Neptune

After three and a half more years of whizzing through deep space, Voyager 2 arrived at the final stop of its grand tour — Neptune — on 25 August 1989.

Neptune’s deep blue atmosphere was imaged in full as Voyager 2 swooped just 3,000 miles above its north pole. Such a close encounter, its closest with another planet since leaving Earth, meant that the craft could get a great view of its gaseous body, its rings and its 14 moons.

The reason for such a close swipe past Neptune was because the team wanted to swing past the larger of the only two moons known to orbit the planet at this point — Triton. This manoeuvre was complex, as Triton was located behind and below Neptune at the time of the fly-by, meaning that the craft had to make a perilously close pass over the top of the planet, but it got a good glimpse of the methane-rich upper atmosphere as it passed by.

Methane absorbs the red light from the Sun, but scatters blue wavelengths back out to space — making the planet appear blue to observers. That gave it its name — after the Roman god of the sea — but only after much astronomical infighting over credit for the discovery that almost saw a French astronomer named Le Verrier name it after himself.

Neptune’s active atmosphere

Neptune is similar in composition to Uranus. It’s slightly smaller, but a little denser and it’s thought to have the same icy, rocky centre. However, its weather is very different — unlike the hazy, featureless view of Uranus from space, the atmosphere of Neptune has active and visible weather patterns. At the time of Voyager 2’s fly-by, the Southern hemisphere was marked by a “Great Dark Spot”, a little like Jupiter’s “Great Red Spot”.

The spot has since disappeared, but is thought to have been an anticyclonic hole in the methane cloud deck — with cirrus clouds above it comprising of crystals of frozen methane. Since then, other holes have appeared and disappeared, most recently including an almost identical one in the northern hemisphere that has remained visible for several years.

The fly-by of Triton, the only one of Neptune’s moons to be spheroidal, yielded yet more surprises for the Voyager 2 team. It’s one of the few moons in the solar system known to be geologically active — its southern pole is covered with geysers that erupt for a year, spewing nitrogen eight kilometres into its weak atmosphere, and volcanoes that flow with lavas of water and ammonia.

Triton

Closer to its equator in the western hemisphere, the moon features a unique surface known as “cantaloupe terrain”, due to its resemblance to melon skin. This terrain consists of water ice that’s formed into depressions between 30 and 40 kilometres in diameter. It’s unlikely that they’re impact craters, as they’re evenly distributed and all of similar size. Instead, it’s thought that they formed due to diapirism — the same process that forms salt domes on Earth and can be seen in lava lamps.

Almost nothing is known about the moon’s northern pole. It was in shadow during Voyager 2’s fly-by and nearly everything we know of Triton comes from this single, brief encounter.

After the success of Voyager 2’s farewell photo of Uranus, a similar shot was scheduled for Neptune as it sped off on its own course out of the solar system — now 48 degrees below the solar plane as a result of its Triton encounter. The result was perhaps even more beautiful — a thin crescent of Neptune with Triton hanging below it, like a mother and her child. The blue colour has disappeared, as the light is no longer being scattered back by the atmosphere — merely reflecting off the top.It remains the last, closest photo taken of the Neptune system.