Mars, the fourth planet in our solar system has two moons, namely Phobos and Deimos.
Deimos was discovered on August 12, 1877, and Phobos on August 18 of the same year by an American astronomer Asaph Hall. The two moons were named after Greek mythological twin personalities.
Phobos, which means fear and Deimos that means terror.
In Greek mythology, Phobos and Deimos accompanied their father Ares (the Greek God of War – or planet Mars) to war.
Scientists believe that Phobos and Deimos are composed of the same material as asteroids, leading them to suspect that the two moons were captured by Mars sometime in its history.
Both of them are tidally locked to Mars, which means that they always show the same side, much the same as our own Moon is tidally locked to Earth.
These moons, however, are tiny compared to our own, making them difficult to see…
But it is not impossible!
Read on to find out how more about the moons of Mars, including how to see them for yourself.
Phobos is only 14 miles wide and orbits Mars at a distance of 3,700 kilometers. What might be more remarkable for such a tiny body is that it hosts the largest known impact crater on any moon called Stickney.
Size and Shape
Phobos is the larger of the two Martian moons. Even so, it is so small that gravity has not pulled it into a sphere.
Since both Martian satellites are not perfectly spherical (Phobos is thought to be potato-shaped), their size cannot be provided in terms of radius. Rather, the dimensions of Phobos are 17 x 14 x 11 miles or 27 x 22 x 18 kilometers.
Phobos is thought to be a C-type asteroid, which is the most common type.
Phobos hosts Stickney, the largest known impact crater on a moon which is nine km wide, almost one-third of Phobos’ width itself.
This impact also knocked loose the surrounding material, which is now seen as grooves on the moon.
Moreover, Phobos, in its tight orbit around Mars, wobbles slightly, causing the material on its slopes to shift continuously.
Deimos is only 7.7 miles wide and orbits Mars at a distance of 12,470 miles. Like Phobos, it hosts innumerable impact craters, out of which only two are officially named.
Size and Shape
Deimos is the smaller of the two Martian moons.
Like Phobos, this moon is not spherical either, so the size cannot be provided in terms of radius. Rather, the diameter of Deimos is 9 x 7 x 6.8 miles or 15 x 12 x 11 kilometers.
Deimos is also believed to be a C-type asteroid and hosts impact craters, the smallest of which is 1.6 miles across.
Unlike Phobos, Deimos does not have grooves on its surface.
Whenever a meteorite impact occurs, material is normally ejected outwards from the impact crater. We see this material in the regions surrounding the crater.
However, Deimos does not exhibit this, suggesting that the gravity of this tiny moon is so low that any material ejected would have escaped into space.
How To See the Moons of Mars
Seeing Phobos and Deimos for yourself is not an easy task. They are small and close to their host planet but, with the right equipment and conditions, it is possible.
Albedo and Size
As we just learned, both Phobos and Deimos are C-type asteroids.
They are dark and have a low albedo, which means that they do not reflect much of the incoming radiation into space.
This, along with their small size makes observation difficult and, even if you do see them, they are mere points of light.
Visibility and Magnitude
Before we can see Phobos and Deimos, we should know the optimal conditions to observe Mars. This happens when the red planet is closest to us during oppositions, such as in October 2020 and December 2022. Oppositions offer the best views possible during its orbit.
Phobos has a brightest magnitude of 11.7 and has a tight, tidally locked orbit around Mars.
It circles the red planet every 7 hours 30 minutes, which means it zips around it three times a day when seen from Earth.
Moreover, its proximity to Mars, which is much larger, might not always make it visible – it hovers only 21.5 arcseconds away from the planet at its farthest.
The chances of spotting Deimos, which revolves around Mars once every 30 hours, are much higher since it orbits farther away from Mars with a maximum separation of 72 arcseconds.
However, Deimos is even dimmer and is never brighter than magnitude 12.8. This puts it right on the edge of a smaller telescope’s capabilities.
A minimum of 6-inch planet-spotting telescope will be required to view Phobos and Deimos.
Additionally, since Phobos is less visible than Deimos, a narrow field eyepiece works best to locate the moon.
Your best chance of seeing them is near Mars’ opposition since that is when the red planet will be closest to Earth, thus giving good views of the moons as well.
Other useful times to attempt the viewing are when the moons reach eastern or western elongation since this is when they are at maximum separation from Mars (so that glare from Mars doesn’t drown out the moons’ faint glow).
Another way to beat the Martian brightness is to use an occulting bar. This sits across the center of your eyepiece and is used to block out the planet from view, leaving you to hunt for the nearby moons. You can make your own using a narrow strip of aluminum foil across the back of your eyepiece.
Couple this with a deep blue filter to reduce the glare from Mars itself, which is 800,000 times brighter than its dimmest moon, and why collimated optics are a must while observing the moons.
We’ve learned that the two moons of Mars, Phobos, and Deimos, are both gravitationally captured asteroids.
In fact, Phobos is currently on a collision course with Mars, where it is getting closer by six feet every year. In the next 50-100 million years, Phobos will come so close to Mars that it will either break into pieces or crash into the red planet.
Deimos, on the other hand, orbits much farther away and is slowly escaping from Mars’ gravitational pull.
Both the moons are primarily made up of the same material and have low albedos, which is why they are hard to spot.
The best views can be obtained when Mars reaches opposition, where it is the closest to Earth. These views require a telescope with an aperture of a minimum of six inches.
While challenging, the resulting views are definitely worth the time and effort!
Written by Adam Kirk