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The Scars Of Evolution

Dave North

Big badda boom.

Most of the moon was made in just that way: chunks o' stuff slamming into other chunks until you get a fairly chunky thing, then other stuff hits it and it just keeps getting bigger (except when a lump or two gets blown of by a big big badda boom).

This time of year I can't help but think about big impacts, because it's usually the best time to try to see the terminator crossing Mare Orientale, the best example of a well-preserved Big Honking Lunar Impact.

Since it's so close to the western edge of the Moon, it's hard to see. The ideal conditions are a strong libration of that limb toward us, with the terminator just a bit before full.

This will happen near midnight on September 12 (if I got the numbers right) but the libration won't be spectacular.

Still, it might be worth a shot. If you succeed, you'll see something at least as captivating as a good edge-on galaxy, and much harder to see.

But what if you don't? Is there anything else to see?

Of course! There's always Mare Nectaris, for example, which is the second-best impact boomer on the moon. Perhaps you've heard of the Altai Scarp: it's a shock ring from the Nectaris impact.

Okay, wait a minute.

I love this subject.

Say you get out your scope in the general vicinity of September 2nd, and happen to look up at the moon near sunset. Whaddya know, what's that big line of mountains near the terminator...?

If you miss it, just try again about two weeks later and you'll see it even higher in the sky near midnight (but you don't have to wait quite that long if you're sleepy).

The idea is to spend just a little while thinking about the size of what you're seeing. The Mare itself is the main impact zone, and the Altai Scarp is the first of a series of mountain rings thrown up by the shock.

As you can see, this actually represents a substantial piece of the lunar surface, and an even bigger percentage of the diameter of the moon... which is about 2000 miles.

That gives you some idea of the size of this impact.

It's not a particularly big one.

In fact, all the Maria are probably scars left from this kind of "evolution" on the moon, including the much larger Mare Imbrium to the northwest of Nectaris, and very probably all of Oceanus Procellarum - which almost looks like it takes up half the face of the moon (it doesn't. Not even close. But still, it's BIG).

Back in the wooly old days, folks used to think the craters, basins and seas were all much deeper, eventually filling in after the impact.

That turns out to be, well, probably wrong.

Most impacts are caused by objects far smaller than the scar they leave: about 10 percent or less in diameter. Often far less.

What counts isn't mass, but energy, and the momentum depends also on the speed. Some of those "chunks" were moving pretty fast relative to the moon.

As a result, most of the impacts were not too much deeper than they are right now. Some of the newer craters (such as Copernicus, Tycho or Theophilus) are frozen in much the same condition they were in shortly after they were formed.

The impacting body barely even touches the moon. There's so much energy involved that, for the most part, it's vaporized on impact - along with a fair amount of the surface below "ground zero."

The larger craters and basins are formed by the explosion travelling outward from this zone, and the crater wall is essentially where most of the big shock energy ran out.

The blast was pushing material ahead of it (and warping the surface of the Moon). Where that stops, you get a raised area ... the crater wall.

In the case of the really big badda booms, the shocks die off in stages and you get multiple rings, such as with Orienatale.

But wait: there's more!

Of course, a bunch of stuff gets tossed out of the crater while all this is going on. That's probably what ray systems are: a thin coating of lightly colored fine matter thrown out at impact.

But there are also boulders and globs of heavier stuff that makes a more obvious mark.

The Imbrium impact was so big it's usually called an "event" in the emotion-packed parlance of science. And you can see globs of Imbrium blast remnants all over the place.

One of my favorite areas of Imbrium stuff is around the ruined Julius Caesar. Check it out.

It's aptly placed, Caesar, scarred and bleeding, nearly gone.

But we are all of us the product of such process.

Mail to: Dave North
Copyright © 2000 San Jose Astronomical Association
Last updated: July 19, 2007

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