Habitable atmospheres - Earth and other planets. Yes, Pluto counts!

Putting the science in fiction - Dan Koboldt, Chuck Wendig 2018

Habitable atmospheres
Earth and other planets. Yes, Pluto counts!

by Lynn Forrest

Within a few episodes of the revived Doctor Who series, I found myself sucked into the Doctor’s universe. When he didn’t travel to some place or time on Earth—particularly a version of London—he was on a different world. A world that, in general, was warm or cool enough with the right kind of atmosphere to wander around without special gear.

As much as I enjoy the show, I can’t suspend belief enough in those episodes to think, “Sure, humans and Time Lords can breathe there just fine.” (I could write a full chapter on what’s wrong with the ATMOS burn-the-atmosphere-to-cleanse-it story line.) Unfortunately for the factual science fiction writer, multiple factors have to come together to let humans or human-like people live on the surface of a planet, and those things can’t coincide on a whim.

Let’s break down the key ingredients for a habitable (Earth-like) atmosphere on a given world:

1. sufficient planetary mass to provide the gravity that keeps the atmosphere in place,

2. a magnetic field,

3. a reasonable distance from its parent star (not too close, not too far), and

4. oxygen in the atmosphere itself.

The importance of gravity

Your characters probably don’t want to float away from the planet they’re on, and neither should your atmosphere. Every object in the universe attracts other objects with a force proportional to its mass and its distance from those objects. This applies to the molecules within the atmosphere, too. On Earth, the acceleration due to gravity—the gravitational force between objects on Earth and the Earth itself—is about 9.8 meters per second squared, often referred to as 1 g.

By comparison, Venus’s surface gravity is about 91 percent that of Earth, Mars about 38 percent, and Earth’s moon about 16 percent. Which of these celestial bodies has an atmosphere? The ones with higher gravity. The moon lacks an atmosphere altogether, and the average atmospheric pressure at the Martian surface is about 0.6 percent of Earth’s (the complex topography of Mars strongly affects the pressure at any given location). On the other hand, Venus boasts a surface pressure ninety-two times greater than Earth’s.

Clearly, gravity isn’t the only reason these planets have or don’t have an atmosphere. Venus provides a shining example of the runaway greenhouse effect: Any oceans it may have had boiled away millions of years ago, creating an oppressive wasteland that destroyed a NASA lander less than an hour after it reached the surface. Mars lacks a magnetic field, a topic covered in more depth in the next section. But a planet must have some gravity to pin its atmosphere in place.

Magnetic fields: a planet’s shield

Thankfully, the Earth has a magnetic field that’s produced by a complicated process involving Earth’s solid core—solid due to high pressures, not low temperatures—surrounded by molten rock. Since the Earth rotates quickly on its axis, the spinning core creates what some call a geodynamo. (That’s as far as I’m comfortable explaining it. Find a friendly geologist for the full scientific details.)

This magnetic field is important because it acts as a shield, protecting Earth’s atmosphere—and us—from the Sun’s solar wind. The highly charged particles that make up the solar wind get deflected by the magnetic field. Without it, the solar wind would strip away part of our atmosphere, including the ozone layer protecting us from ultraviolet radiation. Some theorize that Mars, which has a very weak magnetic field, lost much of its atmosphere due to the solar wind even though it’s farther from the Sun than Earth is. Imagine how much less atmosphere we’d have without our magnetic field!

Conversely, Venus doesn’t generate a magnetic field at all. As a result, the solar wind has stripped molecules from Venus’s atmosphere for millions of years, taking away its hydrogen and thus removing its ability to create water. Why hydrogen? It’s the lightest element and can more readily escape Venus’s gravity.

The ingredients for breathable air

When the air is dry, Earth’s atmosphere is about 78 percent nitrogen, 21 percent oxygen, 1 percent argon, and 0.04 percent carbon dioxide (and, sadly, rising). It has trace gases as well, such as methane—yes, the stuff cows produce—and water vapor when the relative humidity is above 0 percent, which is almost always. For us to breathe without assistance, the air should be at least 19.5 percent oxygen. (Note that levels above 22 percent are considered “oxygen enriched.” Since fire consumes oxygen, I shouldn’t need to explain why a largely or entirely oxygen atmosphere would be Armageddon levels of bad.)

Where does the oxygen come from? Thank the nearest plants for breathable air. Some oxygen was present when the solar system formed, but photosynthesizing organisms generate far more of this molecule necessary for life as we know it. Basically, photosynthesis uses energy from the Sun to pull carbon from carbon dioxide and releases oxygen as a by-product. (The air is the biggest source of carbon for plants!) On Earth, cyanobacteria produced the initial influx of oxygen into our atmosphere. Volcanoes contribute some oxygen as well, but not enough to bring us to our current 21 percent concentration.

I’d argue a habitable atmosphere is a breathable one, but we could get away with living on Mars if we lived inside pressurized domes and carried oxygen tanks with us when we walked outside. If you’d like to learn more, Robert Zubrin’s excellent book The Case for Mars (Free Press, 1996) delves into the science and logistics behind a feasible Martian colony. Unfortunately, the average Martian surface temperature is a frigid -58° F (-50° C). Which brings me to the next point. …

The goldilocks zone

Yes, there is such a thing as the Goldilocks zone. It has more respectable titles such as “habitable zone” or “life zone,” but the Goldilocks term will always stick in my head. True to its title, this is the region in which a planet is neither too hot nor too cold for liquid water.

Liquid water is key for life. Water freezes once the temperature reaches 32° F (0° C) and evaporates at 212° F (100° C). If you want your Earth-like planet to be comfortable for your human or humanoid characters, you want it to be “just right” for vast amounts of liquid water.

Mars could be that way if we added greenhouse gases to its atmosphere. Robert Zubrin proposed the use of fluorocarbons to kick-start the process. Thawing the surface of Mars would release more carbon dioxide, which would add more greenhouse gases to the atmosphere, which would continue warming the atmosphere—a positive feedback. (Great for Mars, but Venus shows why it wouldn’t be great on Earth.) Eventually the Martian surface would become warm enough to maintain liquid water, that water would release oxygen from the soil, and then we could start bringing plants to Mars. Imagine how cool it would be to farm on Mars. “Interplanetary Gardener” has a neat ring to it. (I’m sure Mark Watney from the 2015 movie The Martian would have appreciated a more hospitable climate for his potatoes.)

If the right ingredients are in place, terraforming a world to make it more Earth-like is possible and could make a great story. However, Zubrin’s method is time-consuming and thus couldn’t happen overnight, despite what was implied by the 2000 animated movie Titan A.E.

A recipe for a habitable atmosphere

First of all, start with a decent-sized rocky planet. You need a solid surface for characters to stand on and enough mass to produce sufficient gravity. Then pick a star: it can be like Earth’s or it can be a hotter or cooler star. The key is to slide your planet into an orbit that’s within the Goldilocks zone for liquid water. Give it a sprinkle of photosynthesizing organisms, a generous helping of water, a few billion years, and voilà! Human-friendly planet.

Just keep your characters from showing up early.

This is a highly adaptable recipe. You can always put your characters on a more hostile world where they need protective gear or on a planet where only a small region is human-friendly, such as the poles or along the equator. Planets can have seasons and complex topography, too!