Origin and Significance of Uprange Ray Patterns

Introduction: Crater rays radiate from fresh primary craters on the Moon, Mercury, and Mars. On the Moon, they are related to secondary craters [1,2,3], scouring of the surface [2], or deposition of distal deposits [2]. On Mars, they also can be shown to be related to secondary craters (e.g., [4,5]) or blast winds [6,7]. Observations of arcuate uprange rays emanating from the Deep Impact collision have been interpreted as an evolving excavation flow field [8] based on laboratory experiments [9]. Here we reconsider the significance of different types of uprange crater ray patterns and provide a simple analytical approximation in order to infer their significance. Uprange Ray Patterns: Two different crater ray patterns are found on the Moon, Mars, and Mercury: convex (Fig. 1, Fig. 2a) and concave (Fig. 2b). The former has been described as cardioid pattern (heart shaped, e.g., [8,10]), whereas the latter is described here as an arachnid pattern. Rays extending from secondary craters represent extreme case of the arachnid pattern and form a horseshoe (U-shaped) pattern open downrange (e.g., [10]). Uprange cardioid rays occur on the Moon, whether in the highlands or on the smooth plains. The arcuate uprange rays from the 102 km-diameter lunar crater Tycho represent one of the best-known examples. Other notable examples include Proclus, Petavius B, and Stevinus A on the nearside and Jackson (Fig. 1) on the farside. Secondary crater chains comprising these rays generally point back to their primary crater. Crater rays, however, do not necessarily extend back, appearing to miss the crater entirely. The uprange arachnid pattern is best expressed on Mars (Fig. 2b) but was observed at a certain stage in the evolution of the DI impact as well [8]. This pattern appears to be most commonly associated with craters formed in layered targets (low density layer covering basement). Low-thermal inertia distal rays from small craters in THEMIS-night images [e.g., [5] illustrate this pattern. Both cardioid and arachnid patterns can occur around the same crater (Figg. 2b). Horshoe rays characterize secondary craters around large primaries (>20 km on the Moon). This pattern reflects an absence of uprange material; instead, terti-ary ejecta rays wrap around the lead crater or extend obliquely from a cluster [11]. In addition, there are oblique impacts (uprange and downrange zones of avoidance) with radial rays. These are more typically found on Mercury or at small scales (in the regolith).