Secondary crater

Secondary craters are impact craters formed by the ejecta that was thrown out of a larger crater. They sometimes form radial crater chains. In addition, secondary craters are often seen as clusters or rays surrounding primary craters. The study of secondary craters exploded around the mid-twentieth century when researchers studying surface craters to predict the age of planetary bodies realized that secondary craters contaminated the crater statistics of a body's crater count.

When a velocity-driven extraterrestrial object impacts a relatively stationary body, an impact crater forms. Initial crater(s) to form from the collision are known as primary craters or impact craters. Material expelled from primary craters may form secondary craters (secondaries) under a few conditions:

If ejected material is within an atmosphere, such as on Earth, Venus, or Titan, then it is more difficult to retain high enough velocity to create secondary impacts. Likewise, bodies with higher resurfacing rates, such as Io, also do not record surface cratering.

Self-secondary craters are a those that form from ejected material of a primary crater but that are ejected at such an angle that the ejected material makes an impact within the primary crater itself. Self-secondary craters have caused much controversy with scientists who excavate cratered surfaces with the intent to identify its age based on the composition and melt material. An observed feature on Tycho has been interpreted to be a self-secondary crater morphology known as palimpsests.

Secondary craters are formed around primary craters. When a primary crater forms following a surface impact, the shock waves from the impact will cause the surface area around the impact circle to stress, forming a circular outer ridge around the impact circle. Ejecta from this initial impact is thrust upward out of the impact circle at an angle toward the surrounding area of the impact ridge. This ejecta blanket, or broad area of impacts from the ejected material, surrounds the crater.

Secondary craters may appear as small-scaled singular craters similar to a primary crater with a smaller radius, or as chains and clusters. A secondary crater chain is simply a row or chain of secondary craters lined adjacent to one another. Likewise, a cluster is a population of secondaries near to one another.

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