re: Could it really have been mud seen earlier by Spirit?
This abstract to the upcoming LPSC conference reports on experiments that show the evaporation rate of liquid water on Mars might be as low as 1 mm per hour:
STABILITY OF WATER AND GULLY FORMATION ON MARS.
Lunar and Planetary Science XXXVI (2005) 1496.pdf
"We measured evaporation rates at 7 mb under
conditions much closer to martian than previous
work and eight determinations yielded a value of
1.04 ± 0.14 mm/h [7]."
http://www.lpi.usra.edu/meetings/lpsc2005/pdf/1496.pdf
Bob Clark