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Marc Rayman

Dawn Journal: Science on Ceres

Posted by Marc Rayman

01-01-2016 7:02 CST

Topics: pretty pictures, mission status, asteroids, Dawn, asteroid 1 Ceres

Dear Transcendawnts,

Dawn is now performing the final act of its remarkable celestial choreography, held close in Ceres’ firm gravitational embrace. The distant explorer is developing humankind’s most intimate portrait ever of a dwarf planet, and it likely will be a long, long time before the level of detail is surpassed.

The spacecraft is concluding an outstandingly successful year 1,500 times nearer to Ceres than it began. More importantly, it is more than 1.4 million times closer to Ceres than Earth is today. From its uniquely favorable vantage point, Dawn can relay to us spectacular views that would otherwise be unattainable. At an average altitude of only 240 miles (385 kilometers), the spacecraft is closer to Ceres than the International Space Station is to Earth. From that tight orbit, the dwarf planet looks the same size as a soccer ball seen from only 3.5 inches (9.0 centimeters) away. This is in-your-face exploration.

Exploring Ceres

NASA / JPL-Caltech / UCLA / MPS / DLR / IDA

Exploring Ceres
During the course of 2015, Ceres grows from a small and unremarkable disc to a complex and intriguing world in this selection of Dawn’s pictures. It starts with the first view on Jan. 13 at a distance of 238,000 miles (383,000 kilometers) and concludes on Dec. 10 only 240 miles (385 kilometers) away. The keen-eyed observer will notice several pictures with the unmistakable glow in Occator crater (discussed below), already evident here on Feb. 4 (and in other pictures taken even on Jan. 13), as well as the towering conical Ahuna Mons on June 6, Aug. 19 (Aug. 18 PDT) and Oct. 14.

The spacecraft has returned more than 16,000 pictures of Ceres this year (including more than 2,000 since descending to its low orbit this month). One of your correspondent’s favorites (below) was taken on Dec. 10 when Dawn was verifying the condition of its backup camera. Not only did the camera pass its tests, but it yielded a wonderful, dramatic view not far from the south pole. It is southern hemisphere winter on Ceres now, with the sun north of the equator. From the perspective of the photographed location, the sun is near the horizon, creating the long shadows that add depth and character to the scene. And usually in close-in orbits, we look nearly straight down. Unlike such overhead pictures typical of planetary spacecraft (including Dawn), this view is mostly forward and shows a richly detailed landscape ahead, one you can imagine being in — a real place, albeit an exotic one. This may be like the breathtaking panorama you could enjoy with your face pressed to the porthole of your spaceship as you are approaching your landing sight. You are right there. It looks — it feels! — so real and physical. You might actually plan a hike across some of the terrain. And it may be that a visiting explorer or even a colonist someday will have this same view before setting off on a trek through the Cerean countryside.

Limb of Ceres

NASA / JPL-Caltech / UCLA / MPS / DLR / IDA

Limb of Ceres
Dawn had this view of Ceres at 86 degrees south latitude on Dec. 10, only three days after completing its descent to an average orbital altitude of 240 miles (385 kilometers). Click on the image and allow yourself to be pulled into the scene (and you might meet this writer there). Full image and caption.

Of course, Dawn’s objectives include much more than taking incredibly neat pictures, a task at which it excels. It is designed to collect scientifically meaningful photos and other valuable measurements. We’ll see more below about what some of the images and spectra from higher altitudes have revealed about Ceres, but first let’s take a look at the three highest priority investigations Dawn is conducting now in its final orbit, sometimes known as the low altitude mapping orbit (LAMO). While the camera, visible mapping spectrometer and infrared mapping spectrometer show the surface, these other measurements probe beneath.

With the spacecraft this close to the ground, it can measure two kinds of nuclear radiation that come from as much as a yard (meter) deep. The radiation carries the signatures of the atoms there, allowing scientists to inventory some of the key chemical elements of geological interest. One component of this radiation is gamma ray photons, a high energy form of electromagnetic radiation with a frequency beyond visible light, beyond ultraviolet, even beyond X-rays. Neutrons in the radiation are entirely different from gamma rays. They are particles usually found in the nuclei of atoms (for those of you who happen to look there). Indeed, outweighing protons, and outnumbering them in most kinds of atoms, they constitute most of the mass of atoms other than hydrogen in Ceres (and everywhere else in the universe, including in your correspondent).

To tell us what members of the periodic table of the elements are present, Dawn’s gamma ray and neutron detector (GRaND) does more than detect those two kinds of radiation. Despite its name, GRaND is not at all pretentious, but its capabilities are quite impressive. Consisting of 21 sensors, the device measures the energy of each gamma ray photon and of each neutron. (That doesn’t lend itself to as engaging an acronym.) It is these gamma ray spectra and neutron spectra that reveal the identities of the atomic species in the ground.

Some of the gamma rays are produced by radioactive elements, but most of them and the neutrons are generated as byproducts of cosmic rays impinging on Ceres. Space is pervaded by cosmic radiation, composed of a variety of subatomic particles that originate outside our solar system. Earth’s atmosphere and magnetic field protect the surface (and those who dwell there) from cosmic rays, but Ceres lacks such defenses. The cosmic rays interact with nuclei of atoms, and some of the gamma rays and neutrons that are released escape back into space where they are intercepted by GRaND on the orbiting Dawn.

Unlike the relatively bright light reflected from Ceres’ surface that the camera, infrared spectrometer and visible spectrometer record, the radiation GRaND measures is very faint. Just as a picture of a dim object requires a longer exposure than for a bright subject, GRaND’s “pictures” of Ceres require very long exposures, lasting weeks, but mission planners have provided Dawn with the necessary time. Because the equivalent of the illumination for the gamma ray and neutron pictures is cosmic rays, not sunlight, regions in darkness are no fainter than those illuminated by the sun. GRaND works on both the day side and the night side of Ceres.

Ceres (animation)

NASA / JPL-Caltech / UCLA / MPS / DLR / IDA

Ceres (animation)
These animations of Ceres rotating and a flyover of Occator crater are from photos Dawn took in its second mapping orbit at an altitude of 2,700 miles (4,400 kilometers). The false colors are used to highlight very subtle differences in color that your eye generally would not discern but which reveal differences in the nature of the material on the ground. As explained below, the bright areas tend to be slightly blue. Full animation and caption.

In addition to the gamma ray spectra and neutron spectra, Dawn’s other top priority now is measuring Ceres’ gravity field. The results will help scientists infer the interior structure of the dwarf planet. The measurements made in the higher altitude orbits turned out to be even more accurate than the team had expected, but now that the probe is as close to Ceres as it will ever go, and so the gravitational pull is the strongest, they can obtain still better measurements.

Gravity is one of four fundamental forces in nature, and its extreme weakness is one of the fascinating mysteries of how the universe works. It feels strong to us (well, most of us) because we don’t so easily sense the two kinds of nuclear forces, both of which extend only over extremely short distances, and we generally don’t recognize the electromagnetic force. With both positive and negative electrical charges, attractive and repulsive electromagnetic forces often cancel. Not so with gravity. All matter exerts attractive gravity, and it can all add up. The reason gravity — by far the weakest of the four forces — is so salient for those of you on or near Earth is that there is such a vast amount of matter in the planet and it all pulls together to hold you down. Dawn overcame that pull with its powerful Delta rocket. Now the principal gravitational force acting on it is the cumulative effect of all the matter in Ceres, and that is what determines its orbital motion.

The spacecraft experiences a changing force both as the inhomogeneous dwarf planet beneath it rotates on its axis and as the craft circles that massive orb. When Dawn is closer to locations within Ceres with greater density (i.e., more matter), the ship feels a stronger tug, and when it is near regions with lower density, and hence less powerful gravity, the attraction is weaker. The spacecraft accelerates and decelerates very slightly as its orbit carries it closer to and farther from the volumes of different density. By carefully and systematically plotting the exquisitely small variations in the probe’s motion, navigators can calculate how the mass is distributed inside Ceres, essentially creating an interior map. This technique allowed scientists to establish that Vesta, the protoplanet Dawn explored in 2011-2012, has a dense core (composed principally of iron and nickel) surrounded by a less dense mantle and crust. (That is one of the reasons scientists now consider Vesta to be more closely related to Earth and the other terrestrial planets than to typical asteroids.)

Mapping the orbit requires systems both on Dawn and on Earth. Using the large and exquisitely sensitive antennas of NASA’s Deep Space Network (DSN), navigators measure tiny changes in the frequency, or pitch, of the spacecraft’s radio signal, and that reveals changes in the craft’s velocity. This technique relies on the Doppler effect, which is familiar to most terrestrial readers as they hear the pitch of a siren rise as it approaches and fall as it recedes. Other readers who more commonly travel at speeds closer to that of light recognize that the well-known blueshift and redshift are manifestations of the same principle, applied to light waves rather than sound waves. Even as Dawn orbits Ceres at 610 mph (980 kilometers per hour), engineers can detect changes in its speed of only one foot (0.3 meters) per hour, or one five-thousandth of a mph (one three-thousandth of a kilometer per hour). Another way to track the spacecraft is to measure the distance very accurately as it revolves around Ceres. The DSN times a radio signal that goes from Earth to Dawn and back. As you are reminded at the end of every Dawn Journal, those signals travel at the universal limit of the speed of light, which is known with exceptional accuracy. Combining the speed of light with the time allows the distance to be pinpointed. These measurements with Dawn’s radio, along with other data, enable scientists to peer deep into the dwarf planet.

Although it is not among the highest scientific priorities, the flight team is every bit as interested in the photography as you are. We are visual creatures, so photographs have a special appeal. They transport us to mysterious, faraway worlds more effectively than any propulsion system. Even as Dawn is bringing the alien surface into sharper focus now, the pictures taken in higher orbits have allowed scientists to gain new insights into this ancient world. Geologists have located more than 130 bright regions, none being more striking than the mesmerizing luster in Occator crater. The pictures taken in visible and infrared wavelengths have helped them determine that the highly reflective material is a type of salt.

Bright areas on Ceres

NASA / JPL-Caltech / UCLA / MPS / DLR / IDA

Bright areas on Ceres
This map of Ceres shows the locations of about 130 bright areas (indicated in blue). Most of them are associated with craters, likely because the reflective material was excavated when the craters were formed. The insets at the top show the two brightest regions, Occator crater on the left and Oxo crater on the right. Full image and caption.

It is very difficult to pin down the specific composition with the measurements that have been analyzed so far. Scientists compare how reflective the scene is at different wavelengths with the reflective properties of likely candidate materials studied in laboratories. So far, magnesium sulfate yields the best match (although it is not definitive). That isn’t the kind of salt you normally put on your food (or if it is, I’ll be wary about accepting the kind invitation to dine in your home), but it is very similar (albeit not identical) to Epsom salts, which have many other familiar uses.

Scientists’ best explanation now for the deposits of salt is that when asteroids crash into Ceres, they excavate underground briny water-ice. Once on the surface and exposed to the vacuum of space, even in the freezing cold so far from the sun, the ice sublimes, the water molecules going directly from the solid ice to gas without an intermediate liquid stage. Left behind are the materials that had been dissolved in the water. The size and brightness of the different regions depend in part on how long ago the impact occurred. A very preliminary estimate is that Occator was formed by a powerful collision around 80 million years ago, which is relatively recent in geological times. (We will see in a future Dawn Journal how scientists estimate the age and why the pictures in this low altitude mapping orbit will help refine the value.)

As soon as Dawn’s pictures of Ceres arrived early this year, many people referred to the bright regions as “white spots,” although as we opined then, such a description was premature. The black and white pictures revealed nothing about the color, only the brightness. Now we know that most have a very slight blue tint. For reasons not yet clear, the central bright area of Occator is tinged with more red. Nevertheless, the coloration is subtle, and our eyes would register white.

Haulani crater, Ceres

NASA / JPL-Caltech / UCLA / MPS / DLR / IDA

Haulani crater, Ceres
Dawn captured this picture of Haulani crater in cycle 6 of its third mapping orbit at 915 miles (1,470 kilometers). (Haulani is one of the Hawaiian plant goddesses). The crater is 21 miles (34 kilometers) in diameter. Its well-defined shape indicates it is relatively young, the impact that formed it having occurred in recent geological times. It displays a substantial amount of bright material, which the latest analyses indicate is a kind of salt, as explained above. The same crater as viewed by Dawn from three times higher altitude is here. Dawn’s next view should be four times as sharp as this photo. Full image and caption.

Measurements with both finer wavelength discrimination and broader wavelength coverage in the infrared have revealed still more about the nature of Ceres. Scientists using data from one of the two spectrometers in the visible and infrared mapping spectrometer instrument (VIR) have found that a class of minerals known as phyllosilicates is common on Ceres. As with the magnesium sulfate, the identification is made by comparing Dawn’s detailed spectral measurements with laboratory spectra of a great many different kinds of minerals. This technique is a mainstay of astronomy (with both spacecraft and telescopic observations) and has a solid foundation of research that dates to the nineteenth century, but given the tremendous variety of minerals that occur in nature, the results generally are neither absolutely conclusive nor extremely specific.

There are dozens of phyllosilicates on Earth (one well known group is mica). Ceres too likely contains a mixture of at least several. Other compounds are evident as well, but what is most striking is the signature of ammonia in the minerals. This chemical is manufactured extensively on Earth, but few industries have invested in production plants so far from their home offices. (Any corporations considering establishing Cerean chemical plants are invited to contact the Dawn project. Perhaps, however, mining would be a more appropriate first step in a long-term business plan.)

Ammonia’s presence on Ceres is important. This simple molecule would have been common in the material swirling around the young sun almost 4.6 billion years ago when planets were forming. (Last year we discussed this period at the dawn of the solar system.) But at Ceres’ present distance from the sun, it would have been too warm for ammonia to be caught up in the planet-forming process, just as it was even closer to the sun where Earth resides. There are at least two possible explanations for how Ceres acquired its large inventory of ammonia. One is that it formed much farther from the sun, perhaps even beyond Neptune, where conditions were cool enough for ammonia to condense. In that case, it could easily have incorporated ammonia. Subsequent gravitational jostling among the new residents of the solar system could have propelled Ceres into its present orbit between Mars and Jupiter. Another possibility is that Ceres formed closer to where it is now but that debris containing ammonia from the outer solar system drifted inward and some of it ultimately fell onto the dwarf planet. If enough made its way to Ceres, the ground would be covered with the chemical, just as VIR observed.

Gaue crater, Ceres

NASA / JPL-Caltech / UCLA / MPS / DLR / IDA

Gaue crater, Ceres
Dawn observed Gaue crater in cycle 5 of its third mapping orbit. (Gaue is a goddess who was the intended recipient of rye offerings in Lower Saxony.) The crater is 50 miles (80 kilometers) across and appears to have a relatively fresh rim and a smooth floor. What may once have been a central peak, common in large craters, apparently collapsed, leaving the central pit evident here. Impact ejecta from Gaue has coated the surrounding terrain, muting the appearance of older features. Full image and caption.

Scientists continue to analyze the thousands of photos and millions of infrared and visible spectra even as Dawn is now collecting more precious data. Next month, we will summarize the intricate plan that apportions time among pointing the spacecraft’s sensors at Ceres to perform measurements, its main antenna at Earth to transmit its findings and receive new instructions, and its ion engine in the direction needed to adjust its orbit.

The plans described last month for getting started in this fourth and final mapping orbit worked out extremely well. You can follow Dawn’s activities with the status reports posted at least twice a week here. And you can see new pictures regularly in the Ceres image gallery.

We will be treated to many more marvelous sights on Ceres now that Dawn’s pictures will display four times the detail of the views from its third mapping orbit. The mapping orbits are summarized in the following table, updated from what we have presented before. (This fourth orbit is listed here as beginning on Dec. 16. In fact, the highest priority work, which is obtaining the gamma ray spectra, neutron spectra and gravity measurements, began on Dec. 7, as explained last month. But Dec. 16 is when the spacecraft started its bonus campaign of measuring infrared spectra and taking pictures. Recognizing that what most readers care about is the photography, regardless of the scientific priorities, that is the date we use here.)

Mapping
Orbit
Dawn code
name
DatesAltitude
in miles
(kilometers)
Resolution in
feet (meters)
per pixel
Resolution compared to HubbleOrbit
period
Equivalent
distance of
a soccer ball
1 RC3 April 23 –
May 9
8,400
(13,600)
4,200
(1,300)
24 15
days
10 feet
(3.2 meters)
Survey June 6-30 2,700
(4,400)
1,400
(410)
73 3.1
days
3.4 feet
(1.0 meters)
HAMO Aug 17 –
Oct 23
915
(1,470)
450
(140)
217 19
hours
14 inches
(34 cm)
LAMO Dec 16 –
end of mission
240
(385)
120
(35)
830 5.4
hours
3.5 inches
(9.0 cm)

Dawn is now well-positioned to make many more discoveries on the first dwarf planet discovered. Jan. 1 will be the 215th anniversary of Giuseppe Piazzi’s first glimpse of that dot of light from his observatory in Sicily. Even to that experienced astronomer, Ceres looked like nothing other than a star, except that it moved a little bit from night to night like a planet, whereas the stars were stationary. (For more than a generation after, it was called a planet.) He could not imagine that more than two centuries later, humankind would dispatch a machine on a cosmic journey of more than seven years and three billion miles (five billion kilometers) to reach the distant, uncharted world he descried. Dawn can resolve details more than 60 thousand times finer than Piazzi’s telescope would allow. Our knowledge, our capabilities, our reach and even our ambition all are far beyond what he could have conceived, and yet we can apply them to his discovery to learn more, not only about Ceres itself but also about the dawn of the solar system.

On a personal note, I first saw Ceres through a telescope even smaller than Piazzi’s when I was 12 years old. As a much less experienced observer of the stars than he was, and with the benefit of nearly two centuries of astronomical studies between us, I was thrilled! I knew that what I was seeing was the behemoth of the main asteroid belt. But it never occurred to me when I was only a starry-eyed youth that I would be lucky enough to follow up on Piazzi’s discovery as a starry-eyed adult, responsible for humankind’s first visitor to that fascinating alien world, answering a celestial invitation that was more than 200 years old.

Dawn is 240 miles (385 kilometers) from Ceres. It is also 3.66 AU (340 million miles, or 547 million kilometers) from Earth, or 1,360 times as far as the moon and 3.72 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take one hour and one minute to make the round trip.

Dr. Marc D. Rayman
4:00 p.m. PST December 31, 2015

 
See other posts from January 2016

 

Or read more blog entries about: pretty pictures, mission status, asteroids, Dawn, asteroid 1 Ceres

Comments:

Atom: 01/01/2016 01:49 CST

Thanks Dr. Marc, I especially liked the "Limb of Ceres" image. It is so reminiscent of the Lunar Orbiter 2 image of the lunar crater Copernicus taken almost 50 years ago in 1966. Since both images are full of wonderment, it will be interesting to see what missions may follow Dawn. May you and Dawn have a very productive 2016!

MichaelJMcFadden: 01/01/2016 04:19 CST

Dr. Rayman, thank you for a WONDERFUL summary of the mission so far! You covered just about everything a curious lay science/astronomy/space geek could want! One final piece of info would be appreciated: you noted how we love pictures... so can you fill us in on when we might see more? When I saw Dec. 16th's I figured there'd be dozens before the end of the year, Purty pics may not have the sci-value of some other stuff, but we DO love them! :) - MJM

Marc Rayman: 01/01/2016 05:14 CST

Atom: I had exactly the same thought as you! I seriously considered mentioning that famous view of Copernicus but for several reasons decided not to. It's neat, however, that I'm not the only one who was reminded of it. Michael: I very much understand your eagerness for the images. And although they are not our *top* priority, the pictures certainly do have significant scientific value, some of which I will describe in a Dawn Journal in the next month or two. At any rate, the need for accuracy and scientific review of the data sometimes slows the release of some products, but all of the data are released to the public after the science team has performed the necessary analysis and interpretation for scientific publication. I don't actually know specifically when we will begin releasing more images from this low orbit, but I am confident the flow will begin quite soon and it will be worth the wait. Thanks to both of you for your nice comments!

Torbjörn Larsson: 01/02/2016 08:51 CST

Water ice, water ice, everywhere. And concomitant phyllosilicates! This is easily the most interesting results since the Vesta campaign highlights. It expands the set of bodies where water and those clays are correlated, and while the latter is important for some theories of life emergence water surely is for all of them! Since I am interested in astrobiology I couldn't help but note the similarly helpful ammonia finds as well. It will be interesting to see if the source can be narrowed down. Since Ceres naturally turns up in pebble planet formation scenarios (as I remember it), and we suspect that volatiles have been transported inwards to resupply our crust, my naive layman guess would be that it happened to Ceres too - born in place, nurtured by the outer system.

Galileo 7: 01/02/2016 09:08 CST

Thanks for another excellent update, Marc! I find the entire process so very fascinating - it's a scientific detective story, collecting data, correlating to known materials, generating and updating theories, and so on. Who says science isn't exciting! That low-angle image is simply fantastic! Wish we had more images like this from all of the different missions to other worlds. It offers a completely different perspective from the typical overhead-looking-down views, and really does make the viewer feel as if they're in a ship in orbit. Amazing. I look forward to the next update. Dawn rocks!

dougforworldsexplr: 01/02/2016 08:19 CST

Thank you for another informative article, Marc. I appreciate that you mention again the importance of the data from the different radiation detectors although I also like the pictures including the new one on an angle. I also remember the Apollo missions and some pictures of the Moon on a similar angle but hope that the next president will be more open to including our Moon as a key immediate destination for the Space Launch System instead of a rock from an asteroid. Anyway my main reason for this new entry is to ask you why one of my books and some internet articles being quite certain that some Earth meteorites were from Vesta even before Dawn or any other space craft got near there and it is still not certain if any meteorites on Earth come from Ceres? Also has much progress been made yet to determine the nature of the surface material on Ceres including what organic materials are there to better determine if any Earth meteorites are from Ceres or not and if so what ones? Also on what basis has it been determined that the bright spot in Occator crater a salt probably magnesium sulfate, has it been at all from radiation detectors or only because of the brightness or other characteristics of the photographs?

Marc Rayman : 01/02/2016 08:35 CST

Torbjörn: Yes, Ceres certainly is interesting from an astrobiological point of view. I mentioned the possibility of prebiotic chemistry in my December 2014 Dawn Journal (search for "Pardawn"). Galileo 7: I won't go into the details here, but in my next Dawn Journal I will describe how we are going to take more "low-angle" (or, as well call them, off-nadir) images in this mapping orbit. Now I should mention that we took quite a few pictures of the limb at higher altitudes (you can see four examples in my June Dawn Journal — search for "Evidawnce-Based"), and we looked several degrees off-nadir for much of our third mapping orbit in order to get stereo images for topography, but the larger angle and lower altitude together make the pictures so much cooler. In fact, the earlier ones were taken in order to accomplish specific scientific and engineering objectives. The reason we are taking them in this final orbit is that they are an affordable bonus and we just may get some terrific views! We'll see. I'm glad you're following along.

Marc Rayman: 01/02/2016 08:51 CST

Doug: The initial connection between the meteorites and Vesta was made in 1970, and Dawn's data made it very, very much stronger. I referred to it in my Sept. 5, 2012 Dawn Journal (search for "Marvestalous") but I didn't explain it. It's a long and fascinating story, but a major part of it is the application of the same principle of infrared spectroscopy that I described above. There are other aspects as well, including the recognition of efficient astrodynamical mechanisms for transporting rocks ejected from Vesta to Earth. I may write more about this some time. No meteorites from Ceres have been identified. Indeed, the moon, Mars, and Vesta are the only specific identified sources of meteorites, and we have far, far more from Vesta than from the moon or Mars. (Even accounting for Apollo samples, we have much more material from Vesta than we do from the moon, although the sampling from Vesta was not quite as careful or as controlled as the lunar samples.) The determination of the Occator composition was based on the color photographs at visible and near-infrared wavelengths. Analyses of the full infrared spectra are ongoing. As I've written in comments here in previous months, I do not expect GRaND to provide direct information on the bright regions. Its vision is not sharp enough to distinguish such a small area from surrounding material.

Anonymous: 01/04/2016 12:44 CST

Marc, Thank you for another chapter in the Dawn Journal--it has been quite a ride along with you on the space craft, I would enjoy a stroll with you through the Cerean landscape in the "rim shot" photo. I was really drawn to the lighter patches on the rim of the crater in the centre foreground of the image. Do you care to suggest the nature of what they might be? Perhaps the salts or solid ammonia (Azane)?

Chris Landau: 01/04/2016 05:08 CST

Thanks Marc Great informative blog. I liked the 12 year old kid history bit, morphing to the engineer who now overseas it all. I think that what is being left out in all these photos of Ceres is that Ceres is like Vesicular lava, full of burst bubble holes that you would get in swiss cheese, bubbling boiling mud flows, scoria, coking coal(particular apt as an analogy if Ceres is mostly a Carbonaceous Condrite body)and slag from a blast furnace. Many of the perfectly shaped ,'burst bubble' craters on this dwarf planet and many on our other solar system moons should be reevaluated in this context. I think impact craters to explain everything lacks scientific imagination. It is easy to explain a collapsing central crater peak if one imagines a warm bubbling Ceres, that is warm right now, not in some ancient geologic past. Also there are so many obvious ' bubbling flow structures' that it seems too obvious to ignore. Your thoughts? Chris Landau(geologist) January 4, 2016

Marc Rayman: 01/04/2016 09:42 CST

Anonymous: I'm ready for that stroll and will be more than happy to take it with (almost) anyone who would like to join the two of us! I too really like the bright patches on the the foreground crater. At this point, I think the most likely explanation for the composition is salt, as appears to be the case for the bright material in Occator. But we'll see. After all, there is much more data still to be analyzed, and we will continue to collect even more. (Or when we take our stroll, we'll analyze the material in situ.) Chris: I very rarely mention anything so explicitly about myself in my Dawn Journals, so I'm glad you liked that. Thank you. I don't see the same things you do in the pictures, but I'm not the best judge. For such an interesting and different interpretation of Ceres and indeed an overarching reassessment of many solar system bodies, I think a case much stronger than can possibly be squeezed into a blog comment (no matter how insightful) needs to be made. The current geological descriptions of solar system bodies are based on a huge volume of work that includes extensive quantitative analyses of observational data from telescopes and spacecraft, sophisticated mathematical models, laboratory studies, and more.

Chris: 01/05/2016 04:59 CST

I appreciate the time you take to write these every month, and the extra time to answer all these questions. My one quick question: how are the reaction wheels doing so far?

Marc Rayman: 01/05/2016 07:42 CST

Thank you, Chris. The reaction wheels are operating fine and the rate of hydrazine expenditure is very good. As long as both wheels remain healthy, they will continue to help conserve hydrazine. As I mentioned in my November Dawn Journal (and others), if either one fails, we will continue exclusively with hydrazine control.

QubitsToy: 01/06/2016 07:34 CST

Why are you keeping 2,000 photos to yourselves since descending to its low orbit? The @NASA_Dawn twitter kept us all interested in this new low altitude vantage point, then,... you send out a few far away angle shots of the limb? It is a question worth answering.

Marc Rayman: 01/06/2016 09:16 CST

QubitsToy: I answered this question in my comment on Jan. 1 (above) as well as in comments in previous months with regard to pictures from the higher altitude orbits. The answer is unchanged. I understand your eagerness, and your patience will be rewarded soon.

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