Young Martian crater Gratteri and its secondary craters

Abstract

In response to questions that have been raised about formation and effects of secondary craters on crater chronometry techniques, we studied properties of the secondary crater field around the young Martian primary ray crater Gratteri (diameter 7 km). The crater has an estimated age of 1 to 20 Myr, based on counts of small craters on flat interior surface, consistent with a likely age for a young crater its size (Hartmann et al., 2010). The following are among our findings: (1) We identify an unusual class of craters we call “rampart secondaries” which may suggest low‐angle impacts. (2) We measure size distributions of secondaries as a function of distance from Gratteri and used these data to reconstruct the mass‐velocity distribution of ejecta blasted out of Gratteri. Our data suggest that crater density in rays tends to peak around 120–230 km from Gratteri (roughly 20–30D) and reaches roughly 30–70 times the interray crater density. (3) Comparable total numbers of secondaries form inside rays and outside rays, and about half are concentrated in clusters in 2% of the area around Gratteri, with the others scattered over 98% of the area out to 400 km away from Gratteri. (4) In the old Noachian plains around Gratteri, secondaries have minimal effect on crater chronometry. These results, along with recently reported direct measurements of the rate of formation of 10 m to 20 m primaries on Mars (Daubar et al., 2013), tend to negate suggestions that the numbers and/or clustering of secondaries destroy the effectiveness of crater counting as a chronometric tool.

Size-frequency distribution of different secondary crater populations: 1. Equilibrium caused by secondary impacts

“…It cannot be ruled out that the rollover in the SFD of secondaries may be partly caused by the rollover in the fragment SFD at certain diameters [ Quantin et al ., ]. However, the geological context of the distant secondaries shown in Figures b and e suggests that substantial mutual destruction and interruption had occurred when the secondaries were formed.…”

“…However, interpretations about the dominance of background secondaries in small crater populations have been different, e.g., Dundas and McEwen [] versus Quantin et al . [].…”

“…Secondaries around a primary crater can be roughly classified into four populations by referring to their occurrences and distances to the parent crater: (1) self‐secondaries that are located on top of the continuous ejecta deposits [ Shoemaker et al ., ]; (2) near‐field secondaries beyond the continuous ejecta deposits, which are composed of connected chains and clusters of secondaries [ Schultz and Singer , ; Xiao et al ., ]; (3) distant secondaries (i.e., far‐field secondaries) that occur over ~5 radii from the rim of the parent crater, and many if not most of them occur as isolated patches or clusters within impact rays [e.g., Quantin et al ., ]; and (4) background secondaries, which are a subpopulation of distant secondaries and occur as isolated craters that cannot be easily distinguished from same‐sized primaries [ Shoemaker , ]. Although the different populations of secondaries may exhibit substantial variations in the morphology [ McEwen and Bierhaus , ], density [ Dundas and McEwen , ], and spatial distribution [ Bierhaus et al ., ], the high efficiency of forming secondaries [ Robinson et al ., ; Speyerer et al ., ], their widespread occurrences [e.g., Neish et al ., ], and the enhanced erosional effect on the local topography [ Lucchitta , ; Bandfield et al ., ] must have affected the evolution of local crater populations.…”

Dependence of secondary crater characteristics on downrange distance: High‐resolution morphometry and simulations

“…Schultz and Gault [] identified this as a feature of clustered subsonic laboratory impacts and suggested that it could be used to recognize distal secondaries. Numerous asymmetric ejecta secondaries were identified by Calef et al [] (from many of the same primaries as addressed in the present study) and Quantin et al [] (from Gratteri crater). Highly asymmetric ejecta blankets can form even for moderately oblique low‐velocity impacts (see Moore [, Figure 2], impact angle 45°).…”

“…Significant attention has focused on the size and spatial distribution of secondary craters to understand how these influence the chronology based on crater counting statistics [ Bierhaus et al , ; McEwen et al , ; McEwen and Bierhaus , ; Dundas and McEwen , ; Hirata and Nakamura , ; Werner et al , ; Xiao and Strom , ; Xiao , ]. Some of this work has involved detailed mapping of the secondary crater fields of large primary craters [ Preblich et al , ; Robbins and Hynek , , ; Chuang et al , ; Quantin et al , ]. The challenge of identifying relatively recent secondary craters was helped by the work of Tornabene et al [, ], which described features common to well‐preserved complex primaries on Mars.…”