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Marc Rayman

Dawn Journal: History of Ceres

Posted by Marc Rayman

29-12-2014 13:25 CST

Topics: history, mission status, asteroids, Dawn, asteroid 1 Ceres

Pardawn Me, Dear Readers,

Far away from Earthlings who look forward to a new year, Dawn looks forward to a new world. On the far side of the sun, the interplanetary explorer is closing in on Ceres, using its advanced ion propulsion system to match solar orbits with the dwarf planet.

Since breaking out of orbit around the giant protoplanet Vesta in September 2012, the spaceship has patiently flown in interplanetary cruise. That long mission phase is over, and now Dawn is starting the Ceres chapter of its extraordinary extraterrestrial expedition. Configured for its approach phase, the craft is following a new and carefully designed course described in detail last month. In March it will slip ever so gracefully into orbit for an ambitious and exciting exploration of the alien world ahead.

Over the past year, we have provided previews of the major activities during all the phases of Dawn’s mission at Ceres. This month, let’s take a look at Ceres itself, an intriguing and mysterious orb that has beckoned for more than two centuries. Now, finally, after so long, Earth is answering the cosmic invitation, and an ambassador from our planet is about to take up permanent residence there. Over the course of Dawn’s grand adventure, our knowledge will rocket far, far beyond all that has been learned before.

There can be two accounts of Ceres: its own history, which dates back to near the dawn of the solar system almost 4.6 billion years ago, and its history in the scope of human knowledge, which is somewhat shorter. Both are rich topics, with much more than we can cover here (or in the first log for this entire mission), but let’s touch on a few tidbits. We begin with the latter history.

In 1800, the known solar system contained seven planets: Mercury, Venus, Earth (home to some of our readers), Mars, Jupiter, Saturn and Uranus. This reflected a new and sophisticated scientific understanding, because Uranus had first been noticed in telescopes not long before, in 1781. (The other planets had been known to ancient sky watchers.) Even before William Herschel’s fortuitous sighting of a planet beyond Saturn, astronomers had wondered about the gap between Mars and Jupiter and speculated about the possibility of a planet there. Although some astronomers had searched, their efforts had not yielded a new planet.

Giuseppe Piazzi

Osservatorio Astronomico di Palermo

Giuseppe Piazzi
Giuseppe Piazzi points the way to his discovery, the planet Ceres. (Dawn’s route there is more complex than Piazzi might have guessed.)

Astronomer Giuseppe Piazzi was not looking for a planet on Jan. 1, 1801, but he spotted an unfamiliar dot of light that moved slowly among the stars. He named it for Ceres, the Roman goddess of agriculture, and if you had cereal this morning, you have already had an etymological connection with the goddess.

The Dawn project worked with the International Astronomical Union (IAU) to formalize a plan for names on Ceres that builds upon and broadens Piazzi’s theme. Craters will be named for gods and goddesses of agriculture and vegetation from world mythology. Other features will be named for agricultural festivals.

Because Ceres was fainter than the other known planets, it was evident that it was smaller. Nevertheless, many astronomers considered it to be a planet too.

It is worth noting the significance of this. Modern astronomy had chanced upon only one other planet, so Piazzi’s discovery was A Big Deal. When a new chemical element was found a couple of years later, it was named cerium in tribute to the new planet Ceres. (Uranus had been similarly honored with the 1789 naming of uranium. That element’s peculiar property of emitting radiation would not be known for another century.)

In the six years following the discovery of Ceres, three more bodies were detected orbiting between Mars and Jupiter. (One of them is Vesta, now known in spectacular detail thanks to Dawn’s extensive exploration in 2011-2012.) There then ensued a gap of more than 38 years before another was found, so for well over a generation, the sun’s family of planets was unchanged.

So if you had been reading about all this 200 years ago, there would have been at least two important differences from now. One is that your Internet connection would have been considerably slower. The other is that you might have learned in school or elsewhere that Ceres was a planet.

In 1846, a planet was discovered beyond Uranus, and we call it Neptune. Nothing else of comparable size has subsequently been seen in our solar system.

With scientific knowledge and technology progressing in the middle of the nineteenth century, new objects were glimpsed between Mars and Jupiter. As more and more were seen over the years, what we now know as the main asteroid belt was gradually recognized. Terminology changed too. One of the great strengths of science is that it advances, and sometimes we have to modify our vocabulary to reflect the improved, refined view of the universe.

By the time Pluto was sighted in 1930, Ceres had long been known as a “minor planet” and an “asteroid.” For a while thereafter, Pluto enjoyed planetary status similar to what Ceres had had. In fact, in 1940, scientists named two more additions to the periodic table of the elements neptunium and plutonium. While the histories are not identical, there is a certain parallel, with more and more objects in Pluto’s part of the solar system later being found. Terminology changed again: Pluto was subsumed into the new category of “dwarf planets” defined by the IAU in 2006. Ceres was the first body to be discovered that met the criteria established by the IAU, and Pluto was the second. (Spacecraft are now on their way to both dwarf planets: Dawn to orbit Ceres 214 years after its discovery and the wonderful New Horizons mission to fly past Pluto 85 years after it was found.)

Four views of Ceres from Hubble

NASA / ESA / J. Parker (Southwest Research Institute) / P. Thomas (Cornell University) / L. McFadden (University of Maryland, College Park)

Four views of Ceres from Hubble

Four views of Ceres as it rotates, as seen with Hubble Space Telescope, are the best we have so far. (The brightest feature has been exaggerated here.) As summarized in October, Dawn’s pictures will surpass this quality within a month, revealing details never seen before.

We discussed this new nomenclature in some detail shortly after it was adopted. We understand that the designation then, as now, is controversial among some scientists and the public, and there are strong emotions on this topic. We will not delve into it here (nor in the blog comments below), preferring instead to focus on the extraordinary successes of science, the great power of the scientific method and the thrill of bold adventures far from home. The Dawn team remains both unperturbed and confident in what to call this intriguing and alluring world: we call it “Ceres.” And our goal is to develop that faint smudge of light amidst the stars into a richly detailed portrait.

One of the advances of science was the recognition that Ceres really is entirely different from typical residents of the main asteroid belt. It is a colossus! There are millions upon millions of asteroids, and yet Ceres itself contains roughly 30 percent of the mass in that entire vast region of space. By the way, Vesta, the second most massive body there, constitutes about eight percent of the asteroid belt’s mass. It is remarkable that Dawn will single-handedly explore around 40 percent of the asteroid belt’s mass.

With an equatorial diameter of about 605 miles (975 kilometers), a value that Dawn will refine very soon, Ceres is the largest body between the sun and Pluto that a spacecraft has not yet visited. It is occasionally described as being comparable in size to Texas, which is like comparing a basketball to a flat sheet of paper. Ceres has a surface area 38 percent of that of the continental United States, or more than four times the area of Texas. (Nevertheless, until Dawn shows evidence to the contrary, we will assume Texas has more rodeos.) It is nearly a third of the area of Europe and larger than the combined lands of France, Germany, Italy, Norway, Spain, Sweden and the United Kingdom. Such a large place offers the promise of tremendous diversity and many marvelous and exciting sights to behold. Earth is about to be introduced to a fascinating new world.

How did Ceres come to be? And why is that being phrased as a question instead of a more declarative introduction to the history and nature of this dwarf planet? For that matter, why is this paragraph composed exclusively of questions? At least this sentence isn’t a question, right? OK, really, shouldn’t we stay more on topic?

At the dawn of the solar system almost 4.6 billion years ago, the young sun was surrounded by a swirling cloud of dust and gas. Sometimes some particles would happen to hit and stick together. Then more and more and more particles would stick to them, and eventually these agglomerations would grow so large that their gravity would pull in even more material. It was through mechanisms like this that the planets formed.

But when massive Jupiter developed, its powerful gravity terminated the growth of objects nearby, leaving bits and pieces as asteroids. Ceres and Vesta, already sizable by then, might have grown to become even larger, each incorporating still more of the nearby material, had Jupiter not deprived them of such an opportunity. Not having made it to full planetary proportions, Ceres and Vesta are known as protoplanets, and studying them provides scientists with insight into the largest building blocks of planets and into worlds that are intriguing in their own rights.

Formation of the Solar System

William Hartmann, Courtesy of UCLA

Formation of the Solar System
Dust and gas surrounding the sun coalesced into planets and asteroids, as depicted in this artist’s view.

Ceres apparently formed far enough from the sun under conditions cool enough for it to hang on to water molecules. Indeed, scientists have good reason to believe that water (mostly in the form of ice) may make up an astonishing 30 percent of its mass. Ceres may contain more water than Mars or any other body in the inner solar system except Earth. (Comets, of course, have high proportions of water too, but they are so minuscule compared to this behemoth that each one harbors a quite negligible amount of water when measured against Ceres’ huge inventory.)

Although some of the moons of the outer planets also are ice and rock, and they display very interesting characteristics to the impressive and capable spacecraft that have flown past (in some cases repeatedly, as the craft orbited the host planet), no probe has had the capability to linger in orbit around any of them. Dawn’s in-depth exploration of Ceres will yield more detailed and complete views than we have obtained of any icy moon.

Radioactive elements incorporated into Ceres when it was forming would supply it with some heat, and its great bulk would provide thermal insulation, so it would take a very long time for the heat to escape into space. The sun, faraway though it is, adds still more heat. As a result, there may be some water warm enough to be liquid. (The concentration of any chemical impurities in the water that affect its freezing point, as salt does, may make an important difference in how much is liquid.) This distant, alien world may have lakes or even oceans of liquid water deep underground. What a fantastic possibility!

There will be no liquid on the frigid surface. Even ice on the surface, exposed to the cold vacuum of space, would sublimate before long. But ice could be just beneath the surface, perhaps well less than a yard (a meter) deep.

Ceres then may have a thin, dusty crust over a mantle rich in ice that might be more than 60 miles (100 kilometers) thick. Its warmer core is likely composed mostly of rock.

As heat dissipated from Ceres’ interior over the eons, it may have undergone convection, with the warmer material rising and cooler material sinking very slowly. This is reminiscent of what occurs in pot of heated water and in Earth’s interior. Even if it did occur at some time in Ceres’ history, it probably is not happening any longer, as too much heat would have been lost by now, so there would not be enough left to power the upward movement of warm material. But the convective process might have written its signature in structures or minerals left behind when ice sublimated after being pushed to the surface. Dawn’s photos of geological features and measurements of the composition may provide a window to forces in the interior of the protoplanet sometime in its past.

Even if convection is no longer occurring, Ceres is not entirely static. We have very tantalizing information from a marvelously productive far-infrared space telescope named for the only known astronomer who found a planet before Piazzi made his discovery. The Herschel Space Observatory recently detected a tiny amount of water vapor emanating from the distant dwarf planet. Scientists do not know how the water vapor makes it into space. It might be from ice sublimating (possibly following a powerful impact that exposed subsurface ice) or perhaps from geysers or even erupting cryovolcanoes (“cold volcanoes”) powered by heat that Ceres has retained since its formation. In any case, Herschel saw water, albeit in very, very small quantity.

It is not certain whether water vapor is there all the time. It is unknown whether, for example, it depends on solar heating and hence where Ceres is in its somewhat elliptical orbit around the sun (not as circular as Earth’s orbit but more circular than Mars’), which requires 4.6 years to complete.

Even if the water vapor is present during Dawn’s 1.3-year primary mission in orbit, it would be extremely difficult to detect. Herschel made its findings when our ship was already far, far from Earth, well along its interplanetary itinerary. The probe’s sensors were designed for studying the solid surfaces of airless bodies, not an exceedingly tenuous veil of water molecules. For context, the water vapor Herschel measured is significantly less dense than Earth’s atmosphere is even far above the International Space Station, which orbits in what most people consider to be the vacuum of space. Dawn will not need windshield wipers! Nevertheless, as we saw in February, the Dawn team, ever creative and dedicated to squeezing as much out of the mission as possible, investigated techniques this year that might be effective in searching for an exceptionally thin vapor. They have augmented the plan with many hours of observations of the space above Ceres when the spacecraft is over the night side during its first science orbit in April and May at an altitude of 8,400 miles (13,500 kilometers). It is possible that if there is some water vapor, the instruments may pick up a faint signature in the sunlight that passes through it.

Regardless of the possibility of detecting traces of water from Ceres, Dawn will focus its measurements on the uncharted surface and the interior, as it did at Vesta. Vesta displayed landscapes battered by craters from impacts during more than 4.5 billion years in the rough and tumble asteroid belt. Ceres has spent most or all of its history also in the asteroid belt, but it is possible it will not show its age so clearly. Ice, although very hard at such low temperatures, is not as hard as rock. So it may be that the surface gradually “relaxes” after an impact, just as your skin restores its shape after pressure has been removed. Craters older than a few tens of millions of years may have slowly disappeared. (That may sound old, but it is a small fraction of Ceres’ lifetime.) Near the poles, where it is colder so ice is harder, the scars of impact craters may be preserved for longer.

Ceres has more than water-ice and rock. It probably contains organic materials, some produced by chemical processes with the minerals already there and some delivered by asteroids that fell to its surface. This is noteworthy, because water and organic chemicals are ingredients for life. The combination of Ceres’ internal heat and the weak but persistent heating from the sun provides energy, which also is essential for life. Even if the possibility of life itself there is extremely remote (and it is beyond Dawn’s capability to detect), the conditions for “prebiotic” chemistry would be tremendously interesting. That is why, as we explained in August, we want to protect the special environment on the ground from contamination by the terrestrial chemicals in our orbiting spacecraft.

Dawn's view of Ceres

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

Dawn's view of Ceres
Dawn acquired this picture of Ceres on Dec. 1, as described last month.

While there is more known about Ceres, there is much, much more that is unknown. Dawn seeks to discover many of the secrets of this unfamiliar, fascinating member of the solar system family. One of the measures of its success would be if, upon answering many of our questions about Ceres, we are left with even more questions. Now on the threshold of an old world which will be new to us, we do not have long to wait for the great rewards of new knowledge, new insight, new thrills and new mysteries to solve.

Dawn is 382,000 miles (614,000 kilometers) from Ceres, or 1.6 times the average distance between Earth and the moon. It is also 3.77 AU (351 million miles, or 564 million kilometers) from Earth, or 1,500 times as far as the moon and 3.84 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take one hour and three minutes to make the round trip.

Dr. Marc D. Rayman
8:00 a.m. PST December 29, 2014

See other posts from December 2014


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


Dustin: 12/29/2014 09:26 CST

Thanks Marc for these great updates. I look forward to more as Dawn approaches Ceres in the coming months. However, may I suggest cutting down on the purple prose. The flowery language and heavy use of adjectives is at times distracting. Love the information, though. Thanks again.

Marc Rayman: 12/29/2014 11:40 CST

Thank you for the feedback, Dustin. Some people actually like my writing style, difficult as that may be to believe. Others are less fond of it. To be brief, part of my style choice is intended to appeal to readers who are not necessarily comfortable with material in science and engineering. I suspect that is a very small fraction of the readership here at The Planetary Society site, but there are many other readers who may consider such topics somewhat intimidating. My intent is to show them that it can be fun and interesting and not as difficult as they might expect. Still, I do appreciate your suggestion.

Ceriously: 12/30/2014 12:36 CST

Can we expect another photo to be released soon, similar to the one taken on 1 Dec?

Marc Rayman: 12/30/2014 01:25 CST

What a great name, Ceriously! Our next photo will be acquired on Jan. 13, and we will release the view shortly thereafter. I summarized the approach imaging plan in my October Dawn Journal (posted here on Nov. 2).

Skip Morrow: 12/30/2014 05:17 CST

I liked the writing style. I wouldn't want every article on Planetary.Org to be written like this, but it is nice to read different styles. Thanks! I think this will be a very exciting year, with Dawn and New Horizons reaching their targets. Dawn is expected to stay at Ceres until early 2016. What do you think will happen after that? Extended missions at Ceres? Or find another target in the asteroid belt?

JohnBurch: 12/30/2014 07:23 CST

I actually look forward to Marc's Dawn journal, mainly because of his writing style which I find refreshingly different from the more dry, technical guest posts that we sometimes get here. Remember that this is outreach and a very impressive form of the art at that.

Ab Oort: 12/30/2014 07:32 CST

Thank you Dr Mark, for your equally excellent explanation, of Dawn's extraordinary extraterrestrial expedition

PatrickN: 12/30/2014 07:45 CST

Thanks for another informative and entertaining article. I'd say Ceres is the dark horse for most astonishing discovery of 2015 - it's remarkable how little we know, how much we will learn so quickly, and what surprises we may find in the bargain. (Thanks for the stylistic flourishes, btw. For me, because your content is so good, they work just fine.)

Marc Rayman: 12/30/2014 10:06 CST

Thank each of you for the further feedback. As JohnBurch observes, this is indeed outreach. I write plenty of dry prose in my technical publications ranging from fundamental physics to engineering of flight operations. I have devoted a very great deal of effort to assessing how to communicate with the broader public. It's much too complicated to discuss here, but I think too much outreach is not only dry but also not accessible to as large an audience as it could be. There's more to it as well. But the feedback I receive on my public writing and speaking is overwhelmingly and enthusiastically (even if not unanimously, with genuine respect to Dustin) in favor of the style I use. Skip, Dawn will remain in its low altitude orbit at Ceres following the conclusion of its primary mission. I explained the reason as well as its extended mission possibilities and its fate in my August Dawn Journal (posted here on Sep. 2).

alektos: 12/31/2014 07:04 CST

Well, I'm following the progress of the Dawn mission with interest, as well as the progress of the New Horizons mission, and I hope that the IAU will, before long, agree that the geophysical definition of planet makes more sense: Pluto is still a planet.

Bob Ware: 01/01/2015 04:32 CST

Thank you for the latest info. and good luck on "seeing" the water vapor we know is there up close and personal. Does NASA's PAO plan any coverage of the orbital approach and insertion at some point in the flight path time line?

Marc Rayman: 01/01/2015 09:10 CST

Bob, We don't actually know if there is water vapor there. We know there was water vapor some (not all) of the times Herschel looked. The details are too involved to delve into here, but as I tried to suggest in my text above, depending on the mechanism responsible for its production, it may or may not be there when Dawn is. (Ceres will be at 2.90 AU during our prime opportunity to look for it in four months; that's closer to aphelion than perihelion.) JPL/NASA will release images and other products throughout approach, and there likely will be one press conference. Of course, there will be even more after intensive science observations commence in late April. As I've explained in a number of my blogs (at Vesta and for Ceres plans), orbital insertion itself, while vitally important, is entirely different from that of missions using conventional chemical propulsion. It doesn't lend itself to the sort of coverage most people expect. See the end of my blog posted here on 02/02/2014.

Devin: 01/04/2015 06:49 CST

Marc, you write much like you speak. I met you at the JPL open house in October and we talked at length. The way you explained things was so engaging, like you were able to adjust how technical your explanations were based on who you were talking to. Your enthusiasm was also inspiring, as I g you questions was one of the highlights of my visit. Had I realized you were a contributor to the planetary society I would have mentioned it. Thanks again!

Bob Ware: 01/05/2015 08:52 CST

Thanks for returning back on path with my water vapor comment Dr. Rayman! A bit to much wishful thinking when I wrote that thought! I guess I should stop to much wishful thinking! : ) Thanks for the other info as well. I hope you all fly right down the center of the plot board to your destination! Good luck to you and the team!

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