Ceres: Close Look at Occator Crater

by Paul Gilster on March 7, 2017

We’ve looked recently at the possibility of cryovolcanism on Ceres with regard to the unusual feature called Ahuna Mons (see Ice Volcanoes on Ceres?). Now we have further evidence that outbursts of brine from beneath the surface have been occurring over long periods of time, and that some of these eruptions have been recent. The work comes out of analysis of data from the Dawn mission by scientists at the Max Planck Institute for Solar System Research (MPS), and moves the debate to the unusual crater called Occator.

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Image: This view of the whole Occator crater shows the brightly colored pit in its center and the cryovolcanic dome. The jagged mountains on the edge of the pit throw their shadows on parts of the pit. This image was taken from a distance of 1478 kilometers above the surface and has a resolution of 158 meters per pixel. NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.

Dawn’s Low Altitude Mapping Orbit (December 2015 to September 2016) took the spacecraft to within 375 kilometers of the surface, allowing highly resolved images of Ceres’ surface to be obtained. The geological structures on display within Occator, tracked by the Dawn Framing Cameras and its infrared spectrometer, include smaller, younger craters along with fractures and avalanches, all presenting us with a look into the crater’s evolution.

The bright dome in Occator crater’s central pit, now known as Cerealia Facula, is one of Ceres’ most intriguing features. Infrared data have shown it to be rich in carbonates. Beyond the pit itself, we also find other bright areas called Vinalia Faculae, considerably paler and evidently thinner, though likewise containing carbonates along with dark surrounding material. The MPS researchers, led by Framing Camera lead investigator Andreas Nathues, have evaluated images of Occator from various distances and different angles of view.

It took more than a single impact to produce what we see in Occator crater, though that impact was likely the trigger for subsequent cryovolcanism. As explained by Nathues:

“The age and appearance of the material surrounding the bright dome indicate that Cerealia Facula was formed by a recurring, eruptive process, which also hurled material into more outward regions of the central pit. A single eruptive event is rather unlikely.”

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Image: False color mosaic showing parts of Occator crater. The images were taken from a distance of 375 kilometers. The left side of the mosaic shows the central pit containing the brightest material on Ceres. It measures 11 kilometers in diameter and is partly surrounded by jagged mountains. In the middle of the pit a dome towers 400 meters high covered by fractures. It has a diameter of three kilometers. The right side of the mosaic shows further, less bright spots in Occator crater. NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.

What’s especially interesting here is the age issue, for this is the first time scientists have determined the age of the bright material. As this MPS news release explains, Nathues and team believe the central pit in Occator, which contains a jagged ridge, is all that is left of a former central mountain that would have formed as a result of the impact that created the crater about 34 million years ago. The age estimate comes from studying smaller craters formed from later impacts, and the Dawn images are so highly resolved that the crater count — and the age estimates that emerge from it — are the most accurate to date. The bright material of the dome is made up of much younger material, about four million years old.

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Image: This 3d-anaglyph for the first time shows a part of Occator crater in a combination of anaglyphe and false-color image. NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.

Domes like this, interpreted as signs of cryovolcanism, appear on the Jovian moons Callisto and Ganymede, and the new work argues that the same process is happening on Ceres. The original Occator impact would have caused subsurface brine to move closer to the surface, allowing water and dissolved gases to form a system of vents. Fractures on the surface formed, from which brine began to erupt, depositing salts that led to the formation of the dome.

The process may still be active, though at a much lower level. Imagery of the area at certain angles has revealed what appears to be haze, a phenomenon the researchers explain as the result of sublimating water ice. Imagery from Dawn taken at distances as far as 14000 kilometers have shown unmistakeable variations in brightness that follow a diurnal rhythm. Says Guneshwar Singh Thangjam (MPS):

“The nature of the light scattering at the bottom of Occator differs fundamentally from that at other parts of the Ceres surface. The most likely explanation is that near the crater floor an optically thin, semi-transparent haze is formed.”

I should add, though, that there is a great deal we have yet to learn about this haze, as explained in one of two papers on this work:

The available data are insufficient for an analysis of the optical properties of this haze, but it is most likely an optically thin and semi-transparent layer forming at near-surface level. Diurnal albedo variations that correspond to Occator’s longitude have been detected by radial velocity changes (Molaro et al. 2016) supporting the existence of a temporarily varying haze layer. The detection of water-ice signatures at Oxo [a second crater at which haze has been detected] further supports ongoing sublimation activities on Ceres. Although we expect the maximum haze concentration above the central spot in Occator, it is remarkable that our data also indicate activity at the secondary spots.

The water sublimation, and thus the intensity of the haze, varies with the degree of sunlight. This explanation makes Ceres the closest body to the Sun that experiences cryovolcanism. It is the age difference — the bright deposits are 30 million years younger than the crater — as well as the distribution of the bright material within the crater itself, that tells us how active Occator crater has been and, on a much smaller scale, continues to be to this day.

The paper is Nathues et al., “Evolution of Occator Crater on (1)Ceres,” The Astronomical Journal Volume 153, Number 3 (17 February, 2017). Abstract available. See also G. Thangjam et al., “Haze at Occator Crater on Dwarf Planet Ceres,” The Astrophysical Journal Letters Volume 833, Number 2 (15 December, 2016). Abstract / Preprint.

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{ 18 comments… read them below or add one }

Tulse March 7, 2017 at 15:23

Does cryovolcanism suggest potential conditions for life? How salty would the water have to be to liquid in this instance?

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xcalibur March 8, 2017 at 9:47

It could potentially provide a habitat for microbes.

I’m certain that cryovolcanism is genuine, but as always we need more data.

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Michael March 8, 2017 at 10:44

The salt dome could not hold too much water or it would collapse under its own weight, I would think it would be in the form of hydrated carbonates and sulphates, so a few percent.

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Harry R Ray March 8, 2017 at 10:48

Yes! Albeit HOW briny, liquid water exists underground on Ceres. Extremophiles on Earth thrive in liquid water. What suggests potential conditions for life EVEN MORE is the discovery of TAR-LIKE aliphatic organic compounds around Ernutat crater, which shows no evidence for cryovolcanism NOW, but; in the distant past, WHO KNOWS?

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EricSECT March 8, 2017 at 7:44

It would be great to send a sister of the Curiosity rover on Mars to this interesting little world, we seem to have a good, robust rover design. Mass produce these rovers and send them everywhere! Mercury and Luna polar regions, Callisto, Enceladus…..

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Michael March 8, 2017 at 10:40

There must be trillions of tons of metals on the surface on this world, we could build huge structures and a nice magnetic field to protect the surface from particle radiation.

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Harry R Ray March 9, 2017 at 10:43

I know that this is a LITTLE BIT OT, but since Michael brought it up, here goes! A NASA scientist has recommended the construction of a dipole magnetic field generator to be placed in the LaGrangian point between Mars and the Sun. MANY QUESTIONS!!!! Question one: What would be its radius? Question two: If it were EXCEPTIONALLY LARGE, could either JWST or PLATO detect a TRANSIT of it either just prior to or just after the transit of a PLANET(CASE IN POINT: TRAPPIST-1h probably has a very weak magnetic field AT BEST. It it has volcanoes spewing Hydrogen, its surface COULD be VERY Mars-like)? Question three: If transits WERE detected, could FOLLOW-UP observations with the High Definition Space Telescope(should it EVEER be built)produce a spectra of it that would confirm BEYOND A SHADOW OF A DOUBT that jt WAS an ARTIFICIAL construct, and NOT a natural object, like an exomoon?

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Harry R Ray March 9, 2017 at 11:43

A POSSIBLE answer to question one below. It would be an “inflatable structure” like ECHO 1. If (and ONLY if) the illustration is to scale, it would be roughly 1/3 the radius of Mars, and located at the L1 point. For details, log on to https://www.manyworlds.space.

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Harry R Ray March 8, 2017 at 10:50

Sorry, I meant “liquid briny water”.

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John Freeman March 9, 2017 at 8:35

An interesting target in the hunt for habitable envirinments and life – given that Ceres needs a high delta v to reach, but has much lower surface gravity, would it be easier to do a sample return mission from Ceres than Mars?

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Michael March 9, 2017 at 10:24

Wonder if we could do a bounce, grab and burn back to orbit manoeuvre.

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Michael March 10, 2017 at 11:48

Perhaps a few sticky surfaces on the outside of the craft to pick up material.

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Alex Tolley March 9, 2017 at 12:21

I’m intrigued by this finding of carbonates. I think this implies that Ceres had a lot of CO2 during formation and that these combined in water with metals to form the carbonate salts found. On Earth, most carbonates are found in rock and supposedly are the sink from the dense, CO2 rich, primordial atmosphere. In Ceres case, I would assume no atmosphere, but rather frozen CO2 such as we get at the poles on Mars.

Can we expect to find limestone and similar rocks on Ceres beneath the surface?

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ljk March 9, 2017 at 19:07

Volcanoes be like erupting all over the Sol system…

http://www.astronomy.com/news/2017/03/volcanoes-in-space

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Juraj March 14, 2017 at 22:38

Cerealia Facula and Vinalia Facula crack me up every time, who came up with these names and what do they mean (really cereals and wine)?

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ljk March 15, 2017 at 13:30

Ceres was the main Roman goddess of agriculture:

http://www.ceresva.org/Goddess/Ceres.htm

https://en.wikipedia.org/wiki/Ceres_(mythology)

Her Greek counterpart was Demeter.

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Juraj March 18, 2017 at 16:54

Yes I knew about Ceres/Demeter, asked abut the faculas. And it turns out that both Cerealia and Vinalia are (obviously-themed) Roman festivities.

Hope this will lead to some funny NASA posters!

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ljk March 19, 2017 at 15:24

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