Pluto isn’t the only dwarf planet that recently hosted a spacecraft from Earth. NASA’s Dawn Spacecraft has been hanging out in orbit around the dwarf planet Ceres for almost a year, beaming back data. One of the most puzzling discoveries has been the source of bright white spots on Ceres’ face. Now researchers can finally say what caused those shiny patches: salt.
As the Dawn probe slide neatly into orbit, the spacecraft's cameras captured close-ups of the bright spots. Some experts hoped that the patches were the first sign of water or ice, reports Rachel Feltman for The Washington Post. But the mission’s principle investigator, Christopher Russell, told The Post in July that highly reflective salt fields were more likely the cause.
Now scientific research backs up this educated guess. Two recent papers published in Nature provide a complete picture, telling a new story of Ceres' formation and concluding that the spots are indeed salt. But the salt on Ceres isn't like what you'd find on a dinner table, it's made of magnesium sulfate.
Ceres sports more than 130 bright areas, most hiding within the outlines of impact craters. These shiny patches are in the same spots where water vapor was previously detected, co-author on the first paper Vishnu Reddy says in a statement. So scientists believe that the salts are remnants from salty water ice sublimating. The dwarf planet may hide a layer of this briny water ice beneath its surface, unearthed by striking asteroids.
Though the spots seem to glow white in photos, they are only reflecting about half the sunlight that strikes them, making them "similar in brightness to fresh asphalt," reports Story Hinckley for The Christian Science Monitor. Even so, that’s enough reflection to make them stand out against the dull surroundings of the dwarf plant. Perhaps answering the bright spots' mystery can lay to rest some rumors that they were signs of alien civilization.
One of the new reports also retells Ceres' birth. The discovery of ammonia-rich clays on the dwarf planet suggests that it might not have been born in the asteroid belt between Mars and Jupiter, where it now resides. The current location of the dwarf planet is too warm for ammonia to have solidified and remain in clay. It would have evaporated, Hinckley writes.
It's more likely that Ceres—or some of its parts—formed in the outer solar system. Material could have drifted in from the outer solar system, collecting on the dwarf planet. Alternatively, Ceres itself may have once hung out closer to Pluto and picked up these ammoniated compounds.
Later this month, Dawn will dip lower, just 240 miles above the surface, reports Feltman for The Post. Stay tuned for the likely discovery of even more surprising features on this foreign world.