NASA researchers have spotted something weird on the surface of the dwarf planet Ceres—a pair of mysterious shiny patches that reflect sunlight.

The bright spots were imaged by the Dawn spacecraft, which is whizzing through space on its way to Ceres, and should arrive in orbit on March 6. It'll be the first spacecraft to study a protoplanet at such close range.

"As we slowly approach the stage, our eyes transfixed on Ceres and her planetary dance, we find she has beguiled us but left us none the wiser," said Chris Russell, principal investigator of the Dawn mission. "We expected to be surprised; we did not expect to be this puzzled."

Two shiny patches have been spotted so far—the first from a distance of 83,000 kilometres and the second, a dimmer companion, from 46,000 kilometres. Both are so far too small to resolve properly with the camera.

Ceres is the largest object in the asteroid belt that lies between the orbits of Mars and Jupiter, and was discovered in 1801. It's about 950 kilometres wide, and prior observation tells us that it consists of a mix of rock and frozen water. It is suspected that the latter might be the cause of the shiny patches.

Most of Ceres' ice is hidden below the surface, but when it bumps into other objects in the asteroid belt it could expose patches of this type. That's the working hypothesis, anyway. Though it's countered by the fact that the shiny patches are only reflecting about 40 percent of the light falling on them. Ice should reflect nearly 100 percent. That's being explained away as a result of Dawn's distance from the target.

Another option is that the bright spots are due to cryovolcanic eruptions driven by radioactivity deep inside the protoplanet, which spew ice, rather than lava, out onto the surface. Water vapour measurements back this hypothesis, but they could also have come from ice.

A third possibility is that it's not ice at all, but actually a class of minerals called magnesium silicates that we've detected on other asteroids. We haven't yet seen any evidence of this on Ceres, making it the least likely possibility.

"We will have to wait for better resolution before we can make such geologic interpretations," said Russell, referring to the distance from the protoplanet. The spacecraft will take photos as it approaches, and then many more once it arrives in orbit and properly orients itself to face the surface.

In short, this is one astronomical mystery that likely won't go unsolved for long.