Around 50 K of surface warming is required on Mars to raise annual average low- to mid-latitude temperatures to above the melting point of liquid water. Mars’s current atmosphere is too thin to significantly attenuate ultraviolet (UV) or to provide greenhouse warming of more than a few kelvins. However, observations of dark spots on Mars’s polar carbon dioxide ice caps suggest that they are transiently warmed by a greater amount via a planetary phenomenon known as the solid-state greenhouse effect, which arises when sunlight becomes absorbed in the interior of translucent snow or ice layers. The solid-state greenhouse effect is strongest in materials that are partially transparent to visible radiation but have low thermal conductivity and low infrared transmissivity. Although carbon dioxide and water ices are common on Mars, they are much too volatile to make robust solid-state greenhouse shields for life. Silica has more favorable in properties in that it is chemically stable and refractory at Martian surface temperatures. Solid silica is transparent to visible radiation, but opaque to UV at wavelengths shorter than 200–400 nm and to infrared at wavelengths longer than ~2 μm.
Via the solid-state greenhouse effect, regions on the surface of Mars could be modified in the future to allow life to survive there with much less infrastructure or maintenance than via other approaches. The creation of permanently warm regions would have many benefits for future human activity on Mars, as well as being of fundamental interest for astrobiological experiments and as a potential means to facilitate life-detection efforts. The solid-state greenhouse warming concept also has applications for research in hostile environments on Earth today, such as Antarctica and Chile’s Atacama Desert.
It will be important to investigate the ease with which traditional silica aerogel manufacturing techniques can be adapted to conditions on Mars. However, given the ability of life on Earth to modify its environment, it is also interesting to consider the extent to which organisms could eventually contribute to sustaining Martian habitable conditions themselves. On Earth, multiple organisms already exist that utilize silica as a building material, including hexactinellid sponges and diatom phytoplankton. Diatoms in particular can grow up to several millimetres in length, produce frustules from ~1–10 nm-diameter amorphous silica particles (smaller than the mean pore diameter in silica aerogel networks) and are already known to have high potential for bionanotechnology applications in other areas. It could therefore be interesting in the future to investigate whether high-visible-transmissivity, low-thermal-conductivity silica layers could be produced directly via a synthetic-biology approach. If this is possible, in combination with the results described here, it could eventually allow the development of a self-sustaining biosphere on Mars.
There is the potential for Mars to be made habitable to photosynthetic life in the near to medium term, important ethical and philosophical questions must be considered. Most obviously, if Mars still possesses extant life today, its survival or detection might be hampered by the presence of Earth-based microorganisms. However, no mission has yet detected life on Mars, so if it does exist, it is likely to be confined to very specific regions in the subsurface. The approach studied here would not result in the survival of Earth-based life outside solid-state greenhouse regions, so it should be unlikely to pose a greater risk to the search for Martian life than the presence of humans on the surface. Nonetheless, the planetary protection concerns surrounding the transfer of Earth-based life to Mars are important, so the astrobiological risks associated with this approach to enabling Martian habitability will need to be weighed carefully against the benefits to Mars science and human exploration in future.
The low temperatures and high ultraviolet radiation levels at the surface of Mars today currently preclude the survival of life anywhere except perhaps in limited subsurface niches4. Several ideas for making the Martian surface more habitable have been put forward but they all involve massive environmental modification that will be well beyond human capability for the foreseeable future9. 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 2–3 cm-thick layer of silica aerogel will simultaneously transmit sufficient visible light for photosynthesis, block hazardous ultraviolet radiation and raise temperatures underneath it 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.
SOURCES – Nature
Written By Brian Wang, Nextbigfuture.com