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That brown dust blown by Starhopper is silt that was washed from the US Great Plains & Midwest down the Mississippi River then pushed by shore currents in the Gulf of Mexico along the coast of Texas to Boca Chica. Such transport processes don’t exist on the Moon. As a result.../1
2/...the lunar soil is “unsorted”, meaning that all the different sized particles are mixed together. The dust or silt-sized particles are mixed in bulk with sand, gravel, and rocks. 
3/ We spent a long time trying to figure out which sizes of lunar soil get blown by a landing rocket. (Like in this view out the Lunar Module window in Apollo 15). We finally showed that all sizes are being lifted into the gas. For example...
4/ here is an example of a fist-sized rock we found in one of the landing videos. It is traveling about 15 meters per second (35 miles per hour). Also...
5/...we found these phenomena we called "dust tails". They appear then disappear after 1 or 2 frames in the video (1/30 to 1/15 of a second). When they persist more than one frame in the video, we see that they didn't move; they are fixed on the surface. So what are they?
6/ We didn't know until we did some field tests in Hawaii, when we fired a rocket thruster on the volcanic ash and took high speed videos. We saw dust tails! So we were able to study them.
7/ It turns out the dust streaks occurred because rocket exhaust has to flow around rocks that stick up in the flow. This causes "obstacle scour" (localized faster erosion), emitting dust more into the gas. The dust streak ends after that rock has been undercut & liberated.
8/ The dust tails we see in the Apollo landing videos are more cases of rocks being undercut by the gas then liberated and blown away. We can't see the rock itself under the dark sheet of dust, but we can see the dust tail as it is being liberated.
9/ We can see the dust, and we can see the rocks, but we can't see the sand-grain sized particles in-between. They are too small for the resolution of the camera, and they don't have as much optical density as the dust. But we know the sand sizes are eroding, too, because...
10/...first, they have less weight than rocks and less cohesion than dust, so theory says they are the MOST erodible particle sizes. And second, we measured how much dust blew. If the sand didn't erode, then the surface under the Lunar Module would have run out of dust.
11/ So we found out that all particle sizes including up to rocks are being separated and entrained into the gas flow; it is not just dust. We had to use computer simulations to estimate the velocity they travel. It depends on a lot of variables, but a crude summary is below.
12/ In the Starhopper launch and landing you can see the dust billows around the pad. This is because the rocket exhaust mixes with the surrounding air and stops, so the dust stops with it. On the Moon's vacuum, the gas spreads into vacuum and the particles keep going full speed.
13/ This plot shows an example trajectory for a dust particle traveling at a typical 1900 m/s. It travels all the way around the Moon.
14/ These analyses were for the Lunar Module, with landing weight about 5 tons, burning Aerozine/NTO. Aerozine/NTO makes exhaust gas that travels about 3.05 km/s, a bit faster than lunar escape velocity at 2.38 km/s, so this is why dust goes so far. Some gets blown off the Moon.
15/ For landers that use either LOX/Methane or LOX/Hydrogen (what we will likely use in the future), the exhaust gases go 3.8 km/s or 4.5 km/s, much greater than lunar escape velocity (2.38 km/s). So bigger particles will go globally & more will be blown completely off the Moon.
16/16 To read more about lunar (or Mars) lander plume effects, check out the pages at the @CLASS_UCF Planetary Landing Team website: sciences.ucf.edu/class/landing-…
