Hi alb
This is a really good question. The answer lies in
the same reason we have high and low tides on Earth.
The mass of the Earth tugs on the Moon in the same
way the moon pulls on the Earth. Since the Earth is
so much greater in size it is able to distort the
sphere of the moon. The Earth draws mass from the Moon
towards it. This elongation of the Moon reduces its
rotational velocity and converts it to heat. Eventually
the spin is nearly nullified.
The elongation is drawn along an axis pointing to the
Earth. It results in permanent tidal bulges which makes it
dynamically most stable if one end of the bulge is always
pointed towards the Earth.
This process is called Tidal Locking. A very gradual
process. The dynamics of it are explained in detail
here:
"[21.06] Tidal Despinning Timescales in the Solar System"
C.F. Chyba (SETI Institute and Stanford University), P. J.
Thomas (U. Wisconsin, Eau Claire)
[ http://www.aas.org/publications/baas/v30n3/dps98/320.htm ]
"Planets and satellites in the Solar System despin to a
spin-evolved end-state due to tidal dissipation. The usual
derivation for the despinning timescale sets the change in spin
angular momentum equal to the gravitational torque acting on
the object's tidal bulge (MacDonald 1964, Goldreich and Soter
1966, Peale 1974, 1977). The despinning timescale is found to
be proportional to the difference between the initial and final
spin angular velocities, and is finite. However, this
approximate derivation ignores the orbital mean motion n of the
despinning object, and is less and less satisfactory as the
object's spin angular velocity w approaches n. We have instead
calculated tidal despinning times by applying the formalism of
Peale and Cassen (1978) to calculate tidal energy dissipation
due to tides raised on a non-spin-locked object. Tidal heating
in the latter case is larger than tidal heating in the spin
locked case by a factor (1/7)[(w-n)/n](1/e2), where e is the
orbital eccentricity. This factor is initially greater than 104
for many objects in the Solar System. Calculating despinning
times from energy loss, we find that the despinning timescale
includes a previously neglected term that goes to infinity
logarithmically as w approaches n. In this sense all despinning
timescales are in fact infinite. We therefore define an
effective despinning timescale as the time required for despin
tidal heating to fall below tidal heating due to orbital
eccentricity. For many satellites in the Solar System,
including such major moons as Io and Europa, the neglected term
in the despinning timescale is in fact the dominant term. For
some especially short-period satellites, such as Phobos or
Amalthea, the resulting despinning timescales are one to two
orders of magnitude longer than those previously accepted."
-=o=--=o=--=o=--=o=--=o=--=o=--=o=--=o=--=o=--=o=--=o=--=o=-
Since that's a big chunk of info here is a simpler explanation:
"Tidal locking, or Orbit-spin resonance" by Anthony Lawson
`Physics of the Solar system'. Feb 9, 1999
[ http://www.astro.soton.ac.uk/~ajl/course/mag/node9.html ]
"Although the Moon changes phase throughout a month if we look
carefully we can see that it actually keeps the same face
directed towards us the whole time. This doesn't mean that it
is not rotating, but that it has a rotation period the same
length as its orbital period (this is the sidereal period, 27.3
days, not the synodic period). The Moon is in orbit-spin
resonance, or tidally locked, with the Earth. How did this
happen?"
"As seen above the Earth creates a tidal bulge on the Moon which
lies on the Earth-Moon line. However, if we imagine a time in
past when the Moon was rotating faster, the rotation would tend
to carry the bulge away from the Earth-Moon line. As it did so
the Earth's gravity tried to hold it back slowing the rotation
of the Moon ever so slightly. Over a long period of time this
would slow the rotation of the Moon until eventually the spin
period of the Moon matched that of its orbital period. When
this point is reached the tidal bulge is always pointing
towards the Earth and the rotation rate remains constant. This
is the situation we see today. There are a number other
examples of this tidal locking in the Solar system. Just like
the Moon, the Galilean satellite Io has a spin period which is
equal to its orbital period around Jupiter."
This chart shows the orbital data of satellites in our
Solar System.
[ http://www.solarviews.com/eng/data1.htm ]
Please ask for a clarification if there is a point you do
not understand.
I have provided a few search links to do additional research.
"tidal locked" OR "tidally locked" moon
[ ://www.google.com/search?hl=en&lr=&ie=UTF-8&oe=UTF-8&q=%22tidal+locked%22+OR++%22tidally+locked%22+moon
]
"tidal locked" OR "tidally locked" satellite
[ ://www.google.com/search?hl=en&lr=&ie=UTF-8&oe=UTF-8&q=%22tidal+locked%22+OR++%22tidally+locked%22+satellite
]
"Tidal Despinning"
[ ://www.google.com/search?hl=en&lr=&ie=UTF-8&oe=UTF-8&q=%22Tidal+Despinning%22
]
thanks
-AI |
