Terraforming is the concept of engineering planets to make them habitable for terrestrial life. This idea was born out of fiction. Olaf Stapledon, in his 1930 novel Last and First Men, first described a project undertaken to make Venus suitable for humans which began by placing photosynthetic plants on the surface to release oxygen. In 1942, Jack Williamson coined the term “terraforming” in his short story "Collision Orbit."  In 1961, Carl Sagan wrote the first academic paper on terraforming and published it in Science, moving the terraforming to the realm of serious scientific pursuit.

What are the basic requirements for life? What primary specifications should we give a terraforming engineer? There are two primary organs on Earth necessary to support life.

  1. Atmosphere. An atmosphere containing oxygen is necessary to support aerobic life (like animals), while the presence of carbon dioxide is necessary to support anaerobic and semiaerobic life (like some microbes and plants, respectively). In addition to supplying metabolites, Earth’s atmosphere acts as an insulator, trapping energy from the sun using the greenhouse effect and heating the planet. It also protects against ultraviolet and cosmic radiation that can damage organic material (for example, inducing DNA damage; this is why exposure to direct sunlight can cause cancer). Oxygen and ozone absorb wavelengths shorter than 300 nm.

  2. Liquid Water. Liquid water can only exist within a certain range of temperature and pressure. This means that the surface temperature must exceed 273 K (0.01 °C) and the atmospheric pressure must exceed 610 Pa (.6% of Earth’s atmospheric pressure at sea level). Simultaneously, the temperature and pressure should not surpass the critical point, 647 K (374 °C) and 22.064 MPa (218 times Earth’s atmospheric pressure at sea level).  

With these two requirements in mind, planetary engineers can begin searching for worlds to transform. Since the nearest star, the red dwarf Proxima Centauri, has no known companions and is 4.22 light years from Earth, making travel within a single generation impossible with current propulsion technology, perhaps we should look closer to home. Indeed, we have two candidates for terraforming in our own solar system: Venus and Mars. Current conditions on both these planets are inhospitable, but great scientific minds have considered their potential.

Venus is similar in size to Earth, but it has a dense atmosphere composed primarily of carbon dioxide with a pressure of 9.122 MPa, about 90 times greater than that on Earth, and an average surface temperature of 465 °C. Carl Sagan proposed a method for terraforming Venus in his seminal paper. Microorganisms would metabolize carbon dioxide in the atmosphere, fix carbon, and expel oxygen. Removing carbon from the atmosphere would reduce the greenhouse effect, and thus reduce the temperature. Unfortunately, this plan is not feasible, partially because Venus’s atmosphere does not contain vaporized water, as was suspected at the time. In addition, the process Sagan proposed would leave the planet extremely flammable; a thick layer of carbon, fixed in the form of graphite, would coat the planet and a purely oxygen atmosphere would replace the carbon dioxide. If this new world caught fire, the graphite would oxidize to reform a carbon dioxide atmosphere. I think that the issue of a purely oxygen atmosphere could be combated by the introduction of aerobic organisms after a sufficient concentration of oxygen has accumulated.

In Our Cosmic Future, astrophysicist Nicolas Prantzos reviews other ideas on how to make the Venus atmosphere friendlier for humans. Impacts from asteroids exceeding 700 km in diameter can expel almost one thousandth of Venus’s atmospheric mass into space, but thousands of asteroids would be needed to make the needed difference. Physicist Freeman Dyson theorized that a reflective panel ten times the diameter of Venus could block solar energy, cooling the planet and allowing carbon dioxide to condense into the liquid phase. But then the oceans of carbon dioxide would need to be incorporated into solids and prevented from evaporating again while a small portion of it is used by photosynthetic organisms to produce oxygen, and water would need to be imported from the moons of Saturn and Jupiter.

Terraforming projects will likely begin on the much friendlier Mars long before they begin on Venus. First of all, the Martian day is only about 35 minutes longer than an Earth day, making it easy for circadian rhythms to adapt, and photosynthesis can occur there despite the fact that the Martian surface only receives 43% of the sunlight that Earth’s surface receives. Also, strong evidence shows that liquid water previously flowed on the surface, and that large amounts of ice exist under the surface near the south pole. Like Venus, carbon dioxide comprises most of the atmosphere. Unlike Venus, the Martian atmosphere also contains oxygen and water vapor, is 100 times less dense than Earth’s, has a pressure of .7 kPa (.7% of that on Earth), and has an average temperature of -60 °C. Mars already contains the basic building blocks of life; carbon, nitrogen, hydrogen, and oxygen all exist in usable forms, although the quantities of these components remain unknown. Microorganisms may even live on Mars today; one study found that certain methanogens can grown in a simulated Martian soil if fed carbon dioxide, molecular hydrogen, and varying amounts of water. Carbonate globules, magnetite, and complex hydrocarbons, all likely of organic origin, embedded within a meteorite of Martian origin suggest a history of life on Mars.

The first challenge in creating a new history for life on Mars comes in the form of atmospheric temperature, which must be increased about 60 ºC. In their famous 1991 review, McKay et al. suggest that we may begin this process by introducing chlorofluorocarbons (CFC’s) into the atmosphere until frozen carbon dioxide (dry ice) in the polar caps and the regolith (the soil layer) begins to sublimate. Later, it was suggested to use perfluorocarbons (PFC’s) instead of CFC’s because they would have a less harmful effect to any future ozone and they could be produced on the surface while CFC’s would need to be transported. The PFC’s act as super-greenhouse gasses, and at levels of parts per million, could trap enough energy from the sun to raise the average atmospheric temperature by 20 ºC. Then, a run-away greenhouse effect will occur as carbon dioxide sublimates, increasing the greenhouse effect, and leading to increased sublimation and increased heating. As little as a 4 ºC increase in temperature at the poles may be enough to induce this change and eliminate the polar dry ice caps. This positive feedback process will end only when the supply of surface carbon dioxide, an amount unknown today, is exhausted and the regolith can no longer release stored carbon dioxide. The resulting atmosphere would be one that closely resembles that hypothesized to exist on early Earth: warm and carbon-rich, an ideal breeding ground for anaerobic microbes. While many have thought the Martian atmosphere to be in stable equilibrium, where small deviations in temperature and pressure are corrected by negative feedback mechanisms, Mars may actually be undergoing global climate change, as evidenced by shrinking surface ice and carbon dioxide reservoirs in mid-latitude regions.

With these changes already occurring, human intervention may not need to occur on the scales previously hypothesized; smaller a push towards increased temperatures and pressures may be needed to induce a runaway carbon dioxide accumulation. Other hypothesized methods of heating Mars and initiating the run-away greenhouse effect include pointing massive mirrors at the polar caps, seeding the atmosphere with heat-absorbing dust or volatiles from comets, and altering the orbit, spin axis, or precession cycle to increase solar intensity and flux. While the technologies to accomplish these feats remain in the realm of science fiction today, the future may see their use as terraforming tools.

After releasing PFC’s and carbon dioxide and warming the atmosphere so that liquid water can exist, a process that McKay and Marinova estimated to take about 100 years (C. P McKay&Marinova, 2001), Mars, now with an atmospheric pressure of 10-40 kPa and an average surface temperature of -10 ºC, may be suitable for the seeds of life. The ecosynthesis of Mars has been likened to the ecological changes that occur with decreasing latitude or altitude. In the early stages, organisms that can survive on Mars will be like those that currently live in the Dry Valleys of Antarctica. These organisms thrive just below the surface of rocks, where the environment is warmer and wetter, but enough light for photosynthesis can still penetrate. Photosynthetic anaerobes from this region, like the primitive cyanobacterium Chroococcidiopsis, will likely pioneer the colonization of Mars. Cyanobacteria, in addition to green algae and lichens also found in the Dry Valleys, will establish a nitrogen cycle, taking nitrates in the regolith and converting them to diatomic nitrogen to expel into the atmosphere. The presence of gaseous nitrogen as a “buffer” is critical for respiration in terrestrial animals because high levels of both carbon dioxide and oxygen have toxic effects.

These anaerobic organisms would also fix carbon and expel oxygen. Because the surface of Mars is already oxidized, any oxygen produced by colonizing microbes would enter the atmosphere rather than be absorbed by the planet. In fact, as the planet warms, the oxygen stored in the regolith will bubble out and enter the atmosphere. This will make the transition from an anaerobic plant to an aerobic plant much shorter than the transition that occurred on Earth.

With warming temperature (above freezing for part of the year) and increased oxygen, a polar desert ecosystem could form. In this ecosystem, bryophytes, relatively simple green plants like mosses, will dominate. Over the course of a few hundred years, these mosses would sequester carbon dioxide and produce more oxygen. Eventually, atmospheric conditions would reach a balance of carbon dioxide and oxygen at which higher plants could photosynthesize and grow more efficiently than bryophytes. At this stage, grasses and other flowering plants similar to those in cool polar regions on Earth would begin to dominate the ecosystem. Then, as oxygen levels and temperature continue to increase, more flowering plants and shrubs from mid polar regions would arrive, and eventually trees like birch, willow, and spruce from boreal forests would call Mars home.

Finally, when temperatures in equatorial regions exceed 10 ºC for at least 120 days per Earth year, temperate forests and grasslands like those of North America could survive on Mars. At this time, the beginnings of agriculture could be established, as well, although low levels of oxygen would prevent terrestrial animal life for many years more.

By the time that Mars becomes habitable for humans, thousands, perhaps even tens or hundreds of thousands, of years may have passed. Even faced with this time scale, Mars remains the best starting option. Mercury is too close to the sun for it to provide a stable home, and the outer planets are gaseous so they cannot support life. The moons of the great planets may have potential; Jupiter’s moon Europa contains a liquid ocean layer underneath a frozen shell and Io has enough internal heat to warm the atmosphere. These moons and other bodies in our solar system may be the next stops for life on Earth following the colonization and terraforming of Mars and Venus. One day, life with its origins on Earth may even venture out into interstellar space and other solar systems.

Before initiating terraforming events on any body, however, we would have to first get there and colonize. Currently, projects include directed research into the possibility of travelling to Mars under modern technology, and improving technology to make the journey more comfortable for astronauts. A study on living conditions for the approximately 1.5 year journey has just been initiated. Upon reaching Mars, a self-sustaining colony of engineers would need to be established on or near the surface. This step alone requires technology that does not currently exist.

Terraforming is about more than simply creating new homes for the human race to expand its empire; it is about learning more about our solar system, physics, geology of other planets, Earth’s own geology, evolution, biological capability, and biological diversity. It’s about pushing the perceived boundaries of human capability and science. Robert Zubrin, aerospace engineer and Mars exploration advocate, says in his book The Case for Mars that “failure to terraform Mars constitutes failure to live up to our human nature and a betrayal of our responsibility as members of the community of life itself.” Zubin’s words have more than a twinge of manifest destiny, though. What right do we have to invade foreign lands and repurpose them for our own use? Does it matter if life already exists on these bodies? Should lifeforms reside on bodies we wish to terraform, should we respect their habitat? Should we respect native microbial populations the same way we should respect animal or plant life? Would terraforming somehow destroy any inherent worth in an abiotic world?

Extending the environmental ethics system used on Earth leaves much to be desired, but astrobiologists McKay and Marinova argue that a biologically active Mars would have more inherent value than the seemingly dead planet that exists today based solely on the idea that biodiversity and its well being has inherent worth independent of human usefulness. They also argue that the knowledge gleaned about biospheres and planetary sciences under the wise stewardship ideology outweigh anti-humanism concerns. McKay and Marinova suggest that reviving any previous Martian life or fostering the evolution and diversification possible existing lifeforms is the healthiest option for Mars; however, they concede that this is unlikely, and declare seeding with human life the next best option.

Regardless of the ethical implications, human curiosity has pulled us into space throughout history, whether in the form of myths or space shuttles. In its own sort of feedback loop, human curiosity will continue to propel technological advances that will propel us deeper into space and force us to confront more questions. The concepts of interplanetary travel and terraforming are moving off the TV and movie screens, out of the books, and into our night sky. Imagine, one day your descendants may look at the Evening Star and see not you looking down on them, but their living cousins waving to them from the fertile promised lands of Venus.

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