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LPSC 2015: First results from Dawn at Ceres: provisional place names and possible plumes

Posted By Emily Lakdawalla

19-03-2015 18:29 CDT

Topics: asteroids, Dawn, explaining science, asteroid 1 Ceres, conference report

Three talks on Tuesday at the Lunar and Planetary Science Conference concerned the first results from Dawn at Ceres. Dawn has only just entered orbit around the largest asteroid, and the spacecraft is currently on its night side. You've already seen in my blog many of the best images that we have so far. So it's early days yet for Ceres science, but what Dawn has seen so far is pretty exciting. As a reminder, here is a series of images that shows how Dawn's point of view on Ceres has changed throughout its approach to orbit insertion:

Approaching Ceres

NASA / JPL / UCLA / MPS / DLR / IDA / collage by Emily Lakdawalla

Approaching Ceres
Dawn took this series of images of Ceres before it was captured into its first orbit. Dawn approached Ceres from the direction of the Sun and passed to its night side before orbital capture. In order from left to right, the images were taken December 1, 2014 (camera calibration); January 13 and 25 and February 4 (optical navigation images 1, 2, and 3); February 12 and 19 (Rotation Characterizations 1 and 2); and February 25 and March 1 (optical navigation images 4 and 5). The next images will be taken in April.

And here's a look at the world rotating:

Rotating Ceres from Dawn, February 19, 2015

NASA / JPL / UCLA / MPS / DLR / IDA / Emily Lakdawalla

Rotating Ceres from Dawn, February 19, 2015
Dawn took 27 photos of Ceres during its Rotation Characterization 2 in order to make this animated view of the dwarf planet rotating. The publicly released version of this animation had been stretched to make Ceres' disk appear circular. Ceres is, in fact, quite oblate, so this version has had Ceres' shape corrected. At full size the animation has been enlarged to about 200% of its original resolution.

And here is  a newly released digital elevation model for Ceres, available from the Planetary Data System:

Topographic map of Ceres as of February 2015

NASA / JPL / UCLA / MPS / DLR / IDA / user "JohnVV"

Topographic map of Ceres as of February 2015
This map is a digital elevation model of Ceres made from Rotation Characterization 2 data gathered on February 19, 2015. Darker areas represent lower elevations, and brighter areas represent higher elevations. The map was originally posted here.

As you can see, Dawn has already observed the entire globe of Ceres, albeit at low resolution. Chris Russell opened the session with an overview of the mission and some early first impressions of Ceres. "This is very much unlike Vesta," he said. The impact craters are quite different. In general, Russell said in response to an audience question, early Dawn measurements of Ceres' shape and physical properties are results are "almost perfectly consistent" with the work done by Peter Thomas and coworkers on Ceres' shape as seen in Hubble images. The major news from Russell's presentation was the announcement of some names for features on Ceres. Back in October, the IAU adopted two naming themes for Ceres: craters will be named after agriculture deities, while other features were be named for world agricultural festivals. Russell showed this map that organizes Ceres' surface into quads, with each quad named after one harvest deity. As they did at Vesta, they will split the quads among the team as they begin to map Ceres, and these names will be applied to prominent craters within each quad. Thanks very much to Paul Schenk for sharing the map with me! Russell pointed out his own favorite quad name: Yumyum, located within Ceres' southern hemisphere.

Provisional quad names for Ceres
Provisional quad names for Ceres

I went ahead and applied these quad names to the digital elevation model. The crater with the bright feature in it is unfortunately right on the boundary between the Palo and Ebisu quads. The flat-floored huge crater I wrote about earlier lies mostly inside the quad called Kumba. The bright splash crater is in the quad named Hobnil.

Topographic map of Ceres, with quad names

NASA / JPL / UCLA / MPS / DLR / IDA / JohnVV / Emily Lakdawalla

Topographic map of Ceres, with quad names

The second talk of the session was most sensational: Andreas Nathues, presenting on early imaging results. At the beginning of his talk, he requested for bloggers not to blog it, but for reasoning I explained here, I'm writing about it anyway, as Alex Witze, Eric Hand, Irene Klotz, and others have already done. Nathues said that there was a variety of terrain on Ceres, ranging from smooth, to lineated, to rough. Many craters have flat floors; perhaps they are relaxed (meaning that the icy mantle has flowed over time to make the preexisting crater shallower, evening out the gravitational potential). He pointed to linear features throughout the surface, and says they already see some scarps. (Linear features and scarps are hallmarks of tectonics: geology driven by internal forces.) He called the large smooth crater the "pillowy-floored basin" and says it measures 270 kilometers in diameter. In general, the transition in crater shape from simple craters to complex ones with central peaks happens near a diameter of 25 kilometers.

Then he focused on the bright feature. It is located in the floor of a crater 80 kilometers in diameter. From its behavior as the globe rotates, he said, the bright feature appears to lie in a depression. The images that have been released to the public from the rotation animation do not show all of the photos of the bright feature, so the next point concerns images that I can't show you. "What is amazing," he said, "is that you can see the feature while the rim is still in front of the line of sight. Therefore we believe at the moment that this could be some kind of outgassing. But we need higher resolution data to confirm this." What he is saying is that as Ceres' globe rotates and the 80-kilometer crater's rim rotates into view, that rim should block our ability to see the bright feature on the floor of the crater. However, the bright feature is already visibly bright as the crater begins to rotate into view. Therefore, it must be vertically above the rim of the crater: it must be some kind of plume. "During the day," Nathues went on, "the feature evolves: it brightens. At dusk it gets fainter; at late dusk it disappears completely. We see this for cometary activity."

He moved to color data, showing a global map of Ceres as seen through different-colored filters. There was a striking asymmetry to the color: one hemisphere was much more red and the other much more blue. The images were taken from too great a distance to resolve the bright spot; it is smaller than 4 kilometers across. So they can say that its albedo is at least 0.4 (meaning that it reflects at least 40% of the light that strikes it), but it could be much higher. The color information over the spot is consistent with an icy surface, but this is not a unique interpretation. The feature has variable brightness with time: its brightness increases strongly as seen through the 550-nanometer filter around local noon.

Obviously, active outgassing on Ceres would be a big deal, if it really exists. Fortunately, Dawn will get much closer and will take much better images, which will hopefully confirm this discovery!

Approaching Ceres: one bright spot turns into two


Approaching Ceres: one bright spot turns into two
Dawn captured this image of Ceres during its Rotation Characterization 2, on February 19, 2015. This photo includes the enigmatic bright spot, which has now separated into two bright spots, both of them still smaller than the resolution of the camera (which is roughly 4 kilometers per pixel at a distance of 46,000 kilometers). This image has been enlarged by a factor of two from the original data.

The last talk on actual Dawn data was given by Francesca Zambon, on the imaging spectrometer data, showing global maps. The spatial resolution of her map is only 11 kilometers per pixel at the moment, which is relatively coarse, but still better than Hubble. As was evident in Nathues' maps, Zambon's VIR maps showed a red hemisphere and a blue hemisphere; the bright spot is located at 20 degrees north, 240 degrees east, near the middle of the blue hemisphere. She showed some early temperature maps of Ceres' surface, and a surprising result: the bright spot showed no obvious temperature contrast with the area around it. But a different bright spot, the splash crater located at 4 degrees north, 8 degrees east, is markedly colder than the area around it. (You would expect cooler temperatures for brighter surfaces, all else being equal.)

The rest of the talks in the Ceres session concerned Earth-based observations, geophysical modeling, and future Dawn work. One of the interesting talks from that part of the session was by Tim Titus, who tried to use modeling to figure out where the "snowline" on Ceres is -- that is, the latitude at which ice is stable at the surface. Ceres has nights and days, of course, so the surface temperature varies with time, but those variations die out as you go beneath the surface. Titus defined locations where the subsurface never gets above 145 kelvins to be the location of the snowline on Ceres. For a variety of possible surface properties, a smooth surface would have a snowline somewhere between 40 and 60 degrees. If that surface is roughened (by, say, impact craters), then the snowline shifts poleward. So near-surface ice is stable near the poles, and comet-like ice sublimation could happen there, especially with help from seasonal heating or a meteor impact. On the other hand, plume activity at lower latitudes -- like the 20-degree-north position of the bright spot -- is happening in a place where near-surface water ice is not stable. Activity there would need to originate in a source of water ice that gets recharged somehow, such as by cryovolcanism. Of course, as Andy Rivkin pointed out during the question period, a sufficiently big impact could expose much more deeply-buried ice; he asked if Titus had modeled how often impacts would be expected to expose such deep ice. Titus replied that this was beyond the scope of his work.

A related talk, by Norbert Schorghofer, concerned a prediction for the GRaND neutron spectrometer results, which will not be able to begin acquiring quality science data until Dawn is in its lowest orbit, much later this year. Schorghofer's models suggest that Dawn GRaND should detect  water ice within half a meter of the surface of Ceres, and he predicted that GRaND would observe this ice poleward of 60 degrees latitude. This assumes Ceres' current obliquity, which is very slight at only 3 degrees. As he spoke, I wondered if Ceres' spin axis has tilted more than this in the past, as Earth's and Mars' do. And later in the talk, he answered this question: even if Ceres' obliquity has varied, the prediction is still that there will be near-surface ice poleward of 60 degrees latitude. "This prediction can go wrong, but then it would mean something," he said. It could mean Ceres has lost a lot of near-surface volatiles due to impacts; or there has been true polar wander; or lots of dessication during an early period of radiogenic heating.

I enjoyed a later talk given by Thomas Davison on the shapes of Ceres' largest impact craters and how they may serve as a sensitive probe of the subsurface structure of Ceres. He modeled Ceres in three ways: with a dry olivine core; with a much wetter, serpentine core; and with a "mudball" core of mixed rock and water. Then he struck his model Ceres with asteroids of different sizes, arriving with speeds of 4 kilometers per second. In general, the mudball Ceres produced incredibly flat-floored craters, and the impact had little effect on the core-mantle boundary. The dry-core Ceres tended to produce the most surface topography, with prominent central peaks. The hydrated-core Ceres had more topography and prominent uplift of the core-mantle boundary underneath the crater. This kind of uplift would be obvious as a concentration of mass beneath the crater in Dawn's gravity data. Now, over geologic time, the original shapes of the craters may have been modified by relaxation, as ice flows from high places to low places to even out the surface; but the different appearance of the core-mantle boundary between the three cases should be visible in gravity maps.

All in all, there is a lot to look forward to on Dawn, and I can't wait for more pictures from closer orbits!

See other posts from March 2015


Read more blog entries about: asteroids, Dawn, explaining science, asteroid 1 Ceres, conference report


quayley: 03/20/2015 05:44 CDT

Fascinating blog, as usual, love the dawn mission, hope the Plumes are still going strong when dawn resumes operations. Wonder if the mechanism is similar to the enceladus Plumes, and Pluto yet to come!

Johnnymorales: 03/24/2015 09:56 CDT

I'm glad to find this site. Ever since the Dawn mission began, I have been surprised at how the team has been extremely stingy regarding what they've learned for no apparent reason beyond they are far more conservative than the typical NASA team in charge of other missions like Cassini or any Mars mission. The parallel striations which are seperate from the grooves that seem to surround the Ceres South Pole to me jumped out as looking like the oceanic ridges that mean tectonics on Earth. So I figured I'd hear at least an observation like that from the Dawn team, but nope. The striking continuous paralell lines that arise from the South Pole and go up almost in unison 1/3 up the globe likewise are a striking feature. There is no other such feature in the Solar System, and only the equatorial ridge on Ipatus is similarly so unusual in its coincidental symmetry, yet no comment yet. I hardly expect an explanation, but learned speculation is in order.

Johnnymorales: 03/24/2015 10:12 CDT

It's almost as if the Dawn team wants to kill the thrill of space exploration by ignoring or dramatically downplaying obvious potential in the Ceres photos which even at this distance tell a story far different than told by any other hard solar body. For example, directly below the 2 white spots just above and slightly to the left of the S. Pole the point where the scarps indicating tectonics begins you see a large low relief (betrayed by the shadow it casts oppositely from how a crater ridge casts its shadow) which is topped by a central plateau which could be a caldera like on one of the Elysium volcanoes of Mars. Yet nothing.

Johnnymorales: 03/24/2015 10:22 CDT

Finally, I don't get why they refuse to enhance the images beyond basic contrast leveling they do have with the advanced, state of the art graphics software NASA does have. Basic, fine grained sharpening (relative to the photo resolution) makes a huge difference in how apparent features are. It makes many hazy features crisp enough to justify putting forth plausible speculation. Other basic graphics enhancements can enhance contrast to further define the largest of the 2 white spots and further define down the size of the object. It makes me wonder if NASA doesn't have an individual dedicated to photo enhancement for its own sake, and instead only enhances photos per request and no more than requested. If they don't step up, even mundane findings for Pluto will eclipse their efforts Too bad.

DavidPalmer: 03/27/2015 10:53 CDT

It has been suggested that the highly anomalous bright spots on Ceres represent cryovolcanic or evaporative plumes, and one of the pieces of evidence presented for this model, has been the fact that they seem to project above the rim of the crater which hosts them. However, the plume model is highly implausible, for three main reasons: 1) A plume would spread out and be diffuse, and not be concentrated in one super-bright example would be the plumes of Enceladus, which are not even visible with the sun to the observer's back (equivalent to the orientation of Dawn when it was photographing Ceres), but rather the plumes of Enceladus are only visible when back-lit. Any plume intense enough to produce the surface brightness of the feature on Ceres, would be expected to spread out over a vast area, similar to what we see with the volcanic plumes of Jupiter's Io (which ARE visible when "fore-lit," appearing as large umbrella or parabola-shaped features rising above the limb) 2) Any plume activity vigorous enough to be visually conspicuous would result in ice crystals settling down (as "snow") on the surface, at least locally, or even globally (as is the case with Enceladus), resulting in a very high surface albedo in at least the crater hosting the bright spots. And yet there is nothing of the sort general, Ceres' surface is a relatively uniform grey, even directly adjacent to the bright spots. 3) We would expect a plume to be variable, whereas the bright spot (albeit completely unresolved) was seen by Hubble years ago.....which makes the case even more strongly, to the effect that the surrounding landscape should by now have a thick layer of snow and be highly reflective, if indeed there are active plumes. As an alternative to the plume model, I would like to propose the following hypothesis: that the bright spots represent cryovolcanic spring mounds which, due to the very low surface gravity of Ceres, have grown to enormous heights....the water flows out of a fissure but quickly freezes, and then more flows out on top of that, and more on top of that....till we end up with a gigantic stalagmite-shaped structure of highly reflective ice, which may be hundreds of meters high, even perhaps exceeding a kilometer. This formative mechanism would be rather similar to that of the black and white smokers on the ocean floor of Earth where, due to the buoyancy of the water, we see an environment that simulates a very low gravity regime, and in which vertical chimneys of precipitated minerals form (which would be unstable in a high-gravity surface environment). If the outflow is liquid (not high-speed ice particles as in the case of Enceladus), then we do not face any of difficulties presented by a plume.....all the water (very quickly turning to ice) would stay in the immediate region of the vent. And while it would freeze quickly, over time it would also sublime at a substantial rate, which likely accounts for the thin water vapor atmosphere detected by Herschel. But because of the low gravity and relatively high temperature (up to minus 35 Celsius), and the comparative lack of atmosphere, this water vapor is quickly lost to space, and so does not coat the surrounding surface, except perhaps the small amount that manages to reach the poles. Of course, still another possible explanation of the bright spots is that they are enormous artificially-constructed towers, probably made of highly reflective glass, but perhaps we shouldn't indulge too much in such speculations until we have higher-resolution photos on hand. David Palmer

zippy: 03/30/2015 01:41 CDT

David, I like your idea of a ice mounds, but this is a planetary body with some gravity so these mounds likely fall down over time, but likley don't fall far and pile up near the source. This would solve all the problems of understanding these spots. Creating a circular mound that has depth and many facets that renews itself over time. This makes the most sense to me. Zippy.

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