The Thorny Ethics of Planetary Engineering

Exploration, habitation, and resource extraction all carry a risk of inflicting environmental damage in space, just as they do here on Earth. But some futurists and space settlement enthusiasts have proposed an even more drastic alteration of the space environment: the transformation of the surface of a planet or moon into a more Earth-like environment via a process known as terraforming.

The atmospheric chemistry, pressure, and temperature inside an artificial space habitat is, by design, Earth-like enough to be habitable by humans, but it requires enclosure by pressurized walls and constant maintenance. Terraforming would affect the entire surface of a planet, rather than just a smaller “indoor” region, and by planetary scientist Christopher McKay’s definition, the environment of a terraformed planet “must be stable over long time scales and must require no, or a minimum of, continued technological intervention.” After an initial input of energy and effort, a terraformed environment would behave like Earth’s natural environment and essentially maintain itself.

For example, in 1961, Carl Sagan speculated on the possibility of the “microbiological re-engineering” of Venus by introducing blue-green algae into its atmosphere. The algae would use photosynthesis to convert the planet’s abundant carbon dioxide into oxygen, which would also reduce the greenhouse effect and lower Venus’s surface temperature. Sagan later turned his attention to the potential for “re-engineering” Mars, a planet now considered to be one of our best candidates for successful terraformation. Mars has the opposite problem as Venus: Instead of harboring a thick, toxic atmosphere with a runaway greenhouse effect maintaining deathly high temperatures and pressures at the surface, Mars lost nearly its entire original atmosphere to solar wind, leaving surface pressures so low that liquid water cannot exist. To terraform Mars, planetary engineers would need to increase its surface temperature and atmospheric pressure while protecting the atmosphere from solar wind. Sagan suggested spreading a dark material, or even growing dark-colored plants, on Mars’s polar ice caps, allowing them to absorb more of the Sun’s heat, increasing the surface temperature while releasing water vapor and carbon dioxide into the atmosphere. Other researchers have explored the feasibility of importing greenhouse gases or building giant orbital mirrors to increase Mars’s surface temperature, constructing a magnetic shield to protect Mars’s atmosphere, and releasing genetically engineered microbes onto the planet’s surface to alter the atmospheric and surface chemistry.

Terraformation is the ultimate example of long-term planning, as even optimistic estimates predict that it would take centuries of effort and patience before a human could walk unprotected on the surface of Mars. Advocates of terraforming Mars or other space environments see it as a crucial step toward creating a truly multi-planet civilization. Robert Zubrin, the founder and president of the Mars Society, an organization that advocates for human Mars exploration and colonization, even claims that the successful terraforming of Mars would demonstrate humanity’s superiority over the physical world: “The first astronauts to reach Mars will prove that the worlds of the heavens are accessible to human life. But if we can terraform Mars, it will show that the worlds of the heavens themselves are subject to the human intelligent will,” he writes in his 1996 book “The Case for Mars.”

Whenever someone waxes poetic about humankind bending the universe to our will, it’s worth taking a moment to consider the ethical implications of the proposal. One major consideration about terraforming is that the process could damage or even wipe out any existing life on the planet being terraformed. If an alien microbe evolved on Mars, it probably would not survive in a more Earth-like environment, so by transforming Mars’s surface into Earth’s, we might exterminate species or entire ecosystems without even detecting their existence. The changes we would make to a cold, dry, relatively airless world like Mars would also introduce physical processes — such as wind, flowing water, and new chemical reactions — that could easily erase or contaminate any evidence that extraterrestrial life ever existed on the surface. If we allow planetary engineering to race ahead of astrobiological research, we could miss our opportunity to make what would be the most important scientific discovery in human history: the discovery of life that evolved beyond our planet. We also risk exterminating the very lifeforms we dream of discovering.

The ethical dilemma of terraforming far exceeds planetary protection concerns about forward contamination by a lander or even a human settlement. The goal of terraforming is to intentionally create an entire ecosystem on a global scale, which would more than likely destroy any existing ecosystem. Terraforming technology might even become feasible before we definitively determine whether extraterrestrial life exists on the planet or moon that we hope to transform. But suppose we do discover evidence of existing microbial life on a planet like Mars. Should this disqualify Mars as a target for terraforming? Should we avoid settling on Mars at all?

Carl Sagan, in his book “Cosmos,” famously argued for exactly this stance: “If there is life on Mars, I believe we should do nothing with Mars. Mars then belongs to the Martians, even if the Martians are only microbes. The existence of an independent biology on a nearby planet is a treasure beyond assessing, and the preservation of that life must, I think, supersede any other possible use of Mars.” Planetary scientist Christopher McKay even argues that if microbial life is discovered on Mars, humans should not simply leave Mars to the microbes, we should “undertake the technological activity that will enhance the survival of any indigenous Martian biota and promote global changes on Mars that will allow for maximizing the richness and diversity of these Martian life forms.” In other words, we should engineer the surface of Mars not to improve its habitability for terrestrial life, but for Martian life!

Space ethicist Kelly Smith finds these types of arguments, that humans should avoid worlds where microbial life might already exist, difficult to defend. “You have to first grant that microbes, as a class of organisms, are somehow on the same level with human beings,” he told me in 2018. “I’m not saying you can’t make an argument to that effect, but it really stretches credulity. It’s an uphill battle.” After all, humans have already demonstrated that we are willing to intentionally eradicate disease-causing viruses like smallpox to prevent human death and suffering. Admittedly, viruses are not unequivocally considered to be “alive,” and there were some ethical concerns during the development of the vaccine that smallpox eradication represented a “new form of genocide.” But given the opportunity, humans would likely jump at the chance to exterminate deadly microbial species like the bacterium that causes cholera or the parasite that causes malaria. Unlike these terrestrial microbes, however, hypothetical Martian microbes currently pose no danger to humanity, or even to individual humans. They may merely someday stand in the way of our off-Earth expansion. Space settlement advocates argue that such an expansion is vital for humanity’s long-term survival, but does this potential for indirect harm justify their extinction?

It may seem premature to debate the ethics of using a technology that does not yet exist to indirectly destroy an ecosystem that may not exist at all. But our potential for inadvertently exterminating a unique species or ecosystem in space might arise long before we develop the technology to terraform entire planets. By the time we come to an agreement about the ethics of terraforming and planetary protection, it might be too late.

Erika Nesvold, an astrophysicist, has worked as a researcher at NASA Goddard and the Carnegie Institution for Science. She is a developer for Universe Sandbox, a physics-based space simulator; cofounder of the nonprofit organization the JustSpace Alliance; the creator and host of the podcast Making New Worlds; and author of “Off-Earth,” from which this article is excerpted.