And so Dad replied:
I have seen this occur with zinc over a time period of several years, and I think it reflects vapor phase transport of the zinc and subsequent crystallization from the vapor. It could be that corrosion of some type (possibly chloride dissolved in drops of water condensate) attack the zinc surface, and when the water drops evaporate, the zinc chloride complex also goes into the vapor phase. The attack on the zinc could also be a galvanic type corrosion. Then, condensation could occur on the end of the whisker, and zinc would crystalize because of a drop in electrical charge due to the conductive nature of the whisker.
I suggest the vapor phase transport since there is no continuously wet surface for solution transport. The fine whiskers look similar to mineral crystals I have seen around volcanic gas vents. In the gas vents, high temperatures cause the volatile metal complexes (often chlorides or oxides) to travel to cooler surfaces where they crystalize as very fine needles or whiskers. The electrical conductivity of the metal would make it easier to get electron transfer and precipitate the pure metal.
I have not looked at the NASA studies, so let me know if they have anything close to my guess.