Given yesterday’s look at the ocean beneath Enceladus’ ice, it seems the right time to examine the recent work on Ceres. We know that the dwarf planet may have had a global ocean of its own, but as with Enceladus, questions abound. Is there still liquid within Ceres? We have two new studies from the Dawn mission to give us some insights. The upshot:
“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,” said Julie Castillo-Rogez, Dawn project scientist and co-author of the studies, based at NASA’s Jet Propulsion Laboratory, Pasadena, California.
Anton Ermakov (JPL) is lead author of the first paper, published in the Journal of Geophysical Research, which examined gravity data measurements from Dawn to analyze the composition of Ceres. This is exceedingly fine-grained work, drawing not only on Dawn data but on Deep Space Network observations of tiny changes in the spacecraft’s orbit. We learn that the craters Occator, Kerwan and Yalode, along with the mountain Ahuna Mons, are all associated with gravity anomalies — differences between observed gravity and the values predicted by our best models of the dwarf planet’s gravitational field.
The variations from the scientists’ models of Ceres gravity and what Dawn actually observed at these four locations can tell us something about structure and composition beneath the surface. Both Ahuna Mons and Occator appear to be associated with cryovolcanism. We also learn that the density of the crust is closer to ice than rock, a puzzling finding given other Dawn studies showing that ice would be too soft to serve as the dominant component in Ceres’ crust.
But there is an explanation. From the paper:
Finite element modeling of Ceres’ topography [Fu et al., 2017] shows that the topographic power cannot be supported by a solely ice rheology [physics dealing with the deformation and flow of matter] over billion year timescales. Using a lower bound for crustal density based on rheology, we derive constraints on the crustal thickness using the assumption of hydrostatic equilibrium. A low-density, high strength mixture is required to explain the inferred crustal density and rheology. The latter does not allow more than 43 vol% silicates assuming 15% void porosity in the crust. Therefore, lower density materials, such as salt or gas (clathrate) hydrates, are required.
Image: This animation shows Ceres as seen by NASA’s Dawn spacecraft from its high-altitude mapping orbit at 1,470 kilometers above the surface. The colorful map overlaid at right shows variations in Ceres’ gravity field measured by Dawn, and gives scientists hints about the dwarf planet’s internal structure. Red colors indicate more positive values, corresponding to a stronger gravitational pull than expected, compared to scientists’ pre-Dawn model of Ceres’ internal structure; blue colors indicate more negative values, corresponding to a weaker gravitational pull. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.
A second study, published in Earth and Planetary Science Letters, delved into that crust, its strength and its composition, by studying Ceres’ topography. We would expect that a crust laden with ices and salts would gradually deform over the age of the Solar System, whereas a crust dominated by rock could remain essentially unchanged. Flow models that Roger Fu (Harvard University) applied to the data show a crust that not only mixes ice, salts and rock, but is also composed of clathrate hydrate, as suggested in the paper above.
The latter is the key: Clathrate hydrate produces a structure far stronger than water ice, although maintaining nearly the same density. Fu and colleagues believe that Ceres once had more well defined surface features that have smoothed out over time. The process would require a deformable layer beneath a high-strength crust, and that deformable layer may well contain liquid. We have the possibility, therefore, of at least a small residual liquid ocean.
The Ermakov paper is “Constraints on Ceres’ internal structure and evolution from its shape and gravity measured by the Dawn spacecraft,” Journal of Geophysical Research: Planets, 18 October 2017 (abstract). The Fu paper is “The interior structure of Ceres as revealed by surface topography,” Earth and Planetary Science Letters, Vol. 476 (15 October 2017), 153-164 (abstract).