Chapter 2: Energy at the Lunar Surface - Lunar and Planetary Institute

PHYSICAL PROPERTIES
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Impact Velocity (km sec-I)
Fig. 2.9. Experimentally determined depth/diameter ratios using projectiles with densities from 1 to
7.3 g cm73 and impact velocities from 3 to 13 km sec-l. The inserted histogram on'the lunar
depth/diameters is based on 70 craters; an empirical curve accounting for the oblique impact is
indicated (from H'drz et al., 1973).
by low velocity impacts - possibly secondary events. However, there is
evidence to suggest that the craters were initially glass lined and the glass has
been spalled off. Possibly more important is another rare type of crater
which has been called a multiple pit crater (Hb'rz et al., 1973). These
microcraters are produced by aggregate structures with a low density and
non-homogenous mass distribution.
Shape
Direct observation of the shape of meteorites provides little useful
information. Mason (1962) presents photographs of a variety of meteorite
shapes, however, most were determined by ablation during entry into earth's
atmosphere and later by weathering on the gfound. However, crater
symmetry is determined to a large extent by the shape of the projectile and
its angle of incidence (Mandeville and Vedder, 1971 ; Kerridge and Vedder,
1972; Mandeville and Vedder, 1973). Most noncircular microcraters on lunar
rocks are elongate and shallow which indicates that they were produced by
an oblique impact rather that by irregularly shaped micrometeoroids (Wdrz
et al., 1973). Thus highly nonspherical micrometeoroids such as rods or
platelets are very unlikely. If the particles are modeled as prolate elipsoids