The two Martian moons, Phobos and Deimos , are not spheres like our Moon. Why? (
ID: 2260469 • Letter: T
Question
The two Martian moons, Phobos and Deimos, are not spheres like our Moon. Why?
(a) Back during the Hadean era (the period of heavy bombardment that lasted from about 4.5 to 3.9 billion years ago) Mars was struck by a large planetesimal which fractured predominately into small debris and two larger pieces, all of which ended up in orbit about Mars. As time went by, the small debris was swept up by the two larger pieces which remain in orbit today about Mars as its two irregularly shaped moons.
(b) They are made predominately of rock, which always takes on an irregular shape.
(c) Phobos and Deimos are most likely small, captured asteroids. They are small enough that gravitation is not the dominant force that shapes them. The electromagnetic forces, or atomic forces, are responsible for binding matter together and in the case of rock, the resulting shape that these atomic forces generate is irregular. When rocky objects are large enough, gravitation overwhelms the atomic forces and the shape becomes spherical as is the case with our Moon.
(d) Initially, Mars had a single larger, spherically shaped moon in orbit about it, much like Earth
Explanation / Answer
(c) Phobos and Deimos are most likely small, captured asteroids. They are small enough that gravitation is not the dominant force that shapes them. The electromagnetic forces, or atomic forces, are responsible for binding matter together and in the case of rock, the resulting shape that these atomic forces generate is irregular. When rocky objects are large enough, gravitation overwhelms the atomic forces and the shape becomes spherical as is the case with our Moon.
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