Actually, Ceres Might Have Formed in the Asteroid Belt After All

Dwarf planet Ceres is the largest planetary body in the Asteroid Belt. For a long time, scientists thought it was born in the outer solar system and then migrated to its present position. Some evidence for that origin lies in extensive surface deposits of ammonium-rich materials on the Cerean surface.

Some of those bright, white and whitish-yellow deposits are found in impact craters on Ceres. Researchers suspect they are the remnants of a brine that seeped to the surface from a liquid layer between the mantle and crust. When impacts whacked the planet, they altered its surface. They also dug up and splattered material from the brine layer. Images and observational data from NASA’s Dawn mission of an impact region called Consus Crater also show bright yellowish-white deposits. Now, thanks to a deeper analysis of Dawn data, their presence could point to Ceres’s origin in the Asteroid Belt.

Peeping Inside Ceres

Ceres is classified as a dwarf planet and its rocky component is very similar to that of carbonaceous chondrite asteroids. At least a quarter of its mass is water ice. The surface is pretty complex, consisting of carbon-rich rocks and something called ammoniated phyllosilicates. Those are minerals that include such familiar substances as talc and mica. There’s also evidence of water ice in various surface regions.

This dwarf planet is an active world, with most of its activity driven by cryovolcanism. The surface has been gardened by impacts. The thick outer crust lies over a salt-rich liquid (that brine layer) and a muddy mantle. There’s a lot of evidence to suggest that the concentration of ammonium is greater in deeper layers of the crust. The few places on the surface of Ceres where those obvious yellowish-bright patches show up are in and near Consus Crater and also within other deep craters.

Planetary scientists have long wondered about exactly where Ceres formed. If it formed in the outer Solar system, then it must have migrated into position billions of years ago. If it formed in place, then that raises the question of how it could have become enriched with the icy ammonium-rich materials.

Clues to Ceres’s Birthplace

Why the differing suggestions about where Ceres formed? Let’s look more deeply at those ammonium-rich deposits for an answer. They tend to form in very cold environments. That’s why people assumed that Ceres formed in the outer Solar System. That’s where frozen ammonium ice is most stable. In warmer environments (such as closer to the Sun), it evaporates. So, it makes sense to think that Ceres formed our where it was colder and then somehow migrated to the Asteroid Belt.

However, if the ice was part of a rocky planetesimal, the location might not matter so much. Inside the rock, the ice would be insulated from solar heating. Such world-forming materials exist closer to the Sun, and certainly out at the location of the Asteroid Belt. So, if they coalesced to form Ceres in situ, their encased ices would have contributed to the subsurface brine layer that today feeds the cryovolcanism. Impacts punching through the surface would release the brine, as well.

Connecting the Dots

A team led by Andres Nathues and Ranjan Sarkar (both Dawn mission scientists), zeroed in on materials sprayed across the surface in the area of Consus Crater. It lies in Ceres’s southern hemisphere and stretches across 64 kilometers (~39 miles). The crater walls are about 4.5 kilometers (~3 miles) high and parts of them are eroded. There’s a smaller crater inside on the eastern half of Consus. Its edges appear to be “painted” with speckles of bright yellowish material, which is also spattered out nearby.

Further analysis of the Dawn data ties the ammonium on the surface with the salty brine from Ceres’ interior. Cryovolcanic activity on this world brings the ammonium-rich brine up toward the Cerean surface. Once there, it seeps into the crust, according to Andreas Nathues, former lead investigator for the Dawn mission. “The minerals in Ceres’ crust possibly absorbed the ammonium over many billions of years like a kind of sponge,” said Nathues.

Nathues and others argue that the dwarf planet’s origin does not necessarily have to be in the outer Solar System simply based on the presence of those ammonium-rich deposits. As mentioned above, they could have been part of the planetesimals in the Asteroid Belt that coalesced to build Ceres. Once it formed, Ceres experienced impacts and cryovolcanism and those actions produced the surface deposits we see today.

Evidence from the Craters

Consus Crater itself was “dug out” between 400 and 500 million years ago by a huge impact. That event exposed material from the deep, particularly the ammonium-rich layers below Consus Crater. A later impact about 280 million years ago created the smaller crater inside. The yellowish-bright speckles to the east of the smaller crater are material ejected by the second event. If those materials always existed inside Ceres, then that supports the idea this dwarf planet formed where it is now, rather than out at the edge of the Solar System. That’s where the impacts become important, since that action exposed deeper layers, according to Dawn researcher Ranjan Sarkar.

“At 450 million years, Consus Crater is not particularly old by geological standards, but it is one of the oldest surviving structures on Ceres,” Sarkar said. “Due to its deep excavation, it gives us access to processes that took place in the interior of Ceres over many billions of years, and is thus a kind of window into the dwarf planet’s past.”

For More Information

Dwarf Planet Ceres: Origin in the Asteroid Belt?
Consus Crater on Ceres: Ammonium-enriched Brines Exchange with Phylosilicates?