Triton: Strange Ice Dazzles 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).

Bewitching Moon

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.

Triton and Pluto sport similar bulk compositions and densities. The duo also show similar atmospheres. In addition, both remote frozen worlds travel in unusual orbits. Pluto sports a highly eccentric orbit that sometimes carries it closer to the Sun than Neptune. Also, Pluto orbits in the opposite direction around our Sun than do the other major planets in our Solar System. Similarly, Triton orbits its big blue planet in a direction counter to that of Neptune. Because of the rather strange attributes of both Triton’s and Pluto’s orbits, as well as the similarities of their bulk properties and atmospheres, astronomers have long thought that there is some sort of historical bond between the two. Indeed, it was once believed that Pluto is really an escaped moon of Neptune. However, this particular theory is now considered unlikely. It is much more probable that long ago Triton, like Pluto, orbited our Sun independently, but was unfortunately snared by its adoptive parent-planet. In contrast, Pluto was left to wander free from any planet’s gravitational embrace.

Triton’s surface is primarily coated with frozen nitrogen. It also possesses a mostly water-ice crust, an icy mantle, and a large core composed of rock and metal. This hefty core accounts for two-thirds of Triton’s total mass. Triton’s mean density is about 2.061grams per centimeter cubed, indicating that it has a composition of about 15-35% water ice.

Triton is one of only a handful of moons in our Solar System that is geologically active. As a result, it has a youthful surface that is pockmarked by few impact craters. Heavily cratered surfaces indicate an old surface, while few surface craters suggest a young surface that has been recently resurfaced (on geological time scales). Intricate icy volcanic (cryovolcanic) and tectonic terrains strongly suggest a complicated geological past. Regions of Triton’s surface display geyers that shoot out sublimated hydrogen gas. These eruptions contribute to a thin nitrogen atmosphere that is less than 1/70,000 the pressure of Earth’s atmosphere at sea level.

Triton is one of the coldest denizens of our Solar System. Indeed, it is so frigid that most of its nitrogen atmosphere is condensed as frost. This gives Triton’s surface an extremely bright, mirror-like appearance that reflects approximately 70% of the sunlight that is able to reach it. Like Earth’s own bewitching large Moon, Triton is locked in synchronous rotation with its planet. This means that one side of Triton always faces Neptune, while the other side is always turned away. However, because of Neptune’s rather strange orbital inclination, both of the moon’s poles take turns facing our Star. Spacecraft images show oval pits and mounds created as the result of cryovolcanic flows, as well as smooth volcanic plains. Triton’s young surface sparkles brightly with a new ice-coating.

There are two types of mechanisms proposed to explain how Triton was captured by Neptune. In order to be gravitationally snared by a planet, a wandering body must lose enough energy to be slowed down to a speed that is less than that needed to escape from these gravitational ties that bind. An early theory, explaining how Triton may have been sufficiently slowed down, suggests that there was an ancient collision with another object–either a moon or proto-moon circling Neptune, or another object that (as ill-luck would have it) just happened to be wandering by Neptune at a bad time. Of the two, the collision with a Neptunian moon or proto-moon is considered the most likely scenario. However, a more recent hypothesis proposes that, before it was snared by Neptune, Triton was a member of a binary system. When this binary wandered close to Neptune, it interacted in such a way that the binary was broken in two, with one member of the binary shot yowling into space–while the other, Triton, was fortunately captured by Neptune. This event would have to have been both brief and gentle, in order to save Triton from collisional disruption. Such events are thought to have been frequent during the formation of Neptune–or somewhat later as it migrated outward in our ancient Solar System. Simulations conducted in 2017 suggest that, after Triton’s capture and before its orbital eccentricity decreased, it probably did collide with at least one other Neptunian moon, as well as triggering collisions between other moons.

William Lassell discovered Triton only 17 days after the discovery of Neptune itself–and the giant planet’s discovery is somewhat complicated. Neptune was supposedly discovered in 1946 by the German astronomer Johann Gottried Galle (1812-1910), who used calculations by the French astronomer Urbain Le Verrier (1812-1877) and the English astronomer John Couch Adams (1819-1892)–thus making the discovery of Neptune a joint British-French-German achievement.

Lassell discovered Triton using a 61 cm telescope that he had built himself. When the English astronomer John Herschel (1792-1871) received news of Neptune’s discovery, he wrote to Lassell asking him to go on the hunt for possible moons. Lassell did this, discovering Triton only eight days after receiving Herschel’s letter. Lassell also claimed to have discovered rings around the planet. However, even though Neptune was later confirmed to have a system of rings, they are so dark and dim that it is unlikely that Lassell actually saw them.

The spacecraft Voyager 2 flew over Neptune in 1989, sending back to astronomers on Earth the first truly revealing images of this beautiful banded blue world, and its entourage of frigid moons. Neptune’s shades of blue result from atmospheric methane, not oxygen. Some of Neptune’s spot-like storms are white and resemble big whirling marshmallows.

Strange Ice

“While the icy spectral fingerprint we uncovered was entirely reasonable, especially as this combination of ices can be created in the lab, pinpointing this specific wavelength in infrared light on another world is unprecedented,” commented Dr. Stephen C. Tegler in a May 22, 2019 Gemini Press Release. Dr. Tegler, of Northern Arizona University’s Astrophysical Materials Laboratory, led the international study based on Gemini Observatory observations. The research results are published in the Astronomical Journal.

In our own planet’s atmosphere, carbon monoxide and nitrogen molecules exist as gases–not ices. They can form their own distinctive ices, or they can condense together to form the icy mixture spotted in the Gemini data. This exotic mixture of ices may play an important role in Triton’s famous geysers, first seen in Voyager 2 spacecraft images as dark windblown streaks tearing through Titan’s otherwise bright, icy surface.

The Voyager 2 spacecraft first detected Triton’s geysers, in the act of erupting in the moon’s south polar region, back in 1989. Since then, numerous explanatory theories have been devised that have suggested the possible existence of a subsurface ocean as one potential source of the erupted material. Or, alternatively, the geysers may erupt when the feeble heat of Neptune’s summer warms the slender layer of volatile ice on Triton’s surface–possibly involving the carbon monoxide and nitrogen ice mixture recently revealed by the Gemini observation. That icy mixture could also travel around Triton’s surface as a result of seasonally altering patterns of distant sunlight.

“Despite Triton’s distance from the Sun and the cold temperatures, the weak sunlight is enough to drive strong seasonal changes on Triton’s surface and atmosphere. This work demonstrates the power of combining laboratory studies with telescope observations to understand complex planetary processes in alien environments so different from what we encounter every day here on Earth,” explained Dr. Henry Roe in the May 22, 2019 Gemini Press Release. Dr. Roe is Deputy Director of Gemini and a member of the research team.

The seasons change slowly on Triton, because Neptune takes 165 Earth-years to make a single orbit around our Star. One season on Triton lasts a little longer than 40 years. Triton passed its southern summer solstice back in 2000, leaving two decades more for astronomers to continue to conduct their research before autumn comes to this distant, dimly-lit and exotic moon-world.

In the future, the team of astronomers expect that their discoveries will shed new light on the composition of ices and seasonal alterations occurring in the atmospheres of other remote worlds beyond Neptune. Scientists have suspected that this strange mixture of carbon monoxide and nitrogen ices also exists on Pluto–as indicated by the recently visiting New Horizons spacecraft. The new Gemini findings provide the first direct spectroscopic evidence that these ices mix together on these two kindred and remote frozen worlds.

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