*Thanks to Geoff Landis for checking some figures. Corrections made.*
The aim of terraforming another planet is to make it a new home for humans and other Earth-life. Mars has long been the target of terraforming speculations in science fiction and, since the 1970s, more serious scientific discussions once we had a clearer idea of conditions on the real Mars. Early post Mariner 9 and Viking (1971-1976) models of Mars suggested an immense amount of carbon dioxide ice was present, and most terraforming concepts in the early 1990s focused on liberating that carbon dioxide to warm the planet via the greenhouse effect. An atmosphere of carbon dioxide alone can’t sustain many Earthly living things, but the usual narrative invoked a slow conversion via living things using natural sunlight. Just what are the parameters of that process? What’s needed to make enough breathable oxygen on Mars?
Mars has a total surface area of about 145 trillion square metres. A breathable atmosphere needs a minimum pressure of about 20 kPa of O2. Mars’ surface gravity is 3.71 m/s2, which means 20 kPa pressure needs a column mass of about 5,400 kilograms of gas over every square metre. Earth’s column mass is 10,332 kg, by comparsion. The total mass of O2 atmosphere we need to create is therefore 782 trillion tons. As CO2 molecules mass 44.01 atomic mass units (amu) and O2 molecules mass 32 amu, thus the total mass of carbon dioxide to convert is a bit over 1,000 trillion tons.
Just how much plant life is then required? There are some pretty solid figures on the total biomass on Earth, thanks to satellite imaging and plain old ground-truthing in the habitats themselves.
Short summary: There’s about 520 billion tons carbon in live plants, 122 billion in litter and 2124 billion in the soil.
The basic chemical reaction involved is:
CO2 + H2O => CH2O + O2
Every molecule of oxygen made, means an atom of carbon stored and removal of a molecule of carbon dioxide and water. The water is ultimately returned to the process if the biomass is ‘carbonised’ – producing what’s known as ‘biochar’ or ‘bio-carbon’.
Solar energy, in the form of photons in certain frequency ranges, is stored in the new chemical bond of the simple sugar (the CH2O). It’s released, to power life processes, via the reverse reaction. There’s a less efficient reaction which uses CO2 reacted with the sugar, which plants and animals both use in some circumstances. For every gram of carbon fixed by plants (as polysaccharides like cellulose and lignin) about 40 kJ is stored in the chemical bonds.
Thus the stored energy in biomass ultimately comes from the Sun. A gigaton of organic carbon, requiring about 3.5 gigatons carbon dioxide to make, is 40 exajoules (40 x 1018 joules) of stored sunlight. About 2.7 gigatons of O2 is released in making it. Mars needs about 5.4 tons O2 for every square metre (it’s 145 trillion square metres in area) for a minimal breathable atmosphere. So making it via photosynthesis means storing about 294,000 gigatons of bio-carbon in the soil. Or about 106 times the total organic carbon alive and dead in the soil on Earth. It’s like burying all of Earth’s live plants almost 570 times over. A total energy of almost 11,800,000 exajoules.
At Mars’ average distance of 1.52 AU, the insolation from the Sun is about 590 watts per square metre. Of course that varies over its orbit, but let’s use it as the average received. Mars is a sphere, so the fraction in sunlight is 1/4 of its total surface. That means Mars receives 675,000 exajoules of sunlight each year. More than enough… except plants aren’t very efficient at making new biomass via raw sunlight alone. They need nutrients and they need a continual supply of raw material. They’d need biologically available nitrogen, magnesium and phosphorus – which we’ll assume they can get from the soil and recycle with each cropping cycle. If maybe half the planet was cultivated – bracketing the equator to maximise sunlight received and minimise respiration during low light seasons – and every crop of fast growing grasses is reduced to bio-carbon and buried (not an inconsiderable energy bill in itself) then we might bury a few tons of biochar per square metre over a few millennia. Remember: we’re burying Earth’s *total* biomass 106 times over.
Thus pyrolysing oxygen out of the bare rocks seems a slightly less herculean task by comparison.