QUESTION:
What is to prevent Pathfinder's or Sojourner's solar cells from being
obscured by the infamous Martian dust, rendering them inoperative?

ANSWER from David Dubov:
Nothing prevents the Pathfinder lander or 'Sojourner' rover solar cells from
being obscured by Martian dust. However, it is currently unknown the degree
to which such dust collects, obscures or otherwise impacts solar array
production. The rover carries an experiment which measures the dust
collected in the vicinity of the solar panel and determines the degree to
which such dust reduces solar panel production. Measurements taken from this
experiment during the course of the mission will help characterize the
impact of dust on solar panels and assist us in designing future Mars
missions.