A new black spot on Jupiter was likely caused by an impact over the weekend that took astronomers by surprise.
Slightly smaller across than Earth, the temporary spot was caused by a recent but unobserved comet or asteroid impact, said Glenn Orton, an astronomer at NASA's Jet Propulsion Laboratory in California.
"It's very clear that this is an impact," Orton said.
First spotted in July 19 pictures by Australian amateur astronomer Anthony Wesley, the blemish appears as a dark gray blob on the gas giant's south pole. (The dark spot is in the upper right of the above picture because the planet appears upside down.)
Astronomer Amy Simon-Miller, of NASA's Goddard Space Flight Center in Maryland, agreed that the spot must have been created by an impact, saying that the evidence "is not consistent with changes in weather patterns."
The bombardment that caused the spot would have been very similar to what happened in 1994, when fragments of the comet Shoemaker-Levy 9 slammed into Jupiter. (See a Hubble picture of comet debris approaching Jupiter.)
The impactor's size is still unknown, but it would have exploded like a bomb as it punched a hole through Jupiter's upper atmosphere, Simon-Miller said.
"If you could look edge-on at the planet exactly where it happened, you would have seen a fireball," she said. "What we're looking at now is not the hole, but burnt up debris raining back down onto the planet."
It's possible more impacts could follow in the next few days, and if so, this time astronomers will be ready to spot the actual explosions.
"We're going to be training all the big telescopes on it as soon as we can," Simon-Miller said.