Here are several views of Neptune and Triton obtained by the already then ageing Voyager 2 spacecraft

Neptune & Triton.

Imaged: Monday 14th August 1989 to Sunday 3rd September 1989. 

Closest approach: Friday 25th August 1989.

Here are several views of Neptune and Triton obtained by the already then ageing Voyager 2 spacecraft. Despite Voyager 2's age, (was over 12 years since launch) the spacecraft performed magnificently and to this date, these are the best views of Neptune and Triton, and will be for quite some time yet. 

Neptne orbits our Sun at an average distance of 30.1 times the Sun to Earth distance or 4.515 Billion KM / 2.804 Billion miles, once every 164 years & 281 days. 

Neptune has a mass of 17 times that of Earth, has a mean diameter of 42,244 KM / 30,580 miles and rotates on it's axis once every 16 hours and 7 minutes. Neptune has a deep atmosphere, consisting of 80% Hydrogen, 18% Helium, 1% Methane (gives Neptune the deep blue colour) and 1% trace gases like Ammonia, Nitrogen, etc. Neptune has an average global density of 1.68 times that of water, suggesting a rocky core, a gigantic mantle of water and compressed hot ice, overlain by cold ice then the deep atmosphere.

At the 1 Bar level (same as Earth's at sea level) in Neptune's atmosphere the temperature is minus 201 Celsius / minus 330 Fahrenheit or 72 Kelvin. Neptune has some of the fastest winds known in the solar system, with some jet streams hosting winds in excess of 2,500 KPH / 1,550 MPH as Neptune radiates more heat from it's deep interior that receives from the very distant Sun.  A huge Dark Spot was present, an area of High Pressure punching an Earth wide hole in the upper cloud deck. Bright ammonia and methane ice clouds quickly formed and disappeared and another large storm was seen further south with towering white clouds in the centre.

Neptune has an axial tilt of 28.32 degrees and each season lasts for about 41 years & 70 days. Northern hemisphere Neptune seasons, equinoxes and solstices for the last Neptune year. Spring: 1880, Summer: 1921, Autumn: 1962, Winter: 2003. Spring again in 2044.

Neptune also has three narrow, faint, icy and dusty rings.

Neptune has several moons, though not nearly as many as the other three large outer planets, 14 at the last count, most are small except for one, the enigmatic large moon Triton.

Triton is large at 2,706 KM / 1,680 miles wide.

Triton is unusual in large solar system moons to orbit the parent planet backwards (east to west direction or clockwise from the north). Triton also orbits Neptune by a 23 degree tilt.

Triton orbits Neptune at a mean distance of 354,760 KM / 220,305 miles once every 5 days. 21 hours and 2 minutes at an average speed of 15,804 KPH / 9,814 MPH. The rotation period of Triton is the same so keeps the same face turned towards Neptune permanently.

Triton certainly did not form around Neptune, and data from the Voyager 2 spacecraft, suggest that Triton formed much further from the Sun, maybe was a former Kuiper Belt Object, a possible sibling of Pluto, Eris, Makemake, Haumea, etc. In fact Triton is only a little larger than either Pluto or Eris. Orbital mechanics suggest that Triton must have had a reasonably large moon of its own prior to capture by Neptune, as forward momentum has to be lost, in this case a relatively large moon of Triton would have been ejected as energy would have been imparted into it. If such a moon existed, chances are it is back in the Kuiper Belt, and at least two objects may fit, KBOs Quaoar & Orcus. 

There are issues, as both these have small moons of their own, if either of these were Triton’s original companions, it would be difficult to see how they would have small moons of their own. A former Triton moon may well have been tidally heated during the ejection and Quaoar shows crystalline ice in its spectrum, ice that had melted then refroze slowly with larger ice crystals. However the presence of the moon since named Weywot orbiting Quaoar makes this scenario difficult. 

During the capture of Triton by Neptune, would have resulted in an initial orbit that was very elliptical. As further orbital energy was robbed from Triton, internal heating would melt the original surface, then when the orbit became circular, Triton would refreeze with a new surface. Some small moons of Neptune present would have been scattered, perhaps some ending up in the Kuiper Belt, some deorbiting into Neptune and some perhaps ending up as comets coming close to the Sun. Neptune does have some smaller moons much closer in, but compared to Jupiter, Saturn and Uranus, Neptune is short of moons, and Triton’s arrival would explain that. 

Only about 40% of Triton has been seen well, well enough for high resolution geological mapping during the passage of the Voyager 2 spacecraft on: Friday 25th August 1989. This is on the leading, southern, Neptune facing side. However, Triton turned out to have a diverse and apparently geologically young surface, with average terrain being only about 50 million years old, some as young as only 6 million years old, with a few areas older, but no more than 500 million years. Triton is highly reflective, with an average albedo (reflectivity) of about 80%, Only the Saturn moon Enceladus and the KBO / dwarf planet Eris are brighter. 

Triton has only 179 recognized impact craters and the largest seen to date is only 27 KM / 17 miles wide since named Mazomba Crater. Some frozen ‘lakes’ like Ruach Planitia may well be larger impact craters that were flooded, then froze, but this is far from certain. 

Triton also has ridges, pressure ridges, a pink frozen nitrogen ice cap, also ice blisters and what appear to be cryocalderas, pits that appear to have erupted slushy ices that then froze solid. Some nitrogen ice geysers were active on the pink nitrogen ice cap, the extremely weak sunlight, just about heating some subsurface nitrogen ice into liquid nitrogen, then bursting through overlying nitrogen ice forming nitrogen ice geysers. A large area known as the ‘Cantaloupe Terrain’ is covered by shallow circular depressions, terrain only seen on Triton. It is not clear what caused the terrain, but is appears to be collapses related to slushy ice eruptions. Two massive ridges were seen to cross this area in an X form, Slidr Sulci (north – south) and Tano Sulci (east – west). Triton has an extremely tenuous atmosphere, some 1/70,000th the density of Earth’s at sea level, however due to the extremely low temperatures hazes were detected in limb views and the nitrogen ice particles in the ice geysers are blown down wind at an altitude of around, 8,000 metres / 26,250 feet.

Triton is a very cold, ice & rock object, average surface temperature of minus 235 Celsius / minus 391 Fahrenheit or 38 Kelvin. This is colder than the average for Pluto and is the second coldest known large object in the solar system, only Eris at minus 257 Celsius / minus 431 Fahrenheit or 16 Kelvin is colder. If our own Earth cooled to the same temperatures, our oceans would freeze almost all the way down and our atmosphere would collapse and freeze into a layer of frozen gasses 11 metres / 35 feet thick.

The density is 2.01 grammes per cubic centimetre or just over twice that of regular water ice. Triton is composed of 70% rock and 30% ice. Gravity tracking of the passing Voyager 2 spacecraft suggested that Triton has a differentiated interior, a rocky core with an ice rich mantle and crust. The variety of geological features visible also support this. 

Text: Andrew R Brown.

NASA/JPL-CalTech. Voyager 2 spacecraft.