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

From: Rich Murray <rmforall_at_meteoritecentral.com>
Date: Sat, 7 Nov 2009 23:00:58 -0700
Message-ID: <FE9A6666F82A425C9B25863E4B4BAF7E_at_ownerPC>

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 ]
Received on Sun 08 Nov 2009 01:00:58 AM PST