Our Solar System Part 6: Neptune

Neptune is a beautiful and violent blue planet, named after the Roman god of the sea.

This is a composite image taken by Voyager 2 as part of its study of the outer solar system.

Neptune is very far from the Sun, about 4.5 billion km away. Earth, in comparison, is about 150 million km from the Sun, that's about 30 times closer than Neptune is. In July 2011, it will have completed just one orbit, one Neptune year, since its discovery in 1846. As a result of its distance it receives only a tiny fraction, about 1 thousandth, of the Sun's energy that we receive here on Earth.

Neptune's Layers

What's most interesting about Neptune is what it's made of, and thanks to Voyager 2's information, we know quite a bit. It's an ice giant, consisting mostly of water, ammonia and methane ices, similar to Uranus. But Neptune's atmosphere is a more vivid azure blue than Uranus's milder cyan colour because it contains a larger trace of methane, which absorbs red light. Neptune likely has clouds of different compositions depending on the altitude. 

Bands of high altitude clouds of condensed methane droplets cast shadows on a lower cloud deck as shown here in this NASA image taken by Voyager 2.

Below the high-altitude cirrus-like clouds are clouds of ammonia, hydrogen sulfide, ammonium sulfide and even clouds of water ice that lie deep within the atmosphere where the temperature approaches 0°C. If a space probe attempted to land on Neptune, it would be buffeted by supersonic winds of up to 2000 km/h as it begins its plunge through a gas atmosphere thousands of kilometers thick. It would then gradually sink through a slushy mantle, all the while enduring pressure that is increasing from the hard vacuum of space to pressure millions of times greater than what we experience on the surface of Earth, where atoms themselves begin to be crushed into each other and exhibit strange physical properties! On its way, it would have to withstand temperatures ranging from -218°C in the cloud tops to up to 5000°C deep within the planet's mantle. If it could survive all of that, it would finally reach a relatively small Earth-size core of superheated iron and nickel. 

The probe would pass through a very strange electrically conductive super-dense water-ammonia ocean. Then it would be bombarded by diamond-hard methane crystal "rain" and finally it would pass through superionic water that glows bright yellow and is as hard as steel. Any probe we could build would be crushed and melted.

The Great Dark Spot

A series of ever-changing dark spots on Neptune are visible through the Hubble telescope. The Great Dark Spot is actually a series of anticyclonic storms that form and dissipate every few years. 

This image of a Dark Spot, taken by Voyager 2, was about the same size as Earth in diameter, around which winds blew up to 2400 km/h, the fastest wind speed ever clocked in the solar system.

The Great Dark Spot is thought to represent a hole in the methane cloud deck that generates large white clouds of frozen methane crystals. These storms may be tied to Neptune's seasons, each one lasting 40 years (Neptune has an axial tilt which creates its seasons, similar to that of Earth). In addition to its extreme winds, Neptune also experiences strong latitudinal wind shear because of the differential rotation of its atmosphere (strong enough to instantly rip apart any manmade probe).

A long-standing question is why Neptune has such ferocious winds and storms even though it receives very little energy from the Sun. Whereas Earth's storms are driven by the Sun's energy, Neptune is too far away to draw much energy. The energy source of its storms comes instead from deep within. In fact, Neptune radiates more than twice the energy it receives from the Sun. There are several theories about this phenomenon, one of which relies on intense radioactivity within the small core. Another theory proposes that, as methane is squeezed under great pressure, it separates into two components: liquid metallic hydrogen and (carbon) diamonds, and as these diamonds seasonally rise and sink, extra energy through friction is created.

Magnetosphere and Rings

Neptune's magnetosphere is also mysterious. It has a lot in common with Uranus's magnetosphere in that it is about the same magnitude (both of these magnetospheres are far larger than Earth's) and it probably comes from the movement of some kind of conductive material, maybe the highly pressurized water within its slushy shell rather than from a molten iron core, as it does in Earth, or from liquid metallic hydrogen as it does in Jupiter and Saturn (Neptune just isn't big enough to generate the immense pressure required to make liquid metallic hydrogen).

Like Uranus, Neptune's magnetosphere is oriented at an extreme tilt, almost 50 degrees. The configurations of Neptune's magnetosphere and the resulting aurora are so complex they are very difficult to model. Mathematical models, in fact, suggest that Neptune's rings may complicate the motion of the particles in its magnetosphere. Neptune appears to have three separate plasmaspheres (inner magnetospheres) instead of one large one (as Earth does).  

Earth's singular plasmasphere is shown here to give you an idea of where it's located.

Neptune's magnetosphere generates faint aurora as well as radio emissions. Voyager 2 "heard" them as a radio hiss as it passed by the planet in 1989.

Neptune has five principle rings, shown here, along with some of its moons.

The rings are named after astronomers that contributed important research on the planet. 

They are much less dense and fainter than those of Saturn and more closely resemble those of Jupiter. Unlike the highly reflective icy chunks of Saturn's rings, these rings consist of dark organic compounds and dust. Like all planetary rings, they are stabilized by resonant interaction with the planet, with each other, and with the moons. They may be the result of a long ago collision with an inner moon that fragmented.

The gravity of Neptune shapes the Kuiper belt, a ring of small icy chunks that extends from Neptune's orbit to significantly past that of Pluto, in much the same way that Jupiter's gravity dominates the asteroid belt.

Formation and Early Jostling Around

Neptune and Uranus are too large to be explained by core accretion alone, a process in which the collapsing mass of a molecular gas cloud forms a protoplanetary disk, shown here, from which the Sun, planets, moons, asteroids and other bodies formed.

There simply wasn't enough matter density that far away from the disk center to account for the mass of these ice giants. A model called the Nice model, based on three papers published in Nature in 2005, is now the most widely accepted explanation of Neptune's early history. It suggests that the giant planets initially formed much closer to the Sun than they are today, and they migrated outward long after the protoplanetary gas disk itself dissipated. This model also successfully explains other solar system phenomena such as the Late Heavy Bombardment of the inner solar system, and the formation of the Oort cloud, the Kuiper belt and other bodies.

This is how they think it worked: Over time, many planetesimals, large chunks of rock and ice, at the outer edge of the protoplanetary disk, approached the outer giant planets, Neptune and Uranus, which were then relatively close to the Sun, between 5 and 17 astronomical units (AU) away (Neptune is now 30 AU away from the Sun). As the planetesimals approached the giant planets, they were scattered in toward the Sun, like sling-shots, by the planet's immense gravitational fields. By doing so they exchanged angular momentum with the giant planets so that the planets gradually moved outward in response. This preserves the total angular momentum of the system. Jupiter, in contrast, and far more massive than Neptune or Uranus, threw these planetesimals into highly eccentric orbits or right out of the solar system altogether rather than inwards, so this planet moved inward slightly instead, again conserving the system's angular momentum.

This process of orbit adjustment took place gradually, over several hundreds of millions years, with these planets exerting all the while what is called mean-motion resonance on each other and further altering each other's orbitals until they eventually stabilized into what we see today. This resonance also scattered what was left of the primordial disk, removing 99% of its mass. The planetesimals that were thrown into the inner solar system created the Late Heavy Bombardment, around 4 billion years ago, 600 million years after the solar system formed, a period of intense impacts on the inner rocky planets, as evidenced by the many impact craters we can see today on our pockmarked moon. The remaining rock and ice chunks organized themselves into the asteroid belt and the Kuiper belt.

Triton

Neptune has 13 known moons. The largest one by far, and the only one massive enough to form a proper sphere, is Triton, named after a Greek god, the messenger of the sea, and son of Poseidon. Triton is the only large moon in the solar system with a retrograde orbit, and this suggests that it was captured rather than formed in place. It was probably once a dwarf planet within the Kuiper belt. 

This is a Voyager 2 mosaic image of Triton.

Triton, far from being an orbiting dead rock, is alive with tectonic activity. It's covered with a thick bright ice composed of frozen nitrogen, water and carbon dioxide which is rhythmically squeezed and pulled like the ocean waters of Earth, a tidal interaction that is very gradually degrading Triton's orbit so that it will eventually either collide with Neptune or break up and form a new ring system about 3.6 billion years from now. After Triton was captured by Neptune's gravity, it remained entirely liquid for a billion years thanks to a very eccentric orbit which caused a great deal of tidal heating of the moon's interior. Triton's orbit eventually stabilized into a near perfect circle and as it did so it froze.

A solid core of metal and rock is surrounded by a mantle of water, which may be liquid and convective thanks to the core's radioactivity. Surrounding the mantle is a surface layer of frozen nitrogen, water ice and dry ice (carbon dioxide) and above this is a very thin nitrogen atmosphere with some wind and a few faint condensed nitrogen clouds.  It's extremely cold on the mostly flat surface, about -237°C. Instead of molten rock volcanoes on Earth, Triton spews regular water flows from its cryovolcanoes as well as nitrogen geysers with plumes up to 8 km high. These geysers are not associated with volcanic activity as they are on Earth. Instead, they occur when subsurface pockets of nitrogen heat up and expand from solar heating. Very little sunlight strikes Triton but only a 4°C rise in temperature could create enough pressure for nitrogen gas to erupt in the enormous geysers observed. These "summer" geysers likely last for many decades as Triton's polar regions take turns facing the Sun for 82 years at a stretch. 

Here is a simulation of what Neptune might look like from Triton. 

The atmosphere is so thin the sky appears black with only a thin rim of haze visible at the horizon. Neptune's other moons are so small they would not even be visible in the sky.

Future Missions

Because Triton may have a liquid water mantle, there is a possibility that life may have gotten a foothold there. This connection between the presence of liquid water and the possibility of life will be explored fully in a future article. A future mission called Neptune Orbiter was proposed in 2005 to launch in 2016, taking just over 10 years to get to Neptune. It would study both Neptune and Triton in detail. An orbiter would analyze Neptune's magnetosphere and image the planet. Two probes would also go into Neptune's atmosphere. Similar to the Galileo probe that studied Jupiter's atmosphere, the probes would study the atmosphere in detail before continuing to descend, sending back information until they were inevitably crushed.  As well, one or two probes would land on Triton to analyze the surface, the interior and atmosphere and to search for liquid water and signs of microscopic life, a plan similar to but more advanced than the Cassini/Huygens mission which studied Saturn and its moon, Titan. Unfortunately  this mission is no longer in the works at NASA, at least for now.

Next we will explore Saturn.