Featured Image - 10/28/2008
Messier and Messier A: Unique Impact Craters

Two of the most unusual lunar impact craters on the Moon lie just east of the Apollo 11 landing site (AS15-M-2045; Figure 1). Messier is an unusual elliptical crater with a very distinctive "butterfly" ejecta pattern. Nearby Messier A is actually a complex structure called a "doublet" crater, or two small circular craters in close proximity to one another.

 
Figure 1. Northwestern Mare Fecunditatis. The positions of unusual impact craters Messier and Messier A are highlighted. [Apollo Metric Frame AS15-M-2405 (NASA/JSC/Arizona State University)].

In order to better understand the crater forming process, scientists fired miniature asteroids into lunar-like targets. Real asteroids hit the Moon at fantastically high speeds, greater than 16 km per second (or 35,000 miles per hour). The vertical gun facility at the NASA Ames Research Center provided the best opportunity to simulate these high-energy events. One type of experiment involved tilting the gun at a very steep angle. As the angle of impact got steeper and steeper, scientists saw no change from the typical circular crater. Finally, when the angle was grazing (or less than 15° from the horizon) elliptical, rather than circular, craters formed. Additional ray patterns were seen to spread out like butterfly wings at these extreme impact angles. The high-velocity gun experiments led scientists to the hypothesis that Messier formed as a result of a low-angle impact.

The origin of Messier A is more enigmatic. It is possible that Messier A formed in the same impact event as Messier, but questions about the origins of the doublet crater feature exist. Suggestions for the doublet include an impact near a pre-existing crater or the near-simultaneous impact of two small pieces that broke off from the same projectile that formed Messier. Messier and Messier A are complex features, and geologic investigation of this region by human explorers will clearly be required to determine how Messier A formed.

The Lunar Reconnaissance Orbiter Camera, launching aboard the United States' Lunar Reconnaissance Orbiter spacecraft in early 2009, will photograph craters like these on the lunar surface at 0.5m/pixel. These new high-resolution images will help us to not only improve our understanding of lunar craters, but will also let lunar scientists plan the exploration traverses that will be performed by the next generation of lunar explorers near interesting features like Messier and Messier A.

For more information:
Apollo over the Moon: The View From Orbit (1978) NASA SP-362, p. 121.
B. Ray Hawke et al. (2004) The origin of lunar crater rays, Icarus, 170, 1, pp. 1-16 (read the PSRD summary here)
D. E. Gault and J. A. Wedekind (1978) Experimental studies of oblique impact, Proc. Lun. Planet. Sci. Conf. 9th, p. 3843-3875.

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