In the summer of 2015, as NASA's Dawn spacecraft approached the dwarf planet Ceres, scientists kept seeing a pair of bizarre shiny dots peering back:
What the heck were those things? Some experts initially thought they might be ice. Others speculated ice volcanoes. Many internet commenters joked that Ceres was secretly the Death Star. Eventually, scientists concluded that the shine was probably due to some sort of salty substance reflecting light from the surface.
Now they've gotten an even closer look. On Tuesday, NASA released the most detailed images yet from Dawn — taken just 240 miles above the surface. Here is Occator Crater, which contains the brightest of the bright spots:
The crater is 57 miles across and 2.5 miles deep. "The latest images," NASA announced, "taken from 240 miles above the surface of Ceres, reveal a dome — with fractures crisscrossing the top and flanks — in a smooth-walled pit in the bright center of the crater."
Here's another image of the crater up close:
"With the latest close views, we can see complex features that provide new mysteries to investigate," said Ralf Jaumann, a planetary scientist and Dawn co-investigator at the German Aerospace Center in Berlin, in a statement. "The intricate geometry of the crater interior suggests geologic activity in the recent past, but we will need to complete detailed geologic mapping of the crater in order to test hypotheses for its formation."
The still-mysterious origins of Ceres's shiny dots
So why is the crater shiny? In a paper published last December in Nature, scientists argued that the reflection may come from a magnesium sulfate called hexahydrite.
The idea is that Ceres has a salty layer of water ice just beneath its surface. At some point in the past, asteroids pummeled the dwarf planet, bringing that mixture to the surface. The water ice then evaporated away in the sun, leaving only the bright-colored hexahydrite behind. And because the rest of the planet is so dark, those bright spots stick out.
Still, this needs further exploration. The existence of subsurface water ice remains one of the central mysteries of Ceres.
Until the Dawn mission arrived in 2015, scientists had never seen Ceres up close. The dwarf planet, a 590-mile-wide ball of rock and ice that lies in the asteroid belt, was the biggest unexplored rock between the sun and Pluto.
Scientists have long wondered — based on measurements of the dwarf planet's mass — if Ceres might have vast amounts of ice in its mantle. Some speculated there might be as much fresh water locked away there as is present on all of Earth:
Dawn is now sleuthing for clues. And there are some tantalizing signs. On Tuesday, NASA also announced that the spacecraft had detected increased hydrogen concentrations in the dwarf planet's higher latitudes. "As hydrogen is a principal constituent of water, water ice could be present close to the surface in polar regions."
If so, that could help explain the dots — and resolve some other longstanding mysteries about the planet.
For now we'll have to wait for further study. "Our analyses will test a longstanding prediction that water ice can survive just beneath Ceres' cold, high-latitude surface for billions of years," said Tom Prettyman, the lead for Dawn's Gamma Ray and Neutron Detector and Dawn co-investigator at the Planetary Science Institute, in Tucson, Arizona.
Read more: 6 reasons why NASA's mission to Ceres is a big deal