Living on Mars -- II

In my last post, I talked about the problems facing humans if we want to live on Mars.  Human ingenuity and technological ability have a way of solving some of these problems.  

So, two biggies: temperature and radiation. 

This interesting article about silicon aerogels discusses a novel solution:

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 niches. 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 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 2–3 cm [one inch]-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.

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 favourable 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, depending on the abundance of impurities such as hydroxy groups. However, the thermal conductivity of solid silica (0.8–1.6 W m−1 K−1)  is too high to allow a strong warming effect.

Silica aerogels, which consist of nanoscale networks of interconnecting silica clusters, are over 97% air by volume and have some of the lowest measured thermal conductivities of any known material (~0.02 W m−1 K−1 at 1 bar pressure or 0.01 W m−1 K−1 at Martian atmospheric pressures)17. Because of these properties, silica aerogels have gained prominence in many fields of engineering, including in the design of passively heated buildings on Earth and even in the Mars Exploration Rovers, where thin aerogel layers were used to provide night-time thermal insulation. Silica aerogels therefore hold excellent potential for creating strong solid-state greenhouse warming under Martian conditions.


We performed experiments to demonstrate the warming potential of silica aerogel solid-state greenhouse layers under Mars-like insolation levels. Our experimental set-up consists of a layer of silica aerogel particles or tiles on a low-reflectivity base surrounded by thermally insulating material. The apparatus is exposed to visible radiation from a solar simulator. The broadband flux incident on the layer is measured with a pyranometer, and temperature is recorded by calibrated glass-bead thermistors.

Figure 3 shows the experimental results for both aerogel particle and tile layers versus received visible flux in the 100–200 W m−2 range. For comparison, Earth’s global mean received flux is 342 W m−2, while that of Mars is 147 W m−2. As can be seen, temperature differences of over 45 K are achieved for aerogel particle layers of 3 cm thickness receiving a flux of 150 W m−2. Aerogel tiles, which have higher visible transmission, cause temperature differences that are ~10 K higher, reaching >50 K at just 2 cm thickness. Our experimental results show that under Mars-like insolation levels, warming to the melting point of liquid water or higher can be obtained under a 2–3 cm-thick silica aerogel layer. The peak obtainable warming is likely even higher, because heat is lost in our experimental set-up via sidewall and base thermal losses and convection. We also measured the transmission of the aerogel particles and tiles in the UV and found strong attenuation of UVA and UVB (280–400 nm) and near-total attenuation of the most hazardous UVC (220–275 nm) radiation (Fig. 4).

Fig 3.
Temperature differences between the surface and top of the layer are shown, for aerogel particles (left) and tiles (right), as a function of the layer thickness. Colours indicate data for different incident visible light fluxes. For reference, the annual mean flux on Mars between 45° S and 45° N varies from about 130 W m−2 to 170 W m−2, with diurnal mean values varying from 50 W m−2 to 250 W m−2 over the course of the Martian year. Error bars indicate the estimated standard deviations of the measurements, which were calculated by combining uncertainties due to the thermistor calibration, data acquisition and signal digitization in quadrature.

Fig 4
Plot of UVA and UVB (left) and UVC (right) transmission by silica aerogel layers (particles and tiles) of thickness varying from 1 cm to 3 cm. Both particles and tiles attenuate UVC effectively, with the transmission of UVC through tile layers of 2 cm thickness or more dropping to below 0.5%.

In short, silicon aerogel can solve the problems of a too-low temperature and a too-high UV influx.  What they can't solve is the issue that at very low air pressure, plants behave as if they are experiencing a drought:

Martian and lunar greenhouses must hold up in places where the atmospheric pressures are, at best, less than one percent of Earth-normal. Those greenhouses will be easier to construct and operate if their interior pressure is also very low -- perhaps only one-sixteenth of Earth normal.

The problem is, in such extreme low pressures, plants have to work hard to survive. "Remember, plants have no evolutionary preadaption to hypobaria," says Ferl. There's no reason for them to have learned to interpret the biochemical signals induced by low pressure. And, in fact, they don't. They misinterpret them.

Low pressure makes plants act as if they're drying out.

In recent experiments, supported by NASA's Office of Biological and Physical research, Ferl's group exposed young growing plants to pressures of one-tenth Earth normal for about twenty-four hours. In such a low-pressure environment, water is pulled out through the leaves very quickly, and so extra water is needed to replenish it.

But, says Ferl, the plants were given all the water they needed. Even the relative humidity was kept at nearly 100 percent. Nevertheless, the plants' genes that sensed drought were still being activated. Apparently, says Ferl, the plants interpreted the accelerated water movement as drought stress, even though there was no drought at all. 

That's bad. Plants are wasting their resources if they expend them trying to deal with a problem that isn't even there. For example, they might close up their stomata -- the tiny holes in their leaves from which water escapes. Or they might drop their leaves altogether. But, those responses aren't necessarily appropriate.

[Read more here]

At pressures below 63 mb (for reference, Earth's sea level pressure is 1013 mb, Mar's average air pressure is 6 mb), 

exposed body fluids such as saliva, tears, urine, blood and the liquids wetting the alveoli within the lungs—but not vascular blood (blood within the circulatory system)—will boil away  (Wikipedia)

But human life will stop long before such low pressures are reached:

The earth's atmosphere contains 20.9% oxygen, but the critical factor is its partial pressure. At sea level (1 Bar) this equates to 150mm Hg [Hg=mercury column], but at 4000m it's down to 70mm Hg. At this altitude (as in some Andean communities) it's just about impossible to have babies, though children will survive. At 8000m as on the world's highest mountains, then life is only sustainable in short bursts. Everest climbers call 8000m + the "Death Zone", because all the time you are there you are dying for lack of oxygen, now down to a partial pressure of 35mm Hg. So assume non-survivable long term at anything over 6000m [20,000 feet] or atmospheric pressures under 0.4 Bar [400 mb].   (The Guardian Answers) 

If we want to work on growing our greens on Mars, without having to wear a spacesuit all the time,  the air pressure in the vegetable-growing domes will have to be over 400 mb, preferably higher.    Air pressure inside a passenger jet is maintained at 1800-2400 metres ( 6000-8000 feet) [800 mb] even when the plane is at an altitude of  11,000-12,200 metres (36,000 to 40,000 feet).  The pressure difference between these two altitudes is roughly 600 mb, and as we know, that requires a strengthened cabin to prevent explosive depressurisation, a fate which grounded the first passenger jet in the world, the de Havilland Comet 1. 

This means that spreading sheets of aerogel across the ground will not be enough, because any build up of air pressure under the sheets to the levels needed by plants, never mind humans, will simply break the aerogel.

However, the aerogel will be very useful as part of a dome.  Imagine an outer shield of hardened glass, a middle layer of aerogel, and an inner layer of either polythene (polyethylene) or of perspex (plexiglass).  Both polythene and perspex are high in hydrogen atoms, and hydrogen is the best absorber of cosmic rays.  Perspex contains  less hydrogen proportionately than polythene, but is however also much more translucent than polythene, which means it could be much thicker, without reducing light inside the dome.  Perspex is also much stronger than polythene.  The perspex panes in Monterrey's underwater aquarium are 32 cms (13 inches) thick to withstand the substantial pressure differential between sea level and 10 metres underwater.   The difference between the pressure at sea level and 10 metres down is about 1000 mb, about the pressure we'd want in our domes.

The use of aerogel sandwiched between two layers of harder material will mean that  domes will be passively heated—i.e., that the sun's warmth will be enough to keep them at a livable temperature, because the aerogel will almost eliminate heat radiating away.  This will reduce the energy needed for heating the domes almost to zero, which is vital as energy will provide a major constraint to human settlement on Mars (more on that in a subsequent blog piece).  Note how in the charts describing the experimental results above, a 3 cm thick layer of aerogel increased temperatures by 50 C.  This should be enough between latitudes 30 or 40 north and south to have air temperatures inside the domes of 20 C.  On a summer's day near the equator, daytime temperatures can reach 20 C but fall to minus 73 C before dawn.  That's a rough average of -16 C.  Increasing temperatures by 50 C would mean that the air inside the domes might actually be too hot!

I have been puzzled for some time about how to create domes covered with translucent material which also protected against UV as well as cosmic rays while keeping the inhabitants of the dome  warm.  The answer is silicon aerogel, strengthened by glass and with an added perspex layer, which will not only keep the domes warm but will also protect against radiation.  We won't after all have to spend all our time underground, which is good news.

I've taken a long time to get here, but I wanted to fully analyse the results of this new research.  Anyone who knows more about this than me,  please comment below.

Living on Mars -- I
Living on Mars -- III