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My letter:
I watched the Eclipse of a week ago - then read about the flash observed, which was ascribed to a meteorite impact. Sky and Telescope published the observation with this map, illustrating the light flashes from 2005 - 2018.
Which made me curious about the rather obvious gap with no strikes in the middle. As noted below, I posted the image on physics forums, asking about the gap.
A poster replied that "...most asteroids are roughly in the ecliptic plane - hence the equatorial distribution (less towards the poles).
Earth shields the Moon (gravitationally) from the radial direction, hence the dearth of hits in the middle..."
and...
"...The consensus (and the math) seems to indicate that the Moon became tidally locked very shortly after forming. So yes, it would have been shielded on the near side for ~4Gy..."
I then asked if the impact record bears that out; if the record above from 2005 - 2018 reported some 400+ flashes in this pattern, then over several billion years, is this pattern reinforced, as reflected by the impact crater quantity?
...Which is where the discussion stands at the moment.
Rob Lerner thought you - with your skill set - might shed some light on the subject, so I though I'd ask for your input.
If you have any information on this, I would greatly appreciate it.
Best,
Stephen Goodfellow
**************
(Reply)
The only things I can add are:
In the late 1950's the USSR sent spacecraft to the Moon. Photos showed that the far side of the Moon doesn't have the large dark "seas" (of solidified lava) that are covering the Earth-facing side. The deduction was that the early liquid interior of the Moon was offset toward the Earth because of Earth's gravitational pull. Thus meteor impacts punched into the liquid that spewed out onto the Earth-facing side of the Moon.
Also, because the Moon doesn't spin because of Earth's gravity, the "left" side of the Moon receives meteors as the left side advances, increasing the velocity of the impact. Whereas, on the "right" side, the meteors need to catch up with the Moon as it orbits away.
China's Chang 3 has just landed on the far side south pole with the goal of finding rocks excavated by large and deep impacts to learn more about the interior of the Moon.
The model of Moon formation dating back 5 or so years is no longer the accepted model (where a Mars-sized "world" collided with the Earth and recollected into the Earth + a ring of debris that then coalesced into the Moon). Rather the Earth and Moon formed out of the same cloud of gas and dust at the same time is based upon the identical composition of Earth and Moon rocks.
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