Astrobiological significance of minerals on Mars surface environment:   UV-shielding properties of Fe (jarosite) vs. Ca (gypsum) sulphates
  The recent discovery of liquid water-related sulphates on Mars is of great astrobiological interest. UV radiation experiments, using natural Ca and Fe sulphates (gypsum, jarosite), coming from two selected areas of SE Spain (Jaroso Hydrothermal System and the Sorbas evaporitic basin), were performed using a Xe Lamp with an integrated output from 220 nm to 500 nm of 1.2 Wm-2. The results obtained demonstrate a large difference in the UV protection capabilities of both minerals and also confirm that the mineralogical composition of the Martian regolith is a crucial shielding factor. Whereas gypsum showed a much higher transmission percentage, jarosite samples, with a thickness of only 500 microns, prevented transmission. This result is extremely important for the search for life on Mars as: a) jarosite typically occurs on Earth as alteration crusts and patinas, and b) a very thin crust of jarosite on the surface of Mars would be sufficient to shield microorganisms from UV radiation. 
Gabriel Amaral, Jesus Martinez-Frias and Luis Vazquez