by Ruth Orr
EARTH — Okay, so you’re probably aware that the Earth has like, a lot of water on it. Almost three-quarters of the planet’s entire surface, 71 percent, is covered in water, most of that in the oceans. There’s actually a view from space of the planet in which virtually no land is really visible— just New Zealand, Hawaii, and the outer margins of North America and Australia, because the Pacific Ocean is big enough to span almost the entire field of view. The point is, we’re the blue marble planet. The watery boys. We’re wonderfully wild and wet. Our water makes us special in the grand scheme of space… right?
Actually no, not really. Sorry kids. I mean yea, sure, okay, our water is pretty freaking neat and is kind of the critical basis for life on our planet, but we’re not the only ones who have it.
In fact, even just in our own solar system, many of our neighbors have at least some water. Jupiter’s moon Europa is one place that scientists have considered a possible home for extraterrestrial life, because as far as we can figure, under its icy crust is a liquid, possibly warm ocean. Saturn has a moon, Enceladus, that is even more likely than Europa to have a warm salty ocean on it. And I hear you now, “what do you mean, warm water? It’s so far away from the sun!” to which I remind you that has nothing to do with it. The interior of our planet is toasty warm, with regular vents to the surface under the oceans, heating the water there and dispersing the temperatures around. The same goes for other places with their own hot centers. The dwarf planet Ceres, which hangs out in our asteroid belt, is mostly ice and slushy water. Heck, even our little red buddy Mars has water, frozen at its poles. Our water isn’t unique.
But we are special for a totally different reason. And no, I’m not going with the obvious “let there be life!” thing. You see, while planet Earth is so often associated with water due to its prevalence here, what we should be associated with is actually… fire. To the best of our knowledge, we are alone, not just in our solar system, but in the entire cosmos, with our fire.
Wait, what? Let’s take a quick step back. What even is fire, anyway? What we call fire, those dancing tongues of yellow-orange-white-blue heat and light, is just the visible byproduct of the process of combustion. Combustion is specifically a chemical reaction that occurs between oxygen and some type of fuel, resulting in products that are completely different from the starting material. To get the fuel and oxygen to combust, you have to add heat until you get to the fuel’s ignition temperature. The reaction will continue as long as there is enough heat, fuel, and oxygen. Remove any one of those three, and the process stops and the fire goes out.
Technically, combustion can only happen between gases. So if you’ve got a solid fuel, like, a log, you have to heat it up until the components that make the log split apart and turn to gas, at which point it combusts. That’s why your log eventually disintegrates into a little pile of ash in the fireplace— the bits of it that were burnable literally got so hot they fell apart, became gaseous, and then reacted with the oxygen in the air to become fire.
Okay, so that’s your Intro to Fire 101 crash course. But none of that explains why I’m trying to convince you we alone have fire. Other planets are documented to have lightning strikes and volcano eruptions. Aren’t those kinda… fiery? No. Lightning is just electricity, and volcanos spew out melted rock, but that’s all it is. Melted rock. Nothing is actually combusting.
Fine, fine. But there’s at least one big, yellow, skin-cancer-causing bit of fire that hangs out in the skies above our heads every day, right? Isn’t the sun just a burning ball of gas?
Nope! It’s easy to see how you might get confused with that. If you look at photos of the sun, you’ll see what looks an awful lot like an oversized version of your backyard barbecue pit. Yellow-orange, emitting heat and light, that’s fire, right? Well, go back to what defines fire and what it actually requires and you’ll quickly see that it isn’t. There is no oxygen in space, and there is no giant cosmic log at the center of the sun that’s been merrily burning away for the last few billion years or so. The sun is not combusting.
Rather, it’s undergoing an entirely different process, called nuclear fusion. Nuclear fusion is essentially the opposite of nuclear fission, which is what we use in our nuclear power plants and bombs. In fission, a larger atom is split apart into smaller ones, releasing energy in the form of heat and radiation. With fusion, atoms combine to form a heavier atom. That process also releases energy in the form of radiation and heat. In our sun’s case, the great ball of hydrogen is being heated and under so much pressure that the hydrogen atoms are being smushed together to form helium atoms. The gamma radiation being emitted by the process eventually becomes the light (and heat) we get here on Earth.
Fine, but other places in the cosmos have at least some semblance of atmosphere, some of them including at least a little oxygen. Several of those moons I mentioned earlier, including Europa and several of Jupiter’s other moons, feature oxygen in their atmospheres. And we know because they have warm oceans that heat can exist all the way out there. So why don’t they have fire?
Well, it’s all down to that third leg of the fire triangle, fuel. And as it turns out, the stuff that makes the best fuel for fire is stuff that has a lot of bonds between carbon and hydrogen. And it just so happens that the naturally occurring material that has the most bonds like that is cellulose, as in the stuff that makes up plants. And since we’ve not yet found another planet that is rich in luscious, burnable life, we stand alone in our fiery inferno.
It also means that fire is a more recent phenomenon on the planetary timescale. Since Earth started out as a lifeless ball of hot rock, it took the long, slow evolution of life, and photosynthesizers in particular, to get to the point where combustion could kick off.
Today, our atmosphere is composed of about 78 percent nitrogen, 21 percent oxygen, and one percent argon. But in the way back once-upon-a-time days, when the Earth was young and life wasn’t really living yet, the planet’s atmosphere was mostly hydrogen sulfide, methane, and carbon dioxide.
About 3.4 billion years ago however, life first kicked off as microscopic blobs floating around in the toxic stewy mess of the early oceans. One brand of the brand new bacteria, cyanobacteria, decided to try something new in an effort to stand out from the crowd. They took a look around at their neighbors and at the world around them, and decided that the sun was being underutilized as a resource. It took guts and determination but our heroic underdogs developed a whole new strategy for getting energy— photosynthesis. And we’ve been dealing with the consequences of their actions ever since. Actually, I wrote an entire column in October of 2022 on the crisis these little bacteria boys kicked off, go check it out on our website (“The first global apocalypse”).
But the relevant part of that for today is that one of the byproducts of photosynthesis is oxygen. After a billion years or so of these bacteria pumping the shiny new gas into the atmosphere, there was enough of it to finally trigger combustion events. And boy oh boy, did that happen… big-time.
It took a long while longer for plants to finally colonize land. But we know they were up and about at least 500 million years ago, and by 430 million years ago they were all over the place. Those ancient plants didn’t necessarily look a lot like the ones we’re familiar with today, but they did catch fire. And we know that, because by the Carboniferous period in history, the planet was covered in plants. But decomposition hadn’t really begun yet, so anytime a plant died, it just kind of… stayed there. Forever. Or at least, until wildfires, probably started by lightning strikes, swept through. With that much fuel lying around free for the taking, and the atmosphere supercharged on extra oxygen (about 35 percent to today’s 21 percent), fires had a golden age. Flames may have licked up to over three hundred feet high, with fire whirls and tornados becoming common. At the end of the Cretaceous period, about 66 million years ago, an asteroid struck the Gulf of Mexico. It made for a real bad day for pretty much everything on the planet, triggering tsunamis, earthquakes, and also a planet-wide wildfire. The impact threw tens of thousands of cubic kilometers of burning hot debris into the air, and as it rained back down, fires started everywhere. The layer of charcoal left behind by the destruction of entire forests can be found around the world.
At any rate, plants and other life have had to evolve to work around our planet’s status as home to fire. Today, it’s just a part of daily life, and there are actually entire species that require fire to propagate.
Next time you’re sitting around a campfire watching your marshmallow burn to oblivion, take a moment to appreciate the fact that you get to witness such an incredible phenomenon. Nowhere else in the entire, infinite universe, can you see fire.
