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Ceres: The Lesson of Occator Crater

We learned some time ago from the Dawn mission just how interesting a place Ceres is. If you’re wanting to dig into the latest research on the dwarf planet, as it is now termed, be aware that a collection of papers has appeared in Nature Astronomy, Nature Geoscience and Nature Communications, all published on August 10. These analyze data gathered during Dawn’s second extended mission (XM2) phase, which closed with a series of low orbits as close as 35 kilometers from the surface. Rather than listing these papers separately, I’ll just offer this link to the entire collection at nature.com.

The upshot is that we’re continuing to learn about a small world that remains surprisingly active. Let’s home in on cryovolcanism, which leverages the temperature differential between a frozen world’s interior water and its frigid surface to produce ejections. These are becoming almost common — think Enceladus, for example, and then remember what Voyager saw at Triton. The thinking has been that some kind of cryovolcanism makes sense in the outer Solar System because the gas and ice giants place gravitational stresses on their moons that warm their interior.

But Ceres? Not only is it the only dwarf planet inside Neptune’s orbit, but it’s also the largest of the main belt asteroids. You would think that orbits between Mars and Jupiter, which are obviously not associated with the tidal forces of a closely orbited gas giant, would produce inactive bodies. The Dawn data and the papers it has spawned indicate otherwise. Dawn, studying Ceres from 2015 to 2018, shows us a world of 950 kilometers diameter that is active and possessed of its own form of cryovolcanism. At the core of these studies is Occator Crater, about 92 kilometers in diameter and possessed of a remarkably bright, white coloration.

Image: Through eruptions of brine from the interior of dwarf planet Ceres, Occator Crater received its current shape over millions of years. Credit: © Nathues et al., Nature Astronomy.

The evidence of Occator Crater argues for the remains of a global, salty ocean, a briny mixture that remains liquid and may still be escaping from the interior. We know that the European Space Agency’s Herschel Space Observatory produced evidence of a sporadic exosphere, thin but containing water. A thin haze was spotted during the Dawn mission over Occator Crater, though only intermittently, and apparently young deposits of salt compounds containing water turn up in analysis of the Dawn spectrometer data, as presented in a paper from the Dawn spectrometer team led by the Istituto di Astrofisica Spaziale e Fisica Cosmica in Italy.

“We assume that Ceres is still occasionally cryovolcanically active,” says Andreas Nathues (Max Planck Institute for Solar System Research, Germany), who led a team evaluating high-resolution imagery of Ceres during the final phase of the Dawn mission. Indeed, the bright white coloration found at Occator Crater was picked up during the approach to Ceres, and yielded to subsequent analysis. “On closer inspection, Occator Crater has a very complex structure with elevations, a large central depression, deposits, cracks, and furrows,” Nathues continues. “In all its details, this became clear only during the final phase of the mission.”

Without a source of internal heating, the deep dive into Occator Crater found in the new papers tells us that this is nonetheless a world rich in water, one containing a brine reservoir believed to be about 40 kilometers deep and several hundred kilometers in width. This would explain Occator’s bright areas, which are found to be deposits made largely of sodium carbonate that would have moved up to the surface only to evaporate, leaving the reflective salt crust behind. It’s the deep reservoir of brine found by studying Ceres’ gravity that helps us parse its interior structure. Significantly, crustal density here increases with depth well beyond the effects of pressure.

Image; NASA’s Dawn spacecraft captured pictures in visible and infrared wavelengths, which were combined to create this false-color view of a region in 92-kilometer-wide Occator Crater on the dwarf planet Ceres (in the main asteroid belt between Mars and Jupiter). Here, recently exposed brine, or salty liquids, in the center of the crater were pushed up from a deep reservoir below Ceres’ crust. In this view, they appear reddish. In the foreground is Cerealia Facula (“facula” means bright area), a 15-kilometer-wide region with a composition dominated by salts. The central dome, Cerealia Tholus, is about 3 kilometers across at its base and 340 meters tall. The dome is inside a central depression about 900 meters deep. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.

As Dawn examined the surface from a range of altitudes, it captured images of two areas within Occator Crater — Cerealia Facula and Vinalia Faculae — whose very brightness was unusual, an indication that they were youthful enough not to be darkened by micrometeorite debris. Subsequent research analyzed in this collection has shown that these faculae (bright areas) are less than 2 million years old. Moreover, the salt compounds of Cerealia Facula — sodium chloride bound with water and ammonium chloride — reached the surface only recently.

“For the large deposit at Cerealia Facula, the bulk of the salts were supplied from a slushy area just beneath the surface that was melted by the heat of the impact that formed the crater about 20 million years ago,” said Dawn Principal Investigator Carol Raymond. “The impact heat subsided after a few million years; however, the impact also created large fractures that could reach the deep, long-lived reservoir, allowing brine to continue percolating to the surface.”

So we have two paths that allow liquids to reach the surface. Again, this is happening in the absence of gravitational interactions with planets, an indication that if Ceres is active, so too may be other ice-rich bodies in the outer Solar System. Small conical hills on Ceres analyzed in this work are another avenue showing the effect of frozen groundwater.

Image; This mosaic of Ceres’ Occator Crater is composed of images NASA’s Dawn mission captured on its second extended mission, in 2018. Bright pits and mounds (foreground) were formed by salty liquid released as Occator’s water-rich floor froze after the crater-forming impact about 20 million years ago. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/USRA/LPI.

So we have brine from below the surface pushed upward, the water evaporating, and bright salty deposits left behind, with the remains of a once global ocean surviving and producing a form of cryovolcanism that is most likely still ongoing. The impact that formed Occator Crater occurred perhaps 22 million years ago, with the subsequent subsiding of the inner part of the crater resulting from the loss of material from the interior.

But what stands out in these papers is that brine continued to reach the surface for millions of years afterwards, and may be part of a process that still occurs, as opposed to being the result of a one-time event caused by meltwater from the initial impact. Another dwarf planet turns out to be surprisingly active, as the once enigmatic Occator Crater demonstrates.

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{ 15 comments… add one }
  • Christian G August 12, 2020, 16:47

    Interesting! Another factor to consider in what places can support life, although perhaps not in where in can come into being. Still, Lithopanspermia is still a possibility – do we have data on whether meteorites from Earth or Mars have reached the Belt?

    Speaking of which – do we have any Expanse fans here? I dropped out a while back but I seem to remember Ceres was pretty thoroughly tunneled through in that series.

  • John walker August 12, 2020, 16:51

    “So we have brine from below the surface pushed upward”

    How? What mechanism causes this upward flow or perhaps due to very low gravity more aptly called outward flow? None of the abstracts list a mechanism for that sustained multi million year continuing slurry movement. Ongoing post impact subsidence or contraction? Given Ceres small size and lack of tidal stresses, what interior forces remain to squeeze the fluid outward?

    • Michael Fidler August 13, 2020, 1:59

      Could smaller impacts cause the up-welling? If the crust is thin enough the impacts from asteroid to small meteorites might cause earthquakes and actual resonance ringing of the crust. This would be a good way that the mush would be pushed out thru cracks. Ceres should have more and larger impacts because of being in the asteroid belt. The impacts could also turn the interior ice to a mush.

      • John walker August 13, 2020, 14:03

        Maybe. Ahuna Mons is antipodal to Kerwen crater. There’s been speculation that there’s a causal relationship. Although Kerwen seems much older than Ahuna Mons.

        In the case of Occator slow freezing out of a deeper liquid brine layer could create the needed outward pressure to get the impact melt slurry to the surface via fractures created by the impactor.

        • Andrei August 14, 2020, 16:21

          I was about to say when reading above, and I think you nailed it.
          Frozen water got a bit larger volume than liquid – that’s why ice floats. So when the brine freeze it could cause pressure, in addition the surface of Ceres might even be top heavy as cometary and asteroid material have gathered on the surface which press the material downward.

  • Enzo August 12, 2020, 19:00

    I seem to remember that orbit insertion around Ceres is relatively hard due to its weak gravity. Once in orbit though, landing should be a lot easier than Mars.
    Assuming that’s true, maybe this could be a possible reason why other space agencies would not go ahead with Ceres orbiters/rovers instead of yet another Mars mission as they all seem to at the moment.

    • Wojciech J August 13, 2020, 14:40

      A couple of years ago, China flirted with the concept of Ceres rover, but things went quiet eventually. I still have hope that it will happen one day.

  • Mike Serfas August 13, 2020, 11:18

    I still remember that o-my-god moment when the first images were released showing the bright white dot. For a few days we could entertain the idea that sometime in the past four billion years, travellers from another star or perhaps inhabitants of a cool young Venus had stopped to mine the biggest asteroid, doubtless leaving behind a factory city filled with more toys than Santa’s workshop.

    The truth is probably more relieving than disappointing. You never really know what spyware is built into to imported gewgaws these days. With liquid water suggesting comfortable conditions below, perhaps humans could build all that they failed to find.

  • Alex Tolley August 13, 2020, 11:20

    My understanding is that this “ocean” is a slush of salt water and rocks.

    Irrespective of the mechanism, the presence of the slush below the surface offers another option for water mining on this body. Low gravity should make both drilling and slush extraction relatively simple. The dome fornations indicating the best places to drill to reach the brines.

  • wdk August 13, 2020, 19:51

    Watched the approach of Dawn to Ceres with great interest, owing to:
    -Solar electric propulsion
    – low thrust trajectories
    – the target being a possible asteroid belt source of volatiles
    And it was all tantalizing. Toward the end there were indications from
    Hubble Space Telescope that the mission had just missed an outburst
    of H2O in some form. Circumstantial evidence pointed toward Occator, but as the mission went on, the site began to seem like an off world version of a Death Valley salt mine, 20 mule team, etc.

    Solar electric exploration has been demonstrated, obviously. The limitations Dawn might have encountered in obtaining low orbit were probably due to low thrust to weight ( mass), which means it takes time
    to impart the change in velocity to lower an orbit and the orbits even though the orbits are slow. For example at pericentron lowering apocentron you run out of time to burn efficiently. Then there are also
    irregularities in the planetary gravity potential that make it difficult to get the exact low orbit you want.

    A crater such as Occator is like a window to the interior. If there is internal heat, then there would be pressure and steam venting which would condense on expansion, similar to a rocket nozzle. I hope that someday both the minerals and the water will be available to space travelers ( mineral water?) and that a human foothold can be obtained there. My guess is that if there were, the export of water would head to the moon rather than the moon being a source of water for the expeditions in the other direction.

  • Michael Fidler August 17, 2020, 19:21

    2020 QG

    2020 QG, also known by its internal designation ZTF0DxQ,[2] is a tiny Earth-crossing near-Earth asteroid of the Apollo group that flew by Earth at a distance of approximately 2,900 kilometres (1,800 mi) (less than one-quarter of Earth’s diameter) on 16 August 2020. Thought to be 12 feet in diameter but may have been as large as 46 feet!

    https://en.wikipedia.org/wiki/2020_QG

    • wdk August 18, 2020, 16:58

      M. F.,
      Judging by the perihelion and aphelion, maybe the lunar refreshment program is already under way. The aphelion matches well with Ceres’
      semi major axis; perihelion is essentially our orbit. Inclinations from the ecliptic are about 5 degrees separate ( 5 and 10 degrees, QG and Ceres respectively). Period is about 990 days. Or, if you are sending a package about 445 days. The period for Ceres is 1683 days…

      While the orbital path of Ceres and that of the Earth were connected several days ago, odds are that Ceres was not in position for a half orbit kick off for Sunday’ event, but odds against what happened are high enough. … We’ll just have to be careful what we say.

      • Michael Fidler August 19, 2020, 9:57

        Another interesting coincidence, could it be part of the same group as 2020 QG? It will be interesting to see what the orbit was of this bolide and if the two may be connected.

        Brightest fireball in years 900 times more intense than the full moon turns midnight sky into day over China.
        By Strange Sounds -Aug 18, 2020

        On Aug. 16th, 2020 the brightest fireball in years exploded in the atmosphere over the city of Linyi, turning the midnight sky blue.

        “The flash was so bright that night turned into day for miles around, and the sound from the explosion was so strong that buildings shook.”
        “The meteor illuminated the whole earth and shook the landscape with a loud sound,” reports CMMO staff member Zhou Kun. “The flash of light, which peaked at 22:59 pm local time, was widely observed across the Shandong and Jiangsu provinces of China.“
        “Scientists have Kun estimates the astronomical magnitude of the flash to be -20, more than 900 times brighter than a full Moon (albeit not as bright as the sun). If so, that would make it a rare fireball indeed.”
        “According to a NASA computer model of Earth’s meteoroid environment, a fireball of that magnitude could be a decade class event.”
        More information about this event, including a possible meteorite debris zone, may become available as the Chinese Meteor Monitoring Organization (CMMO) team analyzes the amazing number of eye-witness reports.

        https://strangesounds.org/2020/08/fireball-bolide-china-video.html

        Meteor Explosion In China, Article Spaceweather.com
        https://www.detectorprospector.com/forums/topic/13602-meteor-explosion-in-china-article-spaceweathercom/

        Fireball.
        Taken by Zhou Kun on August 16, 2020 @ Shandong Qingdao,China
        https://spaceweathergallery.com/indiv_upload.php?upload_id=167571

  • wdk August 20, 2020, 13:15

    2020 QG wikipedia page now shows an animation of the Earth flyby.
    There is a resulting decrease in the aphelion. Had there been previous flybys, there might not be so strong a connection to Ceres. And the Apollo asteroid group are defined mainly by being Earth orbit crossers; so if there were a direct connection to Ceres, we would need to assign such objects (and the recent meteor storm) a new name.

    Actually, about a decade ago before Dawn had reached any of its targets, I had been looking at the volatile transfer from the principal asteroids, anticipating a succession of flybys. Though I once had high hopes for volatiles on the moon based on John O’Keefe’s and others’ hypotheses about tektites originating from the moon, the arguments for large reservoirs of water via other means seemed not that certain either.
    So from such circumstances I calculating how a pipeline of ice bags could be launched to the moon via Mars and Earth flybys, with what seemed like the most likely source point being Ceres. Acrobatics toward the end got elaborate…

    It got as far as a proposal with some associates that was not picked up. But clearly, the reference to QG 2020 and its flyby is jolting.

  • ljk September 1, 2020, 8:29

    https://apod.nasa.gov/apod/ap200901.html

    Astronomy Picture of the Day

    Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.

    2020 September 1

    Salt Water Remnants on Ceres

    Video Credit: Dawn Mission, NASA, JPL-Caltech, UCLA, MPS/DLR/IDA

    Explanation: Does Ceres have underground pockets of water? Ceres, the largest asteroid in the asteroid belt, was thought to be composed of rock and ice. At the same time, Ceres was known to have unusual bright spots on its surface. These bright spots were clearly imaged during Dawn’s exciting approach in 2015.

    Analyses of Dawn images and spectra indicated that the bright spots arise from the residue of highly-reflective salt water that used to exist on Ceres’ surface but evaporated. Recent analysis indicates that some of this water may have originated from deep inside Ceres, indicating Ceres to be a kindred spirit with several Solar System moons, also thought to harbor deep water pockets.

    The featured video shows in false-color pink the bright evaporated brine named Cerealia Facula in Occator Crater. In 2018, the mission-successful but fuel-depleted Dawn spacecraft was placed in a distant parking orbit, keeping it away from the Ceres’ surface for at least 20 years to avoid interfering with any life that might there exist.

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