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Moon and Mars
Earth’s Moon most likely formed as the result of a “big whack” — a collision between the young Earth and another planet. The impact blasted debris into space, which coalesced to form the Moon.
The moons of Mars may have formed in the same way.
Phobos and Deimos are small, oblong chunks of rock. The leading theory has been that they were asteroids that passed close to Mars, allowing the planet’s gravity to lock them into orbit. But not all astronomers agreed with that explanation. The composition of the moons didn’t seem quite right, for example. So the idea that they were born in a big whack has gained support in recent years.
It gained more support from two studies published in July. Researchers concluded that Mars was blasted by a smaller object within a few hundred million years of the planet’s birth. That spewed molten rock, bits of dust, and hot gas into orbit around Mars.
According to the simulations, molten rock close to the planet formed a large moon and several smaller ones, while gas and dust formed Phobos and Deimos farther from Mars. Mars’s gravity quickly pulled the closer moons back down, leaving only Phobos and Deimos. They’ve been spiraling closer to Mars ever since. And within 50 million years or so, Phobos, too, will slam into Mars — leaving the Red Planet with a single moon.
Look for Mars below our Moon this evening. The planet looks like a bright orange star. But you need a good telescope to see its two little moons.