  In the companion paper ("Erosion rate of lunar soil under a landing rocket,
part 1: identifying the rate-limiting physics", this issue) an equation was
developed for the rate that lunar soil erodes under the exhaust of a landing
rocket. That equation has only one parameter that is not calibrated from first
principles, so here it is calibrated by the blowing soil's optical density
curve during an Apollo landing. An excellent fit is obtained, helping validate
the equation. However, when extrapolating the erosion rate all the way to
touchdown on the lunar surface, a soil model is needed to handle the increased
resistance to erosion as the deeper, more compacted soil is exposed. Relying on
models derived from Apollo measurements and from Lunar Reconnaissance Orbiter
(LRO) Diviner thermal inertia measurements, only one additional soil parameter
is unknown: the scale of increasing cohesive energy with soil compaction.
Treating this as an additional fitting parameter results in some degeneracy in
the solutions, but the depth of erosion scour in the post-landing imagery
provides an additional constraint on the solution. The results show that about
4 to 10 times more soil was blown in each Apollo landing than previously
believed, so the potential for sandblasting damage is worse than prior
estimates. This also shows that, with further development, instruments to
measure the soil erosion during lunar landings can constrain the soil column's
density profile complementary to the thermal inertia measurements, providing
insight into the landing site's geology.
