Did Ceres once have an ocean?

Left, Ceres as seen by NASA’s Dawn spacecraft from its high-altitude mapping orbit at 913 miles (1,470 km) above the surface. Right, a map showing variations in Ceres’ gravity field as measured by the Dawn spacecraft. This gravity map supports the idea of an ancient ocean on Ceres. Image via NASA JPL.

Ceres – orbiting between Mars and Jupiter – was classified as a planet when it was first discovered in 1801, until the 1850s when it became known as the largest of the little worlds in the asteroid belt. In 2006, astronomers re-classified it as a dwarf planet. Still, it may seem odd to you to imagine little Ceres, a world only 590 miles (950 km) across, having an ocean. And yet Ceres is known to have water-containing minerals on its surface. Two recent studies explore the possibility of an ocean on Ceres in the distant past, and they shed light on the question of what happened to this ocean, if it existed, and on whether Ceres might still have liquid water today.

Our knowledge of Ceres has vastly increased in the past couple of years, since the Dawn spacecraft began orbiting it in early 2015. Dawn’s mission was recently extended, by the way. NASA said:

The Dawn team found that Ceres’ crust is a mixture of ice, salts and hydrated materials that were subjected to past and possibly recent geologic activity, and that this crust represents most of [an] ancient ocean. The second study builds off the first and suggests there is a softer, easily deformable layer beneath Ceres’ rigid surface crust, which could be the signature of residual liquid left over from the ocean, too.

Julie Castillo-Rogez, Dawn project scientist and co-author of the studies, based at NASA’s Jet Propulsion Laboratory, Pasadena, California, said:

More and more, we are learning that Ceres is a complex, dynamic world that may have hosted a lot of liquid water in the past, and may still have some underground.

Earth-Ceres size comparison. Ceres is little, but it accounts for approximately 1/3 of the mass of the asteroid belt. In 2006, IAU astronomers briefly considered changing the status of Ceres back to that of a major planet, but then opted to make both Ceres and Pluto dwarf planets. The reason is neither Ceres nor Pluto “dominates its orbit.” Both share their orbits with many other smaller bodies, in the case of Ceres, asteroids in what we call the asteroid belt. Image via

Anton Ermakov, a postdoctoral researcher at JPL, led the first study, which is an analysis of measurements made via the Dawn spacecraft of Ceres’ gravity. Such measurements enable scientists to estimate the composition and interior structure of a world like Ceres. This study is published in the peer-reviewed Journal of Geophysical Research. The measurements came from observing the spacecraft’s motions with NASA’s Deep Space Network. The scientists were looking small changes in the spacecraft’s orbit, which indicate gravity anomalies. NASA said:

Three craters — Occator, Kerwan and Yalode — and Ceres’ solitary tall mountain, Ahuna Mons, are all associated with ‘gravity anomalies.’ This means discrepancies between the scientists’ models of Ceres’ gravity and what Dawn observed in these four locations can be associated with subsurface structures …

[Ermakov’s study also found] the crust’s density to be relatively low, closer to that of ice than rocks. However, a study by Dawn guest investigator Michael Bland of the U.S. Geological Survey indicated that ice is too soft to be the dominant component of Ceres’ strong crust. So, how can Ceres’ crust be as light as ice in terms of density, but simultaneously much stronger? To answer this question, another team modeled how Ceres’ surface evolved with time.

Ahuna Mons on Ceres, a mountain about 4 miles (6 km) tall, in a simulated view using NASA’s Dawn spacecraft images. This region on Ceres is associated with gravity anomalies, which helps scientists probe Ceres’ interior structure. Image via NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.

Roger Fu at Harvard University in Cambridge, Massachusetts, led this second study, which is published in the peer-reviewed journal Earth and Planetary Science Letters. It investigated the strength and composition of Ceres’ crust and deeper interior by studying the dwarf planet’s topography. NASA explained:

By studying how topography evolves on a planetary body, scientists can understand the composition of its interior. A strong, rock-dominated crust can remain unchanged over the 4.5-billion-year-old age of the solar system, while a weak crust rich in ices and salts would deform over that time.

By modeling how Ceres’ crust flows, Fu and colleagues found it is likely a mixture of ice, salts, rock and an additional component believed to be clathrate hydrate. A clathrate hydrate is a cage of water molecules surrounding a gas molecule. This structure is 100 to 1,000 times stronger than water ice, despite having nearly the same density.

The researchers believe Ceres once had more pronounced surface features, but they have smoothed out over time. This type of flattening of mountains and valleys requires a high-strength crust resting on a more deformable layer, which Fu and colleagues interpret to contain a little bit of liquid.

The team thinks most of Ceres’ ancient ocean is now frozen and bound up in the crust, remaining in the form of ice, clathrate hydrates and salts. It has mostly been that way for more than 4 billion years. But if there is residual liquid underneath, that ocean is not yet entirely frozen.

This is consistent with several thermal evolution models of Ceres published prior to Dawn’s arrival there, supporting the idea that Ceres’ deeper interior contains liquid left over from its ancient ocean.

Artist’s concept of Dawn spacecraft orbiting Ceres. Our knowledge of this little world has greatly increased, thanks to Dawn, whose mission was recently extended. Yet mysteries remain. Image via NASA/JPL-Caltech.

Bottom line: Two recent studies explore the possibility of an ocean on Ceres in the distant past, and they shed light on the question of what happened to this ocean, if it existed, and on whether Ceres might still have liquid water today.


October 28, 2017

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