Although the Moon appears very bright in our sky, it only reflects about 16 per cent of the sunlight that falls on it, making it about as bright as an asphalt parking lot. The Earth, in contrast reflects about 40 per cent of the light that falls on it, because of clouds, snow and ice, and light colored surfaces like grasslands and deserts. The fraction of light that a planet reflects is called its albedo. Albedo in the Solar System ranges from over 70 per cent for brilliant cloud covered Venus to less than 5 per cent for some asteroids. This figure shows albedo markings on the near side of the Moon.
The Moon shows little color to the eye except various shades of white and gray. The photograph was computer enhanced to bring out extremely subtle variations in color.
The near side of the moon is very different from the far side. The near side is dominated by large dark lava plains called maria (Latin for "seas"). The far side has very few maria. Many of the maria are circular because they are actually enormous craters or impact basins that were later filled by lava flows.
All craters bigger than 50 kilometers are shown on this figure. Young craters are surrounded by bright rays of fresh debris blasted out by the impact that created the crater. The crater at bottom with the very large ray system is called Tycho. It is 85 kilometers in diameter. The Apollo 17 mission collected samples of material from these rays, which were dated on Earth. Tycho is estimated to be 108 million years old, or to have formed during the time of the dinosaurs. Slightly above and left of center is Copernicus, another crater with bright rays estimated to be about 800 million years old. Left of Copernicus is Kepler and above Kepler is Aristarchus, the brightest crater on the Moon. All lunar craters on the near side of the Moon were named for famous astronomers.
Some craters are filled with mare basalt and show up as very dark. One of the most conspicuous is at top center, called Plato (stretching the definition of "astronomer" considerably). There are far more craters in the bright areas of the Moon than in the maria, and many of the craters in the maria formed earlier and were flooded with lava flows. So the history of the Moon is one of early cratering, followed by lava flows filling some of the giant impact basins, followed by very little activity except for occasional impacts that created the bright ray craters.
"Mare" is Latin for "sea." The "seas" are now known to be vast lava plains filling ancient impact basins. The names mean:
Created 20 September 2009, Last Update 14 December 2009
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