Secondary crater

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.[3][4]

Secondary crater chain of Copernicus in Mare Imbrium

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.[6]

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  Group of secondary craters on Mars, as seen by HiRISE

Primary craters form from high-velocity impacts whose foundational shock waves must exceed the speed of sound in the target material. Secondary craters occur at lower impact velocities. However, they must still occur at high enough speeds to deliver stress to the target body and produce strain results that exceed the limits of elasticity, that is, secondary projectiles must break the surface.[2]

It can be increasing difficult to distinguish primary craters from secondaries craters when the projectile fractures and breaks apart prior to impact. This depends on conditions in the atmosphere, coupled with projectile velocity and composition. For instance, a projectile that strikes the moon will probably hit intact; whereas if it strikes the earth, it will be slowed and heated by atmospheric entry, possibly breaking up. In that case, the smaller chunks, now separated from the large impacting body, may impact the surface of the planet in the region outside the primary crater, which is where many secondary craters appear following primary surface impact.[7]

Illustration of projectile fracturing prior to primary impact to show the chronological procedure of the creation of primary and secondary impacts from projectile fractures.

For primary impacts, based on geometry, the most probable impact angle is 45° between two objects, and the distribution falls off rapidly outside of the range 30° – 60°.[8] It is observed that impact angle has little effect on the shape of primary craters, except in the case of low angle impacts, where the resulting crater shape becomes less circular and more elliptical.[9] The primary impact angle is much more influential on the morphology (shape) of secondary impacts. Experiments conducted from lunar craters suggests that the ejection angle is at its highest for the early-stage ejecta, that which is ejected from the primary impact at its earliest moments, and that the ejection angle decreases with time for the late-stage ejecta. For example, a primary impact that is vertical to the body surface may produce early-stage ejection angles of 60°-70°, and late-stage ejection angles that have decreases to nearly 30°.[2]

Mechanical properties of a target's regolith (existing loose rocks) will influence the angle and velocity of ejecta from primary impacts. Research using simulations has been conducted that suggest that a target body's regolith decreases the velocity of ejecta. Secondary crater sizes and morphology also are affected by the distribution of rock sizes in the regolith of the target body.[2][10]

The calculation of depth of secondary crater can be formulated based on the target body's density. Studies of the Nördlinger Ries in Germany and of ejecta blocks circling lunar and martian crater rims suggest that ejecta fragments having a similar density would likely express the same depth of penetration, as opposed to ejecta of differing densities creating impacts of varying depths, such as primary impactors, i.e. comets and asteroids.[2]

Secondary crater size is dictated by the size of its parent primary crater. Primary craters can vary from microscopic to thousands of kilometers wide. The morphology of primary craters ranges from bowl-shaped to large, wide basins, where multi-ringed structures are observed. Two factors dominate the morphologies of these craters: material strength and gravity. The bowl-shaped morphology suggests that the topography is supported by the strength of the material, while the topography of the basin-shaped craters is overcome by gravitational forces and collapses toward flatness. The morphology, and size, of secondary craters is limited. Secondary craters exhibit a maximum diameter of < 5% of its parent primary crater.[2] The size of a secondary crater is also dependent on its distance from its primary. The morphology of secondaries is simple but distinctive. Secondaries that form closer to their primaries appear more elliptical with shallower depths. These may form rays or crater chains. The more distant secondaries appear similar in circularity to their parent primaries, but these are often seen in an array of clusters.[2]