Over the past few months, as Dawn closed in on Ceres — the largest rock in the belt of asteroids that orbits the sun between Mars and Jupiter — the probe has captured these dots in higher and higher resolution. The latest photo, released Wednesday, shows them from 2,700 miles away.
Still, scientists remain baffled by the dots. "The bright spots in this configuration make Ceres unique from anything we've seen before in the solar system," Chris Russell, the mission's principal investigator, said in a NASA statement. "With closer views from the new orbit and multiple view angles, we soon will be better able to determine the nature of this enigmatic phenomenon."
Right now, there are three main theories for what the dots might be:
1) Ice. Ceres's mantle is believed to be primarily made of ice, covered by a dusty crust, so these shiny spots may just be patches of ice, reflecting a bit of sunlight. Slate's Phil Plait suggests that their location in a crater may not be a coincidence: an impact could have exposed the ice, as we've previously seen, for instance, on Mars. Right now, Russell considers this to be the most likely explanation.
2) Ice volcanoes. The mission scientists have also suggested the dots could have a "volcano-like" origin. We've previously seen ice volcanoes (called cryovolcanoes) on Neptune's moon Triton and Saturn's moon Enceladus, among other places.
This could explain why they're so close together: they could be a series of vents from the same underlying ice volcano. It might also explain why Earth-based telescopes have detected water vapor emanating from Ceres, as liquid water ejected to the surface would quickly vaporize. The problem with this is most scientists don't expect Ceres to have tectonic activity (because it's too small).
3) Salt. Mission co-investigator Tom McCord notes that if volcanism led water to vaporize from Ceres's surface, it could also produce shiny spots of an entirely different composition.
"What that would do is leave a residual salt deposit, so these bright spots could be salt deposits that accumulated around vents — volcanos — where the water is coming through," he told Seattle Astronomy.
4) Ceres is actually the third Death Star. Okay, no one actually believes this.
For now, we'll have to wait as Ceres continues to photograph the dots from new angles, then zooms in to orbit the dwarf planet from 900 miles away in early August.