Ceres’ bright spots explained

A frozen sea? Sunlight glinting off an alien spaceship? As NASA’s Dawn space probe closed in on Ceres, the largest body in the asteroid belt, the images it beamed home in early 2015 revealed startling bright spots on the surface of this otherwise asphalt-dark world. Speculation was rife as to what they could be, with NASA creating an online poll so everyone could have their say.

Mission scientists led by Andreas Nathues of the Max Planck Institute for Solar System Research in Goettingen have now revealed the spots are patches of a salt, probably hydrated magnesium sulfates. These are similar to the Epsom salt athletes use in the bath to relieve sore muscles, and gardeners sprinkle on their roses. The finding, published in Nature in December, makes Ceres the first body to be neither asteroid nor comet, but something in between – a sort of comety asteroid.

Images from NASA’s Dawn spacecraft were used to create this false-colour movie of Ceres rotating, followed by a flyover view of Occator Crater, home of Ceres’ brightest area.Credit: NASA Jet Propulsion Laboratory

The Nathues team suspected that Ceres’ bright spots were reflective salt flats, similar to the bright dry lakes in Australia’s Simpson Desert. So they created different types of frozen salt in the lab with varying amounts of water, and measured how they reflected infrared light. The reflective properties of hydrated magnesium sulfate made it the most likely candidate for the bright patches on Ceres.

Dawn’s spectral imaging cameras supported the idea the salt was hydrated. When viewed from close to edge-on, they captured a haze of dust and ice particles during Ceres’ day.

The Nathues team also found that most of the 130 bright spots on Ceres are in impact craters, suggesting the ice is below the surface, and only revealed when the top layer is swept off by meteorite strikes. Most impact craters contained ice, suggesting water is abundant.

Estimates of Ceres’ density had already fuelled suspicions that it contained water, but the new findings are the hard evidence. They also explain earlier images from the Herschel Space Observatory, which captured water streaming off into space and giving Ceres a faint tail.

The emerging picture of a water-rich Ceres with a comet-like tail sitting in the middle of the asteroid belt calls for a rethink of what asteroids (chunks of rock and metal with a smidgen of water) and comets (closer to dirty snowballs) are made of.

Both are leftovers from the formation of the Solar System. Until now, it was believed comets formed beyond the `snow line’, far from the warmth of the Sun, whereas asteroids formed in the inner Solar System, and their water was long ago vaporised away. Water-rich Ceres suggests the idea that ice only exists beyond a line in the Solar System is too simple. A more gradual transition is likely, says Nathues.

It also raises questions about how and where Ceres formed: in the cool beyond the snowline, migrating inward later but retaining its water? Or in the asteroid belt where it slowly accumulated ice through comet strikes?

What we do know is that the Dawn space probe, now in its final orbit less than 400 kilometres above the surface, is in the perfect place to solve these new mysteries.