If they have gained mass it could be because the platinum based ingots have absorbed mercury from the atmosphere or hydrogen from the solvents used to clean the ingots.

BBC: RELATED BBC SITES

An atmosphere of hydrogen and carbon dioxide on Callisto.

CNN: Galileo moves on to Europa

And it's believed that the high chemical reactivity of the surface is due to the deposition of chemically active gases from the atmosphere onto the surface, gases like hydrogen peroxide and ozone.

NPR: Revisiting the Viking Missions to Mars

He thus put together a collection of apparatus that would not have disgraced the set of a mad-scientist horror film, and filled it with a mixture of methane, ammonia, hydrogen and water vapour that matched contemporary ideas of what the primitive terrestrial atmosphere had been composed of.

ECONOMIST: Stanley Miller