No crater should be expected. The Descent Propulsion System was throttled very far down during the final landing. The Moon lander was no longer quickly decelerating, so the descent engine only had to support the lander's own weight, which was lessened by the Moon's gravity and by the near exhaustion of the descent propellants. At landing, the engine thrust divided by the nozzle exit area is only about 10 kilopascals (1.5 PSI).[91] Beyond the engine nozzle, the plume spreads and the pressure drops very quickly. (In comparison the Saturn V F-1 first stage engines produced 3.2 MPa (459 PSI) at the mouth of the nozzle.) Rocket exhaust gases expand much quicker after leaving the engine nozzle in a vacuum than in an atmosphere. The effect of an atmosphere on rocket plumes can be easily seen in launches from Earth; as the rocket rises through the thinning atmosphere, the exhaust plumes broaden very noticeably. To lessen this, rocket engines designed for vacuums have longer bells than those designed for use on Earth, but they still cannot stop this spreading. The Moon lander's exhaust gases therefore expanded quickly well beyond the landing site. However, the descent engines did scatter a lot of very fine surface dust as seen in 16mm movies of each landing, and many mission commanders spoke of its effect on visibility. The landers were generally moving horizontally as well as vertically, and photos do show scouring of the surface along the final descent path. Finally, the lunar regolith is very compact below its surface dust layer, further making it impossible for the descent engine to blast out a "crater".[92] In fact, a blast crater was measured under the Apollo 11 lander using shadow lengths of the descent engine bell and estimates of the amount that the landing gear had compressed and how deep the lander footpads had pressed into the lunar surface and it was found that the engine had eroded between 4 and 6 inches of regolith out from underneath the engine bell during the final descent and landing
