Ceres could have been a cosmic water world billions of years ago — and might still be hiding a secret ocean

Ceres, which is either an asteroid or dwarf planet (depending on who you ask), may appear like an unremarkable orb of icy rock floating around, but the largest object in the asteroid belt might have once been a far-out ocean world. There might still be a part of that world lurking beneath the surface.

Though whatever surface oceans were on Ceres — if it had any at all — have long since frozen into what is now its crust, there might still be an ocean remnant in there somewhere. There is so much ice on Ceres that it is second only to Earth for having the most water in the inner solar system. NASA’s Dawn mission has now uncovered what could be evidence for a past liquid ocean and a hidden subsurface ocean. Astronomers had previously suspected Ceres to have a subsurface ocean, and new research its topography and gravitational field suggests that there really might be one beneath all that ice.

“The surface and internal structure of Ceres show evidence of a global process of aqueous alteration, indicating the existence of an ocean in the past. However, it is not clear whether part of this ocean is still present and whether residual fluids are still circulating in the dwarf planet,” said astronomer and Dawn team member Maria Cristina De Sanctis, who recently co-authored one of several studies published in Nature Astronomy.

What could have happened to Ceres back in the depths of time? The residual fluids De Sanctis mentioned may be oozing through a part of the Ceraelia Facula dome (above and below) in the Occator crater, which seems to be giving itself away with the presence of chloride salts. These minerals are not often found in the Solar system. Liquid water that undergoes hydrothermal activity, like the magma-heated water that shoots out of hot-water vents at the bottom of Earth’s oceans, is required for chloride salts to form. Ceraelia Facula is also an unusually bright spot on Ceres. This means it was created by sodium carbonate, thought to have seeped through ice fractures.

NASA animation of the Ceraelia Facula dome on Ceres.

Hydrohalite is another mineral present on Ceraelia Facula that could support the existence of a subsurface ocean. The thing about hydrohalite, which is basically hydrated salt that exists on Earth but is rare anywhere else in the solar system, is that it evaporates pretty quickly, at least in cosmic terms. That would be tens of thousands of years. 20 million years ago, impact from another object would have brought on heat and the fractures that released the briny liquid which left behind chloride salt deposits.

Dawn beamed back hi-res images that surprised the astronomers back on Earth. These images revealed that Ceres may have last been volcanically active 2 million years ago, and cryovolcanoes, like Ahuna Mons, spewing liquid that long after an impact could mean salty water deep beneath the frozen surface. Liquid from that ocean may still be gushing through Ahuna Mons and other possible volcanoes. Such an ocean could be up to 25 miles deep hundreds of miles wide, and if there is hydrothermal activity, there life could even be thriving on the seafloor.

“Pre-existing tectonic cracks may provide pathways for deep brines to migrate within the crust, extending the regions affected by impacts,” Carol Raymond, Dawn Principal Investigator at NASA’s JPL (Jet Propulsion Lab), said in an adjacent study also published in Nature Astronomy.

How Ceres formed and where it emerged from is still unknown. NASA is already planning to send spacecraft to water worlds Europa, Enceladus and the liquid methane and ethane seas of Titan, and a potential future mission to Ceres may demystify more of its secrets.