Ceres, the largest object of the asteroid belt but also the smallest dwarf planet in the solar system, is hiding a secret stash of water-ice in shadowy craters at its poles.

Two new study published in the journals Nature Astronomy and Science, reveal that Ceres is a wet world, with pockets of frozen water-ice residing in permanently shadowed craters — a phenomenon that has only been observed in three places: the moon, Mercury, and now Ceres.

When we think of planetary demotion scandals, Pluto normally comes to mind. But Ceres has been on the ultimate roller classification roller coaster, as the small world has been called a planet, an asteroid, and ultimately a dwarf planet. Regardless of what you call it, the icy world comprises nearly a third of the total mass that consists of the asteroid belt. Spanning 585 miles in diameter, Ceres is poised to be an intriguing world when it comes to future interplanetary pit stops and asteroid mining.

Ice, Ice Baby

The ion-propelled Dawn spacecraft has been orbiting Ceres since 2015, and it has already detected plumes of water vapor, a potential cryovolcano, as well as exposed water-ice. But Dawn isn’t finished yet. The latest results from its GRaND instrument (a.k.a. gamma ray and neutron detector) hints at broad stores of water ice just below the surface and at the poles.

“On Ceres, ice is not just localized to a few craters. It’s everywhere, and nearer to the surface with higher latitudes,” Thomas Prettyman, principal investigator of Dawn’s gamma ray and neutron detector (GRaND), explained in a news release.

Water at the polar regions on Ceres freezes and becomes trapped.
Water at the polar regions on Ceres freezes and becomes trapped. 

Unlike Earth, which has an axial tilt of roughly 23.4 degrees, Ceres’ axial tilt is only a few degrees, which means its poles are only slightly angled towards the sun. So, as the icy world orbits the sun, most of the polar regions remain in permanent shadow — just like we see on the moon and Mercury — and if there was any volatile material (such as water) in those regions, it would be trapped as ice.

Scientists have suspected that cold traps would be present on Ceres, but now they have definitive proof, thanks to the Dawn spacecraft. The robotic probe imaged over 600 craters on Ceres that are in perpetual darkness. The science team analyzed photos of the extensive crater system, and found that at least 10 contain bright spots. After carefully analyzing the light reflected off of these bright spots, the team concluded that the reflective surfaces contained water-ice.

But the water is not just at the poles. It’s also extensively located in Ceres’s subsurface. Dawn observed evidence of water trapped inside hydrated minerals near its surface and also pockets of it in Ceres’ interior.

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Dawn observed signs of water trapped in minerals on Ceres.
Dawn observed signs of water trapped in minerals on Ceres. 

What About Life?

Early in Ceres’s history, there was enough residual heat left over from the formation of the solar system to heat up the interior. This left over heat was just enough for subsurface liquids to flow, which ultimately helped separate Ceres into layers of rock and ice.

“Liquid water had to be in the interior to see the mineralogy on the surface and the hydrogen at the equator,” Carol Raymond, a planetary scientist at NASA’s Jet Propulsion Laboratory explained during a press briefing at this year’s American Geophysical Union (AGU) in San Francisco.

The research team explained that the discovery of water ice at the poles and below the surface, makes Ceres an interesting place to look for signs of life, but does not mean the icy world is habitable. The polar regions are so cold — about 60 Kelvin or minus 351 degrees Fahrenheit — that even the heartiest of extremophiles probably could not survive.

Raymond explained that the liquids lurking inside Ceres would most likely be briny fluids. When water ice freezes, all the impurities are separated and only the pure water-ice freezes.

“Water is likely sequestered in the surface, and froze out fairly early on,” she said. “However, if there was a substantial water ocean present before the freezing,” she explained, “Ceres would be a very interesting object, mostly equivalent to Enceladus or Europa.”

If there was enough liquid inside Ceres it could have reacted with the subsurface rock layers, making it an interesting astrobiological target, like Europa.
If there was enough liquid inside Ceres it could have reacted with the subsurface rock layers, making it an interesting astrobiological target, like Europa.  

Future Benefits

The team is not sure exactly how much ice is sequestered in the craters at Ceres’ poles as it’s difficult to measure in shadowed regions. But the ice does appear to be several meters thick, which could open up future mining possibilities. As humanity pushes out farther into the solar system, water ice and other natural resources could be harvested and used to generate necessities, such as sustainable rocket fuel.

Photos via NASA, Thomas, et al.