Quote:
Originally Posted by Cold-co
From what I have read Mercury and Venus are close to being perfectly round because they are trapped in a syncronous orbit around the Sun and only rotate once in their year.
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Although you’re correct that both
Mercury and
Venus are nearly spherical, Cold-co, I believe what you’ve either read very out-of-date astronomy texts, or misremember what you’ve read about the orbit and rotation of Mercury and Venus. More up-to-date information can be found innumerable places on the internet, including following the wikipedia links in this post.
The rotation characteristics of Mercury and Venus have been know with great precision since the early 1960s, when radar astronomy allowed direct observation of their surfaces.
Mercury’s rotation was a surprise, because prior to these observations, most astronomers that it would be tidally locked (synchronous) with the sun, with its sidereal day and orbital period the same. Instead, it was found to be in 3:2 resonance, rotating 1.5 times each 87.969
Earth day year. Mercury’s axial tilt, the least of any major solar planet, is close to zero, about 0.035° vs. Earth’s 12.4°
Venus is even stranger, rotating the opposite (retrograde) direction of its orbit at the slowest rate of any major planet, 1/248th the rotational rate of the Earth. Its axial tilt is also small, about 2.7°.
The lack of flattening of Mercury and Venus compared to Earth is almost certainly due to their much slower rotation.
However, the shape and rotation of Earth-like planets is, I think, a red herring (intentional or unintentional) when considering
planetary differentiation (the separation of planets into different layers). The equatorial centrifugal force on a test particle within the Earth is at all depths very small, and even at the boundaries of its various deep layers (of which we know the depth with good precision via seismography), a small fraction of the force due to gravity. Although many important effects, especially their generated magnetic fields, would be much different if the Earth-like planets had much different rotations, no model I’ve seen or can plausibly imagine indicates that they would have differentiated much differently had, for example, Venus had Earth rotation, or vice versa.
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