Sciency Words: Macula

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When Voyager 1 trained its camera on the moons of Jupiter, scientists back on Earth had no idea what to expect. Turned out they were right. Voyager was snapping photos of geological features unlike anything anyone had ever seen before. Which meant it was time to make up some new sciency words!


Last week, we talked about Europa’s lineae: the reddish-brown cracks and fissures crisscrossing this small moon’s surface. But those weren’t the only surprises Voyager 1 observed. Let’s zoom in for a closer look.

Sp09 Macula on Europa

Europa has these peculiar dark splotches on its surface, similar in coloration to the lineae. Scientists came up with the term macula (plural maculae) to describe them. It comes from the Latin word for “spot” or “blemish.” It’s related indirectly to the word immaculate, which literally means “without blemish.”

Although Europa’s maculae were discovered in 1979, it wasn’t until 2011 that anyone could adequately explain them. It seems that Europa’s thick ice shell has a complex relationship with the ocean of liquid water deep beneath the surface, resulting in frequent patterns of melting and refreezing.

Sometimes “lakes” of liquid water become embedded between layers of ice. This causes surface ice to sag and cave in, breaking up into chunky, tightly packed icebergs. Some sort of material (possibly organic material) seeps up with the meltwater, causing the dark discoloration.

Eventually, the lake beneath a macula will freeze. Since ice is less dense than water, this forces the now cracked and broken surface ice to rise above the surrounding landscape. In the process, the already strange-looking maculae transform into even stranger-looking chaos terrain.

The term macula can be used to describe almost any dark, spotty or splotchy feature on a planetary body. That doesn’t mean they have anything in common beyond superficial appearances. For example, while maculae on Europa seem to be caused by melting and refreezing ice, maculae on Titan may be related to some sort of volcanic activity.

For next week’s edition of Sciency Words, we’ll move on to Ganymede. Europa wasn’t the only Jovian moon showing off strange, never-before-seen geological features when Voyager arrived.

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