July 20, 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 by Australian amateur astronomer Anthony Wesley on July 19, the blemish appears as a dark gray blob on the gas giant's south pole. (The picture above appears upside down, and the very dark spot on the planet's equator is the shadow of Jupiter's moon Io.)

Astronomer Amy Simon-Miller, of NASA's Goddard Space Flight Center in Maryland, agreed that it must have been 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.

—Ker Than