Interesting video by @DJSnM on the Chinese Mars rover mission. He mentions the landing scar and offers an interesting theory on the two lightened plumes and how that could happen on a single engine lander. 
2/ While watching this video I was reminded of some recent, new thinking about the fluid flow physics you can see in this picture. See the many radial dust streaks? We have always said they are caused by enhanced erosion around rocks and in craters. However,...
3/ ...the latest thinking says there is something else going on in the physics to create these streaks. One problem with the “rocks and craters” theory is that the streaks are too regularly spaced. Why is the spacing between streaks usually about the same?
4/ It isn’t perfectly consistent spacing, of course, but if you ran an autocorrelation function on the streaks it would mathematically show a peak correlation at a typical amount of spacing between streaks. WHY? Rocks are not spaced uniformly on the ground.
5/ So the latest thinking is that this may be a flow instability. Sand is a “complex fluid”, and gas is a fluid, so when they interact (one going supersonic, the other accelerating from a standstill) it creates interesting patterns of flow instability.
6/ We see patterns of fluid flow instability throughout nature. Example: clouds. Why are they spaced so regularly in the sky? What part of physics does this? earthsky.org/earth/what-are…
7/ Another example. Sand dunes on Mars. Why are they so evenly spaced? mars.nasa.gov/resources/693/…
8/ Another example. Chains of pits in the ice sheets of Pluto. What makes the pits all be the same size with consistent spacing between the chains of pits?
9/ Answer: fluid flow instabilities, which cause flow to natural break up into convective rolls or other regularly spaced flow patterns. This causes condensation to happen in regular patterns, making clouds. Or it causes deposition of sand in regular patterns making dunes.
10/ The size of the visible feature is determined by the size of the invisible rolling pattern in the gas. Gas has a specific viscosity and density, etc., which causes it to roll in cylinders of a particular diameter. (en.m.wikipedia.org/wiki/Horizonta…)
11/ It is possible the pits in Pluto’s ice are caused similarly by rolling flow of ice or liquid under the ice, connecting heat on regular patterns do the ice sublimates in long chains of regular spacing. sciencedirect.com/science/articl…
12/ In each of these cases we see a phase-change phenomenon (condensation of water, sublimation of ice, deposition of sand — i.e., granular phase change) coupled with an invisible gas flow. The invisible gas flow is what creates the regular spacing in the visible features.
13/ So the latest thinking about the streaks we see in rocket landings is that it is another example of a gas flow instability coupled with a phase change phenomenon. This time, the phase change is “granular sublimation”, a fancier term for soil erosion. Example from Apollo:
14/ I was reminded of this while watching Scott’s very interesting video of the Chinese lander on Mars. NASA is doing experiments daily to understand the flow physics of rocket landings on the Moon & Mars.
15/ This gorgeous landing photo also reminds me of the human eye, and of the Eye of Sauron 😆 (Eye credit: wiki. Sauron credit: lotr.fandom.com)
16/16 But to hear Scott’s hypothesis on why there are two lightened plumes around a single-engine Mars lander, you’ll need to watch his excellent video, here!
Appendix: I had intended to mention that clearly some of the streaks come from rocks (circled in yellow), but clearly others do not (circled in blue). Nature is complicated.
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