Does Mars Have Moons?

While Mars has moons, they are very different than Earth's moon. They're small, irregularly shaped and probably are captured asteroids. They also orbit much closer to Mars than the moon does to Earth. Though, they appear extremely small in the sky by comparison, due to their tiny size.

What Do They Look Like from the Surface of Mars?

Phobos has approximately the same brightness in the sky as a half moon. However, it is located much closer at a distance of 3,700 miles from the planet, and it orbits it in 7 hours, 39 minutes. Thus, it does not remain in the sky through an entire night in any location.

Also, it rises in the western sky and sets in the East. It orbits in the same direction as Mars rotates. However, it does this much more rapidly. Thus, it rises in the west and can go from horizon to horizon in about 4 hours. However, it is bright enough to provide significant amounts of light on the surface.

Demios also orbits relatively close to the planet at around 12,000 miles. However, it is smaller and more distant. Thus, it is about as bright as Venus in the sky. It orbits slightly slower than the planet rotates. The moon has an orbital time of 30 hours. Thus, it tends to remain in the sky for much longer. It would remain in the sky for approximately 60 hours.

Both of these moons have phases. However, they progress through all of the phases as they pass through the sky. Thus, you can see Phobos’ phases on one pass through the sky.

What Will Happen to the Martian Moons?

Phobos will either crash into the surface or form a ring around the planet millions of years from now. This impact could create a massive impact basin, which could be 120 miles in diameter. Though, the moon may break into pieces and create multiple small craters.

This impact will likely lead to a temporary (but relatively long) period of significant warming of the atmosphere. It also will melt ice near the poles and cause frozen carbon dioxide to sublimate. This would lead to a thickening of the atmosphere. It could even lead to a runaway greenhouse effect, and this could lead to enough vaporization of water and carbon dioxide to thicken the atmosphere enough to support liquid water and rainfall.

Potentially, it could even reactivate volcanism on Mars. This could raise the atmospheric pressure significantly. In the distant past, Mars had an atmospheric pressure of about 80% of Earth's. This supported liquid water and likely vast oceans on its surface. Volcanism played a primary role in this.

Currently, the interior of the planet has partly solidified. Though, there is still a significant amount of magma very deep beneath the surface (far deeper than on Earth). Olympus Mons may have actually erupted as recently as 2 million years ago, and there may have even been small, temporary streams of liquid water flowing down the mountain's lower flanks. Still, the atmosphere was far too thin to support the long term presence of liquid water at this time (though transient brines may sporadically exist in certain regions of the planet for a few hours or less at a time).

Even with restarted volcanism from Phobos striking Mars, the pressure would likely remain too low to support the long term presence of liquid water. Though, some advocates for terraforming Mars believe that an impact from Phobos may help to kickstart the process.

Could Future Astronauts Explore the Moons of Mars?

It's possible. In fact, the Moons of Mars could serve as an excellent way station to get to the surface of the red planet. Phobos is actually closer to Mars than geostationary satellites are to Earth.

However, they are exceedingly small, unlike the moon of Earth, the gravity on these moons is exceptionally weak. In fact, one could throw a stone fast enough to escape the gravity of either moon and send it into its own orbit around Mars. On Demios, it would even be possible to jump directly off the surface into space. Walking on the surface of either of these moons would feel more like floating than walking.

On Demios and on some parts of Phobos, the gravity is so low that a hard step could send you into space. Though, on most parts of Phobos, it would take a very forceful leap to actually "lauch" off the surface. Even still, it would be easy to jump to great heights (potentially up to 2,000 feet) by jumping. A hard step could send you far feet into the air. So, a tether would be required on either moon for extravehicular activities.

What is the Geology of These Moons?

The regolith of Phobos and Demios is very loosely packed due to the extremely low gravity. They also are heavily cratered. In addition, there are large cracks on the surface of Phobos. This may be due to tidal forces, but it could also have been caused by the effects of an impact.

However, the origin of the cracks is unknown. While they are most likely captured asteroids, they appear to have very circular, equatorial orbits around Mars.

The temperature in sunlight on Phobos and Demios is approximately 25 degrees. So, water ice can exist on its surface. Recent evidence suggests that it has significant amounts of water ice under its surface. Demios may have substantial amounts of water ice as well.

Thus, explorers to either moon could melt this ice to use for their water supply. Also, they could use electrolysis to separate the hydrogen and oxygen atoms in the water molecules, and this could allow the astronauts to have access to breathable air. So, both of these moons may play an integral role in the colonization of Mars one day in the future.