Photosynthesis on Mars within Snow and Ice

Dusty water ice (<~1% dust) is exposed at the surface of Mars by impacts 1, as well as11within steep scarps 2, on pole-facing slopes 3 and at the poles 4,5. On Earth, solar radiation can12transmit down to multiple metres within ice, depending on the ice’s optical properties 6-1113Previous studies found radiatively habitable zones at depths between 0.05 – 4.5 m, where14damaging ultraviolet radiation (0.2 – 0.4 μm) is attenuated to safe levels while the amount of15photosynthetically active radiation (PAR, 0.4 – 0.7 μm) is large enough for photosynthesis to16occur within exposed polar (> 75° latitude) ice on Mars 12-14. However, these polar locations are17too cold for the ice to melt at these depths and produce the amount of liquid water necessary for18photosynthesis 15,16. By contrast, exposed ice in the warmer mid-latitudes (30 − 50° latitude)19might be melting under present-day conditions 3,15-17. At these locations, we find that the depths20of habitable zones range from a few centimetres for ice with dust concentrations between 0.01 –210.1%, and up to a few metres within cleaner ice. Thus, if small amounts of liquid water are22available at these shallow depths, mid-latitude ice exposures represent the most easily accessible23locations to search for any extant life on Mars today