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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. ==Formation== 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: # Primary craters must already be present. # The gravitational acceleration of the extraterrestrial body must be great enough to drive the ejected material back toward the surface. # The velocity by which the ejected material returns toward the body's surface must be large enough to form a crater. 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.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「secondary crater」の詳細全文を読む スポンサード リンク
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