Those mysterious spots have since been confirmed as little more than large salt patches, but astronomers have now revealed that Ceres is hiding an even more intriguing secret - it's appears to be covered in huge craters of water ice, hidden just below its surface.
Yep, our new favourite dwarf planet (sorry, Pluto) has been hiding vast reservoirs of ice from us all along.
It's well known that Ceres is covered in dark, shadowy craters, many of which are positioned so that they never see the light of day. But researchers across two separate papers published last week have discovered that at least one of these craters - and perhaps many of them - are filled with frozen water.
That means the dwarf planet, which is located in the main asteroid belt between Mars and Jupiter, could one day serve as a refuelling base for missions heading deep into our Solar System - not to mention the fact that it just got a whole lot more fascinating for astrobiologists.
"There are over 600 persistently shadowed regions on Ceres," said one of the researchers who worked across both appears, Norbert Schorghofer from the University of Hawai'i, in a press conference last week. "I call it Ceres' darkest secret."
The prediction that Ceres was covered in ice hidden just below the surface was first made by astronomers 30 years ago, but until now, we haven't had the evidence to back it up.
When the Dawn spacecraft started orbiting the dwarf planet in March 2015, the focus was initially on its strange bright spots, rather than its dark craters.
But now, in one of the studies published last week, Dawn's Framing Camera has been able to capture scattered light coming from a crater called PSR2 near Ceres' north pole, and the data show that it's filled with a huge sheet of frozen water.
These dark craters are known as 'cold traps', seeing as they contain temperatures lower than –163 degrees Celsius (–260 Fahrenheit and 110 Kelvin). This means they're so cold that very little ice stored in them is likely to turn into vapour, even over billions of years.
So far, PSR2 is the only crater researchers have managed to look inside using Dawn's infrared mapping spectrometer, but the team also detected bright deposits on the floor of 10 other permanently shadowed craters - which could likely be evidence of more ice.
In fact, in the second paper released last week, the team estimates that Ceres is made up of about 10 percent water, now frozen into ice.
"On Ceres, ice is not just localised to a few craters. It's everywhere, and nearer to the surface with higher latitudes," said lead researcher Thomas Prettyman, from the Planetary Science Institute in Arizona.
"These results confirm predictions made nearly three decades ago that ice can survive for billions of years just beneath the surface of Ceres."
The second study used Dawn's gamma ray and neutron detector (GRaND) to look at the number and energy of gamma rays and neutrons being scattered by the top metre of Ceres' surface.
The way these neutrons and gamma rays are scattered can tell researchers what the surface material is made of.
The team suggests that rather than Ceres having a solid layer of ice covering it, the planet is likely made up of a porous mixture of rocky material, and ice has filled its pores.
Based on this evidence, it's more likely than ever that other asteroids in the main asteroid belt could contain ice just under their surface - which is awesome news for space explorers who want to mine asteroids for hydrogen on the way to more distant planets.
"The evidence strengthens the case for the presence of near-surface water ice on other main belt asteroids," said Prettyman.
The research also sheds light on which types of rocky bodies in the Solar System could have once had water ice - and might still have it.
"By finding bodies that were water-rich in the distant past, we can discover clues as to where life may have existed in the early Solar System," said Carol Raymond, deputy principal investigator of NASA's Dawn mission.
The big question now is where Ceres' ice came from in the first place.
"We are interested in how this ice got there and how it managed to last so long," said Schorghofer in a press release. "It could have come from Ceres' ice-rich crust, or it could have been delivered from space."
Either way, we're eager to find out what other secrets Ceres might reveal to us in the coming months.
Check out an incredible flyover of Ceres' Occator Crater below: