June 30, 2017

Dear Adawnlescents,

Orbiting the only dwarf planet inside the orbit of Neptune, Dawn is healthy and continuing to carry out its assignments at Ceres with the masterful skill to be expected for such an experienced space explorer. As Earth and Ceres took up positions on opposite sides of the sun for the first part of this month, the probe operated for almost two weeks without being able to count on assistance from its human handlers, even if it encountered a serious problem. The powerful interference of the sun could have prevented radio communications. But Dawn had no need. When the changing geometry allowed the radio silence to break, the ship confirmed that all was well.

Dawn’s primary responsibility in this phase of its mission continues to be monitoring cosmic rays. For eight months in 2015-2016, circling closer to Ceres than the International Space Station is to Earth, the probe measured nuclear radiation that contains the signatures of geologically important elements down to about a yard (meter) underground. Since December, when it reached a much greater altitude, it has been listening to the faint hiss of cosmic rays. Scientists will mathematically remove that from the earlier recordings of Ceres. This procedure will allow them to squeeze even more information out of the low-altitude census of atomic species.

Kerwan Crater

Ceres’ largest crater is Kerwan, which fills most of this picture. The crater is 174 miles (280 kilometers) in diameter. That may seem large, and indeed it is large compared to all other Cerean craters and probably also compared to the block you live on and the landscape you can see out your window (unless you happen to have a view of the moon). But as we have discussed before, scientists recognize that Ceres should have even larger craters. Those ancient craters probably were erased by the gradual movement of the ice and rock in the ground as it "relaxes" after a disfiguring impact, just as your skin restores its shape after pressure has been removed. That process is stronger for larger craters and likely contributed to making Kerwan’s features appear softened. Kerwan is noticeably polygonal because the crater walls formed along preexisting underground fractures when the impactor struck, and we will see another example of that below. Dawn took this picture on June 12, 2015, from an altitude of 2,700 miles (4,400 kilometers) in its second mapping orbit. We have seen Kerwan from a different perspective as well as a close-up of one area in it photographed from Dawn’s lowest altitude orbit. (The crater is so large that it took about 50 pictures from low altitude to cover it.) Below is a photo of the center of Kerwan from an intermediate altitude. The crater at the center is Insitor, which is 16 miles (26 kilometers) in diameter. (Insitor was a minor Roman god concerned with sowing crops. Perhaps his being minor is appropriate, as the crater is less than 1 percent of Kerwan’s area.) You can locate Kerwan at 11°S, 124°E on this map. The dark material at the upper right of this picture was blasted out by the impact that formed Dantu Crater, which we will see below. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Dawn had to fly far enough above Ceres that it could measure the cosmic rays alone, rather than the combination of Ceres radiation and cosmic radiation it detected at low altitude. The mission continued to go so well after they had sent the spacecraft to a high altitude, that the team devised more new objectives. To start, they had Dawn photograph some very nice scenes of a gibbous Ceres. Then they guided it through two months of intricate orbital maneuvers, allowing the spacecraft to fly across the line from the sun to Ceres, providing a view of the fully illuminated dwarf planet (like a full moon). In addition to yielding lovely new movies and color pictures, these opposition measurements may help scientists discover details of the material on the ground that would otherwise be impossible to descry from orbit.

That orbit extended so high that it took two months to complete one long elliptical loop around Ceres. The opposition observations worked extremely well, but it’s not a convenient orbit for most other investigations (except the cosmic ray measurements). Therefore, earlier this month, mission controllers instructed the spacecraft to use its ion engine to adjust the orbit again, this time reducing the period for one revolution to 30 days and improving the opportunities for future scientific measurements.

Center of Kerwan Crater

Near the center of this picture is Insitor Crater, which is at the center of Kerwan Crater, as we saw above. Insitor is 16 miles (26 kilometers) in diameter and is at 11°S, 125°E on this map. Dawn captured this view from an altitude of 915 miles (1,470 kilometers) in its third mapping orbit on Sept. 23, 2015. Full image (rotated from the one here) and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

In coming months, we will look ahead to new observations the team is just beginning to consider. It has not been assured that further activities would be possible. For half of the time since it embarked on its extraordinary extraterrestrial expedition, Dawn has managed to complete its work without the use of the full complement of equipment it was supposed to have at its disposal. Even with the failures of three reaction wheels, however, the mission has far exceeded its original objectives and well outlasted its expected lifetime. Nevertheless, the spacecraft’s lifetime certainly is limited, most likely by the dwindling supply of hydrazine, although possibly instead by one of the many risks that are part of the very nature of conducting complex operations in the unforgiving far reaches of space. For now, however, it appears that Dawn has enough life left in it to warrant pursuing even more new goals.

On July 16, as the sophisticated ship from distant Earth continues to carry out its mission, it will celebrate the 271st birthday of Giuseppe Piazzi, the first person to spot Ceres. It was a faint point of light amid the stars, one tiny jewel among too many to count. When the 54-year-old made his serendipitous discovery, which gave him an honored place in the history of science, he certainly could not have foreseen what Dawn has now seen. (And there’s no reason he should have. He was an astronomer and mathematician, not a clairvoyant.)

Occator & Kirnis Craters

Dawn had this view on June 24, 2015, from its second mapping orbit at an altitude of 2,700 miles (4,400 kilometers). The scene displays quite a variety of geological features. Most salient is Occator Crater, with its deposits of sodium carbonate and other salts shining in reflected sunlight, at upper left (although you probably didn’t need that helpful guidance in order to find it.) Occator is 57 miles (92 kilometers) in diameter. It is the youngest major feature here. The largest crater, to the right and slightly below Occator, is Kirnis. (Kirnis was a Lithuanian god of cherries. It isn’t known whether he collaborated with deities of chocolate or of sundaes.) This 71-mile (115-kilometer) crater is old, as shown by its degraded appearance, gradually worn down by the particles large and small that fall from space. (Kirnis is too small to have been affected much by the movement of the crust that erases craters.) Notice how the crater rim seems to consist of straight segments. That is most evident for the lower rim, where there is a clear overlap with one of the long linear structures in the right half of the picture. When an impact occurs in an area with fractures, the resulting crater may be shaped by them, yielding a similar polygonal structure, even if there is no other evidence of those fractures visible on the ground. We saw that with Kerwan as well. The fractures in this picture are collectively known as Samhain Catenae. (Samhain, meaning "summer’s end," is a Celtic agricultural festival marking the end of summer and beginning of winter. Halloween can trace its origins to Samhain.) A catena is usually a chain of craters (and is a Latin word for chain), but the term also is applied more generally to large grooves that can be formed by a variety of geological processes. Scientists have not yet determined the mechanism responsible for Samhain Catenae. We will see another catena below. Lociyo Crater, well below Occator, is 21 miles (34 kilometers) across. (Although he was the god of lightning, Lociyo is fortunate to qualify for the naming convention for Cerean craters, because he was associated with agriculture. When the Zapotecs, in what is now Oaxaca, Mexico, cut the first chili of the harvest, they would sacrifice it to Lociyo.) The impact that excavated Lociyo obliterated half of an older crater of about the same size. This scene is centered on this map at 2°N, 249°E. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

In addition to revealing Ceres’ overall appearance, Dawn has acquired a wealth of pictures and other information that scientists are now actively studying. The mission has shown us mesmerizing bright regions and an extensive network of ground fractures in Occator Crater. The shapes and sizes of many craters provide intriguing clues about the strength and other properties of the interior, and the measurements of the gravity field yield still more insight into the inside. The towering cryovolcano Ahuna Mons rises up as a compelling monument to internal geological forces (which we will discuss below). Organic chemicals spotted in and near Ernutet Crater and elsewhere are of special interest for astrobiology. We see ice on the ground and have determined there is a tremendous amount underground (and there may be liquid underground as well). Piazzi discovered -- and Dawn uncovered -- a truly alien world, and its vastness and diversity are part of what make it so fascinating.

Among the minerals Dawn has found is a group known as carbonates, and they are abundant on Ceres. We see two types there. One, which is omnipresent, is known as dolomite and contains calcium and magnesium. It is mixed with another Cerean mineral, serpentine. A different type of carbonate is prominent in Occator Crater. The sodium carbonate there reflects so much sunlight that it seems almost to be luminous, like a giant spotlight casting its brilliance far out into space, perhaps to show off that it contains the highest concentration of any kind of carbonates known anywhere in the solar system except Earth. Occator’s specific kind, sodium carbonate, has been observed only on Earth and in the plumes of Saturn’s watery moon Enceladus. Interestingly, the carbonates and serpentine are formed by chemical reactions between rocks and water under high pressure. How could these minerals be both widespread and exposed?

Pongal Catena on Ceres

Dawn observed this scene on Sept. 28, 2015, from its third mapping orbit at an altitude of 915 miles (1,470 kilometers). On the lower left is the center and northwestern part of Urvara Crater, Ceres’ third largest. Extending outward from Urvara on its right is a narrow depression 60 miles (97 kilometers) long called Pongal Catena. (Pongal is a popular Hindu harvest festival of thanksgiving, celebrated by the Tamil people.) It may be the result of stresses produced by the tremendous impacts that formed Urvara and the even larger Yalode Crater (which we will see below). We saw other catenae above. This picture is centered on this map at 36°S, 263°E. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

One possibility is that they formed deep underground and were later pushed to the surface by internal geological processes. Just as on Earth, those internal forces are mostly powered by heat from the decay of radioactive elements. The heat is carried away by the motion of the material, just as heating water at the bottom of a pot causes it to rise and then make complex convection patterns. The strength of the forces depends on the rate at which the heat leaks from the deep interior to the ground. That is, heat is a form of energy, and a faster flow of heat energy (and thus of material) would provide a more powerful internal engine to drive minerals to the surface.

Heat flows from hot (far underground) to cold (the surface, which is exposed to space). It is at least 80 degrees Fahrenheit (50 degrees Celsius) colder near Ceres’ north and south poles than near the equator. That means the strength of the geological pressure pushing minerals to the surface should depend on the latitude, which would translate into different compositions at different latitudes. But that is not what Dawn sees. The minerals show up everywhere we look. Their prevalence is a fact that is inconsistent with a deep underground origin followed by a heat-driven movement to the surface. Science tells us we need to formulate a different explanation for why minerals produced in water under high pressure now can be found on the ground.

Scientists recognize a more likely explanation. The minerals may have formed in an ocean early in Ceres’ history, when radioactive elements were so abundant that it would have been warm enough to keep a large volume of water as a liquid. But as Ceres aged, it would have cooled (perhaps some readers have experienced this as well), because the supply of radioactive elements would have gradually been depleted as they decayed. Almost the entire ocean would have frozen, encasing Ceres in a shell of ice. But that wouldn’t be the end of the story.

Ice cannot last long on Ceres (except in special places). Cold though it is on that world, there is enough warmth from the distant sun that ice sublimates, turning from a solid into a gas as the water molecules escape into space. Even as that gradual phenomenon occurred at the microscopic level, ice was lost through a much more dramatic and abrupt process. It was blasted away by asteroids that slammed into it. The rain of rocks that fall onto Ceres over millions of years is a familiar hazard to anyone who has lived in the main asteroid belt for millions of years. In fact, scientists estimate that a frozen ocean three miles (five kilometers) thick could have been lost in only a few tens of millions of years, a blink in geological time. (And even if that ice shell had been much thicker, it would still have been lost on a geologically short timescale.)

Yalode Crater

Dawn took this picture of a part of Yalode Crater from an altitude of 915 miles (1,470 kilometers) in its third mapping orbit on Sept. 27, 2015. At 162 miles (260 kilometers) across, Yalode is the second largest crater on Ceres (and too large to be captured with a single picture even from this high). Note the distinctly polygonal craters, including the largest one on the right, Lono, which is 12 miles (20 kilometers) wide. Below Lono is Besua Crater, with a diameter of 11 miles (17 kilometers). Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Before it froze and dispersed, chemical reactions between the water and rocks would have produced a rich inventory of minerals. As Dawn peers down from its orbital perch, it sees their testimony to that long-lost ocean. And even now there may still be reservoirs of liquid within Ceres, as it is warm enough inside.

None of this could have been imagined by Piazzi on the night he first glimpsed Ceres from his observatory in Sicily. Because he wasn’t prescient, he also did not expect that what he discovered would be known at times as a planet, an asteroid, a dwarf planet and eventually as "home" by Dawn. Nor would he have anticipated the Tunisian-Sicilian War, the extraordinary intellectual achievements in the scientific discoveries of evolution, relativity and quantum mechanics, or the inventions of the safety pin, granola, integrated circuits and remotely controlled interplanetary spacecraft. If Piazzi thought seriously about the unique successes of science or about the nature of exploration, he did not leave much of a record.

Dantu Crater

The largest crater here is Dantu, 78 miles (126 kilometers) in diameter. We have seen other views of this impressive landscape, most recently here. Dawn took this picture on Sept. 24, 2015, from an altitude of 915 miles (1,470 kilometers) in its third mapping orbit. Some of the material ejected by the violent excavation of Dantu is visible in the photo of Kerwan Crater above. This scene is centered on this map at 22°N, 133°E. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

For the perspective of someone who did, let’s go back to a time before Piazzi’s 1801 sighting of Ceres but after the dwarf planet’s formation nearly 4.6 billion years ago. Sometime between 1607 and 1620, the polymath and early champion of modern science Francis Bacon wrote this in Cogitata et Visa (Thoughts and Conclusions):

    It would disgrace us, now that the wide spaces of the material globe, the lands and seas, have been broached and explored, if the limits of the intellectual globe should be set by the narrow discoveries of the ancients. Nor are those two enterprises, the opening up of the earth and the opening up of the sciences, linked and yoked together in any trivial way. Distant voyages and travels have brought to light many things in nature, which may throw fresh light on human philosophy and science and correct by experience the opinions and conjectures of the ancients.

Bacon realized that archaic ideas had such a tight grip that they prevented the expansion of Europe’s intellectual horizons. The startling and exciting discoveries of the explorers who pushed the physical horizons during the century or so that preceded his writings broke that suffocating squeeze. New realizations about the reality of the natural world, and how dramatically it differed from the untested notions of old, inspired an ardor for intellectual exploration as daring and vigorous as what had been undertaken in traversing those distant lands and seas.

The reward has been discoveries by Piazzi and uncounted other scientists who have revealed the staggering richness of nature in all its forms, a universe of such majesty, such beauty, such complexity that it would seem to defy explanation. And yet science not only uncovers myriad mysteries but also lifts the veil, revealing inner workings and showing us why things are the way they are. The ultimate rewards of science are knowledge and understanding.

Dawn is both a beneficiary of and a contributor to the extraordinary successes of science since Bacon’s time. The mission’s "distant voyages and travels have brought to light many things in nature." And its exploration of alien lands and its journeys on interplanetary seas continue to "throw fresh light on human philosophy and science." The real beneficiaries are we ourselves. How fortunate we all are to behold what that light has illuminated!

Dawn is 20,000 miles (32,200 kilometers) from Ceres. It is also 3.67 AU (341 million miles, or 549 million kilometers) from Earth, or 1,400 times as far as the moon and 3.61 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. PDT June 30, 2017