On August 25, 1989, Voyager 2 sailed past Triton, capturing our first real look at Neptune's moon. With half the moon lost in shadow, we only got a glimpse of the icy moon. Now that footage has been restored and enhanced for your viewing pleasure.

The most detailed map of Neptune's moon Triton. Full resolution available here. Image credits: data from Voyager 2 (NASA/JPL), processed by Paul Schenk/Lunar & Planetary Institute.

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August 25, 2014 marks a quarter a century since Voyager 2's flyby of Neptune and Triton, the last major accomplishment of the probe before continuing on to deep space. Inspired by both the anniversary and New Horizons' pending encounter with Pluto next summer, Dr. Paul Schenk at the Lunar and Planetary Institute went through NASA's archival photographs of Voyager 2's flyby of Triton.

JPL billboard marking Voyager's progress. Image credit: Paul Schenk

Schenk has good historical context to go diving into the archives: he'd been an intern during the Jupiter flyby in 1979, and the Saturn flyby in 1981. When the Neptune flyby approached, he working as a post-doc at Jet Propulsion Laboratories, but had no connection with the project. Lucky for him, his advisor pulled a few strings to get them both smuggled in to watch the final flyby along with an entire scrum of press. During the flyby, images were recorded onto tape, and only later relayed back to Earth for eager scientists.

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Monitors displaying Voyager's encounter with Neptune and Triton. Image credit: Paul Schenk

Voyager's visit to Triton was brief, a quick flyby on its way out of the solar system. The spacecraft's speed paired with Triton's slow rotation limited data, with much of the moon lost in darkness or blurred beyond recognition.While he couldn't do anything to peer into the shadowy northern hemisphere, Schenk could re-process the existing images to make the most detailed map of Triton we've ever seen.

Jay Inge creating an airbrush map of Triton. Triton was the last body mapped in this fashion. Image credit: Paul Schenk

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Schenk selected the images, performed radiometric calibration, geographic registration, photometric correction, and then finally assembled the mosaics to make a map of Neptune's moon. Using all we've learned in the intervening decades, he could improve the accuracy of feature locations, sharpen up some features by removing the camera's blurring effects, and improve colour processing by stretching out the contrast. Voyager collected light in orange, violet, and ultraviolet bands; these are mapped to orange, green, and blue with enhanced contrast, so the colours aren't quite true-colour to the moon's natural hues. Schenk noted we don't actually know what true-colour is for the moon:

The color does have a greenish cast in equatorial areas. This seems to be real, but there are 'concerns' with Triton's color. First, the color images were sometimes smeared or noisy, due to long exposures under very low solar lighting intensity for which the cameras were not designed. This explains some of the splotchy color mottling that is apparent in a few areas. Secondly, there are some uncertainties in the photometric properties of Triton. Earth-based spectra of Triton obtained in the 1970's and 80's differ in the inferred visual color of Triton and it was not possible to get an exact color 'calibration' on Triton. We did our best, but the colors may not be only approximate, given the slightly different color sensitivity of the Voyager 2 camera.

At full resolution, the new map of Triton has a resolution of 600 meters per pixel. It's formatted to drop it into Google Earth or similar programs if you want to gaze at it in all its spherical glory.

Image credits: data from Voyager 2 (NASA/JPL), processed by Paul Schenk/Lunar & Planetary Institute.

Triton is a world of extremes. It's by far the largest moon of Neptune, with a radius of 1,350 kilometers, or just 22% smaller than Earth's moon. Triton was an outer system body captured by Neptune, then gravitationally massaged until tidal heating melted its interior. This thermodynamically active history was enough to produce volcanoes, fractures, and other features visible on the icy surface.

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Triton has a relatively high density, more than twice that of water at 2.050 grams per cubic centimetre. It's one of the densest moons in the outer solar system, with only Europa and Io beating it out. This high density suggests a large core of rock and metal beneath the ice.

During its brief flyby, Voyager spied active geysers erupting from Triton, marking it as one of the rare active bodies in the outer solar system. Only two other moons are on that list: Jupiter's Io, and Saturn's Enceladus. Expanding to include planets Earth and Venus, Triton is one of just five places in the solar system with observed geothermic activity. The activity is driven by seasonal heating by the sun.

Global colour mosaic of Triton produced from Voyager 2 data in 1998. Image credit: NASA/JPL/USGS

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At 38 K (-235°C, or -391°F), Triton has the coldest surface in the solar system. It's also only one of three bodies with a nitrogen-dominant atmosphere, joined by Saturn's Titan and the Earth. The bitterly cold surface mean most of the atmosphere is condensed as frost, with frozen nitrogen coating the moon along with carbon monoxide, carbon dioxide, and methane. That icy surface is bright: Triton has an albedo so hight 70% of all light is reflected.

The icy surface is full of slight anomalies. The pink southern polar cap might be methane ice, reacting with sunlight to produce pink and red compounds. Dark streaks crossing the deposits might be a mixture of ice and carbonaceous dust, deposits from the venting geyser plumes. The blue-green band along the equator may be relatively fresh nitrogen frost.

Detailed view of cantaloupe terrain as processed in 1996. Image credit: NASA/JPL

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The icy landforms are more uncertain. The green areas are coated in roughly circular depressions separated by ridges; the features are known only as "cantaloupe terrain." Too regular in both size and spacing to be impact craters, and not found anywhere else in the solar system, their origin is still uncertain.

Topographic model of cantaloupe terrain, with 25x vertical exaggeration. Image credit: NASA/JPL/Universities Space Research Association/Lunar & Planetary Institute

On the twentieth anniversary of the flyby, Schenk melded the Voyager imagery with a new topographic model of the puzzling terrain, releasing it as a digital flyover along the equator. The most recent theory as to how the terrain is formed was outlined in a caption accompanying stillframes from the flyover:

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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.

The plain with irregular pitting may be cryovolcanic features, produced by the eruption liquids from Triton's interior, now frozen. Smooth plains, mounts, and round pits are all icy landforms analogous to volcanic processes and flowing lava. The grooves and ridges are familiar from other icy moons, produced by fracturing and deformation of the surface. Impact craters rarely scar the surface, marking it as relatively fresh and young, at most a few million years old.

Scientists and reporters awaiting incoming Infrared Interferometer Spectrometer data from Neptune. Image credit: Paul Schenk

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To commemorate the flyby, Jet Propulsion Laboratories published The Voyager Neptune Travel Guide (publication 89-24), a book targeted at popular audiences outlining fun facts and describing the encounters.

A flipbook of Voyager 2's orbital manoeuvring. Image credit: Paul Schenk

It also contains one of the organization's first efforts to give its robotic explorers personalities, as a section of the guide includes a diary told from the perspective of the Voyager 2 probe. This is its diary entry from after the Neptune and Triton encounter, as the probe began its trip out of the Solar System:

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Star Date -1.259 (1990) Today I am filled with an intense sorrow. The encounter with Neptune has ended. It is a but a tiny star in the background. Part of my sorrow is from not being able to visit Pluto before my departure, for it is a very mysterious body. . . . I wish that I, or my sister spacecraft, had been the one to unveil some of the mysteries of Pluto. The Project never seriously considered a visit to Pluto because it would have meant foregoing Voyager 1's encounter with Saturn's moon Titan. . . . However, it will be many decades before man will send another spacecraft out to that part of the solar system.

While Voyager 1 and 2 missed visiting Pluto, their trip out of the solar system was interrupted by a whole new class of icy bodies after Neptune. Just three years after the Triton flyby, the first Kuiper Belt Objects were encountered, opening up a whole new classification of astronomical bodies.

Check out Paul Schenk's blog for more photographs and stories of the day of the Triton flyby.