Sciency Words: Photosynthesis (Alien Eyes on Earth, Part 3)

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 expand our scientific vocabularies together. Today’s term is:


This post is mainly for my extraterrestrial readers, especially you extraterrestrials who are observing Earth from a distance and are a little puzzled by what you’re seeing.

By now, you’re aware of Earth’s active water cycle, and you’ve observed an alarming amount of oxygen in Earth’s atmosphere. You may have also noticed there’s a strange chemical spread across Earth’s landmasses, a chemical that absorbs red light.

This chemical also absorbs blue light, but due to an atmospheric scattering effect, the blue absorption signature might not be easy for you to see. Also, there’s more of this hard-to-identify chemical in Earth’s oceans, but between the atmospheric scattering of blue light and water’s ability to absorb red, you probably can’t detect it.

I realize you aliens must be pretty advanced. I mean, you’ve developed interstellar travel, after all. Even so, I bet you’re struggling to identify this strange chemical substance. Let me help you out: it’s a weird, complicated molecule we humans call chlorophyll, and it’s used in a biochemical process we call photosynthesis.

Photosynthesis comes from two Greek words: photo, meaning light, and synthesis, meaning synthesis.

Here on Earth, photosynthetic life forms like plants, algae, and cyanobacteria use chlorophyll to absorb sunlight (specifically the red and blue wavelengths). This light energy is then used for a sort of carefully controlled photolysis of water and carbon dioxide molecules, which are then recombined to make carbohydrates.

Please note: there are alternative versions of photosynthesis here on Earth that do not require chlorophyll. It’s just that these alternatives aren’t very popular. Haven’t been for over two billion years. This despite the fact that chlorophyll-based photosynthesis produces an extremely hazardous byproduct: oxygen. But hey, at least now you know where Earth’s mysterious oxygen atmosphere comes from!

You probably have something like photosynthesis back on your home planet, but I imagine the details must be different. Some other chemical probably does the job chlorophyll does here on Earth. Whatever your planet’s photosynthetic chemical is, I bet we humans would have a really hard time identifying it… just as you guys were struggling to identify Earth’s chlorophyll.

* * *

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.

2 thoughts on “Sciency Words: Photosynthesis (Alien Eyes on Earth, Part 3)

    1. I don’t know. This is something I’d like to research more. I suppose we can’t know what would work as an alternative to chlorophyll until we find it.

      I’ve heard speculation, though, that there could be some kind of black chlorophyll—a chlorophyll-like chemical that absorbs all visible light, not just red and blue.


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