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Every story about humankind’s colonization of space, as well as much of current thinking on the subject, depends on the assumption that we will either find a planet near enough like Earth that we can live on it, or we can find a suitable candidate and create Earthlike conditions by “terraforming.”
Finding a New Earth
The odds are pretty slim. Consider that we humans are exquisitely adapted through evolution to live on the planet where we evolved. Not a surprise, because all living creatures adapt and optimize their structure and chemistry to their environment.
But we can’t even live on the seventy percent of the Earth’s surface that is water unless we have support from a civilization on shore, where we evolved. We need ships or platforms, breathable air if we’re trying to colonize the seabed, and fresh water. Humans can visit the ocean. We can live there for months at a time in habitats and submarines prepared ashore. But we can’t stay without help.
We can’t live on the highest places like the Himalayan and Andean peaks, or in the deepest places like the vast caverns of Pennsylvania and New Mexico, without similar support from a surface civilization. On the peaks we die slowly from suffocation, bleeding, and dehydration. In the caverns we die from lack of light and food.
Consider that the first thing we would look for in an Earthlike world is liquid water. But even on Earth, we can only survive on a fraction of the water that’s present. If we drink from the oceans, the excess salts upset our metabolism. Our cellular functions collapse as the relatively fresher water inside the cells rushes out to dilute the saltier water outside, through the most basic process of osmosis. But neither can we drink absolutely pure H2O because it lacks the electrolytes—the salts—we’re accustomed to. With an isolated diet of distilled water or pure rainwater, our bodies would lack essential minerals. And, in sufficient quantity, chemically pure water will invade and disrupt our cells by trying to dilute the salts and nutrients inside them. No, our bodies were adapted to drink ground water that once fell as rain but has since soaked up some soluble minerals—a narrow slice of the Earth’s habitat.
In every other aspect of an environment—the air, the amount and wavelength of sunlight, the mineral mix, the supporting biosphere—we need exactly what this planet provides in both gross and subtle ways. For example, although the local bacteria are sometimes not our friends, we live symbiotically with a huge array of one-celled helpers. Without their influence in aiding our digestion, breaking down our dead skin cells, and supporting the natural decay of organic products in the world around us, we would die slowly—but much more quickly than we could adapt to an environment without them.
We will certainly colonize the Moon one day, but that will be a camping trip. For all the technology we might bring (short of programmable energy-to-matter conversion á la Star Trek), we will still need contact with and supplies from the home planet for long-term survival. Pre-civilization humans could roam our world and inhabit many climates and islands with just their bare hands and a few tools. But none ever settled Antarctica for its cold and dryness or the upper slopes of Mount Everest for their low air pressure. The Moon will be in a similar category of harshness but with even more difficult logistics.
Creating a New Earth
Things are the way they are for a reason. Over the long haul, planets acquire the conditions they have because of underlying physical causes.
The most likely and immediate object of our potential terraforming projects will be Mars. We expect to live on the Moon in pressurized tunnels and domes forever, never hoping to make an Earthlike world of that rocky ball. But Mars already has an atmosphere, soil, sunlight, and weather. How hard can it be to recreate Earth conditions on that nearby planet?
Mars is a much smaller planet than Earth, with a gravity only 38 percent—or just over a third—of that on our home world. (Compare that with the Moon, which is smaller still and has one-sixth the gravity.) What keeps an atmosphere on a planet is gravity. The lower the gravity, the more easily the jostling, bouncing molecules of its gas layer can reach escape velocity and fly off into space.1 If Mars ever had an oxygen-nitrogen atmosphere like Earth’s, it long ago dissipated into space. Mars has an atmosphere, sure, but it’s at about one percent of Earth’s air pressure. Take a jet up to about 50,000 feet and open a window: you will suddenly be breathing a Mars-rich atmosphere. However, the dominant gas on Mars is the heavy molecule carbon dioxide, because all the lighter gases have bounced away, so you won’t be breathing for long.
If you wanted to terraform Mars, you would have to find a way to release enough oxygen-nitrogen mixture to cover the 89.5 million square miles of the planet’s surface.2 That’s assuming you could find that much gas latent in the planet’s rocks and liberate it in a reasonable amount of time. But it better be a fast process, because the gas you pump in will ride up over the heavier carbon dioxide atmosphere and self-select to be among the first gases to bounce away again.
Let’s run the calculations. Assuming you could somehow remove the existing carbon dioxide atmosphere, then release 89.5 million cubic miles of new oxygen-nitrogen atmosphere, they would cover the planet to a depth of just one mile, or about 5,000 feet. That won’t give you anything like a breathable Earth environment: to create the pressure of 14 pounds per square inch we have on Earth, there’s a layer of air twenty times deeper, pressing down at full Earth gravity. Even if you could come up with the 1.8 billion cubic miles of gas to duplicate this atmosphere on Mars, you still wouldn’t have the gravity to sufficiently pressurize it. Or support it. If you piled up 100,000 feet of atmosphere above the Martian surface, the upper layers would dissipate even faster due to the lower escape velocity. Things are the way they are for a reason.
The second great difficulty with terraforming Mars is dealing with radiation from the solar wind. The core of the planet, whether molten or not—and this has been the subject of some debate—does not create the kind of magnetic field found around Earth, Jupiter, and Saturn. Without a magnetic field, the charged particles of the solar wind3 fall directly on the upper atmosphere, heating and accelerating the loss of gases. And any fast particles that aren’t stopped in the atmosphere reach the surface to ionize atoms and break up the organic molecules of living things.
Earth life in general does require a certain amount of radiation to support the rate of genetic mutation that took us collectively from single-celled bacteria to a planet rich with plants, animals, and humans. No radiation, no mutation, no evolution. But it’s a finely tuned amount we need. Too little radiation, and the various species don’t drift enough to adapt to changing environmental conditions. Too much radiation, and our molecules don’t hold together: we become sick with the resulting molecular poisons and die; or our genetic material disrupts, throwing off damaged proteins and cancers.
Once again, we were adapted for life on this planet under this sun. We might visit other Earthlike planets and breathe their atmospheres for a while and drink their strangely flavored water. We might extend our stay by taking along bottled gases, food supplements, and a long checklist of camping gear. But it’s unlikely that we’ll find a planet so close to Earth’s conditions, or create one so stable in its new pattern, that we’ll have time to take off our space helmets, walk freely in the sunshine, hunt the local game for food, mine the metals we need, and eventually evolve and adapt to what will inevitably be a changing environment.
It’s a nice dream. But I am literally not holding my breath for it.
1. The lighter molecules go first and fastest. On Earth, free hydrogen would have left the planet long ago if it didn’t have such an astounding attraction to oxygen atoms: they would rather form water molecules than escape. Helium, on the other hand, does not bond with other atoms. Most of our helium comes from natural gas wells as a residue of Earth’s formation out of the stellar cloud. We extract the helium by refrigeration for commercial purposes—now mostly festive. When you release a helium balloon, it rises into the stratosphere, bursts, and releases its cargo of fossil gas into the upper atmosphere, where the atoms continue to rise, and one-by-one eventually bounce into space. There they are widely space and so less jostled, but the even-lower gravity environment is less likely to draw them back. Finally, the flying gases encounter the solar wind and are pushed away from the planet altogether.
2. Mars has a surface area about equal to all the dry land on Earth.
3. The solar wind is made up of electrically charged protons and electrons and other atomic particles. The magnetic field catches and deflects them—most visibly to fall into our northern and southern polar regions as the colorful auroras. Fortunately for us, the sun is mostly hydrogen and helium, and so its nuclear reactions don’t kick out many fast neutrons. And the sun lacks sufficient energy to bathe us with a hard dose of gamma rays. If it did these things, the life that evolved in this system—if any—would be very different from our soft, water-soluble bodies.