What explains the Luna impact gap?

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Discussion Overview

The discussion revolves around the observed 'impact gap' on the Moon, particularly the absence of impacts in the central region as noted in a lunar impact map from 2005 to 2018. Participants explore potential explanations for this phenomenon, considering factors such as gravitational shielding by Earth, the distribution of asteroids, and historical impact patterns over millions of years.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • Some participants suggest that the equatorial distribution of asteroids contributes to the observed impact gap, with fewer impacts expected towards the poles.
  • Others propose that Earth's gravitational influence shields the Moon from impacts in the central region, leading to a lower density of craters there.
  • A participant questions whether this shielding effect has been consistent over geological time, implying that more craters should exist on the sides than in the middle.
  • There is mention of a consensus regarding the Moon's tidal locking shortly after its formation, which may have implications for impact frequency.
  • Some participants discuss the potential influence of a 'cosine factor' related to the angle of incoming impacts, suggesting that the geometry of asteroid distribution could affect impact density.
  • Another viewpoint raises the idea that the absence of impacts at the poles could be due to the geometry of the asteroid belt and the distribution of impacts over time.
  • Participants reference the existence of lunar maria and their implications for ancient impacts, suggesting that early impacts may have melted the surface, affecting crater visibility.
  • There is a discussion about the overlay of recent impacts and how they might create a 'shadow' effect on the lunar surface, influencing the observed impact distribution.
  • One participant mentions a study tracking large impacts over the last billion years, noting a lack of discernible grouping patterns.
  • Another participant interprets data suggesting a correlation between crater density and lunar highlands, indicating that older craters may remain visible due to a lack of subsequent lava flows.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the causes of the impact gap, with no consensus reached on a singular explanation. The discussion remains unresolved as various hypotheses are presented and debated.

Contextual Notes

Limitations include the dependence on observational data and the potential for missing assumptions regarding the historical impact patterns and the effects of Earth's gravity on impact distribution.

  • #31
DaveC426913 said:
Surely the shadowing effect is due in very large part to the Earth's gravity well, not the Earth proper.

I'd say it's analogous* to the conjecture that life in a given star system may be greatly dependent on having a Jovian superplanet whose huge gravity well sweeps the system clean of potential extinction-level bodies. It wasn;t Jupiter proper that cleared out the comets...

*not saying it's the same mechanics, simply the implication that gravity wells are a primary factor in the orbital mechanics of bombardment.
The "target" the Earth makes is defined by the impact parameter, which itself depends on the velocity of the approaching object. So for example, if we assume that an object was "sneaking up" from behind the Earth in its orbit at a relative velocity of 10 km/s*, then the impact parameter expands out to 1.5 Earth radii. Drop that incoming velocity down to 5 km/s and it expands out to nearly 2.5 Earth radii

*initial velocity, not taking into account the effect of the Earth's gravity. (though in reality it's a bit more complicated than that. Say we start with such a body coming up behind the Earth. As the Earth's gravity begins to pull on forward on it, it gains orbital energy with respect to the Sun and it climbs to a higher orbit, but a higher orbit is a slower orbit, so its actually loses relative velocity with respect to the Earth. )
 
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  • #32
The 0° longitude gap and the absence of recorded impacts near the poles are artifacts of the observing system . . . they simply did not look there. Please refer to

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20150021386.pdf
lunar impacts gaps.jpg


Thanks to spareine for finding the document

BTW, your concern about the Earth's gravity well concentrating or deflecting impact on the Moon might be applied to the difference between impact counts on the eastern and western hemispheres although the normal flow of NEOs catching up with the Earth-Moon system or the Earth-Moon system catching up with the NEOs may be more important.
 
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  • #33
A few comments regarding lunar impacts since someone mentioned the glancing blow hypothisis I feel the need to argue the strongest model for the moon forming impact occurrence is probably the synestia one since it removes the need for a "glancing blow" freeing up a far larger potential parameter space to reproduce the post impact Earth Moon system but I digress so back to the topic at hand. Yeah the bias here is primarily due to the exclusion of these regions though there is apparently a real measurable ge bias on the Moon as interestingly there does seem to be a significant increase in Asteroid impacts in our solar system over the last 290 million years.
https://www.sciencedaily.com/releases/2019/01/190117142042.htm
 
  • #34
Dragrath the synestia hypothisis is interesting and may be accepted



There are the outliers: Venus backwards, Uranus on it side, us with a large moon, and the double dwarf planet Pluto - Charon L O L

We need giant telescopes (interferometers) in space to study other solar systems.

telescope interplanetary using interferometry.jpg


So far as the lunar impacts central gap and nothing at the poles - Occam's Razor . . . plus the paper from the people recording the meteor impacts

Isn't science "interesting"?
 
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