Good summaries and reviews of the results of the MER missions.
Mostly about the Spirit mission:
Remote Sensing Tutorial Page 19-13a
Missions to Mars during the Third Millenium
http://rst.gsfc.nasa.gov/Sect19/Sect19_13a.html
and:
Remote Sensing Tutorial Page 19-13b
http://rst.gsfc.nasa.gov/Sect19/Sect19_13b.html
which is mostly about the Opportunity mission.
This second also has a link to a report that might offer an explanation for the lack of carbonates at Meridiani:
ACID SEDIMENTARY ENVIRONMENTS ON MARS?: POSSIBLE TERRESTRIAL ANALOGS
http://gsa.confex.com/gsa/2002AM/finalprogram/abstract_36936.htm
Bob Clark