There’s something very odd about our history with the moons of Mars. I also realise I’m not strictly sticking to the practice, as far as possible, of moving outward from the Sun to the edge of the system by putting this post after the one on the asteroid belt, but you kind of needed to know enough about the asteroids before I started to talk about them, plus everything’s whizzing around all the time anyway so who’s to say what order anything is in?
The weird thing about Phobos and Deimos, “fear” and “panic” as befits moons of the Red Planet, the planet of war, is that some people seemed to know about them before they were discovered. In 1752 CE, Voltaire wrote «Micromégas», a short story in which aliens say that Mars has two moons. The reasoning here is that Venus has no moon, Earth one and Jupiter four, so Mars could be expected to have two, and possibly Phæton three. It has been claimed that the largest asteroids are in fact that planet’s old moons, but I digress. It also seems to imply Saturn would have five. However, Voltaire may also have been influenced by Jonathan Swift, who made an even more remarkable claim about them in 1726’s ‘Gulliver’s Travels’. In his voyage to Laputa, the floating island sometimes interpreted as a huge flying saucer, the inhabitants are said not only to have discovered two Martian moons but well, here’s the quote:
They have likewise discovered two lesser stars, or satellites, which revolve about Mars; whereof the innermost is distant from the centre of the primary planet exactly three of his diameters, and the outermost, five; the former revolves in the space of ten hours, and the latter in twenty-one and a half; so that the squares of their periodical times are very near in the same proportion with the cubes of their distance from the centre of Mars; which evidently shows them to be governed by the same law of gravitation that influences the other heavenly bodies.
– Voyage To Laputa, Gulliver’s Travels, Jonathan Swift 1726.
In fact Phobos orbits Mars in seven hours and forty minutes whereas Deimos takes thirty hours and twenty minutes, and the distance of Phobos isn’t far off being right either. Nobody knows how he did this. The moons weren’t discovered until 1877. The fact that both Voltaire and Swift mentioned the moons has led to many of the features on their surfaces being named after them and their works, and there’s also the issue of Asaph Hall. Hall was busy trying to discover the moons of Mars once and became disheartened, so he went home and complained to his wife. She took issue with this and encouraged him to go back to the observatory and not to come back until he’d discovered them. They succeeded, and consequently Phobos’s largest crater is named Stickney, after her maiden name, whereas only its second largest is called Hall.
Although the idea is now rejected, the notion that the further out a planet is from the Sun does have its merits. The Hill Sphere of a planet of a given mass will be greater the more distant it is from the star it orbits. This is the region where the planet’s local gravitational influence exceeds the Sun’s. Hence one might indeed expect Earth to have one moon, Mars two, Phæthon three and Jupiter four, although in fact Jupiter has many more than that and only has four large moons and Phæthon disnæ exist. It isn’t entirely absurd though. A similar concept to the Hill Sphere is the Roche Limit, which is the minimum safe distance a large solid object can approach a body without tidal forces ripping it apart. This is 2.44 times the radius of the larger body, and in Mars’s case this is 8 270 kilometres from the centre or 4 880 kilometres above its surface. The actual distance of Phobos from the centre of Mars averages 9 377 kilometres and is therefore not that far off the limit. It also orbits more than once a sol (Martian day), which causes it to be pulled about by Martian tidal forces and its surface is therefore streaked where Mars has been clawing at it. It will collide with the planet in the next few dozen million years, probably around the time the shovel-horned gigantelope becomes extinct. Nor is this the first time it’s happened. There are chains of impact craters around Mars near the equator which suggest other moons have suffered the same fate. As I’ve mentioned before, Mars may have rings in the future and have had them in the past too.
I’m very inclined, unlike the moons which is an argument against what I’m about to say, to believe that both moons are captured asteroids. Against this is the fact that their orbits are close to being circular and their proximity to the plane of the Martian equator, which suggests they were formed with Mars. If this is true, there is a problem in the fact that we happen to be living at a time within one percent of the planet’s history of Phobos crashing into it, but it’s possible that there were more moons in the past and that this has happened on a regular basis. This resembles Hans Hörbinger’s belief (he’d probably like it to be called a theory but I’m not sure it is) that we’ve had a series of icy moons which crash into us periodically, causing ice ages. I’ve even wondered if the reason there isn’t more complex life on the planet is nothing to do with conditions there generally so much as the possibility that it constantly suffers major impacts with bodies several times as massive as the Chicxulub Impactor. On the other hand, and I don’t want to go on about this too much, it’s also been suggested that Mars is a better place for life to have begun than Earth, so maybe one of these impacts sent life-bearing meteors to Earth and those moons are the reason there’s life on Earth in the first place.
Just to fill in the details, both moons are somewhat irregular although Phobos is unusually close to being spherical for such a small moon. Both are probably carbonaceous chondrites, that is, stony but with a fair content of organic compounds in them. Both have almost circular orbits. Phobos is 20 x 23 x 28 kilometres in size and Deimos 10 x 12 x 16, so both are smaller than the Isle of Wight. Phobos has a mass of 9.6 x 1015 kilogrammes and Deimos 2 x 1015 kilogrammes. Both have a density around twice that of water, which makes them the least dense of all the planets and moons in the inner Solar System. From the Martian surface, Phobos crosses the Sun an average of around twice a day, because since it’s near the equator, the two’s positions coincide quite often. Its width is about a third of the Sun’s, which emphasises how close it really is as the diameter is less than a hundredth of Cynthia’s. It’s about as bright as the morning star, but is invisible from within the polar circles. Deimos stays above the horizon for a day and a half at a time and is visible closer to the poles. It’s about as bright as Vega. Both of them are dark compared to Cynthia, and in fact darker than most of the asteroids in the inner belt. Carbonaceous chondrites are more characteristic of asteroids near the orbit of Jupiter, and this suggests that that was where they originated. Their escape velocities are so low that it would be possible to run off the surface.
On Phobos the largest crater is Stickney, at ten kilometres across. This is a considerable fraction of the moon’s diameter. Due to the low escape velocity, there are no ejecta around the craters as they would just be flung deep into space without any chance of recapture. The aforementioned grooves are about half a kilometre wide each, but they may turn out not to be solid grooves in a hard surface because of the smallness of the moon. Both moons are likely to be rubble piles rather than solid objects. They’re also likely to be undifferentiated for the same reason – their gravity is too weak to pull constituents of different densities into separate layers.
I haven’t done the maths on this, but I think Mars has the smallest total mass of satellites of any planet in the system which still has moons at all. This puts the three bodies taken together at the opposite end of the scale from Earth and its companion, which is the largest total mass compared to the planet.
I haven’t said much about Deimos yet. It has two craters on it called Swift and Voltaire, which are at the corner just before the terminator near the centre of the picture. This photo shows how the craters don’t really have rims in the same way as they do on larger bodies, or central peaks. They’re just dents. The moon looks smoother than Phobos and is less subject to the tidal forces which affect the other moon, and its craters are also smaller. Although the ejecta are not recaptured by gravity immediately after impact, Deimos is moving through a ring of them and gradually accumulates them, leading to its smooth appearance. That’s the accumulation of small particles such as dust which were originally part of the moon. I’m guessing that this means the leading half of the moon is different than the “leeward” side but I don’t actually know. This is the case with some other moons, particularly Iapetus.
I also feel like whereas Phobos is temporary, Deimos is permanent. I think Mars has got through a load of other moons which have ended up breaking up into rings and strewing its surface, the next of which is Phobos, but Deimos has always just quietly got on with things over the æons. This is because Deimos orbits further out and seems to be stable, whereas Phobos, as well as everything else, is being braked by the upper atmosphere even though it’s really far out in that respect and the Martian atmosphere is very thin compared to ours. Another aspect of Phobos and other moons constantly hitting Mars is that considering it’s a carbonaceous chondrite, it will end up, as others may have done, contributing organic compounds not that far off from biochemical ones, and maybe the same has happened here, resulting in the emergence of life, maybe there, maybe here, or maybe there before spreading here.
There used to be a popular fringe idea that Phobos was a space station, possibly because it seems to be unstable over a long period of time, and it was also thought to be even less dense than it is now thought to be, suggesting it was hollow. Maybe it is kind of hollow, in the sense of being “spongy” – consisting of irregular boulders which don’t fit together neatly, making it porous and riddled with caves all the way through.
Right, that’s all I’ve got to say about Phobos and Deimos, except that my brother used to share his house with two cats of those names. Deimos was black, just like the moon is darkish. Next time: Ceres (or maybe several of the largest asteroids).
A Box Of Chocolates 