Posted by On Earth, the Moon is the slow clock of the night. It rises in the east, drifts west over hours, and is so large and bright that you can read by it when it’s full. Stand on Mars and almost all of that goes out the window. Mars has two moons rather than one, neither of them round, one moving backwards across the sky and the other barely looking like a moon at all. Seeing a moon from Mars is, in the most literal sense, a different experience of the sky, and rovers have already given us the first real glimpses of it.
A small lumpy thing, very close
Start with Phobos, the inner moon. It is tiny, only around 22 kilometers across on average, but it orbits absurdly close, about 6,000 kilometers above the ground. That proximity rescues it. Seen from the Martian equator when it’s high overhead, Phobos spans roughly a third of the width of our full Moon. Not a disk you’d mistake for home, but definitely not a point either. You would see a shape, and the shape would be wrong: lopsided, potato-like, with a dark grey surface that reflects only a few percent of the sunlight striking it.
That darkness matters. Even at its brightest, Phobos shines far more weakly than our full Moon does, both because it is small and because its surface is about as reflective as fresh asphalt. A Martian night lit by Phobos would not be a silvery, shadow-casting night. Astronauts would notice it, photograph it and probably grow fond of it. They wouldn’t navigate a rock field by it.
There’s another catch. Phobos orbits so low that, on many of its passes, it slips into Mars’s own shadow for part of the night and simply fades from view for a while. Moonlight on Mars is intermittent as well as weak.
The moon that rises in the west
Here is the genuinely weird part. Phobos goes around Mars in 7 hours and 39 minutes, while a Martian day, a sol, lasts about 24 hours and 40 minutes. Because the moon outruns the planet’s rotation, an observer on the ground sees it rise in the west and set in the east, the reverse of the Sun, the stars and every other object in the sky. It crosses from horizon to horizon in roughly four and a half hours, and comes back around about twice per sol.
Sit outside a Martian habitat for an evening and you could watch it happen. Phobos would climb out of the western horizon, swell slightly as it rose higher (it’s closer to you when overhead than at the horizon, by a noticeable margin), slide across the stars quickly enough that you could track its motion over a few minutes, and sink in the east. Then, a handful of hours later, it would do it again.
Deimos does the opposite, and does it slowly. Its orbit, a little over 30 hours long, is only slightly faster than the planet turns, so from the ground it rises in the east like everything else and creeps west so gradually that it stays above the horizon for around two and a half sols at a time. If Phobos is the second hand, Deimos is the hour hand of a clock that has lost a few minutes.
Deimos, the moon that looks like a star
To the naked eye, Deimos wouldn’t look like a moon at all. It is roughly 12 kilometers across and about four times farther away than Phobos, so its disk is too small for human eyes to resolve. What you’d see is a bright point, often compared to the way Venus looks from Earth, wandering very slowly against the background stars. A good pair of binoculars or a small telescope would show phases and a hint of shape. Without one, it is a star that refuses to stay put.
Latitude decides what you see
One detail that rarely makes it into popular accounts: you can’t see Phobos from everywhere on Mars. Its orbit is so tight and so close to the equator that the curve of the planet hides it from observers at high latitudes, beyond roughly 70 degrees north or south. A future polar base, the kind people propose for access to water ice, would never see Phobos at all. Deimos, being farther out, is visible from a much larger area, but it too vanishes for observers very near the poles.
Even at lower latitudes, Phobos looks different depending on where you stand. Near the equator it passes nearly overhead and looks largest. From the mid-latitudes it stays low and appears smaller and dimmer, dragging along a lower arc of the sky. Two crews on opposite sides of Mars would describe noticeably different skies.
Eclipses that last half a minute
Earth enjoys a cosmic coincidence: the Moon and the Sun look almost the same size, which is why we get total solar eclipses with a black disk and a glowing corona. Mars gets nothing of the kind. Phobos is too small to cover the Sun, so when it passes in front, the result is an annular, ring-shaped eclipse at best, and a fast one. A Phobos transit typically lasts around half a minute.
We don’t have to imagine these. Opportunity and Spirit caught Phobos crossing the Sun in 2004. In 2013, Curiosity filmed a near-central transit from Gale Crater, the lumpy silhouette sliding across the solar disk like a bite being taken out of it. In April 2022, Perseverance’s Mastcam-Z recorded another one in sharp detail, colour and all, a short clip that went around the internet and showed, better than any diagram, just how irregular Phobos really is. Deimos transits are less dramatic still: a small dark dot moving across the Sun, more like Venus crossing the Sun as seen from Earth than anything we’d call an eclipse.
These videos are not just pretty. Precise timings of transits let scientists refine the orbits of the moons, and the slight changes in Phobos’s path over time feed into measurements of how it is slowly spiralling toward Mars. Curiosity also once watched Phobos pass in front of Deimos in the night sky, a moon occulting a moon, which is not something you can see from any spot on Earth.
Against a butterscotch sky
The backdrop is unfamiliar too. Fine dust suspended in the thin Martian atmosphere turns the daytime sky a hazy tan or butterscotch colour. Sunsets, oddly, go blue: the same dust scatters blue light forward, toward the observer, so the glow around the setting Sun takes on a cool tint. Curiosity’s 2015 images of a blue sunset over Gale Crater are probably the most famous pictures of it.
Phobos can be spotted in daylight when it’s high and the air is reasonably clear, much as our Moon sometimes is. During a global dust storm, the kind that silenced Opportunity in 2018, both moons, and most of the sky, would effectively disappear for weeks.
Then there’s the other object in the Martian night, the one nobody has looked at from there with their own eyes. Earth would appear as a bright blue-white evening or morning star, sometimes with the Moon visible beside it in a telescope. Curiosity photographed exactly that pairing in 2014: a bright dot and a fainter one, home.
A sky that won’t last forever
The view is temporary on geological scales. Phobos is losing altitude, and in tens of millions of years it will either break apart into a ring or hit the planet. Future Martians, if there are any, may one day trade their fast inner moon for a faint ring arching across the sky. Deimos, meanwhile, is drifting outward very slowly and will look a little smaller still.
For the foreseeable future, though, the first human to look up at a moon from Mars will see something no one has seen in person: a dark, misshapen satellite rising in the wrong direction, and a wandering star that is actually a second moon. Watching a moon from Mars won’t feel like moonwatching at home. It will feel more like being shown how orbital mechanics actually works, live.
If you like thinking about skies on other worlds, SETIworld has much more on Mars, its moons and the planets beyond. Stop by, read a little further, and tell us what you’d want to see first through a telescope set up in Gale Crater.
