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CaptainToonces
2019-Mar-29, 04:17 PM
1. For how long has the moon's rotation been tidally locked to Earth, keeping the same side of it facing the Earth?

2. How much more likely is the "dark side" of the Moon (the side always facing away from Earth) to be struck by asteroids than the side facing Earth?

profloater
2019-Mar-29, 06:21 PM
there must be a slight effect in the shadowing of the near face by the Earth. So I guess a few more impacts might hit the far side. To estimate the tidal lock we need to know the initial spin rate and I do not know that.

Roger E. Moore
2019-Mar-29, 06:32 PM
Partial answer to #2, from NASA:

https://sservi.nasa.gov/?question=3318

ronin
2019-Mar-30, 12:19 AM
As for question 1, without knowing the initial rotation rate of the moon after it's formation it's very difficult to calculate.

Upper estimate of how long it would have taken according to https://astronomy.stackexchange.com/questions/1859/when-did-the-moon-stop is @16 million years, so in cosmic terms it's been tidally locked almost since it's creation.

WaxRubiks
2019-Mar-30, 05:01 AM
there must be a slight effect in the shadowing of the near face by the Earth. So I guess a few more impacts might hit the far side. To estimate the tidal lock we need to know the initial spin rate and I do not know that.

but for every potential collision object that Earth might shield the moon from, there might be a collision object that is swung around the Earth into the course of the moon...so it might be more complicated.

profloater
2019-Mar-30, 09:26 AM
but for every potential collision object that Earth might shield the moon from, there might be a collision object that is swung around the Earth into the course of the moon...so it might be more complicated.

complicated in minor detail but not in principle because a diverted object can also be directed toward the moon, it is only those that hit Earth that we can deduct from the arithmetic. Just as Jupiter is a sweeper that protects the inner planets.

StupendousMan
2019-Mar-30, 11:16 PM
A paper by Wang and Zhou (A&A, vol 594, p. 52, 2016) suggests that there should be no difference in the cratering rate on the Moon's near and far side .... but there should be a difference between its leading and trailing sides. There's a link to a PDF copy of the paper at the ADS entry for it:

http://adsabs.harvard.edu/abs/2016A%26A...594A..52W

and the abstract follows below.


Context. The cratering asymmetry of a bombarded satellite is related to both its orbit and impactors. The inner solar system impactor populations, that is, the main-belt asteroids (MBAs) and the near-Earth objects (NEOs), have dominated during the late heavy bombardment (LHB) and ever since, respectively.

Aims: We formulate the lunar cratering distribution and verify the cratering asymmetries generated by the MBAs as well as the NEOs.
Methods: Based on a planar model that excludes the terrestrial and lunar gravitations on the impactors and assuming the impactor encounter speed with Earth venc is higher than the lunar orbital speed vM, we rigorously integrated the lunar cratering distribution, and derived its approximation to the first order of vM/venc. Numerical simulations of lunar bombardment by the MBAs during the LHB were performed with an Earth-Moon distance aM = 20-60 Earth radii in five cases.

Results: The analytical model directly proves the existence of a leading/trailing asymmetry and the absence of near/far asymmetry. The approximate form of the leading/trailing asymmetry is (1 + A1cosβ), which decreases as the apex distance β increases. The numerical simulations show evidence of a pole/equator asymmetry as well as the leading/trailing asymmetry, and the former is empirically described as (1 + A2cos2ϕ), which decreases as the latitude modulus | ϕ | increases. The amplitudes A1,2 are reliable measurements of asymmetries. Our analysis explicitly indicates the quantitative relations between cratering distribution and bombardment conditions (impactor properties and the lunar orbital status) like A1 ∝ vM/venc, resulting in a method for reproducing the bombardment conditions through measuring the asymmetry. Mutual confirmation between analytical model and numerical simulations is found in terms of the cratering distribution and its variation with aM. Estimates of A1 for crater density distributions generated by the MBAs and the NEOs are 0.101-0.159 and 0.117, respectively.