Arriving in the upper atmosphere
The sun is dimmer out here, but otherwise the clear sky would look very similar to sky above the clouds on Earth. Like an aeroplane making its descent, you plunge into that red and white cloud.
The top layer of cloud is a fine mist of ammonia crystals two or three kilometres thick, floating at the very top of the sky, hiding the whole planet from outside view.
After a few km, you emerge into clear sky again, but now it is pitch black outside. No sunlight gets through the cloud. Air pressure is the same as on earth, but it is much colder... some 200 degrees below zero. The air isn't really air, but hydrogen mixed up with a little helium.
70,000 km to go - It's raining hard
Temperature and pressure both carry on rising as we go down through two more cloud layers, first of ammonium hydrosulphide ice and then water ice. It's raining heavily here, just like clouds here on earth, but the water raining from this cloud doesn't fall far before tropical temperatures evaporate it back into the clouds.
The air pressure is now unbearable. The pressure! Pressure has risen to 10 bar and rising, at the limits of what deep sea divers endure, but it's going to get much worse.
Jupiter is made 90% of hydrogen, 10% helium, and with tiny traces of other substances -ammonia, hydrogen sulphide, and water in the thin cloud layers, as well as methane (0.3%), ethane, silicon compounds, carbon, neon, oxygen, phosphine, sulphur... Believed to be the same composition as the interstellar gas cloud from which the solar system was born: the 'primordial solar nebula'.
We are still only 100km below the planet's visible surface, no sunlight can get through the clouds above, but there's a glow from below. Hundreds of km below, the sky is so hot that it glows - red hot at first, then white hot, and then bright bluish-white as it reaches 9,700 degrees: five times hotter than the core of a blast furnace. The heat is caused by the huge pressure, from the weight of sky overhead pressing down, hundreds of miles of compressed gas, and the greater gravity on Jupiter, 2.5 times stronger than on earth. This pressure causes something else: a big change.
69,300 km to go - Going super-critical...
700km in to our 70,000km journey to the centre of Jupiter, already it is so hot that the air glows like a blue flame. But it isn't a gas any more. It is so compressed that it behaves more like a liquid. The hydrogen/helium atmosphere in its 'super-critical' phase from hereon down.
It is an ocean of liquid-like gas is 55,000 km deep. Powerful currents of this fluid swirl up and down from the depths of the planet in endless storms of hot liquid raging all the way to the planet's outermost clouds. Travelling down through 55,000 km of this liquid hydrogen storm, conditions get more and more extreme until another change takes place.
16,000 km to go - Liquid metal
55,000 km down, pressure reaches a whopping 3million atmospheres (1 atm = earth's air pressure), where compressed super-critical hydrogen turns into dense metallic liquid hydrogen. 16,000km from the centre, we have entered Jupiter's metallic core, a hot globule of liquid metal hydrogen heated to 25,000K and bigger than the earth.
Jupiter is massive. Its powerful gravity sucks in everything in its path - asteroids, comets, rock and dust. A vast swathe of space has been cleared of rocks and asteroids by this giant. Just like on earth, they are vaporised as they fall into the upper atmosphere, but any dense material such as rock and metal will eventually find its way down centre, melted into the liquid metal core.
So Jupiter has no solid at all, except for the microscopic crystals that make up the icy clouds of the outermost layer of its atmosphere.
So no chance of a space mission to Jupiter, then?
Jupiter is too extreme for anything man-made to survive, and besides, there is nothing to land on. However, the planet has over 60 moons, some of which could make very interesting destinations for future space missions.
Jupiter Fact file
* One tenth the diameter of the Sun, 1000 times lighter
* Ten times the diameter of the Earth, 1000 times heavier
* more facts TBA.
Is Jupiter a 'failed star'?
Jupiter has all the right ingredients to be a star, but it would need to be 80 times heavier before pressure in the centre could ignite the nuclear reactions that power stars. If it got any more massive (heavier) it would actually shrink, due to the increase in gravity of the liquid inner, until it was 4 times as heavy.
Jupiter is shrinking
Jupiter gives out slightly more heat than it receives from the sun. When a gas is compressed, it heats up (this is why a bicycle pump gets hot, and why a spray can gets cold). Jupiter was born from a thin cloud of Hydrogen that slowly pulled itself in under its own gravity until the gases were highly compressed and very hot. As it cools, Jupiter shrinks by about 2cm per year, compressing and heating the gas just a little bit more...