Excerpted from Beyond: Our Future in Space, by Chris Impey (W. W. Norton & Company, 2016). Reprinted with permission from the author.
Greening the Red Planet
Let’s ignore for a moment the evil twin. Venus is closest to the Earth in size and mass, and it has the same inventory of carbon dioxide. But on our planet most of the carbon dioxide is built into rocks and dissolved in the oceans, making them mildly acidic and leaving a moderately thick atmosphere to smooth out daily and seasonal temperature variations.
On Venus, only 30 percent closer to the Sun, the carbon dioxide built up in the atmosphere, triggering a runaway greenhouse effect and raising the surface temperature to a level where lead melts. Whoever named Venus after the goddess of love had a sad history of relationships.
Mars is the misbegotten sibling, the runt. It’s half the size of Earth with a third of the gravity. The next nearest Earth-like planet is tens of trillions of miles away, and unreachable with any current technology. The siblings went on divergent paths. One rusted and turned red, the other got the spark of life and turned green. Mars suffocated and dried out as its water and air leached into space, and it became scoured by dust storms and cosmic rays. Yet it’s at the edge of habitability, not much more inhospitable than a volcanic vent or a plateau in the high Andes. Mars has sunlight and reservoirs of water, carbon, nitrogen, and oxygen. One planet lived and the other died.
Perhaps we can make it live again?
One of the most audacious ideas in science is planetary engineering. Planets don’t stay the same. Geological evolution, combined with the aging of their parent star, can render a wasteland habitable and an Eden uninhabitable. This evolution occurs on geological timescales of hundreds of millions or billions of years.
Here’s how the Earth has changed. It formed 4.5 billion years ago and minerals show that there was liquid water within 100 million years, so conceivably life started then. If it did, it must have survived the “Late Heavy Bombardment” 3.9 billion years ago, when unstable orbits in the Solar System led to a surge of meteor impacts. Life around that time was limited to prokaryotes, or cells without nuclei, and there was no oxygen in the atmosphere. Around three billion years ago, bacteria evolved that produced oxygen as a waste product, which is poisonous to other kinds of bacteria. The oxygen content of the atmosphere rose 1.9 billion years ago and facilitated the evolution of eukaryotes, or cells with nuclei. Life diversified as it became multicellular and began to reproduce sexually. Dramatic episodes of glaciation almost obliterated life 2.7 billion and 700 million years ago. In the last 10 percent of the chronology, life finally became big enough to see without a microscope, plants and animals evolved, they moved onto the land, and a crescendo of evolution led to mammals, primates, and finally us. Dramatic change is normal for a biological world.
More recently, we have been inadvertently altering our own planet through industrial growth and the use of fossil fuel. “Terraforming” is the process by which we might potentially alter a different planet to make it more Earth-like or habitable by terrestrial life forms.
The first step would be to raise the temperature on Mars just enough to release frozen carbon dioxide from the polar regions, triggering a runaway greenhouse effect. The positive feedback of this effect favors terraforming. While the carbon-dioxide atmosphere of Mars has only 1 percent of the pressure of the Earth’s atmosphere at sea level, there is enough carbon dioxide frozen in the soil to raise the pressure to 30 percent of the Earth’s. Robert Zubrin and Chris McKay have outlined several ways to accomplish this. Chris McKay is a NASA astrobiologist who believes we have an obligation to seed life on planets that might be habitable. One strategy is to fabricate a 100-kilometer mirror to direct extra sunlight toward the poles. Even if made from aluminized Mylar, such a mirror would weigh 200,000 tons. Being too heavy to launch from Earth, this would have to be constructed from materials refined on Mars.
Another method is to produce efficient heat-trapping gases on Mars, using industrial-scale facilities. There’s a rich irony in using methods to make Mars habitable that are in danger of rendering the Earth uninhabitable. These two methods would each use as much energy as a city like Denver or Seattle, and they would need hundreds of workers to implement. A clever, less costly idea is to redirect small asteroids to impact the surface of Mars. Carbon dioxide would be liberated by heat energy from the impact, and asteroids can deliver ammonia (a very efficient greenhouse gas) and dust, which will cause Mars to absorb more sunlight.
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The next step is to activate a hydrosphere: raise the temperature by an additional amount sufficient to allow liquid water on the surface. Although still inhospitable, these conditions would allow extremophile microbes such as lichen, algae, and bacteria to be established. Their role is to prepare the regolith for photosynthetic organisms. Microbes used for this will be engineered to be optimally suited for their job. If the heating is done with asteroid impacts, these first two steps might take two to three hundred years.
The last step is to add oxygen to the atmosphere. Since oxygen is flammable, care would have to be taken to also add a buffer gas like nitrogen. Brute force would have to be used to import or create the initial oxygen needed for primitive plants, but when more advanced plants can propagate, they become the engine for oxygen production. It would take 500 to 1,000 years to make an atmosphere suitable for animals or humans.
Terraforming may be possible and it’s exciting at a technical level, but to see life breathed into the idea, we can turn to fiction. Kim Stanley Robinson wrote a science fiction trilogy in the mid-1990s about an overpopulated and dying Earth and the “First Hundred,” a pioneering group of Mars colonists. The books capture the ethical issues we’ll face if we go there, telling of the tensions between the Reds who prefer to leave Mars in its pristine state and the Greens who want to turn the planet into a second Earth.
The storytelling is very entertaining, but the physical descriptions are beyond evocative; they’re mesmerizing. Who wouldn’t want to visit Mars after reading this excerpt from Red Mars, the first book in the trilogy: “The sun touched the horizon, and the dune crests faded to shadow. The little button sun sank under the black line to the west. Now the sky was a maroon dome, the high clouds the pink of moss campion. Stars were popping out everywhere, and the maroon sky shifted to a vivid dark violet, an electric color that was picked up by the dune crests, so that seemed crescents of liquid twilight lay across the black plain.”
 Reading the Rocks: The Autobiography of the Earth by M. Bjornerud 2005. New York: Basic Books; and Life on a Young Planet: The First Three Billion Years of Evolution on Earth by A. H. Knoll 2004. Princeton, NJ: Princeton University Press.
 “Technological Requirements for Terraforming Mars” by R. M. Zubrin and C. P. McKay 1993, technical report for NASA Ames Research Center, online at http://www.users.globalnet.co.uk/~mfogg/zubrin.htm.
 Red Mars by K. S. Robinson 1993 covers colonization; the quote in the following paragraph is from p. 171. Green Mars by K. S. Robinson 1994 covers terraforming. Blue Mars by K. S. Robinson 1995 covers the long-term future of human habitation. All are published by Random House (New York).
Excerpted from Beyond: Our Future in Space by Chris Impey. Copyright © Chris Impey, 2016. All rights reserved.
Chris Impey is a University Distinguished Professor of Astronomy and Associate Dean of the College of Science at the University of Arizona. He has over 180 refereed publications on observational cosmology, galaxies, and quasars, and his research has been supported by $20 million in NASA and NSF grants. He has won eleven teaching awards, and has taught two online classes with over 130,000 enrolled. Dr. Impey is a past Vice President of the American Astronomical Society and he has been an NSF Distinguished Teaching Scholar, Carnegie Council’s Arizona Professor of the Year, and most recently, a Howard Hughes Medical Institute Professor. He has written over 50 popular articles on cosmology and astrobiology, two introductory textbooks, a novel, and eight popular science books.
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