Not all of the puzzling bright spots on the dwarf planet Ceres are alike. The closest-yet images of the gleams, taken from 45,000 kilometres away, suggest that at least two of the spots look different from one another when seen in infrared wavelengths.

The Hubble Space Telescope spied many of the bright spots from afar years ago, but the observations from NASA's Dawn spacecraft—which began looping around Ceres on March 6—are the first at close range. The images were released on April 13 in Vienna, Austria, at a meeting of the European Geosciences Union.

Scientists say that the bright spots may be related to ice exposed at the bottom of impact craters or from some kind of active geology. They glimmer tantalizingly in a new full-colour map of Ceres, obtained in February but released at the conference. The map uses false colours to tease out slight differences on the otherwise dark surface of Ceres.

“This is the first idea of what the surface looks like,” said Martin Hoffmann, a Dawn scientist from the Max Planck Institute for Solar System Research in Göttingen, Germany.

Dawn is beginning to sharpen its view of the bright spots as it gets closer to Ceres. The new infrared images compare Spot 1, near Ceres' equator, with a pair of bright spots collectively known as Spot 5. Some scientists have speculated that the latter could be linked to an icy plume.

Spot 1 appears darker in images from Dawn's infrared spectrometer, said Federico Tosi, a Dawn scientist at the Institute for Space Astrophysics and Planetology and the Italian National Institute for Astrophysics in Rome. That suggests that the area is cooler than the rest of the dwarf planet's surface, supporting the idea that the spot is made of ice.

But for some reason Spot 5—the brightest feature seen on Dawn—does not show up in infrared images. “One possibility is that we still don’t have enough resolution to see it in the proper way,” said Tosi.


Dawn has also shown that some parts of Ceres are pockmarked by impact craters, while other regions seem smooth. So far there seem to be fewer large craters on Ceres than expected, says the mission's principal investigator, Christopher Russell of the University of California, Los Angeles.

At nearly 1,000 kilometres across, Ceres is the biggest asteroid in the asteroid belt, which lies between Mars and Jupiter. Researchers expect a close look at Ceres' surface to reveal clues about the formation of protoplanets in the early Solar System, 4.5 billion years ago. Already, the images show complex craters and long, deep curving features that hint at a violent past, Hoffmann said.

Dawn has been traveling towards Ceres since its 2007 launch, with a side visit to the asteroid Vesta. The spacecraft was technically captured by Ceres’s gravity on 6 March, but has been taking weeks to get itself closer. (Scientists were forced to adopt a longer, more leisurely trajectory to Ceres in September, after an errant cosmic ray hit the spacecraft and temporarily knocked it out of commission.) 

Dawn took its most recent set of images on April 10, but only a small fraction of Ceres’s surface was illuminated at the time and mission scientists have not yet released them. The spacecraft will begin detailed science investigations on April 23, after it settles into permanent orbit around Ceres.

This article is reproduced with permission and was first published on April 13, 2015.