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Ceres: A Residual Ocean?

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).

 

 

{ 7 comments… add one }
  • john walker November 9, 2017, 20:40

    Okay, I’ll say it. In the early days of it’s discovery, I was rooting for the “lights” in Occator to be artificial. I thought Ceres would be a great place for discrete base for an eti observation of the nascent Terran civilization.

    • ljk November 10, 2017, 9:47

      Why would an alien base have such prominent “lights” if they were trying to be discreet? :^)

      I think this also shows our likely limited views of how an advanced ETI operating in our Sol system would be. Bases implies a large physical and probably organic presence, when in fact they would instead operate far better as Artilects. And with Breakthrough Starshot, we are just starting to break through the paradigm of the large, bulky starship full of carbon units.

      Besides, I think salt deposits indicating a global liquid ocean beneath the surface of a world smaller than Texas to be rather exciting in and of itself. We seem to forget this while we keep trying to accommodate our decades-old science fiction fantasies.

  • Alex Tolley November 9, 2017, 23:15

    I would have thought Ceres’ subsurface would have fully frozen solid, although if primarily water, I understand that the material could liquify under pressure. I do wonder if impactors could have provided the energy needed to liquefy some of the interior material to create temporary seas or lakes.

    I don’t see Ceres as an abode for life, but one never knows. Landers inspecting its surface and subsurface seem indicated at some point.

  • ericSECT November 10, 2017, 7:44

    Ceres sounds like a good place to send a rover. Mass produce Curiosity, a proven design, and reconfigure the landing sequence (without parachutes).

  • andy November 13, 2017, 14:57

    If the oceans on worlds like Ceres are habitable, then it would seem likely that they constitute the vast majority of habitable worlds in the universe. It’s interesting then that we find ourselves on an open-sky world around a G-type star, rather than from an underground ocean on an ice world either around an M-dwarf or in interstellar space.

  • ljk November 17, 2017, 14:00

    Ancient interior activity likely formed features on Ceres’ surface

    by Laurel Kornfeld

    November 15, 2017

    A new analysis of surface features on Ceres sent back by NASA’s Dawn spacecraft indicates that at least some of those features were created by the ancient movement of materials in the dwarf planet’s interior. The study centers on linear features; specifically, chains of secondary craters and small pits.

    While primary craters are produced directly by impacting objects, secondary craters are formed from materials ejected from the former and often surround them in clusters or rays.

    However, pit chains are produced by fractures beneath the surface resulting from interior geological activity.

    For the study, scientists produced a map showing all features on Ceres with minimum lengths of 0.6 miles (one kilometer) outside of impact craters.

    The map depicted more than 2,000 such features. Because chains of secondary craters and chains of small pits share many common characteristics, the researchers’ most difficult task was distinguishing between the two categories.

    Full article here:

    http://www.spaceflightinsider.com/missions/solar-system/ancient-interior-activity-likely-formed-features-ceres-surface/

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