Ejecta blanket

Symmetrical apron of ejecta that surrounds an impact crater

An ejecta blanket is a generally symmetrical apron of ejecta that surrounds an impact crater; it is layered thickly at the crater's rim and thin to discontinuous at the blanket's outer edge. The impact cratering is one of the basic surface formation mechanisms of the solar system bodies (including the Earth) and the formation and emplacement of ejecta blankets are the fundamental characteristics associated with impact cratering event. The ejecta materials are considered as the transported materials beyond the transient cavity formed during impact cratering regardless of the state of the target materials. In some cases, the excavated fragment of ejects materials can form secondary craters. The materials of ejecta blanket come from rock fragments of crater excavation, materials due to impact melting, and outside the crater. Immediately after an impact event, the falling debris forms an ejecta blanket surrounding the crater. An ejecta blanket is deposited in the interior regions of the crater rim to the final crater rim and beyond the crater rim.

Ejecta blankets are found on the terrestrial planets (e. g., Earth, Mars, and Mercury) and satellites (e. g., Moon). Many of the ejecta blankets of Mars are characterized by fluidized flowing across the surface. In contrast, the ejecta blankets and proximal ejecta deposits of Moon and Mercury (or on airless bodies) are attributed to ballistic sedimentation. Similar to the fresh lunar craters, the Mercurian impact craters also form continuous ejecta deposits of blocky and high albedo materials. The low gravity and lack of atmosphere (air less bodies) favors the formation of the impact cratering and associated ejecta black on the surface of moon and Mercury. Although a thick atmosphere and relatively higher gravity of Venus reduce the likelihood impact cratering, the higher surface temperature augments the efficiency of the impact melting and associated ejecta deposits. Ejecta blanket is a common feature to be seen on the martian impact craters specifically around fresh impact crater. One third of the martian impact craters with ≥ 5 km diameter have discernible impact ejecta around. Layered ejecta blanket are plentiful on the surface of Mars as around 90% of ejecta are characterized as layered materials.

Ejecta blankets have a diverse morphology. Variations in ejecta blanket indicates different geological characteristics involved with the impact cratering process such as nature of target materials and kinetic energy involved with the impact process. These information also give an idea about the planetary environment e. g., gravity and atmospheric effects associated with the impact cratering. Studying impact ejecta is an excellent sampling environment for the future in situ lunar exploration. Many factors determine the extent of ejecta blanket that ranging from size and mass of impactor (meteorite, asteroid, or comet), surface temperature, gravity and atmospheric pressure of target body, the physical characteristics of target rock. The martian ejecta blankets are categorized broadly into three groups based on the observed morphology identified by spacecraft data:

a. Layer ejecta pattern: the ejecta blanket seems have formed by fluidization process and composed of single or multiple partial or complete layers of sheet of materials surrounding the crater. Sometime eolian modification are also common. b. Radial ejecta blanket: the ejecta materials are emplaced by the secondary materials ejected along a balletic trajectory. This radial patterns are also found around the lunar and Mercurian craters. c. Combination of layered and radial ejecta pattern.