A montage of ultraviolet images taken during several Venus Express orbit with the Venus Monitoring Camera (VMC) (each orbit is 24 hours long). Credit: ESA/MPS

A sudden bright spot that appeared in the clouds of Venus just days after a comet left a bruise on Jupiter has scientists stumped as to its cause.

Venus' bright spot, first noticed by amateur astronomer Frank Melillo of Holtsville, NY on July 19, is not the first such brightening noticed on our cloudy neighbor, said planetary scientist Sanjay Limaye of the University of Wisconsin-Madison.

"We have seen such events before," he told

This time is a little different though because the brightening is confined to a smaller region, Limaye said. It also came in the wake of Jupiter's own new (dark) spot, believed to be the result of a comet impact — Limaye attributes the fortunate confluence of the two events for the attention Venus is now getting in the astronomical community.

After Melillo reported the spot, other amateur astronomers and the European Space Agency's (ESA) Venus Express spacecraft confirmed the presence of the blemish.

The new Venus Express images show that the bright spot actually appeared in the planet's southern hemisphere four days before Melillo saw it and that it has since begun to spread out, becoming stretched by the wind's in Venus' thick atmosphere.

But just what caused the brightening is still a mystery. Theories have abounded, from a volcanic eruption to solar particles interacting with the planet's atmosphere.

Limaye says the volcano explanation is unlikely, for several reasons: Volcanoes on Venus seem to be less likely to blow their tops in Mount St. Helens-type fashion, instead behaving more like the oozing lava factories of Hawaii, so their eruptions wouldn't likely produce huge clouds of ash and steam. Also, it is unlikely that the explosions would have the power to push through to the other layers of Venus' extremely dense atmosphere.

Limaye doesn't completely rule out the possibility, however. "It's possible, we just don't know," he said.

Another explanation is that a coronal mass ejection (an energetic plume of plasma from the sun's corona) or the solar wind could have interacted with the clouds of Venus.

These "could cause something, we don't know what," Limaye said.

Yet another possibility is some internal change in Venus' atmosphere that could alter cloud particles and make them more reflective (and therefore brighter as viewed from space).

"Clearly something in the cloud properties changed," Limaye said.

Even though these events have been seen previously, most notably in Jan. 2007, our limited knowledge about the workings of Venus' atmosphere and lack of enough spacecraft to comprehensively study the planet hasn't narrowed down the list of possible causes, Limaye said.

"Right now, I think it's anybody's guess," he said.