re: Take a broom to Mars
TES results are not the end all. Look at the Pathfinder Rover APXS - Martian rocks has so much dust on them the APXS could not identify them accurately. So what do you think a low rez instrumenet like TES from Martian orbit can tell about dust covered rocks? Nothing much.
Vast reserves of carbonates could easily be under a few centimeters of Martian dust. I'll wait until the spinning brushes and other such instruments are employed on Mars before I rule out carbonates.
BD