SJAA Ephemeris October 2002 | SJAA Home | Contents | Previous | Next



Dave North


Only one question came up this month (at least that I remember) and it's a good one: what's the relationship between the size of a crater (or mare, for that matter) and the thing that caused it (the "impactor")?

For quite some time, I've been quoting an old paper I read that claimed the impactor size clustered around approximately 10 percent of the diameter of the result.

The general argument was, the bigger the impactor, the bigger it was in relationship to the diameter of the result.

Or look at it like this: if the crater were 100 meters across, maybe the slammer was 7 meters. If it were 1000 meters, maybe the impactor was 100 meters. If the crater were 10Km, perhaps a 1.5 meter object hit there.

But it looks like all that is wrong-o.

Definitely for earth, anyway. Here are some examples from Dr. Dan Durda:

Meteor Crater in Arizona has a diameter of 1.6Km, and the impactor was probably 25 meters in diameter. Chesapeake Bay: 85/4. Chicxulub, 175/10.

The percentages work out to: about 1.6, 4.7, and 5.7 percent. So the cluster is actually around 5 percent, and the scale of their relationship sure looks like a flattening curve (though that's a pretty small sample).

Now it may seem that the Moon should be a different case. Why? At least two reasons occur immediately: there is no atmosphere, and there is less gravity.

It turns out, however, those are probably unimportant. The first is simply skewed by the fact that the impactor's size is calculated from the time of impact, not while it was in space. So if part of it burned off in the atmosphere, that part just wouldn't count toward the total diameter.

But what about the energy lost to the atmosphere? True, there is some, but it's not significant compared to the overall velocity of an impactor large enough to make a crater we'd be interested in.

Gravity is also not much of an issue in the size of a primary crater. Very little (if any) of the energy of the impactor comes from gravitational attraction to the "target body" (the Moon, in this case). Mostly it's the energy already inherent in their differing velocities in the solar system, much as your gravity has little effect on the bullet that hits you. Or, hopefully, not.

There is some argument about whether the gravity plays a role in the size of the resultant crater because there is less compaction of the soil, and "stuff" is more easily tossed about. This may be - in part - where the original 10 percent figure I tossed about was sourced.

But no, that appears to be more important in the placement of secondary craters and the dispersal of fine ejecta resulting in, among other things, rays.

Overall, the current consensus is the cluster point for size of impactor vs size of crater, both here and on the Moon, is around five percent by diameter, not ten as I have been citing.

My apologies to all those I've unwittingly misled.

Of course, all this is based on some limited experimentation and attempts to measure residual mass from known craters, and extrapolating from there.

The term for this kind of thing is "on the order of," which means the expected five percent could be off by double or more in either direction without surprising anybody.

I won't, however, hide behind that. Best data is somewhere around 20-1 on average, dropping to 40-1 on smaller bodies, and getting to 10-1 only on the very largest.

Those, of course, are the most interesting, and yield up results like Mare Orientale and Mare Imbrium. The latter is roughly 800 miles across, and could be expected to have been produced by something around 80 miles across. That's still a boggling idea.

Of course, in trying to straighten this matter out, I came across a few other interesting factoids.

One is something I've noticed before, but never seen codified by someone who actually knows something: almost all the bodies in the solar system show cratering considerably more heavily in one hemisphere than the other. This is simply anomalous at this time: unexplained.

Further (and this was not included in the paper for some reason) one hemisphere tends to be "higher" (further above the datum) than the other. Notable examples are the Moon and Mars. In both cases, the "higher" hemisphere is also the more heavily marked "highland" area.

I have no explanation. I do speculate that it may be a case of simply showing less markings because the crust is thinner and more basaltic flows occured to cover the craters on one side.

Why would it be thinner on one side? Again, I don't know. Possibly the last extremely major impacts happened on that side, blowing off differentiated crust and at the same time leaving a heavy mass deposit underneath.

Just guessing.

The other interesting thing was various speculations about the "Big Impact" theory of the formation of the Moon. In a nutshell, the idea is something about the size of Mars hit Earth, blew off a bunch of stuff, and some of that accreted to form the Moon.

The major objection is, it requires gyrations almost as obtuse as the Ptolemaic/Copernican epicycles to make the numbers work out, and any such theory lacks both likelihood and elegance.

I prefer to personally file that one under "unknown" and let it go at that for the moment.


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