Expert: Tom Whiting - 4/4/2004

Question
Hello Mr. Whiting,
I don't quite understand how even if the moon is spinning, why is it not subject to tidal braking?

and what is different about mercury that causes its spin rate to be locked differently from the moon's?

thanks a lot

-raichu

Answer
Raichu,
It was, and they are...in fact, they both are subject to tidal braking...actually, everything is subject to slow
tidal braking, even the Earth,
and there are 2 events which eventually take place....the moon is one example and Mercury is the other.

Our moon (and most of the other satellites in the Solar
System) became locked in gravitational resonance with its
"heavy side" always facing the Earth....of course, this
takes millions of years.  Eventually, in a few billion
years, the same will happen to the Earth as we slow down
our spin too....someday the same side of the Earth will always face the moon, and the two will dance around the sun,
orbiting a common center of gravity as they do now, always with the same side facing each other, and with a
rotation rate of about 25-30 days.  (The Earth has already
slowed down its spin rate about 4 hours in the past
65 million years...its been estimated that the dinosaurs
had a 20 hour day).  All due to tidal braking with the moon.

With Mercury, something different happened (perhaps due to
its much slower rotation rate, or much longer revolution rate of 88 days, or much more elliptical orbit, or much greater gravitational field, OR perhaps a combination of all four situations...who knows?)
(No one could possibly know exactly why because we don't know what the actual starting conditions were of 4.6 billion years ago).


So Mercury's slow multi-billion year evolution produced what is called 'spin-orbit coupling' where the heavy side of Mercury faces the sun on every other orbit. Notice the rotation rate is exactly 2/3 the revolution rate.

Hope all this helps,
Clear Skies,
Tom Whiting
Erie, PA