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.
* * *
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.
Planet Pailly 
Wow. Great post. I remember reading (skimming? staring blankly at?) That paper when I came across it while I was doing research for a completely unrelated computer science project I didn’t want to do. I probably still have a copy of it kicking around here somewhere. Thanks for the tiny taste of nostalgia.
LikeLiked by 1 person
You’re welcome! It’s an older paper, but it gets cited so often that I figured I should go ahead and read it.
LikeLiked by 1 person
From what I understand, the spectral signatures that show the amount of oxygen in our atmosphere might also be something that catches their interest, at least if they’re interested in studying biospheres.
Of course, given that this can probably be detected across vast distances, it always leads back to the question: where is everyone?
LikeLiked by 1 person
I’m covering oxygen and ozone for part 2. They’re definitely things that would be noticed, although Sagan offered an interesting non-biological hypothesis for where Earth’s oxygen might be coming from.
LikeLiked by 1 person
I’d be interested in hearing about Sagan’s non-biological hypothesis on the presence of oxygen. Oxygen is so reactive that you have to keep producing it or it would keep disappearing over time.
LikeLiked by 1 person
It certainly took me by surprise. Sagan and the other authors of the 1993 paper manage to rule out this alternative explanation, at least in the case of Earth. But still, I found it interesting that oxygen does not necessarily indicate the presence of life.
LikeLike