Hi, Robert.

Even though some rocks are covered, most of the white powdery substance seems to have been *uncovered* by Spirit's wheels. However, the white substance is not deeply buried by Martian dust.

"The liquid water could be atmospheric".

Yes, probably this way:

[Borax is directly deposited in arid regions from the evaporation of water in intermittent lakes called playas. The playas form only during rainy seasons due to runoff from adjacent mountains]

From: Borax.

Could the white power be related to Borax? How could it be preserved or even be altered under Martian soil and atmospheric conditions? Mars is frustrating.