Their cloud tops are among the coldest places in the solar system, too: –371 degrees Fahrenheit (–224 degrees Celsius) for Uranus and about –361 F (–218 C) for Neptune. Only the surface of Pluto is colder.
But despite receiving so little light from the Sun, Neptune has weather — and what weather! Wispy white clouds scoot above the planet, and in 1989, Voyager 2 clocked winds near a strange, previously unseen dark spot on Neptune, reaching 1,000 mph (1,609 km/h) — the strongest of any in the solar system. This spot, dubbed the Great Dark Spot, was a massive spinning storm the size of Earth. Since its discovery, the storm has faded, but new ones have appeared elsewhere on the planet. By studying these dark spots, scientist might find a window to Neptune’s lower atmosphere.
Both ice giants have atmospheres made of mostly hydrogen and helium, with small amounts of methane. It is the methane gases, however, that give Uranus its beautiful aquamarine color, as methane absorbs red light. Neptune’s color, on the other hand, is a more vivid blue. While methane contributes to that, another elementary component is likely the cause of such an intense blue — but exactly which one remains uncertain.
Beneath the atmospheres of both planets, the mantles are mostly super-hot, high-pressure global oceans of water, ammonia, and methane — essentially a liquid electrical conductor. Inside their mantles, there may exist a deep layer where water is broken down into a soup of hydrogen and oxygen ions. Thousands of miles beneath their surfaces, the pressure is so great that methane splits apart and hardens its carbon compound into diamond crystals that sink to the planets’ cores. Yes: It could be raining diamonds.
The solid core of both planets is made of iron, nickel, and silicates. Neptune is approximately 17 times Earth’s mass and has a core weighing only 1.2 Earth masses. Uranus’ core is small, only 0.55 Earth masses, while the planet’s overall mass is around 14 Earth masses.
While these facts are all well known, the internal heat of both planets presents much more of a conundrum. Uranus hardly radiates any heat at all compared to other planets in the solar system. Neptune, on the other hand, despite being 10 astronomical units (AU; where 1 AU is the average distance between Earth and the Sun) beyond Uranus, radiates 2.61 times as much energy as it receives from the Sun. The explanation for this could have to do with an ancient impact from a protoplanet which expelled most of Uranus’ heat. This would also explain the planet’s extreme tilt. But astronomers still don’t know if internal heat released by Neptune (or Uranus) varies seasonally. Another visiting spacecraft could provide more data.