Chris Peterson wrote:Leon M. Green wrote:
Along with the farside crust being thicker, would it not be that there is more cratering because of taking the meteors that might have hit earth?
No, that effect is small. Even at the time of the bombardment, when the Moon and Earth were closer, meteoroid shadowing by the Earth wasn't significant (the Earth only blocked a small part of the sky), and in addition, the Earth acted as a gravitational lens, so it pulled in some bodies that wouldn't have otherwise impacted.
Even
without any gravitational lensing effect (which is angularly convergent
but
radially divergent making it probably only a 2
nd order effect):
- with a diameter of just 2º in the lunar sky the Earth
physically blocks only 0.0076% [=0.5 x {1- cos(1º)}] of incoming meteors.
Chris Peterson wrote:Leon M. Green wrote:
And I find it curious that in the new "Cosmos" program, Neil DeGrasse Tyson very calmly took the position that the moon was formed concurrently with the Earth, and not the result of a very large impact on the Earth that then remained captured in orbit.
I'm not sure if he did or not. That bit caught my ear, as well. I watched the segment a couple of times, and it's just badly written. What he says and what the graphics show aren't quite the same. The explanation for the Moon is quite ambiguous. So I don't know if Tyson holds some non-standard belief about how the Moon formed, or if he just explained it badly.
Tyson was obviously trying to emphasize the planetesimal
accretion hypothesis of
Viktor Safronov(; see below).
A dramatic representation of the presumed special collisional formation
of the Earth-Moon "binary planet" would have been a distraction from the main point.
http://en.wikipedia.org/wiki/Planetesimals wrote:
<<A widely accepted theory of planet formation, the planetesimal hypothesis of
Viktor Safronov (
11 October 1917 [my mom's birthday! -ACN] - 18 September 1999), states that planets form out of cosmic dust grains that collide and stick to form larger and larger bodies. When the bodies reach sizes of approximately one kilometer, then they can attract each other directly through their mutual gravity, enormously aiding further growth into moon-sized protoplanets. This is how planetesimals are often defined. Bodies that are smaller than planetesimals must rely on Brownian motion or turbulent motions in the gas to cause the collisions that can lead to sticking. Alternatively, planetesimals can form in a very dense layer of dust grains that undergoes a collective gravitational instability in the mid-plane of a protoplanetary disk. Many planetesimals eventually break apart during violent collisions, as may have happened to 4 Vesta and 90 Antiope, but a few of the largest planetesimals can survive such encounters and continue to grow into protoplanets and later planets.
It is generally believed that about 3.8 billion years ago, after a period known as the Late Heavy Bombardment, most of the planetesimals within the Solar System had either been ejected from the Solar System entirely, into distant eccentric orbits such as the Oort cloud, or had collided with larger objects due to the regular gravitational nudges from the giant planets (particularly Jupiter and Neptune). A few planetesimals may have been captured as moons, such as Phobos and Deimos (the moons of Mars), and many of the small high-inclination moons of the giant planets.>>
[quote="Chris Peterson"][quote="Leon M. Green"]
Along with the farside crust being thicker, would it not be that there is more cratering because of taking the meteors that might have hit earth?[/quote]
No, that effect is small. Even at the time of the bombardment, when the Moon and Earth were closer, meteoroid shadowing by the Earth wasn't significant (the Earth only blocked a small part of the sky), and in addition, the Earth acted as a gravitational lens, so it pulled in some bodies that wouldn't have otherwise impacted.[/quote]
Even [b][u]without[/u][/b] any gravitational lensing effect (which is angularly convergent
but [u]radially divergent[/u] making it probably only a 2[sup]nd[/sup] order effect):
[list]with a diameter of just 2º in the lunar sky the Earth
physically blocks only 0.0076% [=0.5 x {1- cos(1º)}] of incoming meteors.[/list]
[quote="Chris Peterson"][quote="Leon M. Green"]
And I find it curious that in the new "Cosmos" program, Neil DeGrasse Tyson very calmly took the position that the moon was formed concurrently with the Earth, and not the result of a very large impact on the Earth that then remained captured in orbit.[/quote]
I'm not sure if he did or not. That bit caught my ear, as well. I watched the segment a couple of times, and it's just badly written. What he says and what the graphics show aren't quite the same. The explanation for the Moon is quite ambiguous. So I don't know if Tyson holds some non-standard belief about how the Moon formed, or if he just explained it badly.[/quote]
Tyson was obviously trying to emphasize the planetesimal [b][u]accretion[/u][/b] hypothesis of [url=http://en.wikipedia.org/wiki/Viktor_Safronov]Viktor Safronov[/url](; see below).
A dramatic representation of the presumed special collisional formation
of the Earth-Moon "binary planet" would have been a distraction from the main point.
[quote=" http://en.wikipedia.org/wiki/Planetesimals"]
<<A widely accepted theory of planet formation, the planetesimal hypothesis of [url=http://en.wikipedia.org/wiki/Viktor_Safronov]Viktor Safronov[/url] ([b][color=#0000FF]11 October 1917 [my mom's birthday! -ACN][/color][/b] - 18 September 1999), states that planets form out of cosmic dust grains that collide and stick to form larger and larger bodies. When the bodies reach sizes of approximately one kilometer, then they can attract each other directly through their mutual gravity, enormously aiding further growth into moon-sized protoplanets. This is how planetesimals are often defined. Bodies that are smaller than planetesimals must rely on Brownian motion or turbulent motions in the gas to cause the collisions that can lead to sticking. Alternatively, planetesimals can form in a very dense layer of dust grains that undergoes a collective gravitational instability in the mid-plane of a protoplanetary disk. Many planetesimals eventually break apart during violent collisions, as may have happened to 4 Vesta and 90 Antiope, but a few of the largest planetesimals can survive such encounters and continue to grow into protoplanets and later planets.
It is generally believed that about 3.8 billion years ago, after a period known as the Late Heavy Bombardment, most of the planetesimals within the Solar System had either been ejected from the Solar System entirely, into distant eccentric orbits such as the Oort cloud, or had collided with larger objects due to the regular gravitational nudges from the giant planets (particularly Jupiter and Neptune). A few planetesimals may have been captured as moons, such as Phobos and Deimos (the moons of Mars), and many of the small high-inclination moons of the giant planets.>>[/quote]