Mars and its Moons
This app shows a real-time display of Mars and its two moons, Phobos and Deimos.
The data for the above display was derived from the NASA's JPL website and covers the period 1900 to 2200 AD. Outside of that time frame the positions of the moons shown is an approximation.
With the app at the default settings the image (orbits and size of the planet) are all to scale.
The Martian System
The martian system is comprised of Mars and its two small moons which orbit very close to the planet - both completing their orbits in 7 hours and 30 hours respectively.
From the martian surface the two moons have been filmed passing each other by the Curiosity Rover.
Phobos, the larger of the two moons shown above, appears at about 1/3 the width that our own moon appears in the Earths sky.
Phobos is a small, irregularly shaped object with a mean radius of 11 km, and is seven times more massive than Deimos. Phobos orbits 6,000 km (3,700 mi) from the Martian surface, which is closer than any other known planetary moon. It is so close that it orbits Mars faster than Mars rotates, and completes an orbit in just 7 hours and 39 minutes. This means that when viewed from the surface of Mars it rises and sets twice in each 21 hour day.
Just like Earth's Moon, Phobos rotates at the same rate as it orbits its planet and so always shows the same side to Mars.
Phobos is heavily cratered and has many grooves cut into it which can be up to 30 meters deep, 200 meters wide and 20km long. These grooves are each thought to be Crater Chains; the result of multiple tiny impacts when other objects orbiting Mars got close to Phobos, were broken up by gravitational forces, and then the debris impacted leaving a closely bunched line of craters.
It is thought to have a covering of around 100 meters of dust and broken rock - the result of collision debris - and has a density that is too low for its core to be of solid rock. It would appear to be a rubble pile in which many various sized rocks are held together with gravity, the voids between the rocks being filled with nothing or with loosely packed dust.
Deimos is very similar to its big brother Phobos in that it has a smooth surface due to a blanket of fragmental rock/dust and littered with impact craters.
The color variations are probably caused by exposure of surface material to the space environment, which leads to darkening and reddening. Brighter and less-red surface materials have seen less exposure to space due to recent impacts or downslope movement of surface dust.
Deimos is around half the size of Phobos at about 6 kilometers (3.7 miles) radius and has an orbital period of 30 hours, 17.9 minutes. Like Phobos its rotation is locked to its orbital period ans do it always shows the same side to Mars.
Discovery and Names
Both Moons were discovered by Asaph Hall in August 1877 at the US Naval Observatory in Washington, D.C. Deimos was discovered first and Phobos 5 days later.
They are named after two characters from Greek mythology - Phobos (panic/fear) and Deimos (terror/dread) who accompanied their father Ares, god of war (known to the Romans as Mars), into battle.