The World Ceres

Title nicked from Asimov.

On the first day of the nineteenth century CE, the astronomer Giuseppe Piazzi pointed his telescope at an area of sky in the hope that Bode’s Law wouldn’t fail him, and indeed found the first independently-orbiting body within the orbit of Saturn since ancient times. This was in spite of an organised posse of astronomers, the “Celestial Police”, searching the heavens for such a planet. They were later to find more, but Piazzi, who had actually been considered for membership of this group, beat them to it. This was the world later to become known as Ceres.

Bode’s Law is the rather unfairly titled principle which appears to determine the distances of the planets from the Sun. It was actually first arrived at by Johann Titius some time before. It uses the sequence 0, 3, 6, 12, 24, . . . , to each of which four is added, giving 4, 7, 10, 16, 28, and has been fairly successful in predicting the positions of various planets. It was popularised by Johann Bode, hence the name. The units amount in this case to tenths of an AU, which is the distance between Earth and the Sun, as is seen in Earth’s position in this series at 10. The series isn’t perfect. For instance, it’s anomalous that it starts at zero and Uranus doesn’t fit, although Neptune does. Nonetheless, astronomers noticed there seemed to be a gap at 28. Mars is 1.524 AU from the Sun on average, with an aphelion of 1.666, whereas Jupiter averages out at 5.204. Astronomers used the sequence as evidence for another planet, and they found it.

However, the planet they found was rather odd compared to the others known at the time. The smallest known planet in the eighteenth century was Mercury, now known to have a diameter of 4 879 kilometres. Ceres is much smaller than this at 946 kilometres. During my lifetime this figure has been revised several times, so I imagine it was different in the early nineteenth century too, but in astronomy books at the time, Ceres is clearly shown as much smaller than the other known planets, yet still acknowledged as one, before the asteroids were discovered.

Over the next few years, a number of other bodies were found between Mars and Jupiter, and the planets were split into the categories of major and minor planets to account for them. Ceres was relegated to the status of a minor planet or asteroid for a long time, up until the decision to redefine planets in 2006 as mentioned here, at which point it was put in the same category as Pluto, a “dwarf planet”. As I’ve said before, I’ve never really understood why there needed to be such a category when that of “minor planet” already existed, but it did at least put Ceres in the same pigeonhole as Pluto, which was some kind of progress. It’s an interesting history though, because it means its tale with us began as a planet, stopped being one and then became one again. Also, in the light of what I’ve said previously, nowadays it could even simply be seen as a planet.

Ceres is not like the asteroids, even though it orbits among them. It conforms to the second 2006 criterion of planethood in being round due to its gravity. No other asteroid is so close to being spherical and the margin is actually quite sharp. The next closest seems to be Hygeia. Taking all known bodies in the system into consideration, the smallest round one is Mimas, which orbits Saturn and has a diameter of 396 kilometres, although it has an enormous crater which prevents it from being perfectly round. It isn’t “lumpy” though. Hygeia is actually larger than Mimas with a diameter of 444 kilometres, and is in fact a candidate dwarf planet in itself. There could be much smaller asteroids which are round, but if so this wouldn’t be due to their gravity.

The planet, for that’s what it is really, is the smallest in the system which orbits the Sun independently, but it also contains the bulk of the mass of all bodies between Mars and Jupiter, at about 30%. This means that even if the hypothesis about a lost, shattered planet there had been correct, or if Jupiter was in a different place and the mass of the asteroid belt had been able to assemble itself into one, it would still be smaller than Mercury or even Cynthia. Because it’s long been dismissed as an asteroid, Ceres has occupied a kind of second-class place in the system for a long time and consequently I for one, and presumably most other people who have learned abut these things, can’t easily reel off a list of statistics and facts about the planet as I would with, say, Uranus or even Pluto. I know its day lasts nine and a bit hours, that it has a very thin atmosphere indeed, not really even worth mentioning, but I don’t know its largest craters, axial tilt, how long it takes to orbit the Sun, highest peaks, climate or any unusual features. I do know that it has more water ice as part of its actual internal structure near one of the poles and that it has some water ice on its surface.

The distance from the Sun is kind of “unusual”. In fact it isn’t unusual at all as the zone Ceres occupies in its orbit is the most crowded of any in the system. However, because we haven’t tended to think of Ceres as a planet, and to be fair it is still something of an outlier as far as planets directly orbiting the Sun are concerned, we haven’t considered what happens at this distance. The main consequence is that it has an unusual range of surface temperature, between -163 and -38°C, which means that at its warmest its temperature overlaps with Earth’s. In other star systems there are probably larger planets in this kind of orbit because of other characteristics being different, such as no giant planets or giant planets in different positions, but for our system this is notable for being intermediate between the coldest (on average) terrestrial planet and the warmest gas giant. If it had the same atmospheric pressure as Earth, Ceres would be able to have liquid ammonia on its surface which could both freeze and evaporate, and the chances are there’d be an ammonia cycle like our own water cycle, along with rivers, lakes, rain and even snow and glaciers. However, in reality there’s practically no atmosphere. Even so, ammonia is rich on the surface and participates in the planet’s geochemistry, which suggests that it originated far out in the outer system where the compound is more abundant. There are clays rich in ammonia and ammonia salts in some of the craters. There is also the intriguing ammonium ion, NH₄⁺. This is distinctive in both bearing a single positive charge and being about the same size as some alkali metal ions, meaning that it behaves as if it’s a metal ion like sodium in sodium chloride, and can even form amalgam with mercury and sodium like solid metallic elements. In other words, it can form into metallic alloys even though it isn’t itself a metal. Due to all this, the geology of even the surface of Ceres is unique, at least for the more reachable part of the system. I may be wrong about this but I think of it as a clay-covered place, except that instead of water making it moist, ammonia does that job instead, and also unlike water (although the hydronium ion is common in the Universe, which is to water as ammonium is to water) in that it behaves a little like an alkali metal.

The asteroid belt divides the five inner terrestrial from the five traditional outer planets (gas giants plus Pluto) of the outer. Hence Ceres can be thought of as the middle planet of the Solar System, or to put it another way, central to it. This is not literally true because as the Titius-Bode Series shows, the planets are each almost double the distance of their predecessors from the Sun counting outward. This means that its composition and temperature are intermediate. It may or may not have a global ocean under its crust. This may have existed but will now have frozen. It would be possible to detect because it would be salty and this would make it detectably magnetically.

There is a single remaining extinct cryovolcano on the surface called Ahuna Mons, which is five kilometres high. At some point I will need to address what counts as height on planets without bodies of liquid on their surfaces. In this case there’s a clearly visible crater next to the mountain, Occator Crater, and it wouldn’t be sensible to assess its height from the bottom of that crater although it might influence its structural integrity. There are white streaks on the slopes like lava flows, and also like the white patches elsewhere on Ceres, all of which are probably salt. Incidentally, although “salt” brings sodium chloride to mind, I can’t find out whether this is the salt in question or whether it’s ammonium chloride, which is also white, or something else. It could be a mixture, but that’s my speculation. There are also possible traces of smaller volcanoes. There’s a concentration of mass about thirty kilometres below it, which suggests it was formed from a plume of mud rising from the mantle (which was probably watery). There’s also sodium carbonate (washing soda) on the slopes, which is found on Earth in desert regions as the mineral natron, used in the Egyptian mummifying process and to make glass. Ahuna is almost exactly opposite to the largest impact crater, Kerwan, suggesting that it may have resulted from shock waves moving around the planet and concentrating on the other side, where they fractured the crust. This happens a lot with large impacts. For instance, Caloris Planitia on Mercury is opposite so-called “chaotic terrain” on the other side, and in fact this is making me wonder right now what was opposite Chicxulub when the impactor hit, killing the larger dinosaurs.

Occator, next to Ahuna, has the largest concentration of bright spots. I have to say, looking at images of all the large bodies in the Solar System, Ceres is distinctive in having small white areas fairly sparsely distributed across its surface. These have been named faculæ, meaning “little torches” in Latin, a name first used to refer to bright spots on the Sun’s photosphere. They’re near ammonia-rich clays and are rich in magnesium sulphate, which is Epsom salt, so the whole planet has a kind of domestic chemical theme going on. These are on a hill in the centre of the crater called Cerealia Tholus, and at this point it’s worthwhile mentioning the name. Ceres is named after the Roman goddess of arable farming, after whom cereals are named. Ceres is known substantially for her daughter Proserpina, more often known by her Greek name Persephone, who was forced into marrying Pluto and living in the underworld, but finding that she could return provided she didn’t eat any food there. However, she ate three pomegranate pips and is therefore condemned to spending a third of the year there. Ceres mourns this by causing winter, and celebrates her return to the upper world with spring. The Greek equivalent of Ceres is Demeter, after whom a moon of Jupiter is called although this was renamed in 1975. Thereby hangs a tale, incidentally: Jupiter’s smaller moons all got renamed in the mid-’70s. The whole domestic flavour of the place is once again confirmed by the mention of cereal. This is a planet made of washing soda, ceramic (kind of) and Epsom salts named after the goddess of cereal! The rare earth metal cerium, discovered two years later and now used in lighter flints and the subject of an essay by Primo Levi, is named after the planet, rather like uranium, neptunium and plutonium.

Occator is unusual in having a central hill. This is normal on many craters on other bodies, but Cerean craters tend just to have dents in the middle. The largest crater is the previously mentioned Kerwan, one hundred and eighty kilometres in diameter. It isn’t clear if it had a central peak because a smaller impact has created a crater where that would be. It’s named after the Hopi cereal nymph, this time for sweetcorn.

Zooming out a bit and treating it as a planet like any other, as opposed to the asteroid it was formerly presumed to be, Ceres averages 2.77 AU from the Sun, approaches it most closely at 2.55 and has an aphelion of 2.98, which makes its orbit slightly less elliptical than Mars’s at 0.0785. It takes somewhat over four and a half years to orbit the Sun and is inclined 10.6° to the ecliptic, which is greater than any other planet out to Neptune unless you count the moons of Uranus (see the post on planet definitions if you don’t get why I’m calling them planets rather than just moons), though less than twice that of Mercury. Looking at the three planets Earth, Mars and Ceres as a, well, series, there is a trend of reducing size. Mars bucks the apparent trend of increase in size up to Jupiter followed by a decrease in size out to Pluto, but if Ceres is included a new possible tendency is revealed, also reflected in reducing density as Earth is over five times as dense as water, Mars and Cynthia around three times as dense and Ceres a little over twice as dense. This may just be playing with numbers, but it’s also possible that Earth hogged all the material, only leaving a few leftovers for the planets closer to Jupiter’s orbit. As for density, the closer planets to the Sun would have been warmer when they formed and this seems to have caused the icier components, or simply those with higher melting and boiling points, to evaporate off. However, Ceres seems to have formed in the outer system. It has an axial tilt of only 4°, so ironically the planet named after a goddess closely associated with the seasons has no seasons of its own. Surface gravity is less that three percent of ours, so if I went there I’d somewhat exceed my birthweight but only because I was small for dates.

Looking at the planet and knowing that most of what I’m seeing is clay puts me in mind of the idea that Ceres has an affinity with the various planets which show up in claymation shows. I can imagine its appearance turning up on someting by Aardman Animation, and it makes me wonder what the Clangers planet was originally made of. However, this is largely in my mind. It’s all very well looking at an image of Ahuna Mons or the planet as a whole in full knowledge that it’s mostly salty clay and seeing it like that, but on the other hand many of the craters are æons old and don’t seem to have sagged in all that time, although they do lack the central mounts found elsewhere. It may be more accurate to think of the planet’s surface as being made of frozen clay rich in ammonia, and it also isn’t clear what clay’s like if it’s mixed with liquid ammonia and well below freezing point as opposed to the stuff we make pots out of. I think Ceres may be the kind of place where our intuitions based on how things are here, or even in the outer system, may mislead us. That said, the edges of the craters are less well-defined and the floors are smoother, and when it was actually being hit by something it would presumably have melted or boiled the material, so at that point maybe it does behave like clay or go through a phase of clay as we know it as it cools down.

Although it doesn’t have an iron core, the planet is likely to have a core high in metals, but also in silicate rocks. The pressure on it will be far lower than on Earth’s core. Our planet is close to 6 371 kilometres in radius, more than twice as dense as Ceres and has thirty times the gravity. Put all of those together and it makes the pressure at the core something like (and these are back of the envelope calculations) what it would be only ten kilometres down in our own crust, or even less. This is only the level of an ocean trench and only a few times deeper than the deepest mines. Consequently the settling out effect of the originally molten planet is milder and not so influenced by pressures beyond easy imaginings. Outside that core is a mantle of silicate rock which may have squeezed out the water and ammonia, or they could have separated out due to being lighter. Above that is a probably frozen solid ocean, and finally on the surface lies the clay-rich crust with salty deposits. All this notwithstanding, it’s also been accurately described as “icy, wet and dark”, i.e. it has a dark surface. It isn’t particularly dark as far as sunlight is concerned.

There are several more ways in which Ceres is special. It’s a survivor from the early Solar System, in that it’s a protoplanet. Near the beginning, there would’ve been hundreds of small planets like this, large enough to undergo interior melting, which mainly happens due to radioactivity, and therefore stratification like Ceres has, but many of them would have collided with each other and stuck together, possibly been thrown out of the system entirely by close encounters with others accelerating their movement. Along with Vesta, which is more battered and smaller, Ceres is a surviving relic from shortly after the Sun formed. It’s also the closest dwarf planet to Earth, the first dwarf planet to be visited by a space probe, the first time a space probe had orbited two bodies on its mission and the largest body except Pluto-Charon not to have been visited up until 2015.

The spacecraft which visited it is also quite interesting. It’s called Dawn, and was actually launched at dawn one day in 2007. It used Mars to accelerate its path and visited and orbited Vesta, also a first, in May 2011. Vesta is interesting in itself, and I’ll be covering that soon as well. It then left Vesta and made its way to Ceres, becoming the first spacecraft to actually orbit two bodies in the Solar System unless you count the orbits made of Earth before some spaceships have headed off into the void. It’s still orbiting Ceres but its mission is now over. Dawn was also the first craft to use ion drive, an idea for a very efficient but slowly accelerating engine which can accelerate vehicles so fast they could cover the distance between us and Cynthia in less than two hours, without using gravitational assist, which is the usual reason space probes are accelerated to this velocity and beyond.

There is plenty more to say about Ceres, but I want to finish as I started: with the pun. Isaac Asimov used to be very fixated on puns, and several of his short stories were only written to make puns. In the case of his article ‘The World Ceres’, published in 1972, he may have been primarily motivated to write it just because he could use a good pun in the title. I have read it but I don’t remember how much detail he went into. It doesn’t seem likely that much was known about it at the time, but I may be wrong. It might be interesting to compare factual articles on astronomy before and after they were visited by probes. For Ceres, this period was a lot longer than usual, but also occurred only 206 years after it was discovered, which is pretty good going.