As the NASA spacecraft Dawn finally approaches Ceres, the first mystery for it to explore has been unveiled; a curious white spot visible in the images beamed back a fortnight ago.

Credit: NASA/JPL Caltech/UCLA/MPS/DLR/IDA Dawn's observations over the course of an hour

The spot is visible from Earth, or at least from Earth’s orbit, showing up in photographs taken by the Hubble Space Telescope in 2003 and 2004. However, these images, like those so far provided by Dawn, are far too small to reveal anything about the spot other than its existence.

Credit NASA. Ceres as seen by the Hubble Space Telescope in 2003-2004 (composite image)

"We do not know what the white spot is, but it's certainly intriguing," Dawn Mission chief engineer Mark Rayman told Space.com. "In fact, it makes you want to send a spacecraft there to find out, and of course that is exactly what we are doing! So as Dawn brings Ceres into sharper focus, we will be able to see with exquisite detail what [the white spot] is."

The most obvious explanation is that the spot is ice filling the bottom of a crater. However, no one knows why this one spot, at high but not quite polar latitudes, should be more ice prone than anywhere else on Ceres. A quite recent impact crater is another possible theory. Younger craters on the Moon, such as Tycho, are much lighter than their surroundings, since the exposed material has not been darkened by exposure to solar radiation and micrometeorites.

Randall Munroe has a theory,

...but his former colleagues at NASA remain non-committal.

Dawn will go into orbit around Ceres on March 6, but will be providing progressively better views of the dwarf planet as it closes in. Early images were lower resolution than what Hubble was able to provide, but from now on we should be able to see the largest member of the asteroid belt in unprecedented detail.

Before travelling to CERES, Dawn spent time orbiting Vesta, the second most massive resident of the asteroid belt. Although Vesta has no bright feature of similar size to Ceres's spot, the mission did reveal several much smaller white spots, mostly a few hundred meters across, which may have similar causes.

Before we get a really good look at the great white spot, we should learn whether spots similar to those on Vesta, too small to be resolved by Hubble or in the most recent set of Dawn images, also exist on Ceres.