Feature five on Ceres is made up of two bright dots (Image: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA)

The unidentified bright spots on dwarf planet Ceres have become more mysterious. The spots on the surface were first glimpsed close-up just a month ago, and now infrared images reveal that they have different thermal properties.

NASA’s Dawn spacecraft is currently in orbit around the dwarf planet, which sits in the asteroid belt between Mars and Jupiter. Mission scientists presented the latest results from the spacecraft at the European Geosciences Union General Assembly in Vienna, Austria, today.

Two spots on the surface, labelled feature one and feature five, show up in visible light images as very bright in comparison to the rest of Ceres’s dull grey, leading to speculation that they could be the sites of watery volcanoes on the dwarf planet, also known as cryovolcanoes.


Now Federico Tosi, who works on Dawn’s Visible and Infrared Spectrometer, has presented infrared images of the two spots, measuring their thermal properties. “What we have found is that bright spot number one corresponds to a dark spot in the thermal image,” he said at a press conference today. In other words, the bright spot is much cooler than its surroundings.

In comparison, feature five, which appears as two separate bright spots next to each other in visible images, didn’t show up in the infrared images. “Spot number five shows no distinct thermal behaviour,” he said, meaning it is the same temperature as its surroundings. At the moment Dawn is too far away from Ceres to determine whether this is due to the bright spots being made from different stuff, or due to a different structure on the ground.

Hide the volcanoes

The spacecraft has just come out from behind the dark side of the dwarf planet, and a few days ago took a new image showing a sliver of the illuminated surface. Over the next couple of months Dawn will be zooming in for a closer look at the bright spots and the rest of the surface. It will also look for signs of watery plumes launching from the surface, as previously seen by the Herschel space telescope.

The current lack of high-resolution data means Tosi and his colleagues are hesitant to draw firm conclusion about the nature of these bright spots. “The current indication is there might be bright spots on the surface of Ceres behaving differently,” he said. “Before invoking cryovolcanoes or something strange going on, we have to be prudent and rule out the easy possibilities.”

Dawn has also uncovered other intriguing details about Ceres, such as a seeming lack of large craters on the surface. The spacecraft previously visited a smaller asteroid, Vesta, and the relative number and sizes of craters on both bodies should be similar – but they aren’t, says Christopher Russell who leads the Dawn mission.

“When we compare the size of the craters with those we see on Vesta, we are missing a number of larger craters,” he said. “That’s something we’ve got to learn more about when we take the next stage of science data.”