Unlike the Planet Zanussi, we have no rings circling our home planet. Glancing at the rest of our Solar System, we appear to be able to discern a pattern. The inner planets Mercury, Venus, Earth and Mars have no rings. The outer gas and ice giants Jupiter, Saturn, Uranus and Neptune do have rings. From this it would be easy to conclude that rings are the exclusive preserve of either the outer Solar System or large planets. This is, however, probably not so, and will certainly not be so a few million years from now.
Until the late 1970s the only known ringed planet was Saturn, whose rings are particularly bold and striking. In 1977, a star was observed to blink on and off a number of times before Uranus passed in front of it, and again the same number of times on the other side at the same intervals. Later, one of the Voyager probes managed to get a good photo around the time it was leaving the orbit of Saturn, of the whole ring system. By that point, the same spacecraft had discovered that Jupiter too had rings, although much less substantial ones than Saturn. Then there seemed to be a long period, relative to my life anyway, when it was just those three, and it seemed odd that Neptune wouldn’t have them but there was not yet any evidence. After the detection of rings around Uranus, astronomers started to look for Neptunian ones but were unlucky with the placement of the planet during the 1980s because it was in a particularly blank part of the sky, and since it takes around a hundred and sixty-five of our years for it to get round the Sun the opportunities to observe occultations of stars are rare compared to Uranus, which moves against the dark background more than twice as fast. That said, there was an occultation, but it was a freak event caused by a tiny moon called Larissa which wasn’t discovered until Voyager got there. It’s under a hundred kilometres across, so the chances were minute.
It has also emerged in the meantime that some asteroids have rings. This seems to be because they tend to be “rubble piles” – bodies consisting of lumps of solid matter held together by a weak gravitational field but not compacted into a single solid body. Amalthea is probably like this too. Consequently, particles can get kicked up from their surfaces by various gravitational events or collisions which then orbit and end up forming rings due to collisions among themselves or momentum from the spin of the asteroid. That’s my guess incidentally. I have no idea if it’s actually true. For the same reason, the anomaly of Earth having an unusually large moon for its size may remain anomalous, because although there are many smaller bodies with relatively large satellites, notably Pluto, this may reflect the fact that their gravity is lower. Again, that’s me guessing. Pluto’s moon Charon is, though, 12% of Pluto’s mass, which means that if the situation were replicated here our moon would be larger than Mars.
I mentioned previously that there will one day be a ringed planet in the inner Solar System. That planet is Mars. Mars is the only planet other than Earth inside the asteroid belt with its own satellites, in the form of Phobos and Deimos. I’ve long thought of them as captured asteroids, and I’m sure they are, but the reality seems a bit more complicated. Mars may have had rings in the past and may have them again in the future. I need to explain the Roche Limit.
We’re accustomed to thinking of objects orbiting, say, Earth as having no gravity. We think of them as being in free fall. On the scale of something like the ISS, this is so close to being true it’s probably not worth considering that it isn’t. In fact, if a rigid body is orbiting a planet, only its centre of gravity is likely to be at zero G because the rest of it isn’t following the exact orbit. The further from the centre of gravity you get, the stronger the gravity becomes. This would be noticeable on a human scale if an astronaut was orbiting a neutron star, for example, because in that case the effect is so extreme that the gradient of increasing gravity is too, and a person in that situation would be quickly torn to pieces. This effect is also true of Earth in both the lunar and solar gravity fields. The core of the planet, more or less, is orbiting the Sun, but the surface is always slightly deviant. This puts minor strains on the crustal rocks, which move up and down by a few metres twice a day, but since they’re solid the effect is quite small. The same does not apply to liquids, notably the ocean, and consequently the level of the sea goes up and down twice a day, somewhat influenced by the Sun’s gravity but much more by lunar gravity. These are of course tides, and this kind of gravitational influence is called “tidal”.
If a body, such as a moon, has a sufficiently large orbit it can be large and hold together under the tidal forces applied to it by the planet, but if it’s within about 2.44 times the planet’s radius, known as the Roche Limit, it will be pulled apart by the tides. Amalthea is somewhat affected by this. The ends of the moon in the orbit have such a low gravity that rubble and dust from the surface is constantly leaving and entering into orbit around Jupiter, forming a ring. Clearly if Cynthia (“the Moon”) were sitting on the surface of this planet, assuming it wouldn’t crash through the crust, which is what would actually happen, it’s easy to see that it would break up and turn into a pile of rocks, which would still be too high and therefore collapse under its own weight until it covered Earth’s surface in tiny moon fragments.
Something similar to this seems to have happened on one moon in our Solar System. Iapetus, also distinctive in being almost black on one side and abruptly changing to almost white on the other, has a huge ridge running round its equator which is thought to be a collapsed ring.
The Martian moons are in highly unstable orbits. Over a period of many millions of years, the orbits of all planets, dwarf planets, asteroids and moons in the Solar System are unstable, but this is particularly true of Phobos and Deimos. Phobos is covered in streaks where Martian gravity tears at it. It will be torn apart and form a ring in something like fifty million years. This is actually something like the sixth or seventh time this has happened, because it appears that this is a regular process, and each time Phobos moves outwards, having lost most of its mass to form an increasingly tenuous ring.
This is crucial because it shows that small planets in the inner Solar System could in fact have rings like their big sisters. Mars is in a sense a special case because it’s next to the asteroid belt and can more easily capture asteroids which then get smashed up by tidal forces, but it’s also the second smallest planet and this is significant. It’s also significant that ring systems are generally temporary. We happen to be living at a time when Mars lacks rings and Saturn has particularly visible ones. The lower reaches of the rings are braked by the planet’s upper atmosphere and constantly rain down onto the clouds at a rate of about 2 500 tonnes a minute. This could explain the Crêpe Ring, which is a fainter inner ring closest to the atmosphere, clearly losing particles. They’re due to be gone completely by 300 million years from now, which is around seven percent of the age of the Solar System, suggesting that they haven’t always been there. On the other hand, Mars does intermittently have rings but they regularly appear and disappear. It’s more or less mere chance that we happen to be around at a time when Saturn has prominent rings and Mars has none. That said, Saturn does have unusually bright and extensive rings, which is because they’re made of ice and reflect sunlight well. Incidentally, I’m going to mention this here although it really belongs on homeedandherbs, my home education and herbalism blog: herbs are traditionally categorised into different planetary governances and zodiacal signs according to their features, so for example plants who vigorously defend themselves with spines or stings are governed by Mars. Plants with prominent rings are governed by Saturn, which seems to make sense until you realise that they were in fact already considered Saturn’s before Galileo discovered them in 1610.
Two rival theories about ring formation have been offered in the past. On the one hand, they could result from bodies which are torn apart because they entered the Roche Limit of the planet concerned, which at the time was Saturn because no other planetary rings were known to exist at the time. On the other, it could simply be that planets gather planetesimals (small chunks of potential future planets and moons) around themselves early in the history of star systems which fail to coalesce because of these tidal forces. This second theory probably isn’t correct as an explanation of the rings which currently exist, but it may nonetheless have been true in the early Solar System. Perhaps all the planets originally had rings, including Earth.
The transient nature of ring systems has another consequence: Jupiter may have had more obvious rings in the past or acquire them in the future. Moreover, these rings could be made of ice and brighter than Saturn’s because they would both be closer and reflect more powerful sunlight. A substantial Jovian ring system would have a diameter of 341 160 kilometres, considerably larger than Saturn’s 270 000 kilometre system, and at closest approach would be about half the distance to Saturn, making them reflect 60% more sunlight, be twice as large and therefore four times as bright anyway, which is over six times brighter. However, unlike Saturn, Jupiter’s axis is almost perpendicular to its orbit and we would only see them side-on. They would be detectable but not spectacular from Earth, although they would be large enough to be visible to the naked eye as a separate structure from the disc of the planet. Moreover, this may well have been the case in the past because Jupiter is close to the asteroid belt and many of its moons do in fact appear to be captured asteroids, and since it’s the largest known solar planet it stands the best chance of developing a substantial ring system, and in fact has done so. In a way, we are living at a slightly anomalous time in Solar System history because only the second largest planet has the most vivid rings.
The development of rings at this stage in the history of the system is most likely to be caused by planets capturing substantial bodies within their Roche Limits which do not immediately impact on their surfaces. The probability of this happening, assuming an even distribution of asteroids and the like, which is of course not so, increases with the size of the planet. Saturn has relatively weak gravity due to its low density, which is less than that of water, but the radius of its Roche Limit is large, so it’s relatively likely to acquire rings even though it doesn’t have much oompf. The Roche Limit is also a volume – effectively a hollow sphere with a planet at the centre – so the relative probability of rings can be calculated as the cube of the radius of the Roche Limit relative to another planet. This means that Saturn is about 760 times more likely to get rings over the same period of time than Earth, but also that Jupiter is 73% more likely to get them even if it occupied Saturn’s orbit rather than sitting next to the asteroids. It’s actually a bit of a freak occurrence that Jupiter’s rings are so much fainter than Saturn’s.
Our home world is the largest planet in the inner Solar System and all other things being equal is therefore the most likely planet of the four to develop rings. Venus is also quite likely, which would lead to a fairly spectacular view even from here, mainly in the form of a brighter Venus. All ring systems are likely to be similar in several ways. They will be the same relative width compared to the disc of the body at their centre, they will have gaps in them corresponding to the distances of large satellites, they will be circular and they will encircle the equator. They would, however, differ in other ways. The four gas giants are cold and therefore likely to have icy rings. Inner planets could also have this for a short period of time if they capture an icy body descending from the outer system, but this will be very temporary, only lasting a few decades at most in Earth’s case. They are more likely to have fairly dark rings made primarily of rock, carbon or iron, or a mixture. However, this doesn’t mean that the picture at the top of this post is unrealistic because although Cynthia is fairly dark, the surface still reflects enough sunlight to be clearly visible during the day and very bright at night. The rings would also be much closer and larger than our satellite, and therefore much brighter.
Earth’s rings could be seen as a kind of compensation for not having auroræ. The view near the Equator would be magnificent, and they would continue to be visible as one moved towards the poles, then as they disappear below the horizon, the auroræ hove into view. A ringed Earth would have a beautiful sky from everywhere.
Before considering the possibilities of how it might happen, it’s worth taking a break to smell the flowers at this point and have a go at imagining the situation of our own planet having rings in detail. From the equator, the rings would be invisible. Planetary rings are extremely thin. For example, Saturn’s are only ten metres thick. In order to get a good view, we would have to be away from the Equator, and with increased latitude two things would happen. The full width of the rings would become more evident and the rings would approach the horizon. The best view would be in places like Aotearoa/New Zealand, Argentina, Chile, the Mediterranean and the northern states of the US and southern Canada, which are all around halfway between the Equator and the poles. It would be a bit like having a permanent rainbow in the sky, although a less colourful one oriented east-west rather than somewhere near north-south, and quite a bit larger. Near the poles they would be very close to or below the horizon. There might also be a division in them, like the Encke and Cassini Divisions of Saturn’s. These are caused when particles orbit in resonance with a large moon. There would be a radius within the rings which would orbit ten times a month, and it’s possible that this would at least be sparser than the rest, but the resonance is quite far from the simpler, larger fractions involved which lead to Jupiter causing the Kirkwood Gaps in the asteroid belt or Mimas, the Death Star moon, causing the Cassini Division. Even these are less visible close up, so we would probably end up with a single solid-looking ring in our sky, although since it was so close we’d probably see something like the “record grooves” appearance Saturn’s have when seen from nearby, and there might also be “spokes” moving through them as they do with those. These spokes might be caused by lightning storms in Saturn’s atmosphere or meteorite impacts on the rings, causing static charge to repel some of the particles and make them slightly wider at those locations.
It would have various consequences. They would cast very large shadows corresponding to the seasons. There would be none at the equinoctes and they would be biggest near the winter solstice in the appropriate hemisphere. These would also be colder than the surroundings and there would therefore be winds blowing into them, and in certain places the shadows would be present intermittently for months at a time, notably in the mid-latitudes where the climate would usually be warmer. This would reduce photosynthesis, although since they would also be very bright there would be some compensation and this could also ameliorate the cooling effect. They would also occupy the position used on the equator by communications satellites. However, they wouldn’t impede space travel since this can occur away from the Equator.
That, then, is Earth with rings. It seems to be compatible with life but it would have significant effects on our climate, and there could also be a steady rain of meteors into the atmosphere at the equator, though these would mainly vaporise. This in itself might lead to more metal ions in the upper atmosphere, and I’m wondering if this would influence the auroræ and attract them towards the poles, making them brighter and more colourful.
Now the question is, has this or will this ever happen without human intervention?
When Earth first formed, it very probably did have rings like all the other planets, as the planetesimals orbited prior to colliding with the gradually accreting protoplanet. Slightly later on, the Mars-sized planet Theia collided with us, breaking off the outer layers of the planet and forming Cynthia. This too would have shown up as a ring, a very substantial one in fact. Saturn’s rings has a mass of less than half that of Mimas. Earth’s at this point would have had more than one percent of our own mass, which is many thousands of times greater on a planet whose mass is only one percent of Saturn’s. This is another illustration of how out of proportion our satellite is.
There is no evidence in favour of this next bit, but also nothing to rule it out and it’s entirely compatible with established facts about the history of this planet and the Solar System.
A large asteroid collides with this planet every few million years, including, of course, the Chicxulub Impactor which wiped out the non-avian dinosaurs. The Śiva Hypothesis holds that this planet moves through a galactic arm every 27 million years, causing an increase in impact events, although there isn’t a huge amount of supporting evidence for this. There was a period during which the Chicxulub Impact worked so well as an explanation for the extinction of the dinosaurs that cosmic impacts were evoked to explain all six prehistoric mass extinctions, the current one being excluded for obvious reasons, but there are various other events which could also explain them quite well. There are therefore sometimes direct hits by large bodies on this planet. How often does this planet encounter another body without an immediate collision? How often does it get within the Roche Limit, almost striking a glancing blow, and instead of hitting us is ripped apart by tidal forces and forms a ring? I don’t think anything at all rules this out, and in fact I think it must have happened a number of times in our history.
Looking specifically at the Chicxulub Impact, a ten kilometre wide object clearly did hit the future Gulf of Mexico 66 million years ago. However, how do we know that wasn’t simply the biggest or most unfortunate remnant of a larger body which had been orbiting the planet for some time previously? It may have broken up within the Roche Limit and given the planet rings, and also, less controversially, some of the rocks smashed up into space by the impact would have created rings. Maybe Palæocene Earth did have rings after all, and maybe they took millions of years to break up.
I can’t prove any of this of course, and in fact I can’t even think of how someone would go about testing this hypothesis. Even so, I would say that the balance of probabilities strongly supports the idea that this planet does sometimes acquire rings. It’s the largest inner planet, it has associated asteroids and for every impact there are countless near-misses. I think we used to have rings, have probably had them several times, and will one day acquire them again. As to when, who knows?
A Box Of Chocolates 