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Why you see spots

Dave North


“If I don't seem to be horribly overwhelmingly interested in the Moon this month, consider that a sign of sanity with such a terrific Mars opposition forming up.”



One weird coincidence this month: somebody decided to build a scope modeled after my 12.5-inch ( see ) with some interesting refinements — no I won't give you his web address but I will say he's Canadian.

The coincidence? As part of his testing, he decided to shoot a couple of photos of ... the Moon. His first.

As far as I know, he had no idea about my column (a characteristic he shares with most of the universe).

No, 12.5 is not too much for the Moon.

This is a question month, since Bob Ray actually sent one in:

"What are the bright specks of light that come from areas near or in between the craters Cayley and Dionysius and at or near Censorinus?" (+4 deg. N/15deg. E. and -1 deg.S/33deg.)

"There are others, but I don't know what to call them. I can't find them on various Moon maps, but they are visible on photos. I believe you wrote an article about this some time ago..."

Disclaimer: slight editing for column style.

First, I'd like to point out that if I wrote a column on it, I don't remember it. So I certainly won't point the finger at Bob.

I've probably mentioned it, but so what? I never get an explanation right the first time anyway.

It's also possible Bob remembered the thorough and detailed talk Bob Garfinkle gave to the club ... but I don't think so. I know a few Bobs, and they generally stick together, so if he caught that talk he'd have remembered it.

No doubt about it.

So what are they?

First, it's fun how specific this question gets. I had no problem finding these suckers (yes, I did look the night the question came in. Bright spots can be looked at almost any time they're lit, though in general the higher the light, the better. Why is that? Tell you later).

But even as well as the question is defined, I have to assume what we're talking about is ... bright halos and interior zones of various not-too-huge craters.

What are they?

They're bright, light colored stuff.

Okay, great answer Dave. But why is the stuff there? Isn't that what the real question is? Of course.

First I'll point out I should just forward this to Bob Garfinkle. But I'm not going to, so here we go.

As here, the Moon is built of layers. Some are lighter, some are darker. Typically, the closer to black, the more magnesium and iron they contain (and are therefore called 'mafic' from the first letters of Magnesium and Iron. Iron? Okay, it's ferric, right?)

The lighter strata generally have more silicon, and are lighter in more ways than one: they are also less dense.

So the lighter stuff would naturally tend to 'float' on the darker and heavier stuff ... and when they're hot enough to be at least semifluid, that's just what happens.

Okay, how do you get a halo forming, or a bright interior to a crater? Imagine an area where there's a relatively thin layer of dark stuff over a layer of real light stuff.

Now slam something into it.

The dark stuff gets blown away, revealing a light interior for the new crater. Or maybe we slam hard enough to blow some of the light stuff up into the air and it settles nearby — an 'ejecta blanket.'

That's about all there is to it.

(It's likely that lunar rays have a similar cause.)

But you say, okay Dave, if the light stuff floats on the dark stuff, why do you have a dark layer over a light one?

That's the interesting part!

Almost all lavas are initially dark. Lighter flows are usually the result of silicic material being heated by a darker flow and pushed up by it.

Another interesting possibility is when a particularly hot flow blankets an area that's already been separated such that the upper layer is silicic (light).

The blanket is very hot and causes the native (light) soil it settles on to heat up dramatically — hot enough to start "sorting" itself even more than it had been previously. However, the covering blanket has already solidified pretty much, so it has nowhere to go. Still, the lighter material floats up just under the new dark blanket ... and you get a particularly bright layer.

Okay, that's my understanding of the "most likely scenario." And I'm quite aware that there will be at least a few people reading this who are far more qualified to deal with these issues.

My email is — drop me a line and tell me where I've gone wrong, or how you would improve on what I accidentally got right.

And somebody send some decent seeing, would you?

If I don't seem to be horribly overwhelmingly interested in the Moon when you see me this month, consider that a sign of sanity with such a terrific Mars opposition forming up.

By the time you read this, of course, you'll know if the dust storms have turned this into a tragedy or not.

Hope not.


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