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Feature-Filled Full Moon
Observing > What bright object in the night sky this evening has highlands and seas? If you guessed the Moon, you are on the money - sorta.
Tonight the Full Moon shines bright as it rises in the southeastern skies. Its incredible glare lights up the sky so much that it’s hardly worth mentioning other objects up there, seeing as they can’t be seen.
But a look at the Full Moon will reveal some interesting characteristics about our nearest planet-like neighbor.
See there the vast dark areas surrounded by bright whiteness. Some see in the overall view the traditional “man on the moon.” I see a dark rabbit as if it’s being lifted up - head and ears to the “left,” body arching over to the long back legs on the Moon’s “right” side. Don’t see it? It’s OK. Neither do most of the students in my class. Alas!
We might agree, though, that the full Moon is pretty bright. But surprisingly it’s only as bright as the asphalt on your street. It’s albedo, a fancy word that tells us how much light is reflected off of something, is only 0.06. Translation: it reflects only 6% of the light it receives. Earth, that bright beauty with all its reflective cloud cover and polar caps has an albedo of 0.39, more than 6 times that of the Moon.
But why are there the dark parts of the Moon (the bunny) and the light parts? It was a mystery for a long, long time.
Galileo was first to see the detail on the Moon with his fancy new telescope. He could see that the bright areas were covered in crater-shaped “mountain” ranges. The dark parts, which he called seas, or “maria,” were to him craterless.
It wasn’t until this last century that we finally figured out what it all was.
The light parts, heavily pockmarked with craters, were not the result of volcanic activity or mountain-building processes as was widely held. Those were impact craters and their debris, from lots and lots of giant impacts.
This idea didn’t fly well at first since huge impact craters smacked of catastrophe, something that didn’t happen according to the prevailing philosophy of the day, a worldview that said that things happened slowly over long periods of time.
Then what on earth were the Moon’s dark areas made of? Why didn’t the impactors impact all over? They did, in fact. But these dark spots turned out to be lowlands, which filled in with moon lava and covered old craters billions of years ago when the Moon was still geologically active.
These smooth and relatively safe areas, the “seas,” were where the first Apollo missions landed. Hence the famous “Sea of Tranquility,” the landing site for Apollo 11.
It wasn’t until we sent spacecraft to the Moon that we could see the far side of our satellite. Here we discovered just about nothing but crater-filled highlands. The maria turn out to be pretty much a show exclusively for Earthlings.
And all these moon craters, great and small, got scientists to ask the obvious questions. If the Moon, smaller and with less gravity got hit that often, what happened to us? Why aren’t we chock full o’ craters?
It turns out we’ve been hit a lot more than the Moon. But we have this excellent recycling program set up on our planet that resurfaces many of the impacts and buries others with plate tectonics. Our atmosphere also helps to break up many before they hit us, and our vast oceans hide many scars.
But there are still dozens and dozens of impacts above ground like Barringer Crater in Arizona - reminders that overall we’ve got it pretty good here on this beautiful planet of ours.
Go out tonight after sunset and see again, but with new eyes, our amazing satellite.
Posted by Mark Ritter at 2005.05.28 03:01 PM | Comments (0)
