Vesta – Curry World?

Not to be confused with PC World, Vesta is saddled with a problem a number of other celestial bodies also experience of having weird pop culture associations. There’s Pluto, after which the Disney dog was apparently named, and while I’m at it, as observed in ‘Dazed And Confused’, why does a cartoon dog have another cartoon dog as a pet? There’s also Uranus, whose name can be pronounced as either “your anus” or “urine-us”. And getting back to the original subject, there is Vesta.

I don’t know how widely the fame of Vesta curries extends, but certainly in England the name has been substantially associated with the things White people used to get in boxes from the supermarket in the 1970s CE, and one of my friends reckoned that the TV series ‘Adrian Mole’ succeeded in nailing the working class Leicester experience perfectly when they ate a Vesta. Goodness knows what South Asians would’ve thought of them. Having said that, I’ve never tried them and that’s even though I’ve been reduced to buying samosas from Sainsbury’s because of the cultural desert I seem to live in nowadays. A quick Google confirms that they do still exist. I mean, I liked Marvel dried milk and Smash instant mash back in the day, so maybe I’d’ve liked them, I dunno.

Why, though, has Vesta got the same name as Vesta, or for that matter Vesta or Vesta? There’s a car, a box of matches and a world in the asteroid belt, and that last one I will get round to in a minute, but for now it’s in order to mention the original Vesta. Vesta was the Roman goddess of hearth and home, which of course immediately makes me think of Dexy’s Midnight Runners because my brain doesn’t work properly:

This is the surrealist painter Max Ernst’s 1937 painting ‘The Angel Of Hearth And Home’, which will be removed on request. It’s one of his few overtly political paintings and represents the spirit of chaos spreading across Europe in the wake of the Spanish Civil War. The title is meant to increase the sense of unease and disorientation one feels on looking at it. It is a vaguely humanoid figure with a fierce-looking fanged mouth and a seven-fingered hand sprouting from its knee. It’s actually the opposite of what one might expect from an angel of hearth and home, and more like death. Well, this opposite figure is the Anti-Vesta. The main association people make nowadays is of course with Vestal Virgins, who undertook not to have sex for thirty years while tending the sacred fire in Rome, considered to be vital to the city’s security. Hence they were tending the hearth of the whole Empire. This is part of a theme in asteroid naming in the early nineteenth century, where the names of female figures were chosen who were also somewhat domestic in nature. I’ve already mentioned Ceres, there’s Vesta, and also her Greek counterpart Hygeia, Juno goddess of marriage and childbirth as well as rather more outward-going things like the state, Flora, Hecuba (Priam’s wife), Victoria and so on. They also often have their own sigils at this early stage, but the point appears to have come when there were so many of them that they gave up.

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This is Vesta’s sigil, clearly representing the eternal flame. Maybe one day it’ll grace a flag. This is Vesta itself:

I’ve selected this rather dingy picture because it shows two features of the body (I’ll talk about its exact nature in a bit) which are particularly distinctive, namely the streaks and the “Snowman””, which is the cluster of craters on the right hand side of the picture. Once again, then, there’s a body with a number of distinctive streaks, like Phobos.

What, though, is Vesta? Is it an asteroid? Ceres kind of turned out not to be, and Vesta may be the second largest. Whereas Ceres is large enough to eclipse the whole of Great Britain and Ireland, Vesta is only big enough to cover Ireland. It’s also the brightest member of the asteroid belt, bright enough in fact to be visible to the naked eye on occasion, although it wasn’t actually discovered until 1807, which happens on occasion. Uranus is also sometimes visible but wasn’t discovered until the eighteenth century. Then again, for many millions of years there must have been animals on whose retinæ images of Vesta, Uranus and even fainter worlds must’ve registered and influenced their visual cortices, but actually recognising it as something orbiting the Sun is another matter. But in any case, Vesta is the brightest asteroid, if asteroid it be. It’s probably also the second largest body orbiting twixt Mars and Jupiter except that Pallas is very close to it in size and it may therefore not be. It has a diameter of 525 kilometres on average, but is considerably less round than Ceres. This makes it definitely larger than Ireland, and in terms of area it gets harder to work it out, but assuming it to be a sphere, which is definitely not true, it’s slightly smaller than Pakistan. Perhaps surprisingly there is no straightforward formula for working out the perimeter or an ellipse, and therefore I’m assuming that no such formula exists for working out the surface area of an ellipsoid either. It’s larger than Mimas, which I always think of as the smallest round body in the system and as a kind of limit below which I kind of have less respect for objects, which may be unfair. Hence there must be something about Vesta’s substance which enables it to retain non-sphericality at a fairly large size, and I imagine this is linked to its rockiness as Mimas is probably much icier.

Although Ceres is the largest object in the asteroid belt, Vesta is the largest one native to it. The large amount of ammonia on and in Ceres suggests that it was originally in the outer system and only arrived in the belt later. Vesta is not like that and has probably always been there. It takes up nine percent of the mass of the asteroid belt and is quite close to being spherical, but just misses out on being a dwarf planet, although it may be the largest object in the system which is decidedly non-spherical. Unlike Ceres, it actually was discovered by the celestial police force set up to find bodies between Mars and Jupiter, and was the fourth discovered by Heinrich Olbers of Olbers’ Paradox fame (why is the night sky dark rather than bright? This is actually a very important question with massive consequences for the nature of the Universe but I don’t want to talk about it here. It’s basically because space must be expanding). It was in fact the last asteroid to be discovered for a long time, and it’s a little surprising that it was only the fourth to be found because it’s so bright and large. The next one, Astræa, wouldn’t be found until 1845, after all the original discoverers had died, then there was a spate of further revelations after that. Vesta therefore probably counts best as the largest asteroid, unless Pallas is, and traditionally people would’ve said Ceres.

Vesta isn’t like Ceres at all, but it is very much like a number of other asteroids in the belt. Some of these are former bits of Vesta which have chipped off due to impacts, but some have orbits which indicate they could never have been anywhere near it and must therefore have formed separately. It’s also responsible for quite a large number of meteorites which reach Earth, and therefore we actually have samples of it. Some of them are even from quite deep inside the asteroid, so its composition can be ascertained fairly well, and it can be seen from these that the asteroid is layered rather than mixed, as a smaller one would be, meaning that it’s heated and melted internally at some point. Its surface has for some time been known to be basalt, which on Earth comprises ninety percent of igneous rocks. On most rocky worlds in the system, igneous and metamorphic rocks are almost all there is. There are some exceptions, such as the strata on Mars, but on the whole there are no sedimentary rocks and the idea of sedimentary as a category is fairly specific to Earth, although there is, for example, clay and the layers of substance on Io, which aren’t sedimentary but are stratified. However, tuff, which is layered volcanic ash, is sedimentary, so water or any other fluid medium isn’t required.

Vesta and Ceres are kind of in each others’ vicinity. The average distance is 2.36 AU from the Sun compared to Ceres’s 2.77, which is around 61 million kilometres apart, about the same as Earth and Mars. This isn’t particularly close of course and reflects the fact that the asteroid belt is actually pretty sparse, but it is roughly as close as the orbits of Earth and Mars. However, the minimum distance is only five million kilometres, although this can only occur when the orbits are precisely aligned. It wouldn’t happen every orbit or even every thousand orbits, because it would depend on the ellipses shuffling round. Vesta’s orbit is also less tilted than Ceres’s at 7°, so they may not pass as closely to each other as might initially seem. The year is three and two-thirds longer than Earth’s. Vesta actually approached the Sun most closely only a month ago, on 26th December 2021.

Earth is slightly flattened at the poles and bulges at the Equator because of its rotation pulling the substance of the planet outwards during formation, when it rotated much faster and was softer. I’m not sure how much contribution the current centrifugal effect has on it. Nonetheless the deviation from sphericality in our case is only 0.3%. In Vesta’s case, the asteroid is kind of tangerine-shaped and its oblateness is around 22%. Also, its equator is elliptical too. An object whose gravity is so low (2.5% Earth’s, which is somewhat lower than that of Ceres) is able to have higher irregularities on its surface, and therefore Vesta also has a mountain which is almost the highest in the system – Rheasilivia is the biggest crater and unlike those on Ceres has a central peak, in this case two hundred kilometres across and is twenty to twenty-five kilometres high, comparable to the Martian Olympus Mons. The crater surrounding it is relatively enormous too, at five hundred and five kilometres diameter or roughly a “πth” of the circumference. In other words, the crater is actually wider than the asteroid in one of its dimensions, and in a way the asteroid could be looked at as simply the site of the crater. As such the rings of streaks may make a lot of sense as ejecta, although I don’t know for sure that’s what they are.

The streaks, known as fossæ, are troughs in the surface encircling the asteroid at the equator. They include Divalia and Saturnalia, the former being larger than the Grand Canyon and twenty kilometres deep. This scale reflects Vestan low gravity, which allows absolutely larger features which give worlds of this size an almost cartoonish or “cute” appearance, with exaggerated features which look out of scale to humans like the big eyes or other features of an animated or cartoon character. The fossæ are grabens, that is, valleys caused by faulting between which the surface has dropped, caused by the impact of the object which formed Rheasilvia. The central belt of Scotland is an example on Earth. Divalia is around ten kilometres wide and 465 kilometres long, making it four times as long but only a quarter as wide as the Lowlands. The fossæ collectively are in the top twenty largest rift valleys in the system. Earth is actually the world with the most large rift valleys, although the very largest is on Venus. Earth’s largest is the Atlantic. Saturnalia Fossa is associated with Veneneia, a crater overlapping with Rheasilvia and only slightly smaller than it at 400 kilometres diameter. Saturnalia is thirty-nine kilometres wide and 365 kilometres long, possibly longer because its end was lost in shadow when Dawn surveyed the asteroid.

Although Vesta is near Ceres and other asteroids relative to the scale of the system, it’s still pretty remote considering its size. If you were living on Vesta, it would take a lot of resources to bring anything you didn’t already have to you. It’s like a desert island in a way, and has resources of its own. Geologically, it’s stony, unlike Ceres which has a lot of clay stuff going on, and is more like an inner system planet in its composition than Ceres is. It’s like a mini-rocky planet, although it isn’t large enough to be a dwarf planet.

About six percent of meteorites falling here on Earth are from Vesta. This can be determined because they are exactly the same colour, that is, their spectra are identical. This is more common than any other body, even though Cynthia is so close and there are also meteorites from Mars and Mercury, both of which are closer most of the time. The light grey colour of the asteroid can be seen in the meteorites too. Vesta’s brightness is partly due to it being large and close, but it reflects more than 42% of the sunlight falling on it, which is more than any of the large planets except Venus. This is because it hasn’t been subject to “space weathering”, which occurs on bodies with only weak magnetic fields and is caused by the attraction of solar wind particles to the surfaces, where they vaporise iron on the surface, turning it into a dark coating. This means that Vesta is either low in iron or has an appreciable magnetic field. Since samples of the asteroid are readily available, it’s possible to test this by seeing if magnetic specks within the meteorites are lined up, and they do seem to be, meaning that the asteroid must be generating the same kind of dynamo-style magnetic field as we have on our home planet.

This brings up the issue of the innards of the place. NASA’s Dawn mission was able to collect data implying that unlike Ceres, Vesta does indeed have an iron core, which is about 110 kilometres in diameter, which means it must have melted early in its history. There are so many meteorites from the asteroid that it’s possible to mount a similar kind of museum exhibition about its mineralogy as it is of Earth’s, actually better in some ways because its smaller size means relatively deeper samples are available than from Earth. As mentioned previously, the most common such asteroid is known as HED – Howardite-Eucrite-Diogenite. I’ve covered these on the linked post. Incidentally, I love the fact that some are called “diogenites”, which suggests they’re either very messy inside or don’t require much in home comforts. It’s just a shame they aren’t called damoclites, like they’re hanging over us waiting to wreak havoc, although that would be rather geocentric.

I ought to mention the Snowman. This is a short chain of relatively large craters, named from bottom to top of this image, Marcia, Calpurnia and Minucia. Together they form a shape reminiscent of a snowman. The method of relative dating of craters works well here as impacts will cause newer crater borders to impinge on older ones rather than the other way round, making it possible to reconstruct what happened, though without much of a timescale.

Like Ceres, Vesta is a protoplanet, though one not given much chance due to being close to Jupiter. Had it been able to form into a proper planet, what can be seen today would’ve been buried deep within its core, or rather, its substance would’ve been distributed throughout the planet’s interior. It has a relatively short day for an asteroid of five and a third hours and a tilt of around 29°, meaning that again unlike Ceres it has seasons.

One of Asimov’s earliest short stories was called ‘Marooned Off Vesta’. It’s actually his first published story, from March 1939, where a spaceship is hit by a meteoroid, leaving three survivors in a fragment with only enough air for three days but the entire water supply for the spaceliner. They’re near Vesta, where a few people have settled. It was followed up by a later ‘Anniversary’ story twenty years later where the survivors have a reunion and discover something surprising about what they salvaged. It dates from the time when the asteroid belt was thought to be strewn with hazardous débris, which is now known not to be so.

That’s it really. Vesta is the largest proper asteroid, the brightest asteroid and, most remarkably, the source of more meteorites which reach Earth than any other body in the Solar System. That’s it really.

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.