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

VELOCITY DISTRIBUTION
The mass distribution for the lunar meteoritic flux of N particles (per
cm-2 106 yr-1 2n strd-1 ) of mass Mp (g) according to the model is:
for particles with a density of 1 g cm-3 impacting with a root-mean-square
velocity of 20 km s-l . The use of two exponential expressions with a
resultant discontinuity is artificial but was introduced for simplicity. Also
shown in Figure 2.7 is the incremental mass flux in each decade of particle
mass, which shows quite clearly that most of the mass of materials impacting
on the lunar surfacqconsists of particles of the order of 10-6g.
Velocity Distribution
The velocity distribution of meteoroids is bounded on the lower side by
the escape velocity of the earth and on the upper side by the escape velocity
of the solar system (Figure 2.8). The modal velocity of the distribution is
approximately 20 km s-1. This last figure is generally accepted as the
root-mean-square velocity for meteoroids (Gault et a[., 1972). Particles
impacting the moon may have velocities as low as 2.4 km s-1 (the lunar
escape velocity) but the mean is expected to be closer to 20 km s-1 because
of the relatively lower effective cross section of the moon for slower
particles. The velocity-frequency distribution is noticeably skewed towards
the higher velocities with a secondary mode near the escape velocity for the
solar system. The secondary mode undoubtably reflects meteoroids with a
cometary origin. The distribution in Figure 2.8 is based on radio and photo
meteors, however, the curve is generally consistent with inflight velocity
measurements of micron-size particles (Berg and Gerloff, 1970; Berg and
Richardson, 197 1).
Physical Properties
Density
A search of the literature produces a large number of values of the
mean density of the meteoroid flux. Most, however, are just estimates and
have frequently been made to satisfy the needs of the particular study. A
considerable body of information is also available for terrestrial falls.