The Moons of Mars Help (page 2)

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By — McGraw-Hill Professional
Updated on Sep 16, 2011

How The Moons Of Mars Formed

“This might sound like a stupid question,” you say, “but—”

“There are no stupid questions.”

“Okay. You showed me an image of Deimos and Phobos in orbit a while ago.”

The image reappears on the holographic screen.

“All right,” you continue. “The orbits of both Phobos and Deimos look like perfect circles.”

“The orbit of Deimos is essentially a perfect circle. Phobos has an elliptical orbit, but the eccentricity is small, so the orbit is almost a perfect circle too,” says the first officer.

“Can we look at the orbits as seen from the plane of Mars’ equator?” you ask.

“Certainly,” says the first officer. He smiles. “We’ll look at the situation from just outside the equatorial plane so that we can get a little perspective.” The view changes. The moons now appear to be orbiting Mars as seen from slightly above their orbital plane. As you suspected, they both orbit almost exactly above the equator of Mars (Fig. 6-5).

Mars From Venus To Mars How The Moons Of Mars Formed

Figure 6-5. Mars and the orbits of its two moons, drawn to scale and viewed from nearly in the equatorial plane.

“If those moons were originally asteroids and were captured by the gravitation of Mars, why are their orbits both so nearly circular, and why are they both so nearly in the plane of Mars’s equator? That’s quite a coincidence, isn’t it?”

“That,” says the first officer, “is not a stupid question. In fact, it may answer the riddle of how these moons came into existence. I believe that both of these moons formed as the result of one or two major impacts in Mars’s distant past, just as the Earth’s moon is believed, by many astronomers, to have formed. It would have taken only a modest-sized object to blast that much material into orbit.”

“But both moons are made of asteroid-like stuff,” you say.

“Yes,” says the first officer. “I think that one or two large asteroids—much bigger than either Phobos or Deimos—crashed into Mars. This melted the big asteroid, and most of it was absorbed by Mars. However, some of this asteroid was cast into Mars orbit, along with some ‘molten Mars,’ and from that stuff, the moons formed. This is my theory,” says the first officer.

We may never know exactly how these moons were created.

Practice problems of this concept can be found at: Mars Practice Problems

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