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We knew that, if the Webb telescope could make it to the L2 Lagrange Point in one piece and deploy properly, this would be an exciting season for astronomers–but, even so, the parade of stunning new images from outer space are marvelous and demand comment. Today’s treasure is a picture of Planet Neptune and its moons as imaged by the near-infrared camera on Webb. The ice giant Neptune is made of strange cold things with a great pall of methane gas over them. Methane gas is very opaque to infrared light (which it absorbs) and so the planet looks like a frosty, haunted bowling ball with glass rings.

Ever since Pluto got demoted to “dwarf planet”, Neptune is the outermost world of our solar system. Yet the great gas giants…or even the trans-Neptunian objects like Eris and Haumea get all of the attention. No space craft has even visited Neptune since Voyage II rolled by in 1989 (the first and last time a probe entered the Neptune system).

Aside from the spectral rings, the image shows some bright sparks in a line along Neptune’s Tropic of Capricorn (which is not called that, but you get the idea). These bright spots are caused by high altitude methane clouds which are made of methane ice (which reflects infrared light better than methane gas does).

The full Webb photo has a striking focal point! Pulling back we see that Neptune’s largest and strangest moon Triton outshines the giant world it orbits. This is because Triton (which is named for the Greco-Roman deity Neptune’s merman super-son) is covered in a sheet of frozen nitrogen which reflects 70% of the sunlight which strikes it–so Triton glows like an aquamarine star in this photo. Ultimately Triton might well turn out to be be more interesting than Neptune: it is the only large moon in the solar system with a retrograde orbit (an orbit opposite of the planet’s rotation). Such an unusual orbit suggests that the moon was a little world captured by the ice giant long ago.

Triton is larger than Pluto and is one of five moons in the solar system known to be geologically active (the others being Io, Europa, Titan, and Enceladus). Voyager II spotted geysers of nitrogen gas venting from the moon. Clearly cryovolcanic activity is taking place below the strange patchwork of old ice (as explained in this confusing yet compelling map/diagram) and lakes of liquid water may exist below the moon’s crust.

I am going to keep staring at images of our strange far-off neighbor world, but I can’t wait to see what Webb photographs next!


Neptune's Moon Triton photographed by Voyager 2 (NASA)

Neptune’s Moon Triton photographed by Voyager 2 (NASA)

It is absolutely freezing here in Brooklyn—a great vortex of bitter Arctic air has swirled south across huge swathes of the nation.  The temperature here is 9° Fahrenheit (or -13° Celsius).  Imagine how much worse things are in Minnesota, where it is -14° Fahrenheit (or -25° Celsius).  Brrr!  It hurts my fingers to write about it–even in my overheated study (well—bedroom, really).  Now truly stretch your mind from the frozen heartland of America to the edge of the planetary solar system.  The largest moon of the ice giant Neptune is the moon Triton, discovered in 1846 by English astronomer/brewer William Lassell, and named for the son of Poseidon.  On the surface of Triton temperatures plunge to 36…which is to say 36 K (Kelvin). To translate that is -237° Celsius or a bone chilling -395° Fahrenheit.

Artist's rendering of an ice volcano on Triton with Neptune in the background (NASA)

Artist’s rendering of an ice volcano on Triton with Neptune in the background (NASA)

Triton is a strange moon.  It is the seventh largest moon in the solar system and it is the only large moon to orbit its planet in a direction opposite from the planet’s rotation (which is called a retrograde orbit).  Since there is no model for retrograde moons forming from accretion disks, Triton must be a captured object from the Oort cloud—and, indeed, the moon is extremely similar in composition to Pluto and other dwarf planets of the Solar system’s distant periphery.  Despite the extreme cold of Triton’s surface, the moon is geologically active.  Like Earth, the moon is probably differentiated into layers: a core, a mantle, and a crust.  The crust is formed of ice: frozen water, methane, and nitrogen.  A large polar cap covers the southern pole, but much of the rest of the moon is a“cantaloupe” surface of melted and refrozen ice.  The surface is (geologically) young.  Cryovolcanic activity and tidal forces have kept the ice active.  Cryovolcanoes were first spotted on Triton during the Voyager 2 flyby in 1989 (the first time such phenomenon were ever observed).  Because of tidal warming (caused by gravitational interaction with Neptune), Triton may have once had a liquid ocean beneath the crust, but this has likely solidified assuming that there is no radioactive decay from the rocky core.

A size comparison of Earth, Earth's moon, and Triton

A size comparison of Earth, Earth’s moon, and Triton

Triton is closer to Neptune than the Earth’s moon is to Earth…and Neptune is seventeen times more massive than Earth.  This doesn’t bode well for the long term future of Triton.  Within the next three and a half billion years, the moon will either be pulled into Neptune’s surface and swallowed or it will be ripped to pieces and form a spectacular ring structure like Saturn’s.

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December 2022