by Pete » Thu Dec 28, 2006 11:33 pm
Phobos orbits about 6000 km above Mars's surface, which is about one Mars diameter; the planet's already tenuous atmosphere probably wouldn't present much resistance. However, Phobos orbits Mars faster than Mars itself rotates (and, importantly, in the direction of Mars's rotation). Consequently, the tidal bulge on Mars lags behind Phobos because of the planet's relatively slow spin. the gravitational force of the bulge pulls "backward" on Phobos, decelerating the moon and draining it of orbital energy, causing its orbit to decay over time.
In a system where the planet spins faster than the satellite (Earth-Moon is a fine example), the tidal bulge raised on the planet by the moon is shifted in the direction of the planet's rotation; it pulls "forward" on the moon, increasing its orbital energy and expanding its mean orbital radius.
Phobos orbits about 6000 km above Mars's surface, which is about one Mars diameter; the planet's already tenuous atmosphere probably wouldn't present much resistance. However, Phobos orbits Mars faster than Mars itself rotates (and, importantly, in the direction of Mars's rotation). Consequently, the tidal bulge on Mars lags behind Phobos because of the planet's relatively slow spin. the gravitational force of the bulge pulls "backward" on Phobos, decelerating the moon and draining it of orbital energy, causing its orbit to decay over time.
In a system where the planet spins faster than the satellite (Earth-Moon is a fine example), the tidal bulge raised on the planet by the moon is shifted [i]in[/i] the direction of the planet's rotation; it pulls "forward" on the moon, increasing its orbital energy and expanding its mean orbital radius.