Impact Structure: n. A large geologic structure, such as a crater or astrobleme, created by the violent collision between a planet and a space projectile such as a comet or meteor.
Impact cratering is one of the most common geological processes that have happened on planetary objects with solid surfaces (our home planet Earth included).
During a cosmic impact on a rocky planet the passage of the high-pressure shock wave results in shock metamorphism, the progressive breakdown in the structural order of minerals and rocks. The pressures and temperatures in the shock wave are well above the magnitudes of pressures and temperatures occurring naturally on this planet.
 The crater:impactor size ratio ranges from 20:1 to 50:1 (Shoemaker 1963, Baldwin 1963).
 The standard unit of pressure is the Pascal, abbreviated Pa, which is equivalent to 1 kilogram per square meter. A GPa is a gigapascal (giga means billion), a measurement of pressure, and is equal to 10,000 times the atmospheric pressure at the Earth’s surface.
Research has established that near the hypervelocity impact point, initial shock pressures can exceed 100 GPa, resulting in the total melting and vaporization of a large volume of target rock together with virtually all of the impactor. Passing outward, the lower shock pressures produce a series of distinctive effects in the target rocks (Koeberl, French, 2009):
- rock melting (≥60 GPa);
- selective mineral melting (40–60 GPa),
- diaplectic glass phases (30–45 GPa);
- high-pressure minerals – coesite and stishovite (12–30 GPa),
- planar deformation features (PDFs) in quartz (10–25 GPa);
- multiple fracturing (cleavage) and basal Brazil twinning in quartz (5–10 GPa);
- rock fracturing (2–5 GPa);
- shatter cones (≥2-30 GPa);
- slickensides (frictional faulting).
Of these shock metamorphic effects only shatter cones and slickensides can be easily identified with the naked eye. The remainder of these effects tends to be microscopic in size.
Shatter cones are found exclusively in two places on Earth:
- in nuclear (or very LARGE non-nuclear) explosion test sites, and
- cosmic velocity meteorite impact structures.
Slickensides are naturally polished rock surfaces that occur when the rocks along a fault rub against each other, making their surfaces smoothed, lineated, and grooved.
Why is this research important to Astrobiology? Studying the effects of impact events on the ancient biosphere of Earth can help astrobiologists understand how similar events in the future could affect the habitability of our planet. Understanding the “principles that will shape the future of life, both on Earth and beyond” is one of the primary goals defined by the NASA Astrobiology Roadmap. Another goal of the Astrobiology Program is to determine how “past life on Earth interacted with its changing planetary and Solar System environment.”
In this and following articles, I will document the geology used to identify impact structures. I will also describe my amateur observations of various craters that I have visited (on this planet for the time being) and how these observations can and cannot be used as evidence of an impact. Before I get into the recently discovered methods that are used for identification of impact structures, I will first describe how they are created.
Impact crater formation is unlike any other known terrestrial geological process. Impact involves the transfer of massive amounts of energy to a relatively small area of the Earth’s surface, in an extremely short period of time (Kinetic energy). The kinetic energy of an object of mass m traveling at a speed v is mv2/2, provided v is much less than the speed of light.
The cratering process has been divided into three distinct stages, each dominated by different forces and mechanisms:
- Contact & Compression
The cratering process begins when the impactor, travelling at 10 to 75 km/s, makes initial contact with the target body. This starts the contact and compression process which lasts only fractions of a second. During this time the impactor will penetrate up to 2 times its diameter. At this point the kinetic energy of the impactor is transformed into shock waves that radiate into the target body and back into the impactor itself. The energy of the impact produces a spherical expanding envelope of hot gas regardless of the angle of impact. The impactor and the immediate area of the impact are completely vaporized . With few exceptions, the impact explosion almost always produces a circular crater. Elliptical impact craters are known elsewhere in the solar system and are caused from impactors at a very low-angle or obliquity of 10°–15° (Kenkmann 2009). After only a fraction of a micro-second, the compression stage ends with the complete transfer of energy in the form of a shock wave and latent heat.
The crater excavation stage (Melosh, 1980) overlaps somewhat with the compression stage and involves two processes:
- upward ejection (spalling) of large near-surface fragments and smaller ejecta (ejecta curtain);
- subsurface flow of target material to form the transient crater.
In seconds the resulting kinetic energy release creates an explosion that forms the initial transient crater. This energy release is transferred into complex interactions within the resulting shockwaves. The crater now consists of an evacuated zone (forming impact ejecta) and a lower displaced zone (forming crater-fill impactites). The excavation process is complete when the energy in the shock waves can no longer displace target rocks. This process could last up to 90 seconds for a crater of up to a 200 km diameter.
The initial transient crater is unstable and the modification stage commences. Small craters of <4 km (on Earth) are relatively stable after the excavation stage. For larger craters, the impact structure is gravitationally unstable and its modification stage will include uplift of the crater floor and collapse of the unstable steep walls (slumping). These movements will be completed in a few minutes and could result in a complex or multi-ring crater. Minor faulting, mass movement and/or hydrothermal activity in the larger craters could last indefinitely.
 In some rare cases the projectile survives the cratering process, at least in the form of meteorite fragments (apart from geochemical traces in melts at larger structures). At Barringer crater and at small simple impact structures (<< 2km) like Wolfe Creek or Odessa, the meteorite can survive. A large fragment of the impacting meteorite was found beneath the melt sheet at the ≥70 km Morokweng impact structure, South Africa (Schmieder 2010).