Lunar secondary craters: Results for secondary sizes across the Moon, and size-velocity distributions of ejected blocks
Authors: Singer Kelsi , McKinnon William , Jolliff Bradley 
Published: March 2020

&lt;p&gt;Planetary impact events eject large volumes of surface material.&amp;#160; Crater excavation processes are difficult to study, and in particular the details of individual ejecta fragments are not well understood.&amp;#160; A related, enduring issue in planetary mapping is whether a given crater resulted from a primary impact (asteroid or comet) or instead is a secondary crater created by an ejecta fragment.&amp;#160; With mapping and statistical analyses of six lunar secondary crater fields we provide three new constraints on these issues: 1) definition of the maximum secondary crater size as a function of distance from a primary crater on the Moon, 2) estimation of the size and velocity of ejecta fragments that formed these secondaries, and 3) estimation of the fragment size ejected at escape velocity.&amp;#160;&lt;/p&gt;&lt;p&gt;We mapped secondary craters around primary craters ranging in size from ~0.83&amp;#8211;660 km in diameter using Lunar Reconnaissance Orbiter Camera (LROC) Narrow and Wide Angle Camera images. &amp;#160;Identification of secondary craters was based on expected secondary crater morphologies (e.g., v-shaped ejecta, clusters or chains, and elongation in the direction radial to the primary, similarity in degradation state across the secondary field) and secondaries were assigned a confidence level (as to whether they were likely a secondary crater) based on the number of expected morphologies they displayed. &amp;#160;Only the most confident features were utilized in this work, as there is no way to capture all secondary craters within a given lunar secondary field. &amp;#160;Scaling from secondary crater sizes to ejecta fragment sizes was carried out using the Housen-Holsapple-Schmidt formulations.&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&lt;/p&gt;&lt;p&gt;The largest secondaries and those made by the highest velocity fragments (up to ~1.4 km/s) were mapped around the Orientale basin.&amp;#160; The estimated size of fragments that could reach the lunar Hill-sphere escape velocity of 2.34 km/s varies by the size of the impact event, but could be as large as ~850 m for Orientale. &amp;#160;Note that these are not necessarily expected to be coherent fragments, they could also be loosely bound collections of smaller fragments. &amp;#160;However, the fragments/clumps mapped here remained in a form that resembles a single fragment in order to form the distinct secondary craters observed. &amp;#160;For low velocity secondaries, surprisingly, we found features that appear to be secondary craters formed from fragments with velocities as small as 50 m/s around the smallest primary. &amp;#160;&lt;/p&gt;&lt;p&gt;Through this analysis, we confirmed and extended a suspected scale-dependent trend in ejecta size-velocity distributions.&amp;#160; Maximum ejecta fragment sizes fall off much more steeply with increasing ejection velocity for larger primary impacts (compared to smaller primary impacts).&amp;#160; Specifically, we characterize the maximum ejecta sizes for a given ejection velocity with a power law, and find the velocity exponent varies between approximately -0.3 and -3 for the range of primary craters investigated here.&amp;#160; Data for the jovian moons Europa and Ganymede confirm similar trends for icy surfaces.&amp;#160; This result is not predicted by analytical theories of formation of Grady-Kipp fragments or spalls during impacts, and suggests that further modeling investigations are warranted to explain this scale-dependent effect.&lt;/p&gt;