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Back to Basics

Dave North


“In the end, you get a Mare. But why is the stuff that leaks out so dark?”



I write history notes, I write about looking for this rille or that, I write about ... well, the wrong things, I sometimes think. And this has been a month to remind me.

I've been getting a bunch of questions, both at public events and through email. So I thought I'd address some of those questions here, with the idea that lots of folks might be curious, but not ask.

Maybe my favorite was: what are those dark marks on the Moon?

Dark marks? Ah. You mean the maria? The dark parts of the man/bunny on the Moon?


Okay. The maria are not only darker than most of the Moon, they're smoother. And that's because they got rougher treatment!

Basically, they're lava grit. And most lava is made of basalt both here and on the Moon - and it's dark.

Why lava?

Because the maria are situated where Really Big Things hit the Moon back when it was in the latest stages of its formation. Actually, pretty much formed.

When you consider that one of the more recent ones, Imbrium, is 1250km (about 775 miles) across, you get some idea of how big the "incoming" was (probably on the order of 100 miles in diameter at least).

If that hit us now, of course, it would be the end of the world.

So anyway, this big honker touches down, blows what seems to be half the Moon into the sky, and everything settles down, but in the middle we have a big glowing lake of lava.

After a while, that all cools down, but basically the surface is a weakened, sunken, fractured mess. Over perhaps the next few million years repeated lava flows fill in the whole area, layer by layer.

In the end, you get a Mare. But why is the stuff that leaks out so dark?

Iron, basically. It has a higher iron content than the lighter (in both senses of the word) upper crustal matter, which has a lot more silica. The lighter stuff is called silicic, and the darker mafic (from magnesium and iron - ferric being a greekism for iron. Note its symbol in the Periodic Table is Fe).

So that's about it ... wait! You said lava "grit." Why grit?

Okay, it's been there (in most cases) for a few billion years now, and during that time all manner of micrometeorites, cosmic rays, pebbles and other crud have been slowly grinding away at it.

Basaltic lavas are crumbly to begin with (you can crunch them with your foot just walking across a lava flow) and now they've been reduced to dust and grit - but it's still darker than the surrounding silicic "upland" soil.

But wait, Dave! You said "latest stages of formation" and went on to say Imbrium was one of the more recent ... why?

Okay, it's not like this kind of thing didn't happen all along. But when the planetoid was a lot more plastic, it was just like tossing pebbles into a pond. Once it started to "set," they left more of a mark. But even those are mostly obliterated by what came later. Finally, when it was getting really solid, the lighter stuff settled on the top (the brighter, primarily silicic soil). All this flowing and drying kind of erased the early impacts.

So we see now what happened after things settled down enough to let the wounds stick around.

Well, fine Dave. Then if the Maria were caused by big impacts, why don't they look like craters? That's an easy one to answer, but it comes in two parts. The first part is: they do, sort of. A great example is Mare Orientale (the Eastern Sea on the western edge of the Moon - the nomenclature was changed, reversing east and west from the old system, so astronauts wouldn't be confused when they landed. Really!)

But Orientale is hard to see (this is a good time of year to look for it just before full Moon) so here's a better one: Mare Nectaris. You can see it in good light a few days after new and/or full, and nearby you'll see Rupes Altai - or the more prosaic Altai Scarp. That's pretty much like a crater rim, only different.

Different? Well, yeah. Craters act differently depending on what size they are. Little ones will look like the 'cup' after which they are named (crater is descended from the Greek for cup). Somewhat larger ones will flatten out at the bottom and have a raised rim with terraces. A bit larger and they get central peaks. Keep going and you get a mare, which will typically be flattish, but have "shock rings" around it - like the Altai Scarp.

So Dave, did they really think there were seas on the Moon?


Well, I'm not really sure. Maybe I'll try to find out.

So, this is fun. If anybody actually sends me a question - email - I'll try to answer it here! Maybe if y'all tell me what to write, I'll do a better job.

Dave North


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