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Catching Up with Dawn at Ceres

The Dawn spacecraft has reached its final orbital altitude, closing to within 385 kilometers of the asteroid (and yes, I really should start calling Ceres a ‘dwarf planet’ consistently — working on it). We have no observations from this distance yet, but that process begins within days, and should give us images with a resolution of 35 meters per pixel, along with a wealth of data from the craft’s scientific package.

Like New Horizons, Dawn makes history every time it returns observations of places we haven’t seen before, or surface features we’re seeing at higher resolution as the orbit lowers. Unlike New Horizons, Dawn is an orbiter, which makes me long for the idea of a Pluto orbiter, even though New Horizons has amply demonstrated how useful and powerful a flyby mission can be. An orbiter lets you complete the mapping process so essential to making a new world tangible, while there are parts of Pluto that our flyby couldn’t make out at highest resolution.

I found the temperatures Dawn recorded at Ceres a bit startling. The 180 K (-93 °C) on the low side seems about right, but I hadn’t expected equatorial temperatures to reach as high as 240 K (-33 °C), which is a temperature I can recall experiencing several times in Iowa during an unusually tough winter. This JPL news release notes that temperatures at and near Ceres’ equator are too high to support surface ice for long periods, as explained in new work published in Nature to which we now turn.

Maria Cristina De Sanctis (National Institute of Astrophysics, Rome) and colleagues tell us in one of the two new papers that Dawn has turned up evidence for clays that are rich in ammonia on Ceres, using data gathered by the spacecraft’s visible and infrared mapping spectrometer. I hearken back to those temperatures because Ceres is too warm to support surface ammonia ice, but ammoniated compounds (ammonia molecules combining with other minerals) could be stable. Finding these tells us that Ceres may not have formed in the main asteroid belt.

“The presence of ammonia-bearing species suggests that Ceres is composed of material accreted in an environment where ammonia and nitrogen were abundant,” says De Sanctis. “Consequently, we think that this material originated in the outer cold solar system.”

The other possibility: The dwarf planet formed about where it is today but drew in materials from the outer system that had formed near the orbit of Neptune. Another interesting finding: Although carbonaceous chondrites (meteorites rich in carbon) are thought to be similar to Ceres in composition, the data do not match at all wavelengths. Ceres shows absorption bands in its reflected light that match up with the ammoniated minerals described above. Moreover, Ceres shows water content as high as 30 percent, while carbonaceous chondrites normally weigh in at 15 to 20 percent bulk water content, possibly indicating accretion from volatile-rich material.

Image: Dwarf planet Ceres is shown in these false-color renderings, which highlight differences in surface materials. Images from NASA’s Dawn spacecraft were used to create a movie of Ceres rotating, followed by a flyover view of Occator Crater, home of Ceres’ brightest area. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.

Resolving the Bright Spots

While we began seeing a small number of unusual bright spots on Ceres as Dawn approached, it turns out that close study reveals more than 130 areas of unusual brightness, most of them associated with impact craters. The second paper in Nature is the work of Andreas Nathues (Max Planck Institute for Solar System Research, Göttingen) and colleagues. Here we learn that the bright material is consistent with hexahydrite, which is a type of magnesium sulfate. What the paper argues is that the bright spots are areas rich in salt that were left behind when water ice sublimated long ago, having been brought to the surface by an impact.

PIA20180_ip

Image: This representation of Ceres’ Occator Crater in false colors shows differences in the surface composition. Red corresponds to a wavelength range around 0.97 micrometers (near infrared), green to a wavelength range around 0.75 micrometers (red, visible light) and blue to a wavelength range of around 0.44 micrometers (blue, visible light). Occator measures about 90 kilometers wide. Scientists use false color to examine differences in surface materials. The color blue on Ceres is generally associated with bright material, found in more than 130 locations, and seems to be consistent with salts, such as sulfates. It is likely that silicate materials are also present. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.

That, of course, backs up the idea that we’re dealing with a subsurface layer of salty water ice, an idea supported by the global nature of the bright spots. The Occator crater contains the brightest material found on Ceres, and it also appears to be one of the youngest surface features, with an age of about 78 million years. Remarkably, what appears to be a diffuse haze can be seen filling the floor of the crater at certain times of day.

The haze appears to be absent at dawn and dusk, while it can be seen at local noon, making Ceres, in the view of the study authors, something like a comet, where water vapor when warmed can lift particles of dust and residual ice off the surface. The Herschel space observatory reported water vapor at Ceres in 2014, a finding consistent with these observations. Remember, however, that we’re still waiting on the unambiguous detection of water ice on Ceres, so the story of the Occator haze will require more data and further analysis.

The papers are Nathues et al., “Sublimation in bright spots on (1) Ceres,” Nature 528 (10 December 2015), 237-240 (abstract) and De Sanctis et al., “Ammoniated phyllosilicates with a likely outer Solar System origin on (1) Ceres,” Nature 528 (10 December 2015), 241-244 (abstract).

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Comments on this entry are closed.

  • Alex Tolley December 11, 2015, 13:25

    I still hoping that Dawn can tell us whether there is a subsurface “ocean” or not. This is such a fascinating little world, and as expected, much richer in features than my expectations pre-encounter.

  • djlactin December 11, 2015, 13:36

    Carbon, Hydrogen, Oxygen, Sulfur. Now we need only Phosphorus to enable long-term occupation. (Oh, okay trace minerals, but by definition…)

  • Michael December 12, 2015, 5:48

    I don’t think that the bright features are that old as the asteroid belt is quite dusty and would have started to coat the reflective material before long. A large amount of ammonia also means a lower melting point and the greater chance of there being a liquid layer on Ceres. I am quite happy that there is ammonia as we now have nitrogen in abundance, terraformers get your shovels we are moving in. Now as for where Ceres formed I would think in the belt where it is now is just fine as the dust acts as a protective layer reducing ammonia and heat losses.

  • andy December 12, 2015, 6:19

    Now we need only Phosphorus to enable long-term occupation.

    And some way to increase the gravity, to avoid your long-term occupants getting health problems…

  • Mark Zambelli December 12, 2015, 7:54

    Go Dawn.
    Hexahydrite sounds perfect as an explanation for all the reasons outlined above, not just as an alternative to forcing water-ice to refrain from sublimating (especially given the ‘balmy’ equatorial temps).

    I’ll echo Alex’s comments re. a global/partial ocean… wonder when Dawn will be able to show one way or the other?

  • Michael December 12, 2015, 10:35

    @andy December 12, 2015 at 6:19

    ‘And some way to increase the gravity, to avoid your long-term occupants getting health problems…’

    Large rotating torus’s can be built on the surface to simulate 1G gravity. There is plenty said on CD about Ceres, here is one discussion thread.

    https://www.centauri-dreams.org/?p=33990

  • "ammoniated" December 12, 2015, 12:41

    Well nowadays one can right everything he wants if it is well connected.
    Just judging for how some of the work connected to ammonia was done it will actually exclude all together the presence of ammonia minerals. They ovelooked some important spectroscopic surveys like the one of M. Dennis Krohn* and Stephen P. Altaner (GEOPHYSICS, VOL. 52, NO.7 (JULY 1987); P. 924-930, 2 FIGS., 1 TABLE). These authors show clearly how “ammoniated” clays look like. What we see here are spectra that lack all the diagnostic features of NH4 clays or sulfates or whatever other NH4 ranting about Ceres look like.
    That is a disgrace!

  • "Yes, ammoniated" December 13, 2015, 0:21

    @”ammoniated” – or, before shooting one’s mouth off, you could look in a dictionary and see that it is actually a legitimate word.

    http://www.merriam-webster.com/dictionary/ammoniate

    http://www.oxforddictionaries.com/definition/english/ammoniated

  • Michael Spencer December 13, 2015, 8:11

    I wonder, Paul, while looking at those short movies, if the background star field is added in post production or if they are actually present during the imaging?

    I’m recalling all of the moon-hoax nonsense in which deniers pointed out the lack of star field in photos taken by astronauts on the moon surface. Perhaps today’s technology is so different, or more advanced, giving the imagers much wider latitude.

  • Harry R Ray December 13, 2015, 14:31

    Should the haze seen in the images be comfirmed by other means, could micro-organisms be a part of it. and then drop to the surface when the ice sublimates? If so, could they be buried under the salt QUICKLY ENOUGH so as to be protected from decomposition by radiation? If so, a sample and return mission would not even need a rover. Only a DRILL would be necessary. As a result of this, it would be a lot cheaper than ANY planned Mars soil sample and return mission currently being planned!It may even be POSSIBLE for a PRIVATE COMPANY to pull it off without the aid of NASA. Oh; by the way! The pilot for “The Expanse” airs tomorrow night at 10PM EST on the SY FI channel! I can’t wait! The trailer had an excellent Dawn immage of Ceres as a backdrop!

  • Denver December 13, 2015, 19:51

    @Andy

    Now we need only Phosphorus to enable long-term occupation.

    And some way to increase the gravity, to avoid your long-term occupants getting health problems…

    Crispr Andy, Crispr.

  • Harry R Ray December 14, 2015, 11:36

    Oh, by the way, not much mention recently of Ceres’ TWO(and ONLY two) really big mountains, Ahuna(the “pyramid”), and Ysolo, the one with the largest base(but not as tall as Ahuna) near Ceres’ north pole. The reason I Brought yhis up HERE, is that next to the bright spots in the craters, the flanks of Ahuna Mons appear to be the brightest things on Ceres. I wonder if Hexahydrite may be leached down its slopes in a process similar to the recurring slope linea on Mars, but occurring DAILY at noon on Ceres, instead of seasonally on Mars. There does not appear to be a really good image yet of Ysolo Mons. Hopefully that will change soon.

  • RobFlores December 14, 2015, 12:05

    Ceres seems to be in a free fire zone in the asteroid belt.
    Seems to me any colony would have to have the capability to
    ‘JUMP’ temporarily to a high altitude, to avoid the surface shockwaves,
    quakes, and falling debris, should not be hard its gravity is very feeble.

    What determines the thickness of the crust ice on these Ice Worlds,
    Mass, Core Size/composition. Ceres is the closest spherical Ice body of significant size to the sun. Does proximity to the sun influence the size of the crust thickness?

  • Michael December 14, 2015, 13:50

    If you look at this water-ammonia mixture graph you can see how ammonia can have a huge effect on the melting point of the mixture, -100 C in some points, add other salts and the melting point can drop further .

    https://upload.wikimedia.org/wikipedia/commons/thumb/8/88/Ammonia_aqueous.svg/660px-Ammonia_aqueous.svg.png