The hint about Martian stellar occultations tends to make me think that part of the answer has something to do with the Martian atmosphere but I haven't been able to find any references to that or to how it applies to Mercury. There is the finding about possible ice in polar craters on Mercury...

Dave Mitsky

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Chance favors the prepared mind.
De gustibus non est disputandum.
Never attribute to malice that which can be adequately explained by stupidity.