Right now, as you read this, our world is being watched keenly and closely by a nearby alien spacecraft. So far the aliens have only observed circumstantial evidence of life: water, oxygen, and a mysterious light-absorbing chemical (chlorophyll). But the aliens are about to detect something that will prove conclusively not only that there’s life on Earth but that there is intelligent life.

Okay, the content of Earth’s radio broadcasts might not seem all that intelligent, but the existence of such broadcasts is clear, unambiguous evidence of a technologically advanced civilization of some kind.

First off, these radio signals are being affected by Earth’s ionosphere in a particularly telling way. During the day, the ionosphere becomes energized by solar radiation, effectively blocking the planet’s radio emissions from escaping into space. But at night, the ionosphere calms down and allows more radio signals through. Because the aliens detect most of the radio emissions from the night side rather than the day side, it would seem clear to them that the signals originate on the planet’s surface, rather than sources near or directly behind the planet.

Secondly, the radio emissions remain stable at constant frequencies over the course of many hours. Naturally occurring radio emissions would tend to drift significantly from one frequency to another over that time period. This strongly suggests an artificial source.

And thirdly, these signals exhibit “pulse-like amplitude modulation”—in other words, the signals appear to be modulated in such a way as to contain bits of information. I imagine this would present something of a double challenge for the aliens: first the technical challenge of decoding the signals, and then the linguistic challenge of interpreting our language—or rather, our many languages.

Whether or not the aliens could make any sense out of these radio signals, this sort of pulsed amplitude modulation is never observed with naturally occurring radio sources. The only reasonable hypothesis is that there is intelligent life on the planet’s surface.

Tomorrow, in the final post for this “Alien Eyes on Earth” series, the aliens will report their findings back to their home planet, and there’s one word—one particular scientific term—that will feature prominently in that report.

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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.

P.S.: Of course the aliens would pick up more than just pop music. They’d be able to hear our news, educational programming, personal cell phone calls, coded military transmissions, et cetera, et cetera… but something tells me that music in particular would draw their interest. The special combination of mathematics and aesthetics is, in my opinion, one of the strongest indicators of intelligent life.

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.