Amateur astronomer Frank Melillo of Holtsville, New York, first spotted the new feature, which is brighter than its surroundings at ultraviolet wavelengths, on the planet's southern hemisphere on 19 July. That same day, an amateur observer in Australia found a dark spot on Jupiter that had been caused by a meteoroid impact.
The Venus spot was confirmed by other observers, and images from Europe's Venus Express, the only spacecraft in orbit around the planet, later revealed that the spot had appeared at least four days before Melillo saw it.
Observations show that the spot had already spread out somewhat by the end of last week, and astronomers are awaiting more recent observations from Venus Express.
The spot is bright at ultraviolet wavelengths, which may argue against a meteoroid impact as a cause. That's because rocky bodies, with the exception of objects very rich in water ice, should cause an impact site to darken at ultraviolet wavelengths as it fills with debris that absorbs such light, says Sanjay Limaye of the University of Wisconsin-Madison and a member of the Venus Express team.
Another possibility is that a gust of charged particles from the sun could have created the glow by energising a patch of the upper atmosphere. Alternatively, waves in the atmosphere, which trigger turbulence and are thought to carry material up and down, could have concentrated bright material to create the spot.
A volcanic eruption is another suspect. Venus boasts the most volcanoes of any planet in the solar system, and nearly 90% of its surface is covered by basaltic lava flows, although no 'smoking gun' has yet been found for current volcanic activity. But an eruption would have had to be very powerful to punch through a dense layer in Venus's atmosphere to create the spot some 65 to 70 kilometres above the planet's surface.
"It's fair to say something unusual happened on Venus. Unfortunately, we don't know what happened," Limaye told New Scientist.
Two spectrometers on board Venus Express might help reveal the culprit. One directly measures the spectrum of light emanating from the planet, while the other can measure trace constituents in the atmosphere by measuring how gases there absorb sunlight.
These instruments could reveal changes in the size distribution of particles in the atmosphere and higher concentrations of molecules, such as sulphur dioxide, that could suggest a volcanic eruption.
If a volcano is to blame, proving it will be difficult. Even if Venus Express finds higher-than-average levels of sulphur dioxide in the atmosphere, the observation could be explained by non-volcanic processes, cautions Limaye. Sunlight can break down sulphuric acid in Venus's clouds to create sulphur dioxide, which may not be evenly circulated in the planet's atmosphere.
This is not the first time astronomers have spotted bright features in Venus's atmosphere. Bright spots have been seen from Earth for decades, although they have not been clearly explained, Limaye says.
The most recent dramatic brightening occurred in January 2007, when areas in both the northern and southern hemispheres of the planet brightened. Because it is localised in a spot, this new feature looks different, but it is equally mysterious.
"This shows how much we don't know about Venus," Limaye says. In some ways, Venus is a simpler planet than Earth - it has no oceans and because of its nearly vertical spin axis, practically no seasons, he adds. But planetary scientists still do not understand what causes the planet's atmosphere to rotate 60 times faster than the planet itself.
Comment: These are possibilities, of course. One may want to consider this as well: Military Hush-Up: Incoming Space Rocks Now Classified