INSIDE THE GIANT

Heat can travel via infrared radiation (light with wavelengths just a little longer than the light we can see), so by measuring the infrared radiation emanating from Jupiter, we can calculate how much it’s cooling and shrinking. We estimate that the planet shrinks at a rate of a few centimeters (about an inch) per century and cools by about one degree Celsius (1.8 F) every million years.    

1. ATMOSPHERE

Gases: Hydrogen, Helium, Methane, Water, Ammonia
Liquids: Water droplets
Solids: Ammonia ice particles
Pressure: 0-1000 Earth atmospheres pressure
Temperature: -150 to +1000 C

2. OUTER INTERIOR

Liquids: Hydrogen and Helium, small amounts of Methane, Water, and Ammonia
Pressure: 1000-2000,000 Earth atmospheres pressure
Temperature: 1000 to 8000 C

3. INTERIOR

Liquid metallic Hydrogen, with Helium and small amounts of Methane, Water, and Ammonia
Pressure: 2000,000-45,000,000 Earth atmospheres pressure
Temperature: 8000 to 16,000 C

4. CORE

Solid or Liquid?: Oxygen, Carbon, and Nitrogen? Ice? Rock?
Pressure: More than 45,000,000 Earth atmospheres pressure
Temperature: 16,000 to 30,000+ C    

According to most theories, Jupiter has a dense core of heavy elements that formed during the early solar system. The solid core of ice, rock, and metal grew from a nearby collection of debris, icy material, and other small objects like the many comets and asteroids that were zipping around four billion years ago. These bits of matter clumped together due to their mutual gravity, becoming larger chunks called planetesimals, which, in turn, collided and stuck together to form Jupiter’s core.

The core grew big enough so that it had enough gravity to attract even hydrogen and helium, the lightest elements that exist. More and more gas accumulated until it became what we now know as Jupiter. Although most scientists agree on this general story, many details remain unknown. For example, we’re still not sure where all the icy matter comes from.

Another theory, however, suggests that there is no core at all. Instead, Jupiter formed from the large cloud of gas and dust that surrounded the sun soon after its birth. As this cloud cooled and condensed, gas and dust particles lumped together so that some regions were denser than others. One of these dense splotches was able to gravitationally pull more and more gas and dust together, swelling into a full-fledged planet.

By measuring Jupiter’s gravitational and magnetic fields, Juno will be able to determine whether a core exists. If it does, exactly what the fields look like will depend on how big the core is, and knowing it’s size will help determine which of the many theories – if any – are most likely to be correct.

If Juno finds no evidence of a core, that could strengthen the condensed-cloud theory. Another possibility is that Jupiter once had a core, but has since eroded away. Or maybe, whatever Juno finds won’t fit any theory, and scientists will have to come up with completely new ideas.    

You jump from your spaceship and fall toward Jupiter below. In space, the temperature is around -270 degrees Celsius – just about three degrees above absolute zero. But as you reach the tops of Jupiter’s atmosphere, things start to heat up.

Don’t take off your coat just yet, though – it’s still -130 degrees Celsius (-200 F). The wisps of clouds around you produce a pressure of only about a fifth of one bar (the pressure at Earth’s sea level).

The clouds consist of ammonia and ammonium hydrosulfide, and are very bright. Jupiter reflects more than half the sunlight that hits it, making it the solar system’s second most reflective planet after Venus.

As you continue to plummet, the clouds thicken and warm up. After 30 kilometers, the pressure around you is about one bar and the temperature is now a balmy -100 degrees Celsius (-150 F). After another 40 kilometers, you hit a thick layer of clouds made from water.

Soon, the pressure will be crushing and the temperature searing. Fortunately, your special space suit keeps you alive as you descend farther, and after a few hundred kilometers, you reach what’s considered the bottom of the atmosphere.

Earth’s atmosphere ends where the ground begins, but since there’s no surface on Jupiter, its atmosphere ends where the gas – mainly hydrogen at this point – acts less like a gas and more like a liquid. This transition probably happens gradually, as the rising pressure squeezes the hydrogen molecules together. It’s like sinking through increasingly heavy fog that gets wetter and wetter until you find yourself swimming in liquid.

You’ve only traveled a distance less than one percent of Jupiter’s radius, but the pressure is already 1,000 bars – a thousand times what you would feel on a beach on Earth – and the thermometer reads just over 1,000 degrees Celsius (2,000 F). This depth is the limit of Juno’s Microwave Radiometer, which measures the micro- wave-wavelength radiation from Jupiter’s atmosphere.

Maybe around a third of the way down – a depth of more than three Earth diameters – when the pressure climbs to two million bars and the temperature soars to between 8,000 and 10,000 degrees Celsius (18,000 F), something extraordinary happens to the liquid hydrogen around you. The intense pressure squeezes the atoms so tightly that their previously bound electrons break free. The electrons can then flow throughout the liquid, generating electrical current and, we believe, Jupiter’s magnetic field. The liquid hydrogen can now conduct electricity like a metal. Called liquid metallic hydrogen, this form of hydrogen fills the rest of Jupiter’s interior until you hit the core.

At last, you’ve arrived at your final destination: the core. Here, the pressure is probably around 45 million bars – as much as that of 1,000 elephants standing on a stiletto heel. The temperature is about 20,000 degrees Celsius (35,000 F), more than three and a half times hotter than the surface of the sun. But what you see is anyone’s guess. No one knows where exactly the core begins nor does anyone know whether the boundary is well-defined or gradual – yet another mystery Juno hopes to solve.