What are rilles? My only question this month (one I've answered before, here and there, but never straight up).
The simple answer is: probably the most interesting observational feature on the Moon. Challenging, fun, and thoroughly cool. But that's not quite what you expected, is it? How about this:
If you spoonerize this month's title, you get "Frau Burroughs," which is not an unreasonable leadin: the word "rille" has a Germanic origin, and means "furrow."
Of course, Burroughs wrote all kinds of stories about the Moon, Mars, whatever, and all of them were just as silly as they could be. So (it turns out) were most of the original scientific ideas about formations on the Moon. And, just for fun, some are still pretty much a complete mystery.
Such as straight rilles. Basically, there are three kinds of rille: arcuate, sinuous, and straight. They may look in some ways similar, but as far as anyone knows, they're essentially unrelated.
Though the first two do share basaltic lavas as an ingredient. About the third, who knows?
Arcuate rilles are normally found around the edges of maria, the large dark plains that are the most distinctive markings on the Moon.
It's generally accepted that they form as a result of the contraction and/or sinking of the basalts that formed the Maria ... perhaps as part of the long-term cooling process, or maybe just from the slow crushing of the crust under the weight of the lavas.
Maybe that's even right.
Such rilles will form arcs, and are usually pretty easy to spot. Rima Hypatia is a good example, almost letter-perfect. A complex like Rima Hyginus, an extremely weird formation, is probably of the same origin.
Sinuous rilles are almost certainly the best understood. They often begin at what is obviously a volcano, and proceed in a snaky line to their terminus. Sometimes they branch, but they almost always meander.
They are lava tubes on a massive scale.
Lava likes to travel in tubes. This isn't hard to understand: as lavas move away from their source, they cool — from the outside in. So their natural behavior is to form a skin like a sausage, then travel further using this "strategy."
Often, lava stops flowing abruptly. When that flow has been feeding a tube, the lava will simply pour most (or all) of the way out the far end, leaving an empty sausage skin.
A fragile skin. Eventually, most tubes simply collapse. On the Moon, the result is a sinuous rille. Here on earth, where erosion happens much faster, they are often lost — though some can be seen here and there.
But what of the mysterious straight rille? What is its nature and how does it form?
The easiest one to see is Rima Ariadaeus, right smack in the middle of the Moon — long and wide. But my favorite is Rima Sirsalis, banished to the western edge of the Moon and not often observed since it shows up not long before the full Moon.
After pondering how run-of-the-mill rilles are made, sooner or later the Moon observer simply has to notice there are examples that just don't seem to fit in.
Sirsalis qualifies in spades. It starts at the edge of Oceanus Procellarum near a modest crater of the same name and proceeds to directly away from the mare, eventually ending up among the cracks in the floor of — appropriately — Darwin, named for a man who pondered natural mysteries.
In the process it runs through craters, ranges of hills, other small rilles ... in fact, almost every kind of geology on the Moon except mare. Its origin makes it seem as if it were literally shunning maria!
This is particularly strange when you consider that most rilles are actually in maria.
Consider this: if you see a rille running across craters and mountains, the rille must be newer than those features. But it's a depression! So whatever it is, it's something that can cause slumping under all such features, in more or less a straight, smooth line.
Some guesses? One popular theory is "tectonic activity." And indeed that shows promise, since faults are often pretty straight and do cause slumping. There are problems, here: the faulting would have to be newer than most other features, for one thing, and there really doesn't seem to be much tectonic activity happening in recent eons.
Yet another idea is that they are collapsed dikes of some sort. Dikes are igneous intrusions into weak rock — sort of like lava that squirts into a weak seam.
Then, of course, to form a depression it would first have squirted in (raising the rock?), then squirted back out, then the crust above would collapse. And all this would happen in pretty much a straight line.
Does my skepticism show?
The point is, nobody seems to have a handle on this.
And for any observer, that just has to make them even more fun to hunt down and ponder!
So, what are rilles? In the end, they're a mystery.
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