re: At the time...
"Microns" of frost was the standard model. There was even a period when the polar deposits were said to be no more than a few inches thick, with "micron" deposits annually.
Of course the evidence did not support this model...
There is no way to actually measure the deposit at the Viking 2 site, but it does appear to be fairly substantial, not unlike a routine high desert snowfall on Earth. But it's still not certain that snows routinely fall on Mars -- at least outside the polar regions. Even there, some people would deny the possibilty of snow.
I'd love to see snow falling on Mars, though!
BR