The more we find out about the bright spots on dwarf planet Ceres, the more mysterious they become. Scientists now say the spots vary in intensity according to the time of day as well as changing as Ceres rotates, as in the artists' impression based on detailed maps, above.
Ceres is the largest object in the asteroid belt, but is still only 950 kilometres in diameter. We have been receiving unprecedented, high resolution images of it thanks to NASA's Dawn spacecraft, currently orbiting the dwarf.
But the latest observations came, not from Dawn, but from Earth thanks to the High Accuracy Radial velocity Planet Searcher, or HARPS, spectrograph at the European Southern Observatory (ESO) 3.6-metre telescope at La Silla, Chile.
Ceres spins every nine hours and calculations showed that the velocities of the spots towards and away from the Earth due to this rotation would be very small, of order 20 kilometres per hour. But this motion is big enough to be measurable via the Doppler effect.
"The result was a surprise," said Antonino Lanza, co-author of the study that reports on the observations. "We did find the expected changes to the spectrum from the rotation of Ceres, but with considerable other variations from night to night.”
The changes could be due to volatile substances that evaporate when hit by sunlight, the researchers suggest.
"When the spots inside the Occator crater are on the side illuminated by the Sun they form plumes that reflect sunlight very effectively," they write. "These plumes then evaporate quickly, lose reflectivity and produce the observed changes.
"This effect, however, changes from night to night, giving rise to additional random patterns, on both short and longer timescales."
If this interpretation is correct, it would differentiate Ceres from Vesta and the other main belt asteroids, suggesting it is internally active.
Ceres is known to be rich in water and previously suggested explanations for the bright spots have included shiny ice, reflective salt deposits or geysers blasting out water vapour.
In December, astronomers using spectral data collected by the orbiting Dawn probe, proposed in a paper published in Nature, that the bright spots were made of hydrated magnesium sulphate.