No moons have been found for Mercury and Venus. Mars has two moons that are much smaller than the Earth's. One is shown in Fig. 24.16. Phobos is 22 km across, and orbits Mars in 7 hr 40 min. Demos is 15 km across and orbits in 30 hr. Since one moon orbits faster than Mars's rotation period and the other is
slower, from the surface they appear to move across the Martian sky in opposite directions. Since the orbital period of Demos is close to Mars's rotation period, an observer on Mars would see that it takes 137 hr to make a complete cycle of the sky. It takes almost three days to go from rising to setting, and then another three days to reappear.
Each moon has an irregular shape. They appear dark, like certain asteroids. This has led to conjecture that these two satellites were asteroids that were captured by Mars.
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