Are there any photos of the Apollo LM descent engine bell and the lunar soil beneath it disturbed by the engine exhaust?
Most of the Apollo photo libraries have a few shots of the surface under the descent engine bell; I think A14 has some interesting ones:
The disturbance of the soil is very subtle; compared with the surface further out, I see more 1-2cm-sized pebbles, suggesting that smaller particles have been blown away from under the engine.
The lack of a massive blast crater under the LM engine is often pointed to as evidence of a hoax, but there are several factors involved, which are not obvious or not intuitive, which minimize the effect of the engine exhaust:
- At touchdown, the engine is running at only about 25% throttle (~2500 lb-f or 11kN), as the descent fuel tanks are nearly empty, making the LM fairly light;
- The area at the end of the engine bell is about half that of the four LM footpads, so pressure at that point while hovering should be about twice as much as the ground pressure of the footpads with the LM on the surface, which of course displace only a few cm of soil;
- As Schwern points out, without atmospheric pressure constraining the exhaust plume, it spreads out more rapidly than the familiar image of a rocket exhaust flame at liftoff, thus is very diffuse. The LM engine is normally shut off while the footpads are still a meter or so above the surface (the contact probes extend about 1.6m below the footpads, but it takes a moment for the crew to react to the contact light) so the exhaust has quite a distance to disperse before reaching the surface soil. (On Apollo 14, however, per the annotated transcript at 108:15:12, Al Shepard left the engine firing all the way to the surface and for a couple of seconds after touchdown, so this doesn't apply to the above picture.)
There is a very nice Apollo 14 image, a combination of the two images 9254 and 9255:
From the Apollo 14 lunar surface journal, see.
Another Apollo 12 ALSJ image: