|Fig. 1: The cool, welcoming surface of Mars! The goal of terraforming is to change this harsh landscape into something more Earthlike. (Courtesy of NASA)|
SpaceX founder and CEO Elon Musk drew comparisons to a James Bond villain for an idle suggestion made on the Late Show: he proposed bombarding Mars with nuclear weapons. The media had a field day mocking the ridiculousness of the proposal, without much discussion of the reasoning or feasibility. Musk was proposing war on the red planet not to vanquish some martian threat, but to terraform the planet. Terraforming is the process by which an astronomical body is engineered by humans to be more earth-like, and therefore more conducive to human colonization. As Fig. 1 shows, the surface of Mars is currently barren and lifeless. There are a number of compelling arguments for colonizing Mars: to combat overpopulation on Earth, to act as failsafe to preserve the human race in event of mass extinction, etc. There are strong counterarguments as well; protecting possible Martian life is one example. These debates are important and worth having, but before they can be settled we must know: can we terraform Mars, and how practical is it?
At present, Mars lacks many of the traits of Earth that make it habitable. Mars has a surface gravity 38% the strength of Earth's. The surface pressure of its atmosphere is less than 1% that of Earths, and its average temperature is -63°C.  Mars' atmosphere is comprised overwhelmingly of carbon dioxide, with only trace amounts of nitrogen and oxygen. Mars also has no magnetic field, likely because its core has cooled to the point it no longer contains liquid metal.  All of these traits will need to be manipulated to allow Mars to sustain life. It is likely that the low surface gravity will have no long term effects on humans, though that is not certain. The primary goal of terraforming is to increase the temperature and pressure of Mars' atmosphere such that humans can survive on the surface without a pressurized suit. An additional, tangential goal is to modify the composition of Mars' atmosphere to be breathable by humans. A final, often overlooked necessity is to construct some sort of shield to protect inhabitants from solar radiation; most likely a magnetic field or some atmospheric modification (or both).
|Fig. 2: Data from the Mars reconnaissance orbiter detecting carbon dioxide ice deposits on Mars' south pole. (Courtesy of NASA)|
The question remains: how does Elon Musk's plan to bomb Mars fit in with these goals? His reasoning arises from a simplified version of the terraforming problem: what is the fastest way to inject energy into the Martian atmosphere? In that way, Musk deserves credit for his proposal not being the worst idea; however, there are better proposals that involve more passive heating of the planet. It is fortunate that the planetary physics of Mars means humans wouldn't have to do all the work themselves. Ironically, the effect that is most endangering the Earth may be the key to making Mars habitable: the greenhouse effect. Mars has a significant amount of frozen carbon dioxide at its poles, though experts differ on the exact amount.  Fig. 2 shows a recent discovery from the Mars Reconnaissance Orbiter of a large carbon dioxide deposit on Mars' south pole, which could be sublimated to ignite a greenhouse effect on Mars. Evaporating enough of this frozen CO2 could lead to a runaway warming effect. Some scientists estimate a change as small as 4°C is enough to kickstart this warming. This is the motivation behind Elon Musk's proposal; however, it is hardly practical. The largest weapons in the US arsenal is the B-53, with a yield of 9 megatons of TNT (or 38,000 TJ).  To maintain a power of 27 TW at one of the poles (necessary to achieve the desired heating), a weapon would have to be detonated every 20 minutes.  This is assuming that all the energy from the weapon goes into the atmosphere, which is a weak assumption; the real required frequency may be higher. This would need to be conducted on a timescale of decades. Considering that these weapons cost of order $10 million (optimistically), conducting this procedure over 50 years would cost approximately $15 trillion dollars, before even considering the cost of the rockets to get the bombs there.  This number is comparable with the GDP of the United States! Other proposals are less flashy, but are probably more practical.
Other proposals involve intentionally burning asteroids in the atmosphere and building orbital mirrors to reflect more sunlight to the surface. Asteroid collisions are similar in spirit to the plan described above. Several asteroids would be directed toward the planet and intentionally burned up in the atmosphere. Instead of the energy coming from nuclear fusion, the gravitational potential energy of the asteroids is converted into heat. Burning up one asteroid with mass 1015 kg would deposit 1.265 × 1022 J of energy into the system; that is equivalent to 330,00 B-53 bombs as described above! It is also possible to get more out of this procedure by importing asteroids comprised mainly of ammonia ice. Ammonia is a powerful greenhouse gas, much more powerful than CO2. In addition to heating from the collision itself, the burned off gases would accelerate the planet's greenhouse effect. These effects combine such that only a dozen or so collisions would be required to completely heat the planet!  The major cost of this procedure is nudging the asteroids onto a collision course with Mars. If orbital mechanics and gravity slingshots are appropriately used, the "nudge" required can be as small as 300 m/s. For the hypothetical 1015 kg asteroid above, that would require 4.5 × 1019 J of energy. This is a pretty significant amount of energy, but it may be possible to use part of the asteroid's nitrates as fuel.  The other glaring concern of this approach is its destructive potential. It is likely not a major concern while the plant is uninhabited, but if more heating needs to be applied while it is being colonized it will likely not be possible to safely impact an asteroid with the planet.
Constructing an orbital mirror to reflect more sunlight to the planet's surface is another elegant terraforming proposal. In order to achieve the amount of heating required, the mirror would have to quite large: at least 400 km across.  It is most likely to be made of aluminum, which would give it a mass of approximately 200,000 tons. This is far too large to launch from the Earth; it would only be practical to build it in space, with materials mined from asteroids or Mars' moons. There are a lot of unknowns with that sort of procedure, but researchers estimate that construction would require only 3 PJ, the smallest energy requirement of any proposal suggested so far. And once the mirror is in place, it will stay fixed, balanced between radiation pressure and gravitational forces. The fact that is heating is completely passive makes it very appealing. Both this technique and asteroid collisions are technically possible with current technologies, but they are definitely very expensive. It is difficult to make realistic cost estimates, as there is so much that is still theoretical.
Modifying the temperature and pressure of Mars is (relatively) easy to do. Other modifications are much more challenging, but essential to the goal of total Martian terraforming. The above techniques would allow humans to walk freely on the surface without a pressure suit; they would still need a breathing apparatus, as the atmosphere would still be toxic. There are very few proposals for making the atmosphere hospitable for humans, because it is very challenging. It is more or less impossible to transport enough oxygen to the planet, as there are no significant reserves in the solar system. The leading plan for this process is to use plants. Once the planet is warm enough, plants could thrive in the CO2 rich atmosphere, and slowly convert the CO2 to oxygen. The timescales involved are thousands of years.  Fortunately, this process is more or less automatic and passive. Sadly, our grandchildren will not be breathing Martian air anytime soon.
Another essential but challenging aspect of terraforming Mars is the construction of some sort of UV shield. Since Mars lacks both a magnetic field and an ozone layer, residents would be at risk from ultraviolet rays and solar flares. It is technically possible to artificially create a magnetic field of similar magnitude to the Earth's. Some scientists claim that clever exploitation of superconductors can tamp down the power requirements to a "modest" 1 GW; however, it would involve encircling the planet with rare superconductors, an expensive procedure indeed.  In addition to the magnetic field requirements, some sort of atmospheric shield similar to the ozone layer would need to be made. Fortunately, many gases that would be suitable as greenhouse gases double as UV shields, but their lifetimes are very short. They would need to be constantly replenished in order to properly shield inhabitants.  It is very likely that life on Mars will always hazardous for colonists due to space weather.
Terraforming the red planet is an interesting and exciting idea. However, there are many aspects involved, and unfortunately most research has been directed at which problems are easily solvable. There are several plans in place to warm the planet and increase the pressure of it's atmosphere. However, there are far fewer plans for building a shield from dangerous space weather, or oxygenating the planet. These concepts are less flashy or elegant as bombing the planet, but they are just as essential. Until these missing pieces are solved, it is difficult to see full Martian terraforming as anything other than science fiction.
© Sean McLaughlin. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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