I guess my question really is in three parts and please let me apologize in advance for any mistakes made in terminology…
Obviously the moon has suffered from a very large number of impacts during it’s history, as evidenced by it’s scarred surface. While an absence of the weathering conditions found here on Earth means even very old scars are still visible, has the moon entered a period of relative quiesence compared to a more violent stage in it’s past? Was it once more in line with an asteroid/meteorite belt and more prone to receiving a large number of impacts?
Shouldn’t the absence of an atmosphere and therefore it’s inability to incinerate many of the meteorites that the Earth’s atmosphere would render ineffective cause it to get hit much more frequently than Earth?
Is the moon’s distance, while seemingly close through a telescope, still just too far away for planetary telescopes to see any but the very largest (and rarest) of impacts?
It just seems to me that the moon should be getting hit by a lot of the same sized meteors that might burn up in our atmosphere and that astronomers would be frequently capturing these events and they’d frequent our news broadcasts. However, for one or more reasons, such is not the case.
Well, for one thing, no one’s watching the Moon 24/7 and the impacts are over in a few seconds, unless they’re spectacularly large. For another, the Moon always has one face towards the Earth, as such we can only see those impacts which have managed to sneak past the much larger Earth–based on flybys, the far side is much more battle-scarred. Finally, “frequently” is a relative thing. The Moon’s surface is a virtually-unchanged recored of impacts almost since its inception ~3.5 billion YA. It looks like a lot when looking at it all at once, but those large impact craters only happen once every few thousand years.
I’d wondered it the Earth did act as a shield and assumed that fact to be true but hadn’t carried the thought to considering how one side faces us and that the other, unprotected one would display an even greater assemblage of craters, etc. That’s pretty interesting.
The Moon, much like Earth, is under constant meteor bombardment… but most of those “meteors” are tiny specks of dust.
And we don’t see meteor impacts on the moon because we have our telescopes trained on more interesting things than the Moon, y’know, neighbours undressing in the window and all that good stuff.
Most of the meteors that burn up in the Earth’s atmosphere are tiny (the size of a pea or less). They wouldn’t leave much of a crater even if they hit the Earth, and they don’t leave much of a crater on the Moon.
It’s believed that there were many more bits of rock flying around the solar system in the very early days. There aren’t so many now because most of them have already crashed into a planet/moon if there was one in the vicinity. They’ve had several billion years to hit something after all.
Whether a meteor burns up in the atmosphere or collides with the moon’s surface, the amount of energy released is the same. So the brightness would be roughly the same. The difference is distance; the meteors we see in the sky are a couple hundred miles away, while the moon is 240,000 miles away.
Also half the moon is in sunlight (on average). The other half is still very close to the sunlit (less than 1/2 degrees), side so it’s difficult to observe really faint flashes of light there.
The huge number of craters on the Moon is not a history of uniform rates of impacts over three billion years. The impacts on the Moon, and on Earth, for that matter are disproportionately old. In fact, it is likely that most of them were made within a few million years of the original formation of the Moon, and most of the balance of them during the following billion years.
First, probable scenarios for the formation of the Moon include a huge number of other particles of various sizes being present in the near vicinity of the Earth/Moon system. As the Moon solidified, those particles began to leave scars from their impacts. That comparatively frequent impact rate dropped off as the system ejected and lost objects by orbital mechanics, and collision.
This same process was also going on for the entire solar system, as well. In the first billion years that the Earth existed, impacts were far more common, and the objects included a few very large bodies, perhaps even a Mars sized object which is thought to have collided with earth, and caused the creation of the moon. That large number of bodies is no longer present, having suffered attrition from impacts, and being thrown out of the solar system by encounters with planets, most particularly Jupiter.
So, the answer is that the Moon is simply not hit as often as its scarred visage makes one think it should. Three and a half billion years is a very long time, and the big bad times of long ago were, well, long ago. Add to that that a grain of sand streaking through the night sky leaves a miles long glowing streak that is very easy to see. The same meteor striking the moon does so instantaneously, and leaves no trail, only a pinpoint of light. The Earth meteor is a few miles over head, the pinpoint impact is nearly half a million miles away.
[QUOTE=Q.E.D.]
For another, the Moon always has one face towards the Earth, as such we can only see those impacts which have managed to sneak past the much larger Earth–based on flybys, the far side is much more battle-scarred. /QUOTE]
I must disagree. The Earth is not a real physical shield to the moon. Imagine that the Earth was the size of a basketball, the Moon would be the size of a softball and would be 18 m away. On this scale, the shielding is minimal.
I’ll leave it to others to discuss whether the Earth gravity attracts or repels meteroids to the moon because the facing side of the moon looks very different than the far side. I believe, the Earth’s gravity redirects meteriods away from the moon which is why the far side as many for impacts.
I must disagree. The Earth is not a real physical shield to the moon. Imagine that the Earth was the size of a basketball, the Moon would be the size of a softball and would be 18 m away. On this scale, the shielding is minimal.
I’ll leave it to others to discuss whether the Earth gravity attracts or repels meteroids to the moon because the facing side of the moon looks very different than the far side. I believe, the Earth’s gravity redirects meteriods away from the moon which is why the far side as many for impacts.
Excellent link, rowrrbazzle, as it describes one thing I was curious about… meteorites that would burn up in our atmosphere but are big enough to create an observable impact if we’re lucky enout to be scoped in.
See, I happen to know Danielle Moser, (she was one of the people who caught that footage) Seems since they started watching, they have seen two impacts of a small scale.
I still think it is kinda cool.