Posted by Stand on Mars for one full sol — 24 hours, 39 minutes and 35 seconds — and the strangest thing overhead is not the butterscotch color of the sky. It is the traffic. The moons on Mars behave nothing like ours: one of them comes up in the west and crosses the whole sky twice while the planet turns once, and the other loiters in nearly the same place for the better part of three days. No person has ever watched this happen. Cameras have, and the footage is odder than any diagram makes it look.
Asaph Hall pulled both of them out of the glare in August 1877, working at the US Naval Observatory in Washington during a close opposition, after he had very nearly given up the search. Phobos and Deimos, fear and dread, the attendants of Ares. For the next century they were points of light with orbital elements attached to them. The view from underneath had to wait for landers.
A Moon That Rises in the Wrong Direction
Phobos goes around Mars once every 7 hours and 39 minutes, from about 6,000 kilometers above the ground — no moon in the Solar System sits closer to its planet. Mars needs more than three times as long to complete a single rotation. So Phobos laps the planet underneath it. It climbs over the western horizon, runs east against the direction everything else in the sky is going, and sets a bit more than four hours later. Do the arithmetic and a typical sol contains two of those crossings, occasionally a third one squeezed in at the edges.
It is not a big object in that sky. Twelve arcminutes across at best, roughly a third the width of the full Moon seen from Earth, and the width does not hold steady. Someone standing directly beneath Phobos at zenith is a whole Mars radius closer to it than someone watching it clear the horizon, which is enough to make the moon visibly swell as it climbs and shrink again as it goes down. Its phase shifts during the same pass. A time-lapse taken from rise to set shows a different lit fraction at the end than at the beginning, because the moon has moved a long way around its orbit in those four hours.
Not everyone on Mars gets to see it. Phobos hugs the planet so tightly that above roughly 70 degrees of latitude, north or south, it never clears the horizon at all. From a polar station, the inner moon simply does not exist.
It also vanishes. Mars throws a long shadow, and Phobos is close enough to fall into it on most orbits, so an object sitting halfway up the night sky can dim out over a few minutes and leave nothing where it was.
Deimos Barely Bothers
The outer moon does the opposite. Deimos takes about 30 hours and 18 minutes per orbit, which is just a little longer than a Martian day, and those two rates very nearly cancel each other out. What is left is a crawl. Deimos drifts east to west like a normal moon, but it needs something close to 2.7 sols to travel from one horizon to the other, and about five and a half days pass between one rising and the next. A rover could photograph it in nearly the same patch of sky on three consecutive nights and struggle to prove it had moved.
Size does not help either. Deimos spans roughly two arcminutes, which is a point of light, not a disc. It would read as a very bright star to a human eye, something in the brightness range of Venus in our own sky, and nothing about it would suggest a moon.
Forty Seconds of Ring
The Sun from Mars is about five-eighths as wide as the Sun from Earth, something near 21 arcminutes. Phobos, at 12 arcminutes on a good day, cannot cover it. What happens instead is a lumpy silhouette sliding onto the solar disc, taking a bite out of one edge, and at the tightest alignments leaving a ring of light around a very irregular black potato. Nobody on the surface would call it darkness. The light drops the way it does under a passing thundercloud, and then it is over.
Perseverance recorded the best version of it so far. On April 2, 2022, sol 397 of the mission in Jezero Crater, Mastcam-Z ran a color sequence of Phobos crossing the Sun that was sharp enough to show sunspots on the solar disc while the moon’s cratered outline moved past them. The whole eclipse lasted around 40 seconds. Curiosity had shot its own versions earlier from Gale Crater, catching a Phobos eclipse in March 2019 and a Deimos transit a few days before it, and Spirit and Opportunity had done the crude first attempts back in 2004 with cameras never designed for the job.
The Deimos transits are a different experience entirely. A dot moves across the Sun. That is the whole event, and it looks a lot like the Venus transits people traveled the world for in the eighteenth century, minus the fanfare.
Why a Rover Spends an Afternoon Photographing a Shadow
These are not postcards. A transit is a timing measurement, and a very good one: the exact second Phobos touches the edge of the Sun, seen from a rover whose position on Mars is known to within a few meters, pins down where the moon actually was at that instant. Predictions used to miss by enough that the event arrived seconds off schedule, and seconds translate into kilometers of orbital error. Every timed crossing since 2004 has tightened the numbers.
What the tightened numbers reveal is that Phobos is falling. Tidal interaction drags it down by something on the order of a couple of meters per century — slow enough to be irrelevant to any mission, fast enough to be measurable once you have decades of precise positions. The rate itself is the interesting part, because it depends on how much the body of Mars flexes as the moon pulls on it. A rigid interior dissipates energy differently than a soft one. In other words, the moons on Mars are being used as a probe of the planet’s insides, which is not what anyone had in mind when the first transit images came down.
InSight added a stranger data point. When Phobos passed in front of the Sun in 2019, the lander’s solar arrays registered the expected dip in power, and the seismometer picked up a faint tilt as the ground briefly cooled and contracted under the shadow. A moon’s shadow, weighed by a seismometer. Nobody planned that measurement in advance.
The Night Side
The obvious question is whether Mars gets moonlit nights. Mostly, no. Both moons are dark objects, about as reflective as charcoal, and neither presents much of a disc. A full Phobos is genuinely the brightest thing in the Martian night sky and does cast a shadow you could pick out on the ground, but it delivers a small fraction of the light a full Moon pours over an Earth landscape. Deimos contributes almost nothing measurable. It is a bright star that happens to be a rock.
So the nights stay dark, and the sky stays crowded in a way ours is not. Two objects moving at obviously different speeds, one of them going the wrong way, one of them occasionally blinking out mid-sky. Then dust season arrives, the atmosphere fills with suspended particles, and the whole show goes soft and orange behind the haze.
What the moons on Mars would look like to a person standing in Jezero or Gale is still, strictly speaking, a guess assembled from robot cameras with different color responses than a human retina. The geometry is nailed down. The impression is not. That gap is worth keeping in mind whenever someone shows you an artist’s rendering of a Martian sunset with a moon hanging in it.
If this kind of thing pulls at you — orbital mechanics that read like a magic trick, rovers doing astronomy on a planet they were sent to study from the ground up — there is more of it waiting at SETIworld, along with people who argue about the details. Come read, and bring your questions about the moons on Mars with you.
