If an alien spaceship were passing by, there are several things the aliens would notice about Earth, even from a distance. Water, for one thing. And not just any old water, but water in solid, liquid, and gaseous phases. And if that weren’t enough to grab the aliens’ attention, a spectrographic analysis of the atmosphere would reveal another big surprise: oxygen. Alarmingly high quantities of oxygen.

We humans are used to thinking of oxygen as a good thing, something beneficial to life. From an alien perspective, that may not be so. Imagine if I told you about a planet with a 20% chlorine atmosphere. Would you want to go there? Probably not. Chlorine is a highly reactive, extremely toxic chemical. It’s an oxidizer. One of the strongest oxidizers on the periodic table.

But as you could probably guess from the name, oxygen is also a really strong oxidizer—almost as strong as chlorine. We humans have evolved in an oxygen-rich environment. We’re used to it, and we’ve adapted to it so well that oxygen has become a benefit to us rather than a liability.

To extraterrestrials, however, a 20% oxygen atmosphere could be as frightening as a 20% chlorine atmosphere would be to us.

So where does all that oxygen come from? The aliens already know about Earth’s water, so they might speculate about water molecules being broken apart by UV radiation from the Sun. This process, known as photolysis, would turn water (H₂O) into atomic hydrogen and atomic oxygen, which could then recombine to make molecular hydrogen (H₂) and molecular oxygen (O₂). Light-weight hydrogen molecules would then escape to space and be swept off by the solar wind, leaving the heavier oxygen molecules behind.

That might account for some of Earth’s oxygen, but Earth has a lot of oxygen. A lot more than can be explained by photolysis alone. So how can there be so much oxygen? What keeps all that highly reactive oxygen from reacting itself away… unless something is constantly replenishing Earth’s oxygen supply?

These aliens observing our planet are good scientists, so they won’t jump to conclusions too quickly. It’s possible, the aliens might say to each other, that all this oxygen is being generated by biological activity. But they can’t know that for sure. Not yet.

Tomorrow on Sciency Words, we’ll look at a scientific term that you have almost certainly heard of before, but that the aliens maybe haven’t.

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Today’s post was inspired by a 1993 paper by Carl Sagan and others. Sagan and his colleagues wanted to know which of Earth’s features can be observed by a passing spacecraft and, perhaps more interestingly, which features cannot.

Aliens are naturally curious about Earth, once they notice it’s there. Earth is covered in water. That by itself isn’t so unusual, but even from a great distance it’s obvious there’s something special about Earth’s water.

Hydrogen and oxygen are the #1 and #3 most common elements in the universe, respectively; therefore, it shouldn’t surprise anyone that H₂O, the simplest hydrogen/oxygen compound, is abundant throughout the cosmos.

In a gaseous form, water is often present in the atmospheres of gas giants. Solid water can be found on many asteroids and comets, as well as on icy worlds like Pluto, Titan, or Europa. Liquid water sometimes exists beneath the surfaces of these icy worlds, and it’s sometimes found on rocky worlds like Mars, typically with salt or other impurities mixed in to help lower its freezing point.

But any passing spaceship observing Earth would notice evidence not just of water, not just of liquid water, but clear evidence of water in all three phases.

• Gaseous water: the characteristic absorption lines of water vapor can be detected in a spectrographic analysis of Earth’s atmosphere.
• Solid water: highly reflective polar regions indicate the presence of ice.
• Liquid water: and then there are specular reflections—glints of sunlight reflecting off very smooth surfaces, surfaces so smooth that they almost certainly must be liquid.

It would be one thing to have liquid water sitting stagnant on the planet’s surface, but Earth’s water appears (even from a distance) to be cycling through multiple phase transitions. Passing aliens might not know what that means, but it’s certainly strange enough to make them curious about Earth. Perhaps curious enough to come take a closer look.

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Today’s post was inspired by a 1993 paper by Carl Sagan and others. Sagan and his colleagues wanted to know which of Earth’s features can be observed by a passing spacecraft and, perhaps more interestingly, which features cannot.

Today’s post is part of a special series here on Planet Pailly called Sciency Words. Each week, we take a closer look at an interesting science or science-related term to help us all expand our scientific vocabularies together. Today’s term is:

SHADOW BIOSPHERE

Crazy Talk

We are not alone on planet Earth. There are aliens among us. Their existence has gone unnoticed and unsuspected for millions of years.

Truth be told, I shouldn’t call them “aliens.” They evolved here on Earth, side by side with what we, in our arrogance, call “organic life.”

They’re everywhere. There’s a whole biosphere of these weird creatures sharing our planet with us. It’s called the shadow biosphere.

Not Crazy Talk

I first heard about the shadow biosphere on an episode of SciShow, and I’ve been seeing the term more and more lately. It seems like some sort of astrobiology buzzword at the moment.

The idea is that an alternative form of life could have evolved here on Earth, and we just haven’t discovered it yet. Maybe it lives in areas totally inaccessible to us, like deep beneath the Earth’s crust. Or maybe it’s so different from us that we don’t yet recognize it as a life form.

Personally, I take this as more of an astrobiology thought experiment than a serious hypothesis about life on our planet. It’s a way of reminding us how limited our understanding of life is and show how difficult it might be to identify alien life should we happen to find it.

You see, to determine if something is alive, we must try to identify ways in which it is similar to other living things. Does it move? Grow? Reproduce? On a more fundamental level, is it cellular in structure? Does it have a carbon-based biochemistry? A DNA-like genetic code?

But all these questions presuppose that newly discovered life forms will be similar to life forms we already know about. What if we’re dealing with a life form totally dissimilar to life as we know it? What if they’re non-cellular, non-carbon-based organisms that don’t have anything resembling DNA?

Why, such organisms might be so strange to us that they could exist all around us, even right here on Earth, and we wouldn’t know it. Or so this type of thought experiment may lead you to conclude.

Back to Crazy Talk

It’s not just a thought experiment. The shadow biosphere is real. It’s real, I tell you! Wait, where are you taking me? No, I don’t want to take my medicine. Are you working for them? Did the pet rocks send you?