Posted by The moons of Mars are not really moons in the way anyone pictures the word. Phobos is about 22 kilometres across, lumpy, dark and covered in dust; Deimos is roughly half that and smoother. Put either of them next to Earth’s Moon and they would look like debris. Yet they carry information about the early Solar System that Mars itself has lost, and after nearly 150 years nobody can say for certain where they came from.
That last part is the interesting bit. Two rocks in plain sight, photographed thousands of times, and their origin story is still contested.
Asaph Hall Nearly Gave Up
In August 1877 Mars came unusually close to Earth, and Asaph Hall was using the 26-inch refractor at the United States Naval Observatory in Washington to look for satellites. He found nothing for several nights and was ready to stop. The story, which Hall himself told, is that his wife Angeline Stickney talked him into continuing.
He found Deimos on 12 August and Phobos on the 18th. The names come from Greek — fear and dread, the attendants of Ares — suggested by an English schoolmaster, Henry Madan. The largest crater on Phobos was later named Stickney.
There is a coincidence attached to this that refuses to die. Jonathan Swift gave Mars two moons in Gulliver’s Travels in 1726, complete with orbital periods, a century and a half before anyone saw them. It is coincidence, arrived at through numerology about the spacing of known satellites, but it is a good one.
Two Very Small Rocks
Phobos orbits closer to its planet than any other moon in the Solar System — roughly 6,000 kilometres above the Martian surface, which is nearer than many satellites orbit Earth. It completes a circuit in 7 hours and 39 minutes, faster than Mars rotates, so from the ground it rises in the west and sets in the east, crossing the sky twice a day.
Deimos sits much further out at around 23,000 kilometres and takes a little over 30 hours, which means it hangs almost stationary in the Martian sky and takes days to drift from horizon to horizon.
Neither is anywhere near massive enough to be round. Both are dark, reflecting only a few percent of the light that falls on them, and both have very low density — Phobos comes in under two grams per cubic centimetre, which for rock means it must be substantially porous. A rubble pile, in other words, held together loosely rather than a solid body.
Curiosity and Perseverance have both filmed Phobos crossing the Sun. It is too small to cover the disc, so what you get is a lumpy silhouette sliding across in an annular eclipse lasting about thirty seconds.
Captured, or Built From Debris?
Here is the argument. Spectroscopically the moons of Mars look like D-type asteroids — dark, carbon-rich objects common in the outer asteroid belt and among the Trojans. That similarity is the strongest evidence for capture: the two bodies wandered in from further out and got caught by Martian gravity.
The problem is orbital mechanics. Both moons travel on nearly circular, nearly equatorial, prograde orbits. Captured objects arrive on eccentric, randomly inclined paths, and circularising them requires some mechanism to shed a great deal of energy — atmospheric drag through a much thicker early Martian atmosphere is the usual proposal, and it has to be tuned quite carefully to work.
The competing model says the moons formed in place, from a debris disc thrown up by a giant impact. Mars has a strong candidate for that impact: the enormous Borealis basin covering much of the northern hemisphere, one of the largest impact structures known. Under this picture Phobos and Deimos are the last survivors of a family of moons that mostly fell back long ago, and their asteroid-like spectra come from the composition of the impactor.
Neither model is comfortable. Capture explains the composition and struggles with the orbits; impact explains the orbits and has to work harder on the composition. Resolving it requires a sample.
Phobos Is Falling
Because Phobos orbits faster than Mars spins, tidal forces are dragging it inward rather than pushing it out — the opposite of what happens between Earth and our Moon. The rate is measurable: roughly two centimetres a year.
Extrapolate and Phobos has something on the order of thirty to fifty million years left. What happens at the end depends on how weak it really is. A solid body would crash. A loosely bound rubble pile is more likely to be pulled apart as it crosses the Roche limit, in which case Mars would briefly acquire a ring of debris that would eventually rain down onto the equator.
Thirty million years is nothing geologically. We are watching a moon in the last few percent of its existence.
Deimos is on the opposite trajectory, drifting slowly outward, and will eventually escape. So Mars is not a system in equilibrium. It is a system in the middle of losing both its satellites by different routes, which is worth remembering whenever anyone treats a planet’s moons as permanent fixtures.
The Grooves Nobody Can Agree On
Phobos is scored with long parallel grooves, hundreds of them, most a few hundred metres wide and running for kilometres. Three explanations compete and none has won outright.
They might be fractures from the Stickney impact — the crater is nine kilometres across on a body of twenty-two, an event that came close to destroying the moon. They might be tidal stress cracks, the surface already beginning to fail as Phobos spirals inward, which would make them a preview of the breakup. Or they might be chains of secondary craters gouged by debris blasted off the surface of Mars by impacts, material that went into orbit and then swept across Phobos.
Each explanation accounts for some of the grooves and not others. The pattern is probably not from a single cause.
Why Anyone Would Send a Spacecraft
The Soviet Union tried twice in 1988 and lost both Phobos probes, the second after returning only a handful of images in March 1989. Russia tried again with Fobos-Grunt in 2011 and never got it out of Earth orbit. It has been a genuinely unlucky target.
JAXA’s Martian Moons eXploration mission is the current attempt, designed to survey both moons, land briefly on Phobos and bring a sample back. That sample settles the origin question in a way no remote sensing can. If the material matches outer-belt asteroids, capture wins. If it looks like Martian crust melted and reassembled, the giant impact model wins. And if it is a captured primitive body, it carries carbon compounds and hydrated minerals from the era before planets finished forming, which is the same reason Ryugu and Bennu were worth going to.
There is a practical angle as well. Phobos sits deep in the Martian gravity well but takes very little energy to land on and leave, which makes it an obvious staging point for anyone planning crewed operations at Mars.
Whichever answer comes back, it constrains how often small planets end up with satellites — and by extension what to expect around rocky worlds elsewhere. SETIworld follows the missions to the moons of Mars alongside the rest of the search for how planetary systems assemble themselves.
