How do the two Martian moons – Phobos and Deimos – look from the surface of Mars? First of all, they don’t look at all like Earth’s moon does from Earth!
The Martian moons are tiny. The larger moon, Phobos, is only about about 14 miles (23 km) across. And Deimos is about half that size. Plus, these little moons orbit Mars more closely than our moon orbits Earth. But, of course, because they’re so tiny, they appear smaller than our moon does from the surface of their home world.
In fact, Deimos, the more distant moon, looks like a star in Mars’ sky. But it’s twice as bright as any starlike object seen in Earth’s sky. Deimos orbits at nearly the same speed Mars rotates, so it needs three Martian days to crawl from one side of Mars’ sky to the other. And, by the way, a day on Mars is about the same length as Earth’s day.
On the other hand, Phobos – the larger and closer of the two moons – zooms around Mars two and a half times every Martian day. Because it out-races Mars’ rotation, Phobos rises in the west and sets in the east. Phobos appears about a third as large in the Martian sky as our moon does in Earth’s sky. What’s more, Phobos isn’t round like our moon. It resembles a shining gray-white potato.
Another odd thing about Phobos – it’s not visible all over Mars. Phobos orbits above Mars’ equator so near the planet that it’s always hidden beneath the horizon in the Martian polar regions. Our moon, by contrast, can be viewed anywhere on Earth.
For observers on the Martian equator, Phobos eclipses the sun nearly every day. Eclipses last only about 30 seconds, so quickly does Phobos race across the sky. Because Phobos covers only a fraction of the sun’s disk, eclipses are never total.
For observers in the north and south mid-latitudes of Mars, Phobos never eclipses the sun – it always moves south of the sun (for northern observers) or north of the sun (for southern observers).
Deimos eclipses the sun much less often – about once a month. Because it’s smaller and farther away than Phobos, it would be barely visible against the sun’s disk.
As seen from Mars, Phobos and Deimos pass through phases, just like Earth’s moon. They pass from new to crescent to gibbous to full to gibbous to crescent to new again. However, Deimos’ phases are not terribly obvious – they are seen only as a slow change in brightness. Phobos’ phases are more apparent. Because Phobos is irregular, however, the phases look strange. For example, crescent Phobos looks squashed and jagged.
The surface material of Phobos and Deimos is rich in dark carbon – they are among the darkest moons in the solar system. They reflect about 5% of the light that strikes them – about half as much as Earth’s moon. To get a sense of how dark that is, keep in mind that our moon is about as reflective as asphalt. Phobos and Deimos, then, are half as reflective as asphalt. From the surface of Mars, however, the two moons still look bright and gray-white against the black night sky.
Bottom line: The larger Martian moon, Phobos, is only about about 14 miles (23 km) across. The smaller one, Deimos, is about half that size. These little moons orbit Mars more closely than our moon orbits Earth, but remember … they’re small. Deimos looks like a bright star in Mars’ sky. Phobos looks like a shining gray-white potato!