Jupiter From the Inside

The spacecraft is named Juno. This is a better joke than NASA usually permits itself. In the myth, Jupiter — king of the gods, serial philanderer — drew a veil of clouds around himself to hide his misbehavior. His wife Juno was the one who could see through it. So we built a machine, named it after the goddess who sees past the cover story, and threw it at a planet that has spent four billion years hiding behind its own weather.

Because that's the thing about Jupiter: everything you have ever seen of it is the outfit, not the body. The stripes, the Great Red Spot, that whole marbled cinnamon-roll of a face — all of it is cloud. The actual planet underneath has never been photographed and never will be, because there may not be a surface to photograph. Go down far enough and the gas just gets denser until it becomes a strange metallic-hydrogen ocean under pressures that would flatten anything we could send. Jupiter isn't a ball you could stand on. It's mostly an atmosphere with delusions of grandeur.

Juno's job was to ask the rude questions. How much water is in there? Does it have a solid core, a fuzzy one, or none at all? What's driving a magnetic field strong enough to make Earth's look like a fridge magnet? These aren't idle curiosities. Jupiter is two and a half times the mass of every other planet combined — the first thing to condense out of the leftover material after the Sun switched on. It's the eldest sibling, the one that set the table for the rest of us. Learn how Jupiter is built and you learn something about the machinery that eventually coughed up the Earth, and therefore, down the long chain of consequence, you.

To do this Juno flies a deliberately reckless-looking orbit — a long ellipse that swoops in close over the poles and then flees back out — threading between the planet and the worst of its radiation belts, which are lethal enough to fry unshielded electronics in an afternoon. Its brain rides inside a titanium vault. And it runs on sunlight, out there where sunlight is roughly one twenty-fifth as strong as it is here, making it the most distant solar-powered thing our species has ever flung into the dark. A machine squinting to read by the light of a Sun gone small, wrapped in armor, staring straight down through the clouds of a god.

The answers took years to arrive, and the strangest of them cut straight to the point of the whole mission. We went hunting for Jupiter's core — the hard seed at the center, the real planet under the costume — and Juno's exquisitely precise measurements of the gravity field found that there may not be one. Not a clean ball of rock and metal, anyway, but a dilute core: heavy elements smeared upward and blended into the hydrogen above them across a huge fraction of the planet's radius, a fuzzy gradient where everyone had expected a boundary. No crisp line where the weather ends and the world begins. You go looking for the body underneath the outfit and find that the outfit goes all the way down.

Think about what that does to a machine named after the goddess who sees through the disguise. Juno looked inside — past the stripes, past the Great Red Spot, down through the metallic-hydrogen ocean toward the true thing beneath the cover story — and what she found was that there is no clean edge between the cover story and the true thing. The veil Jupiter drew around himself wasn't concealing the body. All the way down, past every layer we peeled back, the veil was the body. We built an eye to see through the disguise, and it discovered the disguise has no bottom.

We can't always see the surface of ourselves either. Maybe that's because, like the planet, we were never going to find a clean one. We sent something to look inside Jupiter, and that is what it brought back. Draw whatever conclusions you like.