Triton: Insights into an Icy Surface

Al Jackson reminds me in a morning email that today is the 100th anniversary of the Arthur Eddington expedition that demonstrated the validity of Einstein’s General Relativity. The bending of starlight could be observed by looking at the apparent position of stars in the vicinity of the Sun during a solar eclipse. Eddington’s team made the requisite observations at Principe, off the west coast of Africa, and the famous New York Times headline would result: “Lights All Askew in the Heavens . . . Einstein Theory Triumphs.”

Al also sent along a copy of the original paper in Philosophical Transactions of the Royal Society of London, where authorship is given as “F. W. Dyson, A. S. Eddington and C. Davidson.” This created an agreeable whimsy: I imagined the evidently ageless Freeman Dyson continually traveling through time to provide his insights at major achievements like this, but the reality is that this Dyson was Frank Dyson, then Britain’s Astronomer Royal.

Ron Cowen does a wonderful job on the Eddington eclipse expedition for Scientific American today, if you’d like to read more. And now on to today’s story.

Ices on Neptune’s Largest Moon

Triton has always intrigued me, and in most ways even more than Titan. When I was young, Triton was thought to be much larger than it actually is, and it had the imaginative advantage of being far more distant, so that we had another world almost as large as a planet, we thought, way out there in the farthest reaches of the Solar System. The fact that Triton’s orbit was retrograde also helped, an indication of its likely history as a captured Kuiper Belt object.

We learned a lot once Voyager made the journey, verifying the fact that even though Triton is Neptune’s largest moon, it’s still only about ⅔ the size of our own Moon. We found temperatures near absolute zero and an atmosphere almost 70,000 times less dense than that of the Earth, one laden with nitrogen, methane and carbon monoxide, with a striking mix of terrains.

Image: This view of the volcanic plains of Neptune’s moon Triton was produced using topographic maps derived from images acquired by NASA’s Voyager spacecraft during its August 1989 flyby. Triton, Neptune’s largest moon, was the last solid object visited by the Voyager 2 spacecraft on its epic 10-year tour of the outer solar system. The rugged terrain in the foreground is Triton’s infamous cantaloupe terrain, most likely formed when the icy crust of Triton underwent wholesale overturn, forming large numbers of rising blobs of ice (diapirs). The numerous irregular mounds are a few hundred meters (several hundred feet) high and a few kilometers (several miles) across and formed when the top of the crust buckled during overturn. The large walled plains are of unknown origin, although the irregular pit in the center of the background walled plain may be volcanic in nature. These plains are approximately 150 meters (0.093 miles) deep and 200 to 250 kilometers (124 to 155 miles) across. Credit: NASA/JPL/Universities Space Research Association/Lunar & Planetary Institute.

Triton also offers a striking example of laboratory work leading to insights into distant places. A paper now in process at the Astronomical Journal looks at a specific wavelength of infrared light that is absorbed when carbon monoxide and nitrogen molecules vibrate in unison. Both absorb their own distinct wavelengths of infrared light, but the vibration of an icy mixture of the two produces a unique separate wavelength that the study has identified.

Now we turn to the 8-meter Gemini South Telescope in Chile, which the team used to confirm the same infrared signature on Triton, using a high resolution spectrograph called IGRINS (Immersion Grating Infrared Spectrometer). Stephen Tegler (Northern Arizona University), who led the study, gets across the satisfaction of closing the gap between analysis and observation:

“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 of infrared light on another world is unprecedented.”

The work shows us a Triton where both carbon monoxide and nitrogen freeze as solid ices, but also are able to form an icy mixture that is revealed in the Gemini data. Voyager 2 observed dark streaks on the surface during its 1989 flyby, and we saw geysers at the moon’s south polar region. We may be looking at evidence of an internal ocean providing a source for the geyser material, or possibly heating of the thin layer of volatile surface ices by the Sun.

Image: Voyager 2 image of Triton showing the south polar region with dark streaks produced by geysers visible on the icy surface. Credit: NASA/JPL.

To go any further demands continued investigation, preferably with (one day) an orbiter or lander mission to Triton. For now, though, the fusion of lab work with existing data shows the continuing relevance of the one mission we did manage to fly by distant Neptune, and the ability of Earth-based instruments to build on our emerging understanding of its largest moon.

“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,” adds Henry Roe, Deputy Director of Gemini and a member of the research team. “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.”

Seasonal variation on Triton is slow-moving given Neptune’s 165-year orbit, so that each season lasts about 40 years. Triton’s summer solstice occurred in 2000, meaning we are 20 years out before autumn begins. Patient data gathering will help us see how seasonal variations in the atmosphere and on the surface affect the mixture of ices to be found there. We also now know to look for the infrared signature of mixing ices on other small worlds in the Kuiper Belt.

The paper is Tegler et al., “A New Two-Molecule Combination Band as Diagnostic of Carbon Monoxide Diluted in Nitrogen Ice on Triton,” in press at the Astronomical Journal.