Moon's Center of Mass

Hi,
What is the process which caused the Moon's 2 Km
center-of-mass/center-of-figure offset?
Thanks,
Vvega


Possibly lava (denser than the other rocks) tending to emerge on the
earth-facing side, back when the moon was volcanically active? That'd
explain why there aren't many large "seas" on the far side too. I dimly
remember a theory along such lines being fashionable a few decades ago
anyway...

"AndyK" wrote in message
...
Hi,
What is the process which caused the Moon's 2 Km
center-of-mass/center-of-figure offset?
Thanks,
Vvega


Possibly lava (denser than the other rocks) tending to emerge on the
earth-facing side, back when the moon was volcanically active? That'd
explain why there aren't many large "seas" on the far side too. I dimly
remember a theory along such lines being fashionable a few decades ago
anyway...



I think you mean that lava is less dense than the rocks (otherwise it would
not work its way upwards!).

One reason the lava tended to emerge in the basins and craters of the
nearside is that the crust is much thicker on the farside. When a major
impact event occurred on the farside, the cracking of the surface rocks did
not penetrate into the mantle, but on the nearside this could happen,
eventually leading to upwelling. The maria resulted from much later
flooding of the impact basins (it all took time, millions of years in many
cases).

The displaced centre of mass also affected this. If you think of the
gravitational potential at the surface, a "sea level" if you see the
analogy, the surface on the farside is also a few km higher, so lava also
found it easier to reach the surface on the nearside.

Large impacts occurred on both sides of the moon in about equal numbers, but
only on the nearside could large maria form. The biggest lava plains on the
farside are in Mare Orientale basin and the crater Tsiolkovsky, and by
nearside standards, they are pretty small.

--
Mike Dworetsky

(Remove "pants" spamblock to send e-mail)

"AndyK" wrote in message
...
Hi,
What is the process which caused the Moon's 2 Km
center-of-mass/center-of-figure offset?
Thanks,
Vvega


Possibly lava (denser than the other rocks) tending to emerge on the
earth-facing side, back when the moon was volcanically active? That'd
explain why there aren't many large "seas" on the far side too. I dimly
remember a theory along such lines being fashionable a few decades ago
anyway...



I think you mean that lava is less dense than the rocks (otherwise it would
not work its way upwards!).

One reason the lava tended to emerge in the basins and craters of the
nearside is that the crust is much thicker on the farside. When a major
impact event occurred on the farside, the cracking of the surface rocks did
not penetrate into the mantle, but on the nearside this could happen,
eventually leading to upwelling. The maria resulted from much later
flooding of the impact basins (it all took time, millions of years in many
cases).

The displaced centre of mass also affected this. If you think of the
gravitational potential at the surface, a "sea level" if you see the
analogy, the surface on the farside is also a few km higher, so lava also
found it easier to reach the surface on the nearside.

Large impacts occurred on both sides of the moon in about equal numbers, but
only on the nearside could large maria form. The biggest lava plains on the
farside are in Mare Orientale basin and the crater Tsiolkovsky, and by
nearside standards, they are pretty small.

--
Mike Dworetsky

(Remove "pants" spamblock to send e-mail)

"AndyK" wrote:

Hi,
What is the process which caused the Moon's 2 Km
center-of-mass/center-of-figure offset?
Thanks,
Vvega


Possibly lava (denser than the other rocks) tending to emerge on the
earth-facing side, back when the moon was volcanically active? That'd
explain why there aren't many large "seas" on the far side too. I dimly
remember a theory along such lines being fashionable a few decades ago
anyway...

How about:

If the vocanism was caused by tidal deformations in the days when the
Moon rotated relative to earth, that volcanism would continue for a
few million years once the Moon slowed down and became locked to
earth's rotation. With greater gravitational force on the earth side,
earth side volcanism would continue longer, leading both to bigger
seas earthside and to displacement of the cog.

Seems plausible (and therefore probably wrong.)

Cheers

Martin

--------------
Martin Frey
N 51 02 E 0 47
--------------

"AndyK" wrote:

Hi,
What is the process which caused the Moon's 2 Km
center-of-mass/center-of-figure offset?
Thanks,
Vvega


Possibly lava (denser than the other rocks) tending to emerge on the
earth-facing side, back when the moon was volcanically active? That'd
explain why there aren't many large "seas" on the far side too. I dimly
remember a theory along such lines being fashionable a few decades ago
anyway...

How about:

If the vocanism was caused by tidal deformations in the days when the
Moon rotated relative to earth, that volcanism would continue for a
few million years once the Moon slowed down and became locked to
earth's rotation. With greater gravitational force on the earth side,
earth side volcanism would continue longer, leading both to bigger
seas earthside and to displacement of the cog.

Seems plausible (and therefore probably wrong.)

Cheers

Martin

--------------
Martin Frey
N 51 02 E 0 47
--------------

Hi,
What is the process which caused the Moon's 2 Km
center-of-mass/center-of-figure offset?
Thanks,
Vvega


Possibly lava (denser than the other rocks) tending to emerge on the
earth-facing side, back when the moon was volcanically active? That'd
explain why there aren't many large "seas" on the far side too. I dimly
remember a theory along such lines being fashionable a few decades ago
anyway...

How about:

If the vocanism was caused by tidal deformations in the days when the
Moon rotated relative to earth, that volcanism would continue for a
few million years once the Moon slowed down and became locked to
earth's rotation. With greater gravitational force on the earth side,
earth side volcanism would continue longer, leading both to bigger
seas earthside and to displacement of the cog.

Seems plausible (and therefore probably wrong.)

Cheers

Martin


Well, he asked for a THEORY, not necessarily the correct answer - that's how
I interpreted it anyway :-)

Hi,
What is the process which caused the Moon's 2 Km
center-of-mass/center-of-figure offset?
Thanks,
Vvega


Possibly lava (denser than the other rocks) tending to emerge on the
earth-facing side, back when the moon was volcanically active? That'd
explain why there aren't many large "seas" on the far side too. I dimly
remember a theory along such lines being fashionable a few decades ago
anyway...

How about:

If the vocanism was caused by tidal deformations in the days when the
Moon rotated relative to earth, that volcanism would continue for a
few million years once the Moon slowed down and became locked to
earth's rotation. With greater gravitational force on the earth side,
earth side volcanism would continue longer, leading both to bigger
seas earthside and to displacement of the cog.

Seems plausible (and therefore probably wrong.)

Cheers

Martin


Well, he asked for a THEORY, not necessarily the correct answer - that's how
I interpreted it anyway :-)