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Closeup of part of Cerealia Facula on Ceres

Dawn is going out on a high! Or maybe a low. Rapidly nearing the end of a unique decade-long interplanetary expedition, Dawn is taking phenomenal pictures of dwarf planet Ceres as it swoops closer to the ground than ever before. While the pictures are too new for compelling scientific conclusions to be reached, a clear consensus has already emerged: Wow!!!

Every 27 hours, the bold adventurer plunges from 2,500 miles (4,000 kilometers) down to just 22 miles (35 kilometers) above the alien world, accelerating to 1,050 mph (1,690 kph), and then shoots back up to do it all over again. (Try that, bungee jumpers!)

When Dawn dives low, it takes spectacular pictures, and you can see some of them here and more in the image gallery. But that's not all it does. The spacecraft also collects a trove of data on the nuclear radiation emanating from Ceres (which can reveal some of the atomic elements that are present), the gravity field (which can reveal the distribution of mass underground) and the infrared and visible light (which can reveal the minerals on the ground). Dawn has made all these kinds of measurements before, not only during more than three years at Ceres, the largest object in the main asteroid belt, but also during its 2011-2012 studies of Vesta, the second largest. But prior to this month, Dawn had never been this close and so never had such breathtaking sights and never been able to gather such high-resolution information.

We described the nature of this orbit in the three previous Dawn Journals. It is known as extended mission orbit 7 (XMO7) because Dawn's computer program for generating really cool and dramatic names was offline when it was time to come up with the name. Ever resourceful, the team activated the backup software that generates accurate but uninspiring names.

That kind of resourcefulness has served Dawn very well. Despite critical hardware failures that could have been disastrous for the mission, the flight team has accomplished success after success. The difficulty of flying so low -- only three times your altitude when you travel in a commercial jet -- and actually collecting useful data there seemed unachievable as recently as late last year. And now Dawn is doing it regularly.

Dawn had this exquisitely close-up view of a section of the north wall of Occator Crater from an altitude of only 21 miles (33 kilometers) on June 16. This area is a little east of where the crater to the north intersects Occator Crater, near the 1:00 position. (See this view, for example.) Notice the many rocks that slid part of the way down the wall, leaving a trail behind, and then were stopped by friction. The view here is about two miles (three kilometers) across. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Before XMO7, the spacecraft's lowest orbit around Ceres was 240 miles (385 kilometers), about the same height as the International Space Station is above Earth. Dawn spent eight months in 2015-2016 at that altitude, providing an exquisite view of the dwarf planet. It subsequently flew higher to pursue other scientific objectives.

Now Dawn is observing Ceres from as low as about 22 miles (35 kilometers). That tremendous reduction in altitude, a factor of 11, is the largest of the entire mission. At no other time at Vesta or Ceres did Dawn move in that much closer from its previous best vantage point. For those of you who enjoy the numbers, the table here has the distances for each of Dawn's observations of Ceres before the comprehensive mapping began, and this table shows the altitudes of the four mapping orbits of the prime mission, the last being the lowest. In those tables, we compared Dawn's view of Ceres to a view of a soccer ball. The low point of XMO7 would be like looking at a soccer ball from only one-third of an inch (eight millimeters) away. This is truly in-your-face exploration.

And the jump in resolution is amazing. With the fantastic new details, it seems we are seeing a whole new Ceres. A picture is worth a thousand words, but these pictures also merit a few exclamation points!!!

Dawn was 22 miles (36 kilometers) high on June 17 when it photographed this network of fractures in the southeastern floor of Occator Crater. The scene is about 2.1 miles (3.5 kilometers) across. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Dawn completed ion thrusting to XMO7 on June 6 and began its new observations of Ceres on schedule on June 9. Everywhere the spacecraft looked, it had fascinating new views. But the team had one special site in mind, and you might too. (Maybe it's even the same site.)

One of the bonus objectives was to try to get photos of Cerealia Facula, the mesmerizingly bright center of Occator Crater. We have explained why targeting a specific location is so hard. One of the attractive features of XMO7 was that it allowed two specially targeted attempts, thus increasing the chances that at least one would work. The team worked very hard to devise methods to take full advantage of that, while always quite well aware that it might not work.

Before we proceed, let's recall some terminology and introduce a new term. The high point in Dawn's orbit, 2,500 miles (4,000 kilometers), is known as apodemeter, analogous to the more common term apogee, which applies for orbits around Earth. (Demeter is the Greek counterpart of the Roman goddess Ceres.) The low point, 22 miles (35 kilometers), is peridemeter. Each 27.2-hour orbital revolution has one apodemeter and one peridemeter.

In April we discussed that Dawn travels much faster near peridemeter than near apodemeter, just as a swing moves faster at its low point than at its high point. As a fun fact, which does not bear on any of the following discussion, Dawn spends less than two hours over the dayside of Ceres (including peridemeter) and more than 25 hours over the nightside (including apodemeter). That may be surprising, but if you contemplate the illustrations of the elliptical XMO7 below and in March and think about the constantly changing velocity, it may make sense. (Or you may decide that it doesn't matter, accept it and move on.)

The solid ellipse is Dawn's orbit around Ceres, XMO7, ranging from 22 miles (35 kilometers) to 2,500 miles (4,000 kilometers). The spacecraft orbits counterclockwise from this perspective, going around once every 27. As shown in March, the orbit itself gradually rotates, so the lowest altitude shifts south. Dawn maneuvered to XMO7 early in June. The dashed circle shows the previous lowest altitude, LAMO/XMO1Image credit: NASA/JPL-Caltech

Even as they were excited by the fabulous new pictures and other data, the flight team began the carefully planned campaign to photograph Cerealia Facula when Dawn would be at peridemeter late in the day on June 22 and shortly after midnight on June 24. Navigators measured the orbital parameters very accurately and monitored how they changed. Each time the craft fires its small jets to control its orientation in the zero-gravity of spaceflight (necessary because of the failed reaction wheels), it nudges itself in orbit. The team compared the resulting distortion of the orbital motion with their predictions of this complicating effect in order to improve subsequent predictions. 

Mission planners had windows in the schedule for using the ion propulsion system to adjust the orbit. They instructed Dawn to fire its ion engine for 2 hours and 7 minutes on June 20 as the ship sailed upward. Fifteen hours later, on June 21, after it had crested in its orbit and was descending, it performed a second burn for 1 hour and 11 minutes.

The purpose of this pair of maneuvers was to bring Dawn's flight path at peridemeter right over Cerealia Facula. But the experienced explorers in mission control recognized that even with all their careful planning and Dawn's faithful execution of its assignments, there was a good chance the probe would not fly directly above that unique site as it sped northward. Therefore, they had also worked out plans to quickly determine how far east or west it would be at peridemeter and radio a (nearly) last minute adjustment in the angle it would point its sensors. 

After the second segment of ion maneuvering, Dawn's orbit took it down to peridemeter again on June 21 for another intensive period of close-up observations. Even before it had time to finish radioing those findings to Earth the next day, the team began preparing for the next dive down. On June 22, they made their final calculations of the orbital path and predicted that Dawn would fly a little west of Cerealia Facula that night and a little east of it the next time around. That afternoon, they transmitted instructions to Dawn to aim its camera and spectrometers just a little to the right the first time and just a little to the left the second time. (Sophisticated and capable though Dawn is, the instructions controllers sent were a little more specific and quantitative than the descriptions here.)

The team would have considered their extensive efforts successful if the spacecraft had photographed part of Cerealia Facula once. (Dawn flies so close to the ground that it would be impossible to photograph all of Cerealia Facula on any one orbit; its camera's view is simply not wide enough.) As it turned out, Dawn managed to get pictures of Cerealia Facula on three consecutive orbits, each time seeing different parts, yielding far better coverage of this exotic landscape than we had even hoped for.

Flying to this incredibly low orbit, getting such a wealth of data and even managing to photograph a good portion of Cerealia Facula truly tested the very limits of the mission's capabilities. Dawn has surpassed all expectations, accomplishing feats not even considered when it was designed.

Dawn had this view on June 13 when its orbit took it 24 miles (39 kilometers) over Vinalia Faculae, the diffuse bright salt deposits east of Cerealia Facula in Occator Crater. The exposure was optimized to show details of the bright material (and chosen to minimize smear from the spacecraft's high speed so close to the ground), revealing a complex distribution. The rugged dark terrain appears similar to some terrestrial lava flows, but on cold Ceres, what flowed was mostly a muddy mixture of ice and rock. The picture is 2.3 miles (3.7 kilometers) wide.  Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

In order to prepare for the long shot of attempting to capture Cerealia Facula, Dawn rotated to point its main antenna to Earth relatively often, sometimes after each peridemeter or after two or three. That allowed the flight team to work more closely with the spacecraft. Then it would turn again to bring its sensors to bear on Ceres shortly before soaring through the next peridemeter. But all that turning costs Dawn hydrazine, the resource that limits its operational life to only another few months. (We outlined this situation last month and will delve into it more fully next month.) Now Dawn will observe Ceres on five consecutive orbits, filling its memory with data, and then spend almost two full days, including one peridemeter, transmitting that valuable information back to Earth. While its antenna is trained on Earth, the spacecraft cannot simultaneously direct its sensors at Ceres. That actually yields especially good gravity measurements, which use the Doppler shift of the radio signal, because the signal is much stronger with the main antenna than with one of the auxiliary antennas. Pictures and spectra, however, cannot be acquired on that one peridemeter in every six during which Dawn sends its results to Earth. The flight team determined that the benefit of turning less often and thus reducing hydrazine consumption yields the best scientific return. (This savings was already accounted for when we described the end of the mission as likely being between August and October.)

We saw in March that the latitude at which Dawn reaches peridemeter shifts south with every revolution. That is, the low point of each orbit is about 2° south of the one before. As a result, each time the spacecraft flies over Occator Crater now, it is higher than the previous time. Occator is at 20°N. Now the peridemeter is close to the equator, and soon Dawn's best views of Ceres will be in the region of Urvara Crater.

Dawn observed this landscape on June 10 from an altitude of 24 miles (38 kilometers). Note all the boulders in the crater on the lower left. The crater's average diameter is about 0.9 miles (1.4 kilometers). This scene is around 75 miles (120 kilometers) north of Occator Crater. We described above that Dawn's peridemeter gradually moves south. This early in XMO7, the low altitude occurred well north of Occator Crater, because the team had designed the orbit so the best Occator observations would be later in June. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Firing ion engine #2 on June 21 accomplished more than the orbital adjustment that allowed the ship to spot Cerealia Facula at three consecutive peridemeters. It also completed the planned use of the ion propulsion system for the entire mission. 

Dawn's ion engines have enabled this interplanetary spaceship to accomplish a journey unique in humankind's exploration of the solar system. After departing Earth with the help of a conventional rocket, Dawn used those engines to fly past Mars in 2009, to travel to Vesta and enter orbit in 2011, to maneuver extensively in orbit to optimize its observations there, to break out of orbit in 2012, to travel to Ceres and slip into orbit in 2015, and to perform even more maneuvering there than at Vesta. No other spacecraft has ever orbited two extraterrestrial destinations, and Dawn's mission to do so would have been impossible without ion propulsion.

We summarize the mission's ion thrusting on every Dawnniversary of launch, but since no further use is planned, we can give some final numbers here. Dawn thrust for a total of 2,141 days (5.9 years), or 55 percent of the time it has been in space (and 0.000000042 percent of the time since the Big Bang). The thrusting has achieved the equivalent of accelerating the probe by 25,700 mph (41,400 kilometers per hour). As we have often explained (see here for one of the more extensive discussions), because of the principles of motion for orbital flight, whether around the Sun or any other gravitating body, Dawn is not actually traveling this much faster than when it launched. But the effective change in speed remains a useful measure of the effect of any spacecraft's propulsive work. Dawn has far exceeded the velocity change achieved by any other spacecraft under its own power. (For a comparison with probes that enter orbit around Mars, refer to this earlier Dawn Journal.) 

The engines have done their job admirably, and now we have no further use for them. As a reminder, they are not needed for Dawn to stay in orbit around Ceres, just as the Moon doesn't need propulsion to stay in orbit around Earth and Earth doesn't need propulsion to say in orbit around the Sun. Next month we will discuss what will happen to Dawn's orbit after the mission ends.

Dawn took this picture on June 9, the first time it took high resolution photos from its new orbit, XMO7. The spacecraft was 30 miles (48 kilometers) over this field of boulders inside Occator Crater's eastern rim. This scene is 2.9 miles (4.6 kilometers) wide. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

When the ion engine was programmed to stop thrusting on June 21, some Dawn team members gathered in mission control to mark the occasion. Dawn was busy and was not communicating with Earth at the time. Even if it had been, a radio signal confirming the end of thrust would have taken almost 25 minutes to reach our planet. But the team decided to neglect the limitation of the speed of light and mark the moment (1:15:03 pm PDT) that the blue glow on the distant ship's engine would extinguish for the last time. And at that same moment, the blue lights in mission control were turned off for the last time as well.

It's natural to feel some sadness or loss now that the engines will not fire again. After all, ion propulsion is cool, especially for those of us who first heard of it in science fiction. It is even cooler for those who appreciate its tremendous capability and how valuable that is for deep-space missions. We can feel wistful, of course, but the last use of the ion engines was a direct result of their great success. After a truly stupendous interplanetary mission, we have Dawn right where we want it: in an orbit optimized for getting the last, best data at the endlessly fascinating dwarf planet it has unveiled. We can be grateful the ion engines allowed Dawn to explore two of the last uncharted worlds in the inner solar system and that they captivated our imagination as the distant spacecraft traveled through the solar system on a blue-green beam of xenon ions. Not too long ago, ion propulsion was mostly in the domain of science fiction. NASA's Deep Space 1 put it firmly into the realm of science fact. Building on DS1, Dawn has rocketed far beyond, accomplishing a space trek that would have been impossible without ion propulsion. Its mission was to boldly go where -- well, you know. And it has! Dawn's engines will never emit their cool blue glow again, but their legacy will not fade.

Dawn is 100 miles (160 kilometers) from Ceres (and headed for peridemeter). It is also 3.06 AU (284 million miles, or 457 million kilometers) from Earth, or 1,125 times as far as the Moon and 3.01 times as far as the Sun today. Radio signals, traveling at the universal limit of the speed of light, take 51 minutes to make the round trip.

Dr. Marc Rayman
7:00 pm PDT June 30, 2018

TAGS: DAWN, CERES, VESTA, DWARF PLANET, JOURNAL

  • Marc Rayman
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Yalode -  ts the second largest crater on Ceres.

Now in its third year of orbiting a distant dwarf planet, a spacecraft from Earth is as active as ever. Like a master artist, Dawn is working hard to add fine details to its stunning portrait of Ceres.

In this phase of its extended mission, the spacecraft’s top priority is to record space radiation (known as cosmic rays) in order to refine its earlier measurements of the atomic species down to about a yard (meter) underground. The data Dawn has been collecting are excellent.

As we explained in January, the ambitious mission has added a complex bonus to its plans. The team is piloting the ship through an intricate set of space maneuvers to dramatically shift its orbit around Ceres. They are now about halfway through, and it has been smooth sailing. Dawn is on course and on schedule. (If you happen to be one of the few readers for whom it isn’t second nature to plan how to change a spacecraft’s orbit around a dwarf planet by 90 degrees and then fly it under control of ion engine, last month’s Dawn Journal presents a few of the details that may not be obvious. And you can follow the adventurer’s orbital progress with the regular mission status updates.)

If all goes well, on April 29 the new orbit will take Dawn exactly between the sun and the famous bright region at the center of Occator Crater. Named Cerealia Facula, the area is composed largely of salts. (Based on infrared spectra, the strongest candidate for the primary constituent is sodium carbonate). The probe will be at an altitude of about 12,400 miles (20,000 kilometers), or more than 50 times higher than it was in 2016 when it captured its sharpest photos of Occator (as well as the rest of Ceres’ 1.1 million square miles, or 2.8 million square kilometers). But the objective of reaching a position at which the sun and Ceres are in opposite directions, a special alignment known as opposition, is not to take pictures that display more details to our eyes. In fact, however, the pictures will contain intriguing new details that are not readily discerned by visual inspection. Dawn will take pictures as it gets closer and closer to opposition, covering a range of angles. In each image, scientists will scrutinize the handful of pixels on Cerealia Facula to track how the brightness changes as Dawn’s vantage point changes.

Occator Crater
Dawn took this photo of Occator Crater on Oct. 18, 2016, at an altitude of 920 miles (1,480 kilometers) in extended mission orbit 2. We have seen other views of Occator, from farther, from closer, with exposures optimized for the brightest areas, in color, with the crater on the limb of Ceres and more, but you can never have too many pictures of such a captivating scene. The central bright region is Cerealia Facula, and the collection of others is Vinalia Faculae. (A bright region on a planet is a facula. Here is more on these names.) These are the brightest areas on Ceres. One scenario for how they formed is that underground briny water made its way to the surface through fractures. When the water was on the ground, exposed to the cold vacuum of space, it froze and sublimated (that is, it transformed from a solid to a gas). The dissolved salt was left behind, with sodium carbonate being the likely principal constituent, and that reflective material is what we see here. We will see below that opposition surge measurements may provide evidence to support or modify this scenario. (A recent estimate is that Cerealia Facula may be some tens of millions of years younger than the crater itself. We discussed last year how ages are determined.) Since we can’t have too many views of this exotic scenery, another is below (and it shows the fractures that may have served as conduits for the water). Occator is on this map at 20°N, 239°E. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

We described the opposition surge, in which the reflected sunlight at opposition may be significantly brighter than it is in any other geometrical arrangement. A few degrees or even a fraction of a degree can make a large difference. But why is that? What is the underlying reason for the opposition surge? What can we learn by measuring it? And is the best cake better than the best candy?

Those are all interesting and important questions. We will address some of them here and leave the rest for your own thorough investigation.

There are at least three separate physical effects that may contribute to the opposition surge. One of them is known as shadow hiding. When the sun shines on the ground, tiny irregularities in the surface, even at the microscopic level, will cast shadows. When you look at the ground, those shadows collectively detract from its overall brightness, even if each individual shadow is too small for you to see. The total amount of light reflected off the ground and into your eyes (or your camera) is less than it would be if every point, no matter how small, were well lit. However, if you look along the same direction as the incoming light, then all the shadows will be hidden. They will all be on the opposite side of those tiny irregularities, out of reach of both the incident light and your sight. In that case, anything you can see will be illuminated, and the scene will be brighter. The figure below is intended to illustrate this phenomenon of shadow hiding (and excluding the caption, the picture is probably worth almost 480 words).

Illustration of Shadow hiding
Illustration of shadow hiding. At the bottom is the ground on Ceres with greatly exaggerated crystals of salt pointing in random directions. (Shadow hiding occurs even with very small grains.) The white dashed lines show light from the sun, and each ray traces the light to the tip of a crystal and then to the point beyond. The solid black lines along the ground and the crystals are in shadow. That is, the incoming light cannot reach those places. Therefore, when Dawn is in the position on the right, looking along the same direction as the incoming light, it cannot see those shadows, because there is no line of sight to those hidden locations. In that special position, where Ceres is at opposition, every point on the ground Dawn sees is lit. When Dawn is in the position on the left, it does have a direct line of sight to some (although not all) shadows, as shown by the black dotted lines. Some of the ground it sees is lit and some is not. The difference between these two perspectives is the shadow-hiding component of the opposition surge. (Remember that these crystals are too tiny for Dawn to discern. One pixel in the explorer’s camera would take in this entire scene, so what matters is the total lit surface here, not the fine details.) Now at location 1, there are crystals that happen to point directly at Dawn when it is on the left, and at location 2, there are crystals that point directly at Dawn when it is on the right. You can see that at opposition, the shadows are hidden for both crystal orientations. But when Dawn is on the left, crystals pointing directly at it don’t provide a fully lit scene. Shadows are still visible. So, shadow hiding does not depend on any special alignment of crystals on the ground. It is the special observing location that matters. In summary, the ground appears brighter to Dawn when it is at opposition than when it is elsewhere. Although all crystals here are the same size, different crystal sizes may yield different shadowing and hence different opposition surge signatures. So, with a good measurement of the opposition surge, the crystal sizes may be determined. The self-portrait at right (biceps not to scale) is provided to illustrate your correspondent’s artistic skills. It should help you calibrate the fine details of the rest of the image. There are many simplifications here. In other words, take this diagram with a grain of sodium carbonate. Image credit: NASA/JPL-Caltech

The opposition surge was first described scientifically in 1887 by Hugo von Seeliger, an accomplished astronomer and highly esteemed teacher of astronomers. He analyzed data collected by Gustav Müller when Earth’s and Saturn’s orbits around the sun brought Saturn into opposition, and the brightness of the rings increased unexpectedly. Seeliger realized that shadow hiding among the myriad particles in the rings could explain Müller’s observations. The opposition surge is occasionally known as the Seeliger effect. (Although astronomers had been observing the rings for more than two centuries by then, a careful scientific analysis to show that the rings were not solid but rather composed of many small particles had only been completed about 30 years before Seeliger’s advance.)

Now astronomers recognize the opposition surge on many solar system bodies, including Earth’s moon and the moons of other planets, as well as Mars and asteroids. In fact, it also occurs on many materials on Earth, including vegetation. Scientists exploit the phenomenon to determine the character of materials at a distance when they can make careful measurements at opposition.

For many solar system objects, however, it is difficult or impossible to position the observer along the line between the sun and the target. But thanks to the extraordinary maneuverability provided by Dawn’s ion engine, we may be able to perform the desired measurement in Occator Crater.

3-D Anaglyph of Cerealia Facula
This 3-D image of part of Occator Crater, the brightest area on Ceres, was created with photos from Dawn’s lowest altitude orbit at 240 miles (385 kilometers). The spacecraft took pictures of this scenery from different angles, forming stereo views. To perceive the 3-D, you need colored filters, with red for your left eye and blue for your right. (You can get a 4-D view by looking at it for a while. However, apart from the daily and annual changes in the angle of the incoming sunlight, no changes are expected to be discernible even over a few years.) If you don’t have access to stereo glasses, you can see a more conventional photo here. The bright region on the left, Cerealia Facula, is about nine miles (14 kilometers) across, and the stereo reveals a dome that rises to about 1,300 feet (400 meters). The other bright areas are collectively called Vinalia Faculae. Occator is on this map at 20°N, 239°E. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

It was nearly a century after Seeliger’s description of shadow hiding before scientists realized that there is another contributor to the opposition surge, which we mention only briefly here. It depends on the principle of constructive interference, which applies more in physics than in politics. Waves (in this case, light waves) that have their crests at the same places can add up to be especially strong (which makes the light bright). (Destructive interference, which may be more evident outside of the physics realm, occurs when troughs of one wave cancel crests of another.) We will not delve into why constructive interference tends to occur at opposition, but anyone with a thorough understanding of classical electromagnetic theory can work it out, as physicists did in the 1960s to 1980s. (More properly, it should be formulated not classically but quantum mechanically, but we recognize that some readers will prefer the former methodology because it is, as one physicist described it in 1968, "much simpler and more satisfying to the physical intuition." So, why make it hard?) For convenient use to ruin parties, the most common term for constructive interference in the opposition surge is coherent backscatter, but it sometimes goes by the other comparably self-explanatory terms weak photon localization and time reversal symmetry. Regardless of the name, as the light waves interact with the material they are illuminating at opposition, constructive interference can produce a surge in brightness.

The intensity of the opposition surge depends on the details of the material reflecting the light. Even the relative contributions of shadow hiding and coherent backscatter depend on the properties of the materials. (While both cause the reflected light to grow stronger as the angle to opposition shrinks, coherent backscatter tends to dominate at the very smallest angles.)

Especially sensitive laboratory measurements show that sometimes shadow hiding and coherent backscatter together are not sufficient to explain the result, so there must be even more to the opposition surge. The unique capability of science to explain the natural world, shown over and over and over again during the last half millennium, provides confidence that a detailed theoretical understanding eventually will be attained.

Part of science’s success derives from its combination of experiment and theory. For now, however, the opposition surge is more in the domain of the former than the latter. In other words, translating any opposition surge observation into a useful description of the properties of the reflecting material requires controlled laboratory measurements of well characterized materials. They provide the basis for interpreting the observation.

Occator’s Bright Spots in 3-D
This short animation shows how the illumination of the northern hemisphere changes as Ceres’ axial tilt changes from 2 to 12 to 20 degrees. (In each frame, the lighting is shown on the summer solstice, when the sun reaches its greatest northern latitude.) We have discussed the orientation of the dwarf planet’s axis before. As we saw, it is tipped only 4 degrees, causing much more modest changes in lighting throughout each Cerean year (which is 4.6 terrestrial years) than Earth (and perhaps your planet) experiences. However, the gravitational tugs of Jupiter and Saturn, despite their distance, tip the axis. The angle can change from as little as 2 degrees to as much as 20 degrees in only about 12,000 years, which astronomers consider to be very fast. (Earth’s axis is tilted 23.5 degrees and is stabilized by the moon. Mars, which lacks a sizable moon, also goes through dramatic changes in axial tilt, although much more slowly than Ceres.) The angle of the sun near the poles is an important factor for where ice might accumulate. The animation shows the regions that would stay in shadow throughout every Cerean day of a full Cerean year, with blue for 2 and 12 degrees and red for 20 degrees. (The blue at 12 degrees is difficult to see.) When the sun goes farther north, it shines deeper into craters, illuminating and warming locations that would remain in shadow if the sun could not rise as high in the sky. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

If Dawn accomplishes the tricky measurements (which we will describe next month), scientists will compare the Cerealia Facula opposition surge with lab measurements of the opposition surge. As always in good science, to establish the details of the experiments, they will start by integrating the knowledge already available, including the tremendous trove of data Dawn has already collected -- spectra of neutrons, gamma rays, visible light and infrared light plus extensive color and stereo photography and gravity measurements. In the context of their understanding of physics, chemistry and geology throughout the solar system, scientists will determine not only the mixtures of chemicals to test but also the properties such as grain sizes and how densely packed the particles are. They will perform experiments then on many combinations of credible facular composition and properties. Comparing those results with Dawn’s findings, they will be able to elucidate more about what really is on the ground in that mesmerizing crater. For example, if they determine the salt crystals are small, that may mean that salty water had been on the ground and sublimated quickly in the vacuum of space. But if the salt came out of solution more slowly underground and was later pushed to the surface by other geological processes, the crystals would be larger.

It is an impressive demonstration of the power of science that we can navigate an interplanetary spaceship to a particular location high above the mysterious, lustrous landscape of a distant alien world and gain insight into some details that would be too fine for you to see even if you were standing on the ground. Using the best of science, Dawn is teasing every secret it can from a relict from the dawn of the solar system. On behalf of everyone who appreciates the majesty of the cosmos, our dedicated, virtuoso artist is adding exquisite touches to what is already a masterpiece.

Dawn is 31,400 miles (50,500 kilometers) from Ceres. It is also 3.48 AU (324 million miles, or 521 million kilometers) from Earth, or 1,430 times as far as the moon and 3.48 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 58 minutes to make the round trip.

Dr. Marc D. Rayman
4:00 p.m. PDT March 30, 2017

TAGS: DAWN, BLOG, JOURNAL, CERES, VESTA

  • Marc Rayman
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