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Space Topics: MoonFacts and Pictures
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Map of the near side of the Moon
Credit: NASA / USGS / Wm. Robert Johnston |
The highlands are made of light colored rock and are completely covered with overlapping impact craters, from tiny, hemispherical holes to gigantic, multi-ringed impact basins. The large number of craters suggests that the highlands represent an ancient surface, roughly 4 billion years old. Apollo 14, 15, 16, and 17, as well as Luna 20, sampled lunar highlands.
The maria appear to be dark deposits of a volcanic rock called basalt that fill many (but not all) of the impact basins on the lunar nearside. Surprisingly, very few mare exist on the lunar farside. The maria are clearly younger than the highlands, because they fill parts of impact basins that formed in the highlands. They are also clearly younger because their surfaces show many fewer impact craters than the highlands do. Apollo 11, 12, and 15, and Luna 16 and 24, sampled lunar maria.
Highland Rocks
The highlands are composed of at least three different, distinct types of rock, representing three different types of volcanic lavas. Much of the highlands is covered in anorthosite, an unusual volcanic rock made mostly of plagioclase feldspar. The discovery of anorthosite fragments by Apollo 11 astronauts was very exciting, because so much anorthosite covering so much of the lunar surface could only mean that the Moon was entirely molten at some point in its history. The feldspar crystals that began to crystallize out of this melt would have been less dense than the melt and thus would have floated to the top of the ocean, eventually solidifying into an almost entirely feldspar crust.
A second type of highlands rock is similar to the anorthosite, but it contains more other minerals, like olivine, and is much richer in magnesium, so is called the “Mg-suite.” Scientists haven’t figured out whether the Mg-suite formed at the same time as the anorthosite. If it did, the Moon must have had some way to make two different kinds of magmas simultaneously and keep them separate.
Finally, there is KREEP. KREEP is an acronym that identifies rocks containing potassium (symbol: K), rare earth elements (REE, such as thorium), and phosphorous (P). These chemical elements cannot fit into the crystal structures of the common lunar rock-forming minerals of plagioclase feldspar, pyroxene, and olivine. As a result, when a lunar magma slowly crystallizes, the KREEP elements get left behind in the dwindling reservoir of melt. Rocks formed from such late-stage melts have unusual mineral compositions. Studying deposits of KREEP tells lunar geologists about the final stages of the Moon’s geologic evolution.
Mare basalt
Basalt is a type of volcanic rock that is very common across the solar system. Basalt currently erupts on Earth at mid-ocean ridges and hot spots, such as the volcanoes of Hawaii and Iceland. Throughout the solar system, it is a dark colored rock composed largely of two minerals, pyroxene and plagioclase feldspar, and sometimes contains a green mineral called olivine. Other components are also present in trace amounts
But basalts from the Moon aren’t identical to Earth’s. For one thing, every rock on Earth contains at least some trace of water bound up in its minerals, but the Moon’s rocks are completely dry. A few basalts, within Mare Tranquillitatis and Oceanus Procellarum, contain strangely high proportions of titanium. In false-color images of the Moon from the Galileo and Clementine spacecraft, these high-titanium basalts show up as a brilliant blue, contrasting sharply with the yellow and orange color that represents the ordinary iron-containing basalts of the other maria and the deep reds of the ancient, weathered, iron-poor highlands.
Regolith
Although lunar topography looks sharp-edged from Earth-based telescopes, visits to the Moon revealed landscapes that looked smoothed and rounded. What’s going on? The Moon’s ancient, dormant surface has been battered for billions of years by countless small impact events. The vast majority of impact events are too small to produce new craters that are visible from Earth. But the small impact events do have an effect. Over time, they have fractured the entire upper few kilometers of the lunar crust. The top few meters of the surface, both highlands and maria, consist of a blocky soil called “regolith.”
Regolith is composed of a variety of rock types. Each new asteroid or comet impact onto the moon breaks more rocks into smaller pieces and may wear down another tiny bit of steep topography from an ancient crater. With large enough impact events, the pieces of broken-up rock can be thrown great distances, mixing bits of highland rock into bits of mare basalt and vice versa. Or highland-type material can be dug out of the ground when a meteorite strikes a thin layer of mare basalt.
Large impact events also melt some rock at the point of impact. When this rock melt is sprayed into the sky, it forms small spherical drops that solidify in the cold vacuum before returning to the ground. They form spherical beads of glass that are mixed in with the broken-up rock.
Origin of the Moon
The Moon is a standout in the solar system for its large size relative to its planet. At one quarter the diameter of Earth, it is larger, relative to its planet, than any other moon in the solar system except for Pluto’s moon Charon. The large size of the Moon, the relatively small size of its metal core, its chemical similarities and differences to Earth, and the characteristics of its orbit around Earth, suggest a dramatic theory for its origin.
The impact origin theory for the formation of the Moon states that more than 4 billion years ago, when the solar system was young, a Mars-sized proto-planet collided with the proto-Earth. The collision was a glancing one; a large chunk of the Mars-sized planet sheared off, its fragments captured into Earth orbit, forming a temporary ring. Both planets melted under the tremendous heat generated by the impact. Most of the core of the Mars-sized planet sank into and merged with the proto-Earth’s core. Some of the debris that had been sheared off in the impact eventually landed on Earth. But some of it stayed in Earth orbit and eventually coalesced to form the Moon.
The Earth-Moon System
Today, the Moon orbits Earth at a distance of 380,000 kilometers (240,000 miles). But initially, just after the giant impact that formed it, the Moon would have been much closer to Earth than it is now, probably only 24,000 kilometers (15,000 miles) from Earth’s center. That is less than three Earth radii above Earth’s surface! Tidal forces between Earth and the Moon cause the Moon to recede from Earth over time. Tidal forces are stronger when the bodies are closer, so the recession was fast early in the Moon’s history. By the first few hundred million years, the Moon had moved out to an orbit more than half its present distance from Earth. The recession has not stopped; the Moon is still retreating at about 3.8 centimeters (1.5 inches) per year.
The Moon would have originated with a spin rate that differed from its orbital rate, but tidal forces have acted to slow its rotation over time. Now, the Moon rotates at the same rate that it orbits Earth, a condition called synchronous rotation. At the same time, the same forces have slowed Earth’s rotation rate. Fossil records of the daily and yearly growth patterns of corals prove that there were approximately 400 days in a year, a few hundred million years ago, meaning that Earth’s rotation has slowed at a rate of about 5 milliseconds per year, per year. Earth would have had a 14-hour day at the birth of the Earth-Moon system. If nothing happens to disturb the Earth-Moon system for the next several billion years, Earth, too, will eventually be in synchronous rotation with the Moon. At that point in time, an Earth day and month will be equal, at about 47 current Earth-days, and the Moon’s distance from Earth will be 135% of its current value.
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