The Solar System:


"Fontana del Nettuno" ("Fountain of Neptune") by Giovanni Ceccarini (19th century CE; Italian), from 1822-23 (Piazza del Popolo, Rome, Italy).

Credit: Giovanni Ceccarini; photographed by Jebulon (Wikimedia Commons) [link]

Why so (really) blue?

Like Uranus, methane in Neptune's atmosphere absorbs red light – but that doesn’t completely explain Neptune’s dark blue color.

In addition to its darker color, the atmosphere of Neptune is not as uniformly featureless as that of Uranus.

This image of Neptune was obtained in visible light by Voyager 2 in 1989. The picture shows the Great Dark Spot and its companion bright smudge; on the west limb of Neptune (lower left), the fast moving bright feature called Scooter and the little dark spot are visible. These clouds were seen to persist for as long as Voyager's cameras could resolve them. North of these, a bright cloud band similar to the south polar streak can be seen.

Credit (image and some text): NASA/JPL [link]

Ice? You'll have to be more specific...

As of May 2019, there are 18 known forms of water ice.

  • Ice I = This is normal ice like you put in a glass of water. This is the only kind found naturally on Earth.

  • Ice II – XVII = These forms of ice exist under extreme conditions of temperature and pressure.

  • Ice XVIII = A recently discovered form of “superionic ice”.

The 1963 novel “Cat’s Cradle” by Kurt Vonnegut, Jr., features a fictional (and deadly!) version of ice IX (“ice-nine”). Real ice IX is harmless!

Credit: © 1963 Holt, Rinehart and Winston (HRW) [link]

How do you create ice XVIII?

Blast a droplet of water with the highest powered lasers on Earth and see what happens…

This creates conditions of

      • Pressure = millions of Earth atmospheres

      • Temperature > 2000 K

The water molecules assemble into a mixed solid/liquid state.

      • The oxygen atoms in the water molecules form a lattice (solid)

      • The hydrogen atoms flow within this lattice (liquid)

Superionic ice is

      • dense (4x ice I),

      • hot,

      • and black…

In this time-integrated photograph of an X-ray diffraction experiment, giant lasers focus on a water sample to compress it into the superionic phase. Additional laser beams generate an X-ray flash off an iron foil that allows the researchers to take a snapshot of the water in its superionic ice state.

Credit (image and some text): Millot, Coppari, Kowaluk (LLNL) [link]

The conditions for creating superionic ice XVIII are difficult to produce on Earth, but exist naturally deep in the interior of the atmospheres of ice giant planets like Uranus and Neptune.

Because of this, superionic ice might be the most common form of water in the Universe.

The discovery of superionic ice potentially solves the puzzle of what giant icy planets like Uranus and Neptune are made of. They’re now thought to have gaseous, mixed-chemical outer shells, a liquid layer of ionized water below that, a solid layer of superionic ice comprising the bulk of their interiors, and rocky centers.

Credit (image and some text): Moteh, I., 2019, Quanta Magazine [link]

This Voyager 2 photograph of Neptune shows three of the prominent features seen on Neptune in 1989. At the north (top) is the Great Dark Spot, accompanied by bright, white clouds that undergo rapid changes in appearance. To the south of the Great Dark Spot is the bright feature that Voyager scientists have nicknamed "Scooter." Still farther south is the feature called "Dark Spot 2," which has a bright core. Each feature moves eastward at a different velocity, so it is only occasionally that they appear close to each other, such as at the time this image was obtained.

Credit (image and some text): NASA/JPL [link]

A 1994 observation of Neptune with the Hubble Space Telescope showed that the Great Dark Spot had disappeared. But by 2016, when Neptune was observed again with the Hubble Space Telescope, a new dark spot had appeared.

The full visible-light image of Neptune from the Hubble Space Telescope (left) shows that a new dark feature resides near and below a patch of bright clouds in the planet's southern hemisphere. The dark spot measures roughly 3,000 miles (4,800 kilometers) across. Other high-altitude clouds can be seen at the planet's equatorial region and polar regions.

The image at right shows that Neptune's dark vortices are typically best seen at blue wavelengths. Only Hubble has the high resolution required for identifying such weather features on distant Neptune.

Credit (image and some text): NASA, ESA, and M.H. Wong and J. Tollefson (UC Berkeley) [link]

A cloudy day on Neptune

This Voyager 2 high resolution color image of Neptune shows linear clouds that are stretched approximately along lines of constant latitude and float at high altitudes in Neptune's atmosphere. The bright sides of the clouds which face the Sun (toward the lower left) are brighter than the surrounding cloud deck because they are more directly exposed to the Sun. Shadows can be seen on the side opposite the Sun, cast by the clouds onto a lower layer of Neptune's atmosphere.

Credit (image and some text): NASA/JPL [link]

Hey, guess what? Neptune has rings!

(but they're pretty unimpressive)

This pair of sensitive Voyager 2 images show the full ring system of Neptune. The planet itself would be located in the black strip between the two images on either side. In order to photograph the rings, Neptune has to be blocked out. In the bright reflected sunlight from Neptune, the faint, narrow rings would be washed out and all but invisible.

Credit: NASA/JPL [link]

In Neptune's outermost ring, 23,400 km (39,000 miles) out, material mysteriously clumps into three bright arcs. These arcs are probably caused by gravitational interactions with the shepherd moon Galatea, which orbits just inside the ring. Voyager 2 acquired this image as it encountered Neptune in August of 1989.

Credit: NASA/JPL [link]

Moons of Neptune

  • Neptune has 14 known moons in total.

  • All of the moons are pretty tiny except Triton.

  • Triton might be a captured dwarf planet from the Kuiper Belt (i.e., originally an object like Pluto).

      • Triton is the only large moon in the Solar System with a retrograde orbit (i.e., it orbits Neptune in the opposite direction to Neptune's rotation). This supports the "capture" theory of Triton's origin.

Global color mosaic of Neptune's largest moon, Triton, obtained in 1989 by Voyager 2. With a radius of 1350 km (839 mi), about 22% smaller than Earth's moon, Triton is by far the largest satellite of Neptune. It is one of only three objects in the Solar System known to have a nitrogen-dominated atmosphere (the others are Earth and Saturn's giant moon, Titan). Triton's surface is so cold that most of Triton's nitrogen atmosphere is condensed as frost, making it the only satellite in the Solar System known to have a surface made mainly of nitrogen ice.

The pinkish deposits constitute a vast south polar cap believed to contain methane ice, which would have reacted under sunlight to form pink or red chemical compounds. The dark streaks overlying these pink ices are believed to be an icy and perhaps carbonaceous dust deposited from huge geyser-like plumes. The bluish-green band visible in this image extends all the way around Triton near the equator; it may consist of relatively fresh nitrogen frost deposits. The greenish areas includes what is called the cantaloupe terrain, whose origin is unknown, and a set of "cryovolcanic" landscapes apparently produced by icy-cold liquids (now frozen) that erupted from Triton's interior.

Credit (image and some text): NASA/JPL/USGS [link]

Detail of Triton's south polar terrain photographed by the Voyager 2 spacecraft. About 50 dark plumes mark likely cryovolcanoes or "ice geysers".

Credit: original image = NASA/JPL [link]; reprocessed image = Ilmari Karonen (Wikimedia Commons) [link]



  • Millot, M., Coppari, F., Rygg, J. R., et al., 2019, Nature, 569, 251 - “Nanosecond X-ray diffraction of shock-compressed superionic water ice” [link]

Also see:

      • Bishop, B., 2019 (May 8), Lawrence Livermore National Laboratory (LLNL) - ”Giant lasers crystallize water with shockwaves, revealing the atomic structure of superionic ice” [link]

      • Sokol, J., 2019 (May 8), Quanta Magazine - “Black, Hot Ice May Be Nature’s Most Common Form of Water” [link]