So long scar! Hello graben!

The Alpine Valley has been one lunar feature that has always fascinated me, even when I had little interest in the moon - which was most of the past 50 years ;-) It looks so dramatic - and at the same time, so artificial - almost man made.

I had always assumed that this 80-mile slash through the mountains on the north shore of Mare Imbrium was the result of a grazing strike from some huge asteroid coming in from the north towards Imbrium. When you look at it, it's easy to picture such an event - especially since we now know (as we didn't 50 years ago) that almost all Lunar craters were caused by such impacts - or by secondary impacts of junk thrown out from the first.

But it has slowly dawned on me from various readings that this is probably not how this feature came to pass. The process that formed it was probably just as violent, but more a collapse than a slash. As I prepared to observe last night I decided it was a great time to get serious about the Alps as they would be right on the terminator. So I turned to my new guide, "Discover the Moon" by Jean Lacroux and Christian Legrand, and found the valley described as a "graben." Huh? That sent me to the dictionary.

Turns out "graben" is German for "ditch.' How unromantic! I found several definitions. This one's from the Natural History of NovaScotia web site: " A steep-sided, flat-bottomed valley formed between parallel faults."

Yep - that fits the pictures and my observation last night. But it doesn't say much about how it came to be. Steep sides it has - about 3,000-feet. What fun it would be to walk along the edge of this "ditch." Hmmm. . . would you call the Grand Canyon a "ditch?' Somehow, it sounds better in German - a graben. Yes - that has more dignity to my un-Germanic ears.

But I don't think the Grand Canyon is a graben - though I read the Rhine Valley is. But where did I get the idea the Alpine Valley was formed by an impact, and does being a graben rule ths out? I started looking through my old astronomy books and found the answer in Ernest Cherrington, Jr.'s classic "Exploring the Moon through Binoculars" - a book I frequently referenced before I got serious about the moon. (Oh - and while you may spot the Alpine Valley in binoculars, it's far easier with a telescope. ) Cherrington says:

The observer is strongly impressed by the artificial appearance of this feature. It is inconceivable that the Alps could have been formed in a way that would leave such a broad, straight highway through their center.

(Agreed.)

One is tempted to speculate that the valley was carved out in a later epoch by an enormous projectile that plowed through the range, it's impact energy grinding to dust and heating to vapor the mountain masses before it and scattering the pulverized remains as an undetected blanket over a vast area.

Wow! I like that. I mean, we're talking real mountains here. The White Mountains of New Hampshire go up about 5,000 feet. These Lunar Alps average around 8,000 feet. Picture yourself standing just outside the foothills of such a 50-mile wide range of mountains, then try to imagine something big enough and powerful enough to just plow its way through the whole bloody range. Talk about moving muntains!

Cherrington goes on:

Several eminent authorities tell us that the Alpine Valley probably was so formed. Others maintain that such an explanation is improbable and that this and other valleys are but cracks in the lunar surface.

Oh nice! So the "eminent authorities" favor the projectile theory and the "others" think it's just a big crack - uh, graben - uh ditch.

Well, until someone impresses me otherwise, my "eminnent" authrority is Charles A. Wood and his wonderfully readable "The Modern Moon." Wood actually raises an interesting variation on the projectile theory. He says some guy back in 1895 proposed that it was formed by a projectile thrown out by the huge impact that formed Mare Imbrium. OK - that seems to fit the picture. But then Wood punctures this theory by pointing out that the end of the ditch nearest to Imbrium is very narrow and flanked by two mountains. So whatever was thrown out had to squeeeze through them, then get wider - uh-uh. (Check out the image above.)

Wood concludes - and I believe correctly, but what the heck do I know: . . .

Geologists such as Nathaniel Shaler in 1903 and Josiah Spurr in 1945 recognized the Alpine Valley as a graben, a down-faulted block of crust. They are almost certainly correct. the feature must have formed as a local adjustment to the stresses built up during the Imbrium impact event.

Hmmmm . . . wonder if Shaler and Spurr were among the "others" Cherrington dismissed so easily?

OK, whatever formed Imbrium was BIG and it's not hard to imagine some tremendous upheavals and downheavals resulting, but Wood makes it sound too clinical for me. I mean, I wouldn't describe a slash 7-miles wide and 50 miles long that demolishes whole mountains in its path as "a local adjustment." Adjustment! Yeah -and a hydrogen bomb causes a rearrangement of the landscape and lives beneath it!

Seriously - a scientific and clincical approach is all well and good, but when I look at this valley in my telescope - this graben - this magnificent ditch, I am impressed. And the more Ilook, the more impressed I get. In my mind I can feel the moon shake and see the mountains drop - sort of like one of those modern, well-planned building demoolition where everything turns to dust int he same instant and rops sstraight down - as it most assuredly did. We don't have words, in English or in German, to really capture such an event.

Go here for a nice overview chart of the region - and be sure to move your cursor over fetaures to identify them.

BTW - it was a beautiful night and I didn't spend all my time in the Valley. I went south of Imbrium and explored some other features that caught my eye, including a straight, steep wall that cuts between the craters Murchison and Pallas. Couldn't find anything in Wood on this, so i turned to Antonin Rukl's "Atlas of the Moon." But all I got was this terse description: "Crater with a considerably ruined wall." Honestly, these guys are a big help - I have two copies of Rukl's Atlas and use it all the time - but every time I look at the moon I find stuff that kicks my curiosity up several notches and frequently I can't find any answers to satisfy it - which may be a good thing because I then end up focusing on the raw experience. Sometimes, cool, scientific understatement simply can't compete with the holistic reality of being there.

( I have two copie sof the Atlas, btw, because of the educational observing programs. I like to encourage people to find a feature that interests them while at the scope, then go see if they can identify it using the atlas - when you have several people doing this at once, two copies are abrely enough.)