Sciency Words: Photolysis

Sciency Words PHYS copy

Things have been a bit hectic lately, so welcome to a rare Saturday edition of Sciency Words.

Every Friday (normally), 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:


Last year, when I did my special Mission to the Solar System, I fell in love with one planet in particular: Venus. I guess I’ve always had a soft spot for sciency girls, and Venus is about as sciency as they get. She’s really, really into chemistry.

Jy30 Venus Does Chemistry

Photolysis (also known as photo-dissociation) is one of Venus’s favorite chemistry tools. Photolysis occurs when high-energy photons ram into chemical bonds, causing those bonds to break.

By high-energy photons, of course, I mean light. Specifically ultraviolet light, X-rays, and gamma rays. On Venus, UV rays from the Sun cause the photolysis of sulfur compounds, contributing to the sulfur cycle that causes Venus’s infamous sulfuric acid rain.

Jy30 Photolysis on Venus

The same process breaks apart oxygen molecules in Earth’s stratosphere, allowing them to recombine as ozone, thus generating the ozone layer. Photolysis is also probably responsible for the chemical changes on Jupiter that make the Great Red Spot look red (or sometimes other colors).

And speaking as an artist, photolysis is something I have to guard against. Paints are just a mix of chemicals, and the photolytic break down of those chemicals can, over time, cause paints and other pigments to fade or change color.

Photolysis by any other name…

The term photolysis is sometimes used as a blanket term for similar kinds of chemical bond breaking. For example, very little sunlight reaches the surface of Venus, but sulfur compounds still vigorously break apart and recombine due to the intense heat.

Some academic sources I’ve read still call that photolysis, though I prefer the term thermal dissociation. Calling a chemical reaction that occurs in a near pitch-black environment “photolysis” feels awkward.

Just my opinion.

Photolysis here, photolysis there, photolysis everywhere…

When I first learned about photolysis, I didn’t fully appreciate its significance. I understood only that it (and also thermal dissociation) played key roles in Venus’s extra special chemistry projects.

Then I encountered the word again while researching other planets. And then it popped up in an art textbook I was reading. I’ve gradually come to understand that it is a fundamental concept in science, or at least in chemistry.

Next week, I’ll be revisiting Saturn’s largest and orangest moon: Titan. I have a sneaking suspicion that we will once again see photolysis in action.


Photolysis of Sulphuric Acid as the Source of Sulphur Oxides in the Mesosphere of Venus from Nature Geoscience.

The Sulfur Cycle on Venus: New Insights from Venus Express from the 2009 Lunar and Planetary Science Conference.

One thought on “Sciency Words: Photolysis

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