by Chris Peterson » Tue Sep 18, 2007 1:21 am
craterchains wrote:Think of ALL the smaller objects that are so heavily cratered, but they have very little gravity to cause the hyper velocity speeds.
The gravity of an object doesn't determine the collision speed, but the
minimum collision speed. The dynamics of the all the objects in the Solar System mean that collisions tend to be much faster than this minimum. This is very obvious on Earth, where the average meteor has a speed about four times higher than the minimum speed as determined by the Earth's gravity. Furthermore, the rate of cratering has not been constant. Much of the cratering we see on Solar System bodies occurred about 4 billion years ago, during a period of instability when material was flying around in strongly elliptical or even parabolic/hyperbolic orbits. Such material will be close to Solar escape velocity, which even at Saturn is still very fast (about 14 km/s).
NO, I am not joking, all these craters and NOT ONE IMPACTOR can be found around them. Just fragments?
There is increasing evidence that most asteroidal bodies are probably not rigidly solid, but rather made up of loosely bound collections of material. What would you expect a collision with something like that to look like? And what makes you so certain there isn't a certain amount of impactor debris around some craters? It isn't like we've seen them at the necessary resolution, or been able to sample surrounding material. Parent material is found around craters on the Earth.
[quote="craterchains"]Think of ALL the smaller objects that are so heavily cratered, but they have very little gravity to cause the hyper velocity speeds.[/quote]
The gravity of an object doesn't determine the collision speed, but the [i]minimum [/i]collision speed. The dynamics of the all the objects in the Solar System mean that collisions tend to be much faster than this minimum. This is very obvious on Earth, where the average meteor has a speed about four times higher than the minimum speed as determined by the Earth's gravity. Furthermore, the rate of cratering has not been constant. Much of the cratering we see on Solar System bodies occurred about 4 billion years ago, during a period of instability when material was flying around in strongly elliptical or even parabolic/hyperbolic orbits. Such material will be close to Solar escape velocity, which even at Saturn is still very fast (about 14 km/s).
[quote]NO, I am not joking, all these craters and NOT ONE IMPACTOR can be found around them. Just fragments?[/quote]
There is increasing evidence that most asteroidal bodies are probably not rigidly solid, but rather made up of loosely bound collections of material. What would you expect a collision with something like that to look like? And what makes you so certain there isn't a certain amount of impactor debris around some craters? It isn't like we've seen them at the necessary resolution, or been able to sample surrounding material. Parent material is found around craters on the Earth.