Mars' Moons Vs. Earth’s Moon
February 18, 2013

Mars’ Moons Vs. Earth’s Moon: What’s The Difference?

Rayshell Clapper for — Your Universe Online

Earth has one moon, and it is considerably larger than Pluto. In fact, the relative size of Earth´s moon is quite massive compared to the moons of other planets. We have a fascination with our moon on many levels. But what if Earth had two moons as Mars does?

According to NASA, unlike Earth´s moon, Mars moons are two of the smallest in the entire solar system. Mars moons also have proper names: Phobos and Deimos. Their names come from the mythological sons of Ares, the Greek version of the Roman war god Mars. Phobos means fear or panic while Deimos means flight from defeat. Of the two, Phobos is a bit larger, but neither is particularly massive. And neither of Mars´ moons is round like Earth´s moon. In fact, Phobos has a crater on it that is almost half the width of the moon itself. This crater is 6 miles wide and is called the Stickney Crater.

Earth´s moon orbits the planet about every 27-29 days. By contrast, Phobos, the closer of Mars´ two moons, orbits the red planet three times a day, while Deimos completes an orbit every 30 hours. The difference in orbit has to do with the distances between the moons and their planets. Earth´s moon is about 240,000 miles (385,000 km) away on average, while Mars´ Phobos is only 3,700 mi (6,000 km) — the closest known moon to any planet.

One feature that the three moons have in common is that they all show the same face to their planet. The Mars moons are a bit different in composition, however. Both Phobos and Deimos are lumpy, heavily-cratered and covered in dust and loose rocks. By contrast, Earth´s moon is divided into mountainous highlands and large, roughly circular plains called maria.

The maria are flat or undulating floors covered by a thin layer of powdered rock, which makes them appear darker than the rest of the moon´s surface. The brighter areas consist of the mountainous highlands, which have rough terrain and rocky rubble. Like Phobos and Deimos, Earth´s moon also has craters.

Since the moons have very different masses, they also have very different gravitational pulls. Earth´s moon has a roughly one-sixth the gravitational pull as that found on Earth, meaning that a 150-pound human would weigh only 25 pounds on our moon. Phobos, on the other hand, has about one one-thousandth of Earth´s gravitational pull, which means that same 150-pound person would weigh two ounces or 57 grams.

Mars´ moons may be small, but they are still visible from the surface of their host planet. In fact, NASA´s Curiosity rover has snapped a few pictures of Phobos as it traverses the Martian countryside.

Both the Mars moons were discovered in 1877 by the American astronomer Asaph Hall, who is also responsible for naming them. When the Stickney Crater was discovered on Phobos in the 1970s, astronomers named it after Hall´s wife Angelina, whose maiden name Stickney. While the Mars moons may not be as familiar to most of us as our own moon, they continue to capture the fascination of scientists, astronauts and sci-fi fans everywhere.