Sciency Words: Specular Reflection

Sciency Words BIO copy

Sciency Words is a special series here on Planet Pailly celebrating the rich and colorful world of science and science-related terminology. Today, we’re looking at the term:


A specular reflection is a reflection off a smooth, mirror-like surface, such as glass, polished metal, or a tranquil body of water. The opposite of a specular reflection is called a diffuse reflection, where light strikes a surface and scatters in multiple directions.

Specular reflections are rare in nature. Few surfaces have the perfect, mirror-smooth finish that makes this phenomenon possible. Pools of liquid water are really the best example. Well, pools of liquid—it doesn’t necessarily have to be water.

In the field of planetary science, specular reflections have become extremely important in relation to Titan, Saturn’s largest moon. For a long time, scientists thought Titan might have liquid on its surface. Not liquid water—Titan’s too cold for that—but perhaps liquid hydrocarbons, specifically a mixture of liquid methane and ethane.

And so when the Cassini spacecraft entered orbit of Saturn in 2004, the search was on for Titan’s liquids. Titan’s hazy atmosphere makes it almost impossible to view the moon’s surface in visible light, so Cassini made its observations in other wavelengths, from infrared to radio frequencies.

Dark regions were soon identified on Titan’s surface. Were they lakes of hydrocarbons? No one could be sure until 2008, when Cassini bounced radiowaves off a suspected lake in the southern hemisphere; the radiowaves bounced back, just like a specular reflection.

In 2009, Cassini was again observing Titan in infrared when a glint of sunlight bounced off another suspected lake, this time in the northern hemisphere. Again, it was just like a specular reflection.

Sp10 Titan Sparkles
Cassini continues to investigate Titan’s other… peculiarities.

In fact, these specular reflections turned out to be surprisingly bright. Titan’s lakes must be extremely smooth and still, with hardly any waves at all. This suggests that either Titan’s weather is oddly tranquil or that the methane/ethane mix in these lakes is more viscous than we expected, more like honey than water.

Earth and Titan are the only places in the Solar System where liquid anything flows on the surface. As a result, these two worlds have a surprising amount of stuff in common, from erosion to weather patterns, and maybe even life. More on that next week.

In the meantime, who’s up for a swim?


Smoothness of Titan’s Ontario Lacus: Constraints from Cassini RADAR Specular Reflection Data from Geophysical Research Letters.

Sunlight Glint Confirms Liquid in Titan Lake Zone from NASA.

Saturn Moon’s Mirror-Smooth Lake “Good for Skipping Rocks” from New Scientist.

3 thoughts on “Sciency Words: Specular Reflection

  1. I love that little factoid you just snuck in there that Earth and Titan are the only places where there’s liquid flowing on the surface. It’s one of those “maybe you never thought about this, but…” ones that always seem to catch people’s eye (ear? let’s go with eye).

    Liked by 1 person

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