If you look at the daylight photos of the landed first stage of the Orbcomm G2 Falcon 9 mission, there is all sorts of soot covering stuff in interesting patterns.
What elements of the soot pattern can you explain?
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1$\begingroup$ I'd guess one of those lines is the border between LOX/RP-1 tanks; the cooler LOX might have condensed more of the soot... But I thought the LOX is usually on top and takes more space. Maybe flip that reasoning: some cleaning action because the colder tank condensed some water out of the air? $\endgroup$– Nick TDec 22, 2015 at 18:19
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At the bottom you can see the outline of the legs, which protected the white paint since they extend only in the last 10 seconds of flight or so. (Oddly late, considering how we thought they would have an aero role in landing)
The LOX tank was cold being Liquid Oxygen, but then even colder, so ice likely formed on the white areas, and the soot that was picked up on landing (Flying into your own plume will do that to ya) was picked up on the ice, which fell off or melted protecting the paint job.
Tom Mueller, main designer of the Merlin engines tweeted that with Methane, the soot won't stick to either tank, due to the cold. Tweet by Mueller
The lines near the top of the band are said to be the oxygen vent areas which would of course have been either cold, and left ice behind or else blew off the soot.
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2$\begingroup$ If the legs were extended earlier, the center of pressure would shift dramatically forward and might make the booster unstable. $\endgroup$– Nick TDec 22, 2015 at 18:25
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1$\begingroup$ mhm what shape is the LOX tank? i would have thought that the bottom part is an half sphere and thus i would have expected a rather blurred soot line. $\endgroup$– pastulloDec 22, 2015 at 19:52
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$\begingroup$ The bottom is a half-spherical bulkhead which separates the LOX tank from the RP-1 tank. $\endgroup$– HobbesDec 23, 2015 at 14:00
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$\begingroup$ May second stage ignition cause any soot on the top regions of the stage? $\endgroup$ Jan 1, 2016 at 10:56
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$\begingroup$ @uhoh I was estimating, it may be less, it may be more, but it is very near landing. The legs open very late, compared to most pre-landing video expectations. $\endgroup$– geoffcJan 3, 2016 at 21:17