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New Insights into Ceres’ Bright Spots

One reason for catching up with recent planetary science here in the Solar System is the upcoming arrival of Juno, which enters into polar orbit around Jupiter on July 4. Juno’s arrival is a reminder that the past year has been packed with interesting news from places like Pluto/Charon (New Horizons), Comet 67P/Churyumov-Gerasimenko (Rosetta), and the topic of today’s post, the intriguing dwarf planet Ceres, as studied by the orbiting Dawn spacecraft.

But the recent Ceres news hasn’t just involved Dawn. Paolo Molaro (INAF-Trieste Astronomical Observatory) had led a study looking at the bright spots Dawn found upon approaching Ceres last year. The data Molaro and team drew on came from the European Southern Observatory’s 3.6-meter instrument at La Silla and its HARPS spectrograph, which have shown us not only the motion of the bright spots as Ceres rotates but also variations that indicate volatile material within them. The suggestion is that this material evaporates when exposed to sunlight.

Ceres’ nine-hour rotation produces a small but measurable Doppler effect, with the bright spots expected to affect the spectrum of the reflected light, producing what shows up as a radial velocity variation within the overall Doppler rotational measurement. But the resulting measurements were more complex than expected, indicating a change in the reflectivity of the bright features in Occator crater. Says co-author Antonino Lanza (INAF-Catania Astrophysical Observatory), a co-author of the study:

“The result was a surprise. We did find the expected changes to the spectrum from the rotation of Ceres, but with considerable other variations from night to night.”

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Image: The bright spots on Ceres as imaged by the Dawn spacecraft at an altitude of approximately 1500 kilometers. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.

The changes in albedo vary from night to night, showing up as random patterns on short and long timescales. If, as has been suggested, the reflective material in the bright spots is fresh water ice or hydrated magnesium sulfates, then these changes could be caused by evaporation, forming highly reflective plumes. Still unknown is the energy source that seems to be driving the leakage of material from deeper below.

The work plays interestingly off other recent observations, as the paper notes:

Perna et al. (2015) found variations in the slope of visible spectra at the level of 2-3 percent over 1000 Angstrom with a variation in the relative reflectivity of about 10 percent in the region between 500 and 800 nm. Herschel detected water vapor plumes erupting off the surface of Ceres, which may come from volcano-like ice geysers (Küppers et al. 2014). The recent Dawn observations suggest that the bright spots could provide some atmosphere in this particular region of Ceres confirming Herschel’s water vapor detection (Witze 2015). It has been noted that the spots appear bright at dawn on Ceres while they seem to fade by dusk. That could mean that sunlight plays an important role, for instance by heating up ice just beneath the surface and causing it to blast off some kind of plume or other feature.

This description would account for plumes quickly losing their reflectivity as sunlight conditions change, only to form again in a cycle of evaporation and freezing that is also reflected in the radial velocity measurements from HARPS. Subsequent analysis of data from Dawn has shown bright localized areas that are consistent with hydrated magnesium sulfates, including a bright pit on the floor of the Occator crater that shows probable sublimation of water ice. Haze clouds are thus produced, appearing and disappearing in a regular daily cycle.

This ‘diffuse haze,’ as the authors describe it, fills the floor of Occator and then disappears almost completely at dusk, providing a possible counterpart to the radial velocity variability that Molaro and team have detected. That’s useful, because if both are produced by the same cause, then we have a way to continue to monitor daily activity on Ceres even after the Dawn mission ends. Further radial velocity observations and analysis of the Dawn imagery could confirm this possibility.

The paper is Molaro et al., “Daily variability of Ceres’ Albedo detected by means of
radial velocities changes of the reflected sunlight,” published online by Monthly Notices of the Royal Astronomical Society 7 February 2016 (abstract / preprint).

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Comments on this entry are closed.

  • Harry R Ray June 7, 2016, 14:05

    The KEY QUESTION here ks the GROWTH RATE of the central dome! Is it a micron a year? A millimeter a year? A centimeter a year? I do not believe(correct me if I’m wrong) that the Dawn spacecraft has a laser altimeter as one of its instruments. Determining the rate of growth would give you the LENGTH OF TIME this process has been going on. Could a SIMILAR PROCESS going on for MILLIONS OF YEARS have built Ahuna Mons? Current theory suggests that the solitary mountain on Ceres is an extrusion dome, similar to the lava dome that formed after the eruption of Mount St Helens, but more akin to Mount Kilimanjaro as a SOLITARY volcano instead of being PART of a mountain range. Huge questions remain about this theory(most explicitly why there is ONLY ONE SUCH FORMATION on Ceres, and how could such a massive magma chamber form and be maintained on such a small cold world). A viable ALTERNATIVE theory would ease these concerns.

  • Frank Smith June 7, 2016, 16:40

    The central peak of a complex crater is a rebound feature from the cratering event. It’s not “volcanic”. It would consist of material from deep beneath the pre-crater surface.

    So the central bright spot could be a sample of subsurface material several km deep. The volatile material could be depleted in the older surface material of Ceres. Occator looks very young to me with few superimposed younger craters. I would speculate the bright material is water ice or a ice mixture that is slowly sublimating or is the residue of a volatile material.

  • Michael June 8, 2016, 8:04

    This is a relaxation process after an impact, the fact that is still going on indicates it is fairly young but I don’t know for sure. I would have thought the dust would have covered it if it was over a few million years old.

    Here are some closes ups and it looks like a salt dome.

    https://en.wikipedia.org/wiki/Occator_(crater)#/media/File:PIA20355-Ceres-DwarfPlanet-OccatorCrater-Center-201602-crop.jpg

    I am still thinking in the far future it will be a large fish farm.

    • Michael June 9, 2016, 2:48

      A hopper and orbitor spacecraft would do well here, the central mound looks like a magnesium sulphate deposit. If the mantle is freezing out there is a tendancy for salts to be pushed out, this may be the reason for the salt crust. The salts will also act as a humidefying agent and is most likely the reason for the continued out gassing of water. Here is the most likely place to find life if it gained a foot hold or it may have even started here.

    • Harry R Ray June 10, 2016, 9:31

      Even if it were a few million years old, as long as it remains ACTIVE EVERY DAY AT HIGH NOON, any dust that accumulates in the other 23 hours will itself be completely covered by NEW salt deposits. So, instead of being a salt dome, it could be a completely NEW kind of geological feature: A salt-dust dome! The BIG question would be, over MANY MILLIONS OF YEARS, could it grow to be as BIG as Ahuna Mons?

      • Michael June 13, 2016, 8:07

        ‘In your face…’, so far it is most likely magnesium sulphate.

        http://www.jpl.nasa.gov/blog/2015/12/in-your-face-exploration-at-ceres

        • Michael June 15, 2016, 7:55

          There are some bacteria that use sulphate in their metabolism and there are many organisms that excrete ammonia. Ceres has a strange hint of ammonia and plenty of sulphate.

          Ceres could have come in from the outer solar system having collected ammonia compounds or amassed ammonia compounds where it is now and then they got covered up to keep them stable or could ‘life’ be generating the ammonia.

          When Dawn runs out of fuel it may be a good idea to crash it into one of those mounds to see the debris ejected.

  • Sean Robert Meaney June 8, 2016, 21:57

    Who said they fade by dusk. I watched the footage it gives of light as it rotates out of view. That implies photon energy storage and release from crystaline structures.

  • Harry R Ray June 30, 2016, 10:36

    BREAKING NEWS: We FINALLY KNOW what makes Occator Crater’s bright spots! It is neither ice or Epsom salt, as previously postulated. It is(get THIS)WATER SOFTENER!!!!!!!(AKA Washing Soda, Sodium Carbonate,or Na2CO3). This compound is usually associated with hydrothermal vents on Earth, but let’s let THE IMMAGINATION RUN WILD FOR A SECOND! The alien base needs water softener for their washing machines to get off all of that dark gimey stuff on the surface of Ceres off of their clothes, so they are drilling into Ceres surface until they find a source of highly pressurized water, and the stuff just gushes up with the water, which evaporates, leaving the water softener on the surface. LOL(I HOPE)!