Secondary crater
A secondary crater is made by a block thrown out of a larger impact crater, not by an object from space.
When a large impact blasts out a crater, it throws blocks of rock across the surrounding landscape. Where these blocks land, they make their own, smaller craters, called secondary craters. They often occur in chains or clusters and are usually shallower and less regular than primary craters.
Secondaries can be very numerous. A single large impact on Mars, such as the one that formed the crater Zunil about 10 kilometres across, may have produced millions of secondaries. On the Moon, rays from craters such as Tycho and Copernicus are dotted with them.
Secondary craters cause problems for crater counting, the method of estimating the age of a surface from the number of craters. If secondaries are counted as primaries, a surface may appear older than it is. Planetary scientists try to identify and exclude them, which can be difficult when they are far from their parent crater.
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