The rather breathless reportage that terraforming Mars isn’t possible with current technology – which was arguably true prior to the scientific study in question anyway – is dependent on a particular terraforming concept. Work in the early 1990s by Chris McKay and Robert Zubrin proposed that Mars could be terraformed by liberating supposedly vast amounts of carbon dioxide locked away in the crust and polar caps. To do so would require making several hundred gigawatt nuclear reactors powering fluorocarbon (“super greenhouse gases”) factories on Mars to push the surface temperature a bit away from its present day cold trap more towards a self-warming runaway of CO2 and eventually water vapour. Alternatively the polar caps could be vaporised by a linear nuclear explosion or comets and asteroids dropped onto it. However *none* of those are really present-day options. Thus the article can be called “fake news” (to coin a phrase.) It’s a good scientific study, but it’s a premature headline grabber about terraforming.
Before we start moving asteroids, what does terraforming Mars really mean? Mars lacks several things to make it truly “Earth-like” – chiefly sufficient mass for Earth-like gravity, sufficient water for Earth-like oceans, sufficient oxygen to breathe, sufficient magnetic field and sufficient energy input by the Sun. Traditional terraforming tries to get a two-for-one by increasing a greenhouse gas (carbon dioxide) to warm the planet enough for liquid water on the surface and maybe some terrestrial plant life. Even though plants use carbon dioxide for making sugar to store solar energy in chemical bonds, they also need oxygen to use that sugar (i.e. respire) and too much CO2 inhibits growth in most species. Assuming lots of genetic engineering, it was hoped we might convert Mars, after millennia, to an oxygen-rich atmosphere somewhat like Earth’s.
If we can ignore the gravity issue (we could rotate cities to make our own if we really need it) and make our own magnetic field (big superconductor ring or rings), then what Mars really needs is oxygen and sunlight. Rather than CO2 to warm the planet indirectly – and not very efficiently – what if we increase the available sunlight?
Present day Mars has about 6 millibars of CO2 in its atmosphere, while Earth has about 0.4 millibars. Also the lower gravity on Mars means the column mass is equivalent to 16 millibars worth on Earth. Thus if Mars received as much sunlight as Earth, it’d be *too hot* from its CO2 greenhouse effect. If we increased the available sunlight by ~50%, then it should be about right. If we imagine an annular mirror suspended above Mars, directing light down onto the surface, just how big would it need to be? Its effective area should be a minimum 50% that of Mars, but naturally it needs to be bigger than Mars. Mars presents to the Sun a disk that’s about 4,000 km in radius. The annular mirror needs at least a Mars sized hole – if it’s close to Mars – and then sufficient width to match 50% the area of Mars. Or about 900 kilometres wide and an average radius of about 4,500 km. Immense, but it doesn’t have to be very heavy. However such a structure would ruin the night-sky.
A “mirror-lens” could be parked at the Mars-Sun L-1 point and use solar radiation pressure to help keep itself in place, directing extra sunlight towards the planet, its mirror image making the Sun larger as seen from the surface. In the early stages, the Soletta as such mirrors are called, can be closer to the planet and focus its light into an intense pyrolysis beam to separate oxygen from the metal oxides in the crust directly. No mucking about with plants for millennia required. I’ve calculated in a prior blog-post elsewhere that a 50 petawatt (i.e. 50,000 trillion watt) beam is sufficient to give Mars an oxygen atmosphere in about 6 years. Mars receives 30 petawatts from the Sun, so our “50% Soletta” gives us 15 petawatts to blast oxygen out of the crust with, taking about 20 years. Merely releasing more carbon dioxide, for plants to then use at 0.1% efficiency, is much slower by comparison.
Just how massive is that Soletta? Imagine we can make reflective multi-layer graphene sheets with a surface mass density of 1 gram per square metre (copy paper is 80 grams per square metre, so it’s not even that challenging.) The total minimum area needed is 25 trillion square metres, therefore the mass is 25 billion kilograms or 25 million tons. While graphene is still uber expensive to make, it’s basically carbon, which is uber-cheap. Of course it won’t be shipped from Earth. Carbonaceous asteroids are typically ~5% carbon, so one just 1 km across is sufficient for the job. There are thousands of those in the asteroid belt and near earth asteroids. For comparsion, a 1 gigawatt coal-fired power station uses about 3 million tons of carbon per year, so 25 million tons isn’t really that much carbon.