[meteorite-list] Origin and significance of uprange ray patterns, PH Schultz et al, 2009 2p: Rich Murray 2009.11.07

[Rich Murray]

Origin and significance of uprange ray patterns, PH Schultz et al, 2009 2p: 
Rich Murray 2009.11.07

http://www.lpi.usra.edu/meetings/lpsc2009/pdf/2496.pdf  2 pages

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])....

Horseshoe rays characterize secondary craters around large primaries (>20 km 
on the Moon).
This pattern reflects an absence of uprange material; instead, tertiary 
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)....

Arachnid patterns also have been produced in laboratory experiments for 
impacts into layered targets, thereby indicating a change in the flow field 
with depth.
More generally, the different ray patterns from oblique impacts reflect the 
effect of target/projectile impedance contrasts and the ratio of impactor 
size divided by its speed....

If this interpretation is valid, then arcuate uprange crater rays not only 
reflect impact angle but also the the projectile-to-crater diameter 
ratio....

[ some very helpful photos and diagrams ]