*So named because poster O. T. Rush likened himself to a leopard lying on a limb, waiting to pounce. We miss the leopard's original target, so Roblo, don't be a stranger.
The Leopard's Limb is for leftovers, off-topics, anecdotes, etc. It is sort of redundant, as the former are not at all unwelcome on the LTE page.
Leftover from yesterday: Kit
Yes, Linds. Kit is welcome here, as you are. Anyone within reason is welcome. I will not delineate what is outside of reason. We're all adults. We know what is within reason.
Mooning over the past
From NPR comes a report that the Earth may have once had two moons. Scientists think that our current Moon was created early in the Earth's history when the Earth collided with a Mars-sized planet (such collisions were not uncommon in the chaotic early Solar System). The collision knocked off a sizable chunk of Earth, along with the other planet. At first, the fragments orbited the Earth sort of like Saturn's rings, then coalesced into our Moon, and, if the theory is right, a second moon, which ultimately collided with the larger satellite.
Scientists pose this theory because the "back" side of the Moon, the side we cannot see because the same side of the Moon always faces the Earth, is markedly different from the "front" side. The rear side is mountainous and heavily cratered, while "our" side of the Moon is relatively flat. It is thought that our side is the remnant of the larger moon while the back side is the remnant of the smaller. It is also thought that the collision was relatively soft, as the two moons slowly merged.
No living creatures on Earth saw the two moons. The collision/merger of the two would have occurred while the Earth was still molten.
http://www.npr.org/2011/08/04/138954932/early-earth-may-have-been-orbited-by-two-moons
A "soft" collision would also require two molten bodies. Would the mountains be the result of the collision? Our mountain ranges are the result of plate movement, but not sure if the moon has plates. The relative lack of craters facing the earth makes sense. The earth most likely absorbs the vast majority of meteors heading towards the earth facing side of the moon.
ReplyDeleteHi dotnet!
ReplyDeleteThe mountains are thought to have originated with the "soft collision." There are flat spots ("maria," plural Latin for "seas"; singular is "mare") on the back side, but smaller. The maria were formed by vulcanism (volcanic activity some 500MM years after the collision. Molten basalt smoothed out impact craters from the earlier days. By the time of the vulcanism, most of the debris left from the Solar System's genesis had been swept out of the inner part of the system.
I suspect the side facing us was more volcanically active because it is made of somewhat different material, more earthlike than the other side. I also suspect it is denser, thus more gravitationally attracted. That's why that heavier side ended up facing the Earth when tidal effects braked the Moon's rotation to exactly 1 rotation for every revolution around Earth.
The Earth is not a meteor or asteroid shield. The Earth covers just a few degrees of sky as seen from the Moon, probably about the area of a 50-cent piece as seen at arm's length--that is a guess, but the Moon's diameter is roughly a quarter that of the Earth, and you can block out the full Moon by extending your thumb to arm's length.