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8. Mars: the red planet

The mysterious moons of Mars

Discovery and prediction

Mars has two little moons that are so dark and small that they were undetected for centuries, even after the invention of the telescope. They were discovered in 1877 by Asaph Hall (1829-1907) using a 0.66-meter (26-inch) telescope at the United States Naval Observatory. He named the inner moon Phobos, the Greek word for “fear” and the outer one Deimos, Greek for “flight, panic or terror”, after the attendants of the Greek god of war, Ares, in Homer’s Illiad.

Fig. .. 

These are not the same kind of object as the Earth’s large and spherical Moon. Both moons of Mars are very small, with insufficient gravity to mold them into a spherical shape. They have a battered appearance, with a profusion of craters large and small. In fact, the surface of Phobos has been pounded into a layer of insulating dust at least a meter thick, enough to bury anything that tried to land on the small moon. Eons of meteorite impacts have apparently sandblasted it.

A maverick moon

Phobos is about as close to Mars as it can get. If it came much nearer to Mars, the planet’s gravitational forces would tear Phobos apart. In fact, the orbit of Phobos is steadily shrinking. If it continues to move toward Mars at the present rate, Phobos will either smash into the Martian surface or be torn apart by the planet’s gravity to make a ring around Mars in 50 million years. Because Phobos is about 4.6 billion years old, we are, astronomically speaking, catching a fleeting glimpse of the last few moments of its life.

Origin of the Martian moons

Fig. .. 

The irregular shapes, small sizes, and low mass densities of Phobos and Deimos closely resemble those of the numerous asteroids that orbit the Sun between the orbits of Mars and Jupiter. The two Martian moons and the asteroids have a battered appearance, with a profusion of craters large and small. Moreover, the surfaces of the Martian moons are as dark as some asteroids, known as the carbonaceous C-type, and nowhere near as lightly colored as the surface of Mars. Scientists therefore speculate that Phobos and Deimos were adopted from the asteroid belt.

Fig. .. 

Summary Diagram


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Copyright 2010, Professor Kenneth R. Lang, Tufts University