image credit: NASA, JPL, USGS; image source; larger image
The Galileo spacecraft captured this image as it passed by the moon on its way to Jupiter. See the smooth dark areas? They were created three to four billion years ago when large volcanoes erupted and lava filled in the low-lying regions. Most of the smooth dark areas are round--these started off as enormous craters. Later, volcanoes erupted and filled them in, producing "impact basins." To find out more about how impact basins were formed, visit Impact Cratering.
In the right half of the image above, look at the region close to the edge of the shadow, where the craters stand out most clearly (that's because the angle of the sun is low). Note how the whitish regions of the moon are almost completely filled with craters, whereas the smooth, dark areas have very few. For a possible explanation, called the "late heavy bombardment," visit NASA's The Solar System's Big Bang.
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For an activity on the formation of impact craters, try this NASA activity. Be sure to do this activity with an adult.