From OttawaRasc
Moon … Highlands, Lowlands & Lavas
by Simon Hanmer
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Even with the naked eye, you can easily see that the Moon is made of at least two different kinds of material: (i) big patches of dark coloured rock called Mare (Latin for Sea) and (ii) light coloured rock that makes up the rest. Any telescope will show that the dark Mare are flat like the Canadian Prairies, whereas the light areas either form chains of mountains called Highlands, or are pockmarked with craters.
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The light rock is called anorthosite, principally made of a low density mineral called feldspar. Because the outer part of the Moon, called the crust, is about 70 km thick and is mostly made of feldspar, that’s a lot of lightweight material. Why is the Moon’s crust so light? There’s only one way to do this geologically. At the beginning of its history, the outer part of the Moon must have been completely molten, an ocean of molten rock several hundred km deep! Crystals began to form in this magma ocean as it cooled down and, because crystals of feldspar are so light, they rose like a scum to the top of the ocean, while the heavier crystals like olivine and pyroxene sank toward the bottom. The heat needed to melt the outer part of the Moon probably came from energy released by the impacts of the material that collected together to make the Moon in the first place, about 4.5 billion years ago.
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How did the Moon form? According to current theory, the chemical compositions of the Earth and Moon can be best explained if the Earth was struck by another planetary body about the size of Mars, very shortly after Earth had first formed. The Moon seems to have very little iron in it’s core, so planetologists think that the outer part of the impactor, and some of the outer part of the Earth, were literally volatilised and ejected into orbit. There, the ejected material condensed as a ring system around the Earth, somewhat like Saturn’s rings, except that within a few tens of years, the unstable ring system coalesced to form the spherical body that became our Moon.
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After the Moon had formed, and the light material had floated to the top of the magma ocean to make the crust, the Moon continued to be bombarded by planetissimals that left many of the scars we see on the lunar surface today. These scars are of two kinds: impact craters and impact basins. Here, the focus is on impact basins because they are the key to understanding the difference between the Highlands and the lunar Mare.
What are “impact basins”? They are so big that planetary geologists have argued for quite a time about whether or not they could be the results of impacts, or whether they were the result of the internal geology of the Moon. Their dimensions can be impressive, up to 1200 km across and 12 km deep (for comparison, Everest, the highest peak on Earth, is about 10 km high).
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It turns out that they definitely formed from major impacts. Look at the Orientale Basin (not visible from Earth). It has a bull’s-eye shape with concentric ridges around its centre. These ridges are circular mountains that formed when the force of the impact sent out shock waves that lifted and tilted the anorthosite crust at the edges of the huge hole that formed when material from the centre of the basin was hurled out by the impact.
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Compare this with the Mare Imbrium (the Sea of Rain; easily visible to any Earth-bound telescope), ignoring the dark material for the moment. The Carpathian, Apennine, Caucasus, Alps and Jura mountains form a nearly continuous arc of highlands that are interpreted as the outer-most ridge that resulted from the impact that created the Imbrium Basin in the first place. Lunar planetologists also believe that the LaHire and Spitzbergen Mountains inside the Mare Imbrium form the remnants of an inner ring of impact-related ridges.
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The dark material that fills the Mare is made of basalt lava, very similar to the rocks that form the seabed of Earth’s oceans. It’s dark because it contains a lot of the dark mineral pyroxene that is composed of iron and magnesium, as well as silica. The huge impact basins like the Mare Imbrium were flooded by giant basalt eruptions that filled them, but not quite to the brim. Hence we can still see the Highland crests poking through.
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If you look at a lunar map of the rest of the near side, you’ll see that the Mare Serenitatis (Sea of Serenity) and the Mare Crisium (Sea of Crises) are very similar to the Mare Imbrium, but what about the other large patches of dark material? The Mare Humorum, Mare Nectaris and Mare Fecundatis (Seas of Tears, Nectar and Fertility) are somewhat similar to Imbrium, but have a more corroded look because they are older and have probably been flooded with more lava. However, the Mare Nubium (Sea of Clouds) and the Sea of Tranquility are so old and have been flooded with so much lava that it is difficult see any trace of their original circular Highland rims.
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When were these huge impact basins flooded with basalt lava? The impacts that formed the basins occurred about 4 billion years ago, but the main basalt lavas didn’t erupt until about 3 billion years ago! Why did the lavas erupt so long after the initial impacts? Like Venus, the Moon was always dry. If you check the article on Venus, you’ll see that this means that the lunar interior was very stiff, even when it was young and still warm, so plate tectonics could not operate and enable the Moon to cool down from the heat that was gradually building up due to the radioactive breakdown of elements such as U and K in its interior. Heat built up inside the Moon until melting began at roughly 200-400 km below the surface, and basalt magmas rose to the surface taking the heat with them. However, because the Moon is relatively small, it didn’t contain very much radioactive material, so it couldn’t heat up enough to produce more huge volumes of lava after the Mare had formed.
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Next time the Moon is up, and you can’t see your favourite deep sky objects, take another look at the planetary geology of our nearest neighbour. You’ll be looking at the relics of:
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- A Moon-wide magma ocean and the scum that floated to its surface
- Giant impact structures that formed lunar basins
- and the build-up of radioactive heat inside the Moon that led to melting and huge eruptions of lava that flooded the impact basins and almost buried the Highlands.
