Sciency Words A to Z: Unknown Absorber

Welcome to a special A to Z Challenge edition of Sciency Words!  Sciency Words is an ongoing series here on Planet Pailly about the definitions and etymologies of science or science-related terms.  In today’s post, U is for:


If there’s one thing worth remembering from all this astrobiology stuff, it’s that life begins with chemistry.  All life in the universe, no matter how strange and exotic it may seem to us Earthlings, must depend on chemistry.  And I don’t know many places that are more chemically active than the planet Venus.  So is Venus a good place to go looking for alien life.

To quote from David Grinspoon’s book Venus Revealed, “Where life is concerned, Venus is consistently voted ‘least likely to succeed.’”  Sure, Venus is chemically active, but in a way that will violently tear apart complex organic molecules.

However, Grinspoon has the temerity to go ahead and speculate—and he makes it abundantly clear this is pure speculation—about the kinds of organisms that might call Venus home.  And that speculation focuses on a mysterious substance found in the Venusian clouds, a substance that has long been called the unknown near-U.V. absorber, or simply the unknown absorber.

In the field of spectroscopy, every chemical is known to absorb very specific wavelengths of light.  When light is spread out into a spectrum, as with a prism, you get a sort of unique barcode that you can use to identify chemicals.

A very simple “barcode” representing hydrogen.

If you’ve ever wondered how astronomers know which chemicals are found in space, or on other planets, this is how they do it.

In 1974, NASA’s Mariner 10 spacecraft sent us our first ever close-up photos of Venus.  In the visible part of the spectrum, there were no real surprises, but photos taken in ultraviolet showed that something was absorbing U.V. light like crazy, producing a spectroscopic barcode that nobody recognized.

In his speculation about life on Venus, Grinspoon mentions another chemical with a complex, hard-to-identify spectral barcode: chlorophyll, the chemical that makes photosynthesis possible here on Earth.  I say hard-to-identify… it’s not hard for us to identify, because we already know what it is.  But if extraterrestrial observers were studying Earth’s spectrum, chlorophyll would have them very confused—almost as confused as we were by Venus’s unknown absorber.

So could the unknown absorber be a chlorophyll-like molecule? Could this be the first evidence of air-born bacteria, drifting around in Venus’s cloudbanks, performing their own version of photosynthesis?  Maybe, Grinspoon tells us in Venus Revealed.  But that book came out in 1997.  In 2016, this paper was published identifying Venus’s unknown absorber as disulfur dioxide.

On a personal note, I wrote a blog post about Venus’s formerly unknown absorber before, and my post got the attention of the lead author of that 2016 paper.

But even though the mystery of Venus’s unknown absorber may have been laid to rest, I think this still served as a valuable lesson about what we should be looking for out there in the cosmos. Someday, another unknown absorber, with another weird spectral barcode, may be the thing that leads us straight to the discovery of alien life.

Next time on Sciency Words A to Z, the Martians better watch out.  The Vikings have landed on their planet!

7 thoughts on “Sciency Words A to Z: Unknown Absorber

    1. Are we talking about cleaning up Earth’s atmosphere or Venus’s? Either way, I doubt it.

      Venus has a whole lot of unstable sulfur compounds in its atmosphere. They exist because there’s so much sulfur chemistry going on that even unstable compounds keep forming as rapidly as they break apart. So if we’re trying to terraform Venus, we want all that sulfur chemistry to stop, including the formation of disulfur dioxide.

      As for introducing disulfur dioxide to Earth’s atmosphere, I’m not really sure what would happen. Since it’s an unstable compound, my guess is that it would immediately start breaking apart into sulfur monoxide. After that, my big fear would be sulfur monoxide would start eating up the ozone layer.

      Liked by 1 person

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