Terraforming

In 1836, Charles Darwin stepped onto Ascension Island — a volcanic cinder in the middle of the South Atlantic, 1,600 km from Africa and 2,250 km from South America — and found it almost completely barren. Twenty-five species of small plants, a flightless bird, a land crab. "We know we live on a rock," the residents of neighboring St Helena told him, "but the poor people of Ascension live on a cinder." But Darwin saw something in that cinder. He goaded his friend Joseph Hooker, the intrepid botanist, to convince the Royal Navy to start shipping trees to the island. The idea was breathtakingly simple: trees would capture rain, reduce evaporation, and create soil. The cinder would become a garden.¹

Beginning in 1850, ships deposited motley assortments of plants from botanical gardens in Europe, South Africa, and Argentina. By the late 1870s, eucalyptus, Norfolk Island pine, bamboo, and banana had all run riot on the island's highest peak. Today that peak — Green Mountain, 859 meters — is covered in lush tropical cloud forest, where trees capture sea mist and create a damp oasis amid the surrounding aridity. Species that would never coexist in nature grow side by side. It's a fully functioning, self-sustaining ecosystem, assembled from scratch by the Royal Navy in a matter of decades.²

This is, as Dave Wilkinson of Liverpool John Moores University put it, "really exciting" — because ecosystems this complex normally develop over millions of years through co-evolution. Ascension's cloud forest was cobbled together by trial and error. No one planned which species would thrive together. They just planted everything and let the ones that worked stay. What this tells us is that you don't need to engineer every interaction. You can seed a barren landscape with diverse organisms and let ecological fitting — species finding their own niches in a novel community — do the work. That's a profoundly different approach from the control-everything mentality that dominates most engineering.²

From Islands to Planets

The Ascension experiment is, in essence, the world's first terraforming project. The word itself was coined by science fiction writer Jack Williamson in 1942, but the concept — modifying a hostile environment to support Earth-based life — has been taken increasingly seriously since Carl Sagan proposed seeding Venus's atmosphere with algae in 1961. In 1976, NASA published "On the Habitability of Mars," concluding that photosynthetic organisms, polar ice cap melting, and greenhouse gases could create a warmer, oxygen-rich atmosphere. By the 1980s, biophysicist Robert Haynes was promoting "ecopoiesis" — the fabrication of a self-sustaining ecosystem from a sterile planet — as a staged process: first seed with microbes, then plants, then animals.³

The connection between Ascension and Mars is more than metaphorical. Both are cases of primary succession — the process by which lifeless land becomes a functioning ecosystem. On Earth, this happens naturally after volcanic eruptions or glacial retreat: first microbes, then lichens, then hardy plants, then animals, over centuries. Ascension fast-forwarded the process by skipping to the plant stage, which only worked because Earth already had an oxygen atmosphere, a hydrological cycle, and favorable temperatures. Mars has none of those.⁴

Sarah Goslee, writing in Clarkesworld, lays out the stages a Mars terraforming project would need. First, physical engineering: release CO2 from the polar ice caps to trigger a greenhouse effect and raise surface temperatures until liquid water can exist. Then biological engineering: seed with cyanobacteria, lichens, and cold-tolerant photosynthetic organisms to start converting CO2 into oxygen and rock into soil. Eventually mosses, then insects, then flowering plants. The timescale is centuries.⁴

The parallel to Biogeochemistry is direct. Earth's own atmosphere is a product of biological terraforming — cyanobacteria converted an anoxic planet into an oxygen-rich one over two billion years. The difference is that Earth's transformation was accidental and incredibly slow, while Mars would be deliberate and (relatively) fast. Whether "relatively fast" means centuries or millennia is an open question. As Goslee notes, "right now ecologists don't know nearly enough even to build community types here on Earth where the environment is already hospitable." We're better at destroying ecosystems than building them.

The Venus Problem and Other Targets

Mars gets most of the attention, but Venus is in some ways the more interesting target. It's Earth's twin in size and mass, sitting at the inner edge of the habitable zone, with a surface temperature of 462°C and an atmospheric pressure 90 times Earth's — a runaway greenhouse effect turned up to eleven. The proposals for Venus are correspondingly dramatic: hydrogen bombardment to convert atmospheric CO2 into water and graphite, calcium and magnesium injection to sequester carbon as carbonite, or simply blasting the atmosphere into space with asteroid impacts.³

My favorite Venus idea is the solar shade — placing a massive reflector at the Sun-Venus L1 Lagrange point to cut sunlight and cool the planet. It's elegant because it addresses the root cause (too much solar energy) rather than the symptoms (too much CO2). But it also highlights the scale of the challenge. Venus's atmosphere is 96.5% CO2 at 90 bar. You'd need to remove the equivalent of Earth's entire ocean in CO2 mass. Even with a solar shade, the chemical processing required is staggering.

The outer solar system offers a different kind of challenge. Jupiter's moon Europa and Saturn's moon Titan both have abundant water ice and potentially subsurface oceans. Europa might already harbor life around hydrothermal vents. Ganymede is larger than Mercury and has its own magnetosphere. The problem is energy: these moons receive a tiny fraction of the sunlight Earth gets, so any terraforming would need to either redirect sunlight (orbital mirrors) or find alternative energy sources.³

The Ethics of Making Worlds

Kim Stanley Robinson's Mars trilogy — Red Mars, Green Mars, Blue Mars — remains the most serious fictional treatment of terraforming, and it's notable for taking the ethical questions as seriously as the engineering ones. The preservationist camp argues that even a lifeless planet has intrinsic value and shouldn't be altered. The anthropocentrist camp sees spreading humanity as an evolutionary imperative. Between them sit the zoocentrists (who only value organisms capable of feeling) and the ecocentrists (who value all life but not inorganic nature).⁴

The strongest case for terraforming is existential risk: putting humanity on a single planet is keeping all your eggs in one basket. The strongest case against is humility: we don't understand Earth's ecosystems well enough to reliably manage them, let alone build new ones from scratch. The Ascension experiment offers both hope and caution on this front. Hope, because it worked — a functioning ecosystem was assembled in decades. Caution, because it worked largely by accident, and nobody can explain exactly why those particular species cooperated instead of competing themselves to death.²

There's a deeper question that I think most terraforming discussions skip past: what counts as "working"? Ascension's cloud forest is thriving, but it's nothing like any natural ecosystem. It's a novel community of species that never evolved together, functioning under rules that ecologists don't fully understand. If we terraform Mars and end up with something similarly functional but deeply alien to any Earthly ecosystem, have we succeeded or failed? The answer probably depends on whether your goal is to reproduce Earth or to create something that sustains human life. These are very different goals, and the tension between them is where the most interesting decisions will need to be made.