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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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.