  The low temperatures and high ultraviolet (UV) radiation levels at the
surface of Mars today currently preclude the survival of life anywhere except
perhaps in limited subsurface niches. Several ideas for making the martian
surface more habitable have been put forward previously, but they all involve
massive environmental modification that will be well beyond human capability
for the foreseeable future. Here we present a new approach to this problem. We
show that widespread regions of the surface of Mars could be made habitable to
photosynthetic life in the future via a solid-state analogue to Earth's
atmospheric greenhouse effect. Specifically, we demonstrate via experiments and
modelling that under martian environmental conditions, a 2 to 3-cm thick layer
of silica (SiO2) aerogel will simultaneously transmit sufficient visible light
for photosynthesis, block hazardous ultraviolet radiation, and raise
temperatures underneath permanently to above the melting point of water,
without the need for any internal heat source. Placing silica aerogel shields
over sufficiently ice-rich regions of the martian surface could therefore allow
photosynthetic life to survive there with minimal subsequent intervention. This
regional approach to making Mars habitable is much more achievable than global
atmospheric modification. In addition, it can be developed systematically
starting from minimal resources, and can be further tested in extreme
environments on Earth today.
