From OttawaRasc
Moon: Beyond craters and basins: Visual evidence for the evolution of the Moon’s surface
by Simon Hanmer
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In the "Highlands, Lowlands & Lavas" article I pointed out the two principal features we can see on the visible side of our nearest neighbour in the Solar System, apart from the heavily cratered terrain: light coloured highlands and dark coloured lunar Mare. Let’s change scales of observation and look at some of the surface features that allow us, as amateur astronomers, to determine the kinds of processes that occurred in the Moon’s past, and some of the visual evidence for separating older events from younger ones.
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Valleys
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Lunar craters can be observed with just about any telescope or with steadily-held binoculars, but with a steady telescope, you can see valleys. Lunar valleys come in two varieties: straight and squiggly. The straight ones are usually interpreted as simple rifts whose walls were pulled away from each other so that the valley floor dropped into the resulting space below.
Sirsalis Rill is a nice example of a rift about 450 km long, but it’s not very easy to see from Earth. Other straight valleys are much easier for amateur telescopes, but they’re not quite as simple as suggested by the rift model. For example, the Hyginus and Ariadaeus Rills are made up of bent or off-set segments.
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segments. These features are interpreted as ancient lava rivers. Very hot, runny lava was able to flow over the lunar surface for long distances before it froze. As it cooled, it solidified on the outside first, forming a long, skinny tunnel that still-molten lava could continue to flow through. The chains of holes and the squiggly valley segments are interpreted as the beginnings of collapse of the empty lava tunnel, long after the molten lava had stopped flowing through it, and the long open squiggly valley represents the stage where the entire lava tunnel roof has collapsed. Where was this lava flowing to? Schroter’s Rill is probably the easiest squiggly valley for amateur telescopes, and it shows that lava was flowing from a point of eruption in the Highlands toward the floor of the western part of the Mare Imbrium. This represents good observational evidence that the dark material covering the Mare floors is indeed lava, and you can see it for yourselves!
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The most famous of all lunar valleys is probably the Alpine Valley that cuts across the Alps on the northeastern side of the Mare Imbrium. Most people think of it as a straight valley and interpret it as a rift, but if you look very carefully at this photo, you’ll see that there is a squiggly lava tunnel running all the way down the length of the rift. Although the Alpine Valley is very easy to see when the shadows are just right, I don’t expect to be able to see the lava tunnel with any amateur telescope. But I may be wrong, so let us know when Gary asks for observation reports if you do!
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Scarps and ridges
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Scarps are steep slopes that form when a large fault reaches the lunar surface and the ground on one side drops down with respect to the other side of the fault. The most famous is perhaps is the Altai Scarp that runs parallel to, and faces toward, the Mare Nectaris. It represents a segment of the circular ridges created by the original impact that formed the Nectaris Basin. This scarp is gigantic, rising to an average of 1.5 km high, and locally up to 4 km high! Another very well known example is the Straight Wall that faces toward the Mare Nubium. However, because it is so straight, it is probably a late fault scarp that formed long after lava filled the original impact basin.
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instead of being continuous, ridges in the SE corners of the Mare Humorum and Mare Serenitatis seem to be made of off-set segments, and there are several small craters that lie along the crests of some of the ridges. Also, if the light is right, you can see that the ridges are either symmetrical, or they were scarps that faced towards the exterior of the Mare. But, scarps related to impact basins should face inwards towards the excavated hole! These ridges look to me like late faults that have channeled lava from deep down; molten rock that has reached the surface as volcanic craters located along the fault network. I think these fault ridges are younger than the Mare lavas, but old enough for volcanic activity to still have been possible. While you might see the ridges through your telescopes, I doubt you will be able to see these small craters. Maybe this is a lunar observing challenge?
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Time lines
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Light coloured linear streaks that radiate out from some impact craters can extend half-way round the Moon. These crater rays represent debris ejected from the craters at the moment of meteorite impact. Any land forms that are crossed by the crater rays must be older than the crater that the rays originated from. Two of the best preserved examples are related to the Copernicus and Tycho craters. Copernicus formed about 1 billion years ago, at about the same time as the rocks that make up much of southern Ontario, including the Foymount observing site south of Renfrew and the FLO. Crater rays from Tycho cross all of the Mare on the lunar near-side, demonstrating that Tycho is the youngest large impact crater on the Moon’s surface.It formed about 100 million years ago, when the dinosaurs were still in their prime.
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Make the most the Moon’s proximity to Earth to directly observe for yourselves the evidence for its geological history, and bear in mind that you’ll be observing the scars left by phenomena very similar to those that have shaped the other rocky planets of our inner Solar System.
