Added on Sept. 21: Blood Vessel Remains Are Rarely Found on Mars Soil
Surface
Some people say rock minerals or fracture fills can form these without
http://roadtoendeavour.wordpress.com/ ( post of Sept. 19, 2012)
http://roadtoendeavour.files.wordpress.com/2012/09/untitled-1cs2.jpg
Those are unlikely to be fracture fills, because similar morphology is
rarely found on the surface of Martian soils. They are not geologic
veins in origin, because no minerals form such morphology without
pseudomorphic replacement of curly blood vessel remains smaller than
500 micrometers across.