A Distant Moon World

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In the outer Solar System, far from our Sun’s brilliant golden light and intense heat, there is a beautiful banded blue planet that is orbited by a bizarre moon. The sapphire-blue ice-giant Neptune is both the farthest major planet from our Sun, as well as the smallest of the quartet of outer gaseous giant planets–the other three are Jupiter, Saturn, and Uranus. Triton is Neptune’s largest moon, and this icy, mysterious, and unusual small world may not have been born a moon at all. Triton is thought to be a captured frozen oddball that wandered away from its remote birthplace in the Kuiper belt–only to be snared by the gravity of its adopted parent-planet. In May 2019, a team of astronomers using Gemini Observatory to explore Triton, announced their surprising discovery that for the first time beyond the lab, they have observed the extraordinary marriage between carbon monoxide and nitrogen ices on Triton.

Extreme conditions can cause all kinds of exotic things to form, and carbon monoxide (CO) and nitrogen (N2) are both mixed together and frozen solid on Triton. This new discovery offers important insights into how this volatile witch’s broth can transport material across Triton’s surface via geysers, cause seasonal atmospheric changes, and provide a context for conditions that may be seen on other remote, icy worlds.

While working in a laboratory, an international team of scientists discovered that there is a very specific wavelength of infrared light absorbed when carbon monoxide and nitrogen molecules unite and vibrate together in unison. When carbon monoxide and nitrogen ices are observed individually, each absorbs its own characteristic wavelengths in infrared light. However, the tandem vibration of an ice mixture absorbs at an additional, tattle-tale wavelength identified in this new study.


The same team, using the 8-meter Gemini South Telescope in Chile, announced that they recorded this distinctive infrared signature on Triton. The team’s use of the high-resolution spectrograph dubbed Immersion Grating Infrared Spectrometer (IGRINS) proved to be of central importance. IGRINS was constructed by a collaboration between the University of Texas at Austin and the Korea Astronomy and Space Science Institute (KASI). Both the Gemini Observatory and IGRINS are funded by the US National Science Foundation (NSF).

Triton, as Neptune’s largest moon, was also the first Neptunian moon to be discovered. The discovery was made on October 10, 1846, by the English astronomer William Lassell (1799-1880). Triton also has the distinction of being the only large moon in our Solar System that sports a retrograde orbit. This means that it orbits Neptune in the direction opposite to its parent-planet’s rotation.

At 1,680 miles in diameter, Triton is the seventh largest moon in our Solar System. It is also the sole natural satellite of Neptune that is massive enough to be in hydrostatic equilibrium–and the second-largest planetary moon in relation to its primary, after Earth’s own large Moon.

Because of its tattle-tale retrograde orbit, and a composition similar to that of the ice-dwarf planet Pluto, Triton is believed to have been born a dwarf planet–like Pluto. However, Triton was captured by Neptune’s gravity long ago, when it was jostled out of its birthplace in the distant Kuiper belt, and then wandered too close to the powerful gravity of the giant blue planet. At this point, Triton underwent a metamorphosis from a small, icy dwarf planet circling our Sun, to a large moon of one of the giant planets in our Solar System.The Kuiper belt is the remote home of a multitude of frozen comet nuclei and icy dwarf planets. This region is situated beyond the orbit of Neptune, at approximately 30 to 50 astronomical units (AU). One AU is equivalent to the average distance between Earth and Sun, which is about 93,000,000 miles. Pluto is a denizen of this frigid, twilight realm, where icy objects, both large and small, dance in our Solar System’s distant deep freeze.

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