From the press release:
December 13, 2004
Mars Rovers Spot Water-Clue Mineral, Frost, Clouds
"A portion of Mars' water vapor is moving from the north pole toward the south pole during the current northern-summer and southern-winter period. The transient increase in atmospheric water at Meridiani, just south of the equator, plus low temperatures near the surface, contribute to appearance of the clouds and frost, Wolff said. Frost shows up some mornings on the rover itself. The possibility that it has a clumping effect on the accumulated dust on solar panels is under consideration as a factor in unexpected boosts of electric output from the panels."
http://marsrovers.jpl.nasa.gov/newsroom/pressreleases/20041213a.html
Rover scientists had speculated that some of this frost may melt to liquid to contribute to the clumping. Note that actual SURFACE temperatures can exceed 0 C when the air temperature is below freezing, when the surface is exposed to direct sunlight (it's southern winter at the landing sites now.) This would be especially true of a dark surface like the solar panels specifically designed to absorb sunlight.
What is the expected temperatures of the solar panels during the hottest portion of the day? Do the cameras actually observe the frost on solar panels? If so then we may actually be able to observe the transition to liquid during late morning to early afternoon.
Note that the accumulation of this frost confirms visually what has been found by the Mars Odyssey HEND hydrogen mapper that water accumulates on Mars in near equatorial locations during northern and southern Summer:
Further evidence for current liquid water near the equator on Mars.
Posted by Robert Clark on 11/25/2004 12:39:16 AM
http://habitablezone.com/space/messages/358773.html
This visual evidence from the rovers further supports the suggestion by the authors of the HEND report that this increase is due to atmospheric deposition. Of the various explanations offered by the authors, it supports the suggestion the water is in free form, not bound in sulfate or other evaporite:
47 - EVIDENCE OF THE SEASONAL REDISTRIBUTION OF WATER IN THE SURFICIAL
MARTIAN REGOLITH BASED ON ANALYSIS OF THE HEND MAPPING DATA. R.O.
Kuzmin, E.V. Zabalueva, I.G. Mitrofanov, M.L.Litvak, A.V. Parshukov,
V.Yu.Grin'kov, W. Boynton, R.S. Saunders.
http://www.geokhi.ru/~planetology/theses/47_kuzmin_et_al.pdf
The authors suggest frost or ice. My opinion is that during warm *surface* temperatures this frost can melt to liquid, particularly for low albedo (dark), low thermal inertia (easy to heat up) materials, such as the solar panels.
Bob Clark