Animpact structure is a generally circular or craterlikegeologic structure of deformedbedrock orsediment produced byimpact on aplanetary surface, whatever the stage oferosion of the structure. In contrast, animpact crater is the surface expression of an impact structure. In many cases, on Earth, the impact crater has been destroyed by erosion, leaving only the deformed rock or sediment of the impact structure behind.[1] This is the fate of almost all oldimpact craters on Earth, unlike theancient pristine craters preserved on theMoon and other geologically inactive rocky bodies with old surfaces[2] in theSolar System. Impact structure is synonymous with the less commonly used termastrobleme meaning "star wound".[3]
In an impact structure, the typical visible andtopographic expressions of an impact crater are no longer obvious. Anymeteorite fragments that may once have been present would be long since eroded away. Possible impact structures may be initially recognized by their anomalous geological character orgeophysical expression. These may still be confirmed as impact structures by the presence of shocked minerals (particularlyshocked quartz),shatter cones,geochemical evidence ofextraterrestrial material or other methods.
Bodies with current geological activity such asIo and Earth have very few recognizable impact craters—Io, in fact, is so volcanically active that not a single impact crater, of any size, has been found on its surface to date. The Earth has about 150 recognized craters, but many have been geologically modified and would be difficult to recognize from orbit. On the other hand, geologically inactive bodies with old surfaces, such as Earth'sMoon orCallisto, are covered with impact craters of all sizes.
