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
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.