[Lurker]

According to the simulation results, the difference in melt volume could be readily explained if Chicxulub -- the impact crater that doomed the dinosaurs -- was formed by an asteroid and Sudbury was formed by a comet.

Wouldn't the material being impacted have some effect on that as well? And isn't anything moving at orbital velocity going to make a pretty darned big hole anyway?

L

[Fraxinus]

Wouldn't the material being impacted have some effect on that as well?

How great is the difference in the melting point between limestone and granite?

The difference between an asteroid and comets is that comets can have retrograde orbits (opposite of the direction of earths orbit) and thus are capable of hitting with much greater velocities.

[SunkenCiv]

Lurker: Wouldn't the material being impacted have some effect on that as well? And isn't anything moving at orbital velocity going to make a pretty darned big hole anyway?

The size of the body and its terminal velocity have the most to do with the energy delivered. A smaller factor is how strongly held together the rock(s) or ice or ice/rock lump is. Doublet craters (two or more craters of basically simultaneous origin) result from an incoming body fracturing due to having poor integrity; also atmospheric heating can play some factor, but with these velocities, less so.

Fraxinus: The difference between an asteroid and comets is that comets can have retrograde orbits (opposite of the direction of earths orbit) and thus are capable of hitting with much greater velocities.

Asteroids also can have retrograde orbits. Comets have greater velocities because they are on much more elliptical orbits, so that (under Kepler's laws) they reach their greatest velocity when nearest the Sun. All bodies do that, but with comets it's much more obvious.

Fraxinus: How great is the difference in the melting point between limestone and granite?

I think that's a great point. Also, the three times greater age of the crater means much less evidence around.

Lurker: How would these scientists know which direction the comet or asteroid was traveling? For that matter how do they know what it was that hit? Have they found fragments or something?

They make a guess based on the shape of the crater. Impacts make very round craters regardless of their direction or how steep a trip the bolide took through the atmosphere; however, the crater will be slightly elliptical, with the elongation in line with the approach path. At that point I think it's a matter of which lip of the crater is higher, and in which direction the subterranean fractures are leaning.