Ceres Is Young

Dawn Mission scientists were surprised to see this asteroid (dwarf planet Ceres) with a water cycle and evidence of rapid geological changes.

For planetary scientists accustomed to thinking in billions of years, what the Dawn spacecraft revealed about Ceres must have been a shock. A press release from Jet Propulsion Laboratory says, “NASA Dawn Reveals Recent Changes in Ceres’ Surface.” Previously, the spacecraft had detected carbonates that must have been produced in an ocean. Now, they are seeing changes within the observation period of the mission.

Water ice is not stable on the surface of Ceres over long time periods unless it is hidden in shadows, as in the case of Juling. Similarly, hydrated carbonate would dehydrate, although over a longer timescale of a few million years.

“This implies that the sites rich in hydrated carbonates have been exposed due to recent activity on the surface,” Carrozzo said.

The great diversity of material, ice and carbonates, exposed via impacts, landslides and cryovolcanism suggests Ceres’ crust is not uniform in composition. These heterogeneities were either produced during the freezing of Ceres’ original ocean – which formed the crust – or later on as a consequence of large impacts or cryovolcanic intrusions.

“Changes in the abundance of water ice on a short timescale, as well as the presence of hydrated sodium carbonates, are further evidence that Ceres is a geologically and chemically active body,” said Cristina De Sanctis, VIR team leader at the Institute of Astrophysics and Planetary Science.

Two papers announce the discoveries. One, in Science Advances, speaks of the “detection of water ice in a mid-latitude crater and its unexpected variation with time,” with changes visible in just six months. “The reported variation on Ceres’ surface indicates that this body is chemically and physically active at the present time.”

The smaller a body, the more rapidly it should have cooled and become inactive in 4.5 billion years. Ceres is much smaller than Pluto (which also showed unexpected activity when New Horizons flew past in 2015; see review 19 March 2016), and smaller than Saturn’s moon Dione. The other asteroid target of Dawn, Vesta, also showed evidence of watery youth (28 Oct 2011, 9 Feb 2015). At the end of the paper, the scientists scramble for possible sources of energy to keep Ceres active.

The variation of superficial water ice occurring in the short time frame covered by these observations provides new scenarios for Ceres’ evolution. It indicates an active body and reinforces not only the link with the icy solar system objects but also the uniqueness of Ceres, given its present location in the main belt.

The other paper by Carrozzo et al. in Science Advances tries to understand the presence of carbonates on Ceres.

The correlation of Na carbonates with some extrusive constructs suggests that at least some Na carbonates are transported to, or near to, the surface by ascending subsurface fluids in several areas of Ceres. However, the association of Na carbonate with young craters could be consistent with the material mobilized or created by impact-induced heating. The data are not sufficient to fully discriminate among the possible mechanisms, and likely, the types of Na carbonates and the observed distribution are the results of a combination of different processes.

Whatever happened, the detection of carbonates on the surface “provides major constraints on Ceres’ chemical evolution,” the authors say.

NH₄ salts are speculated to be unstable over geologic time. Hydrated sodium carbonates are not stable on airless surfaces and dehydrate upon exposure to vacuum and irradiation over Myr timescales. Destabilization on the surface involves both dehydration and decarbonation; for example, Na₂CO₃·H₂O in Na₂CO₃ + H₂O(g). The detection of hydrated Na carbonates supports an aqueous origin of Na carbonates followed by their partial decomposition (mainly dehydration) in surface environments. This implies that sites rich in hydrated carbonates have been formed/exposed recently (a few million years), and dehydration of hydrated Na carbonates is still ongoing. This is in accordance with crater counting and modeling that predict recent formation, within tens to hundreds of millions of years.

The different chemical forms of the sodium carbonate, their fresh appearance, morphological settings, and the uneven distribution on Ceres indicate that the formation, exposure, dehydration, and destruction processes of carbonates are recurrent and continuous in recent geological time, implying a still-evolving body and modern processes involving fluid water.

They still make estimates into the millions of years, but those fall far short of the assumed 4.5 billion years for the origin of the solar system, sometimes called “geologic time” (what we dub “Darwin Years“).

Ceres joins the young, active object club, a large and growing list. One wonders if planetary scientists will ever find any object that lives up the assumed age of the solar system. Does this object look billions of years old to you?

DAWN 10th anniversary image (false color) of Ceres, centered on Occator Crater with its bright sodium carbonate deposits. (NASA/JPL)