Thousands of individual craters have been named, however, mostly for great scientists and philosophers Figure 1.
Among the most prominent craters are those named for Plato, Copernicus, Tycho, and Kepler. Galileo only has a small crater, however, reflecting his low standing among the Vatican scientists who made some of the first lunar maps. We know today that the resemblance of lunar features to terrestrial ones is superficial. Even when they look somewhat similar, the origins of lunar features such as craters and mountains are very different from their terrestrial counterparts.
Figure 2. To trace the detailed history of the Moon or of any planet, we must be able to estimate the ages of individual rocks. Once lunar samples were brought back by the Apollo astronauts, the radioactive dating techniques that had been developed for Earth were applied to them.
The solidification ages of the samples ranged from about 3. For comparison, as we saw in the chapter on Earth, Moon, and Sky , both Earth and the Moon were formed between 4. They are made of relatively low-density rock that solidified on the cooling Moon like slag floating on the top of a smelter. Because they formed so early in lunar history between 4.
The highlands have low, rounded profiles that resemble the oldest, most eroded mountains on Earth Figure 3a. Because there is no atmosphere or water on the Moon, there has been no wind, water, or ice to carve them into cliffs and sharp peaks, the way we have seen them shaped on Earth. Their smooth features are attributed to gradual erosion, mostly due to impact cratering from meteorites. Figure 3: Lunar Mountain and Lunar Maria. Note the smooth contours of the lunar mountains, which have not been sculpted by water or ice.
This view of Mare Imbrium also shows numerous secondary craters and evidence of material ejected from the large crater Copernicus on the upper horizon. This suggests that the moon, earth, and other planets all formed rather quickly from this smaller debris that was floating around.
Astronomers call this the planetesimal hypothesis. Although we can't look back in our own solar system We know that there still exists evidence of this earlier state in our solar system Astronomers believe that the entire solar system 4. Under gravity, these smaller objects collected into larger and larger objects by collisions until they reached planetary sizes. It would also explain why the rates of impact would suddenly cease.
As the planets grow from the collection of these smaller objects , there would be less and less of these smaller objects floating around as time went on. The planetesimal hypothesis received another boost when planets exoplanets were discovered orbiting other stars. The count is over 3, in animation The bad news is How did the moon form? Good question All prior models had to account for the fact that the moon has a lower density than the earth, has very little iron and contained less volatile gasses than earth rocks.
Here were some major now obsolete theories: Fission Theory - The earth was initially rotating so fast that a blob of mantle material was flung out into space This idea seems ridiculous but was actually taught in my grade school.
This would explain why the moon had such a low density and accounts for the Pacific Ocean basin. Modern computers allow all kinds of possible simulations and at no time does it offer this solution.
Capture Theory - To account for the differences in density, simply have the moon form somewhere else in the solar system where the planetesimals have a lower density. Then it has to migrate from there and get captured by the earth's gravity as it moves past.
Astronomers have a problem getting the moon dislodged from its initial position and have bigger problems getting it captured by the earth. In computer simulations, the passing moon either crashes into the earth or gets whip lashed to a remote location. Double Planet Theory - The moon and earth formed together, from the same debris, and at the same time. Why, then, would they not have the same density? No adequate theory held up.
It suggests that during the very early process of accretion of planetesimals, two large worlds were forming about 1 AU from the sun. Eventually one object about the size of Mars and known as Theia collided at a glancing angle with the other which was about the size of Earth.
The collision put large amounts of debris in orbit around the earth, which quickly coalesced, This theory explains the similarities between earth and moon rocks. In addition, the energy from the impact depleted any volatile gasses and the ejected material would have been mostly low density, mantle type rocks, This hypothesis is consistent with the idea that collisions were plentiful in the early ages of the solar system.
During later volcanic episodes, liquid magma came to the surface and filled these basins. When it cooled down and solidified, it formed the large flat areas we can still see now. As this happened in comparatively recent times, the number of impact craters is far less than in the highland areas. From the two AMIE images it is possible to see how highlands present a very irregular topography and many craters, while the mare area is comparatively flat and shows a much smaller number of craters.
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