The Strange Surface of Ceres

Physics
By Allison Kubo Hutchison 

Comparison of Earth, the Moon, and Ceres. Image by Gregory Revera NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.

Ceres is the largest body in the asteroid belt. It represents the history of our solar system as a protoplanet, a planetary embryo which formed 4.56 billion years ago. Earth itself is made of the agglomeration of several planetary embryos and in Ceres we can see the early stages of solar system evolution. The gravity of Ceres has pleasantly rounded it unlike many of the smaller bodies in the asteroid belt. Due to its size Ceres was the first object in the asteroid belt to be discovered in 1801 by Giuseppe Piazzi and was originally thought to be a planet. It was later reclassified to an asteroid and then with the reclassification of Pluto in 2006 Ceres finally fell into place as a dwarf planet.

Ceres unlike other solar bodies such as Europa and Earth has no source of internal heat. Earth heat from the decay of radioactive materials in the core while Europa has tidal heating. Tidal heating occurs due to the the competing pulling forces from Jupiter and the large moon Ganymede generating tidal forces 1,000 times stronger than the moon’s effect on Earth. Ceres, situated in the asteroid belt, has none of the sources of internal heating which led scientist of believe it would be a cold, inactive body. In 2015, the NASA Dawn mission arrived in orbit around Ceres after first visiting it’s smaller neighbor Vesta. Dawn returned high resolution mapping of the surface of Ceres and revealed a heavily cratered surface but unlike our Moon it does not have many very large craters. This indicates there are some geologic resurfacing processes occurring at Ceres.

The Dawn mission revealed that Ceres is host to several cryovolcanoes. In the outer solar system, cryovolcanism occurs when liquid water or water vapour reaches the surface from depth. This process is similar to terrestrial volcanism since it represents the interaction of liquid and solid regimes however this occurs at harsh cold temperatures. It is unusual that Ceres would have cryovolcanism since it does not have the typical sources of heat, either a hot mantle or tidal heating as seen in Europa and Enceladus.

The first cryovolcanic edifice identified on Ceres was Ahuna Mons. Ahuna Mons was identified as a volcanic dome. It was formed by the extrusion of thick, viscous muddy brine similar to silicic volcanic domes on Earth as the dome in the crater of Mount Saint Helens. Based on modeling of the relaxation of the dome, workers approximated it’s age to be 210 million years. This is just yesterday when considering geologic processes.

Ahuna Mons is ~ 17-km wide and 4 km high. NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Research published this August has also revealed more recent activity at Occator Crater. Occator Crater is home of one of the mysterious “bright spots”. These “bright spots” represent areas of high albedo, or light reflectivity, and indicate unusual materials on the surface. However, these bright spots have now been linked to cryovolcanism in Occator crater. It is believed that these “bright spots” represent places where brine has reached the surface. This activity is dated to approximately 2.1 million years ago, an eyeblink in geologic considerations. The “bright spots” confirm the presence of a global briney mantle which implies that Ceres once hosted a brine ocean under the crust.

Ceres has been found to be differentiated. Planetary differentiation is a process that occurs as the heavy metallic elements settle to the center of the body and the light silicate elements rise to the surface. Differentiation processes only occur in sufficiently large bodies such as Ceres. In the case of Earth, we have a solid metallic core, silicate mantle, and light silicate crust. The structure of Ceres is thought to be similar but with added water. Ceres is nearly 40% water by volume where Earth is only 0.1% by volume. Ceres may host a metallic

core but the jury is still out however it does likely have a “muddy mantle” of clays and brines and a stronger crust of clay, salts, solid methane, and ice.

These features are so much more than bumps on a body millions of kilometers away. They are important insights into the history and evolution of the solar system. It also emphasizes the power of salts in sustaining the presence of liquid water and reveals a wealth of activity on a cold desolate proto-planet. The presence of the briney mantle and it’s incredibly carbon rich surface may also indicate it was once suitable for life elevating it to a body of utmost significance.

Ceres isn’t dead, it is just salty.

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