A question scientists have had about Jupiter for a long time is: How deep do those banded belts and zones go? Are we seeing something painted on very topmost layers of the planet? Or are they really deep? Is there a different circulation pattern at depth than the one we see at the surface? Or do the wind patterns we see from space continue within the planet and then die out at depth into a solidly rotating interior? That's where Juno's unique gravity measurements come in.

How We Measure Gravity

First, consider the simplest interaction of a spacecraft with a planet: a flyby. When a spacecraft gets close to a massive object like a planet, it accelerates. When the spacecraft speeds away, it decelerates. The amount of acceleration and deceleration is mostly related to the mass of the planet: a more massive planet has a bigger effect. So we can use spacecraft flybys to measure planetary masses.

To measure how the speed of the spacecraft changes as it passes by the planet, we usually use line-of-sight Doppler tracking. The spacecraft broadcasts a radio signal to Earth, a signal that has a very precisely known frequency. (Spacecraft that are expected to do good gravity science are equipped with ultrastable oscillators to generate super-precise radio signals.) The spacecraft's speed toward or away from Earth raises or lowers that frequency slightly. When the speed changes due to the planet's gravity, the frequency of the spacecraft's signal also shifts. By measuring and recording these minute shifts, along with the distance between the spacecraft and the planet (measured using the two-way travel time of a radio signal), we can measure the force that the planet exerts on the spacecraft, and so determine its total mass in just one spacecraft flyby.

Things get a little bit more interesting if you can send your spacecraft sufficiently close to a planet. If you're closer to the planet than a couple of planet diameters, you can watch how the acceleration of the spacecraft changes as it travels across the planet's lines of latitude. Then the motion of your spacecraft is sensitive to how the mass is distributed within the planet. (In more technical terms, you can learn about the body's moment of inertia.) You might be able to figure out, for instance, whether it's internally stratified, with denser stuff in the middle, or not. With some assumptions about composition, you can figure out at what depth those internal stratifications lie. Cassini did this several times with different moons of Saturn. If you'd like to know more, here are some really good lecture notes on the topic by Francis Nimmo.

When you really get to have fun with gravity is if you can put your spacecraft in orbit around the planet, an orbit that gets quite close. The closer your spacecraft is to the planet, the more sensitive its motion is to lumps and bumps in the gravity field caused by excess or missing mass. That's why Juno orbits so much closer to Jupiter than any previous mission, getting within 5,000 kilometers of the cloud tops.

Here's where I admit that until last week, I didn't know there was more to gravity at Jupiter than just measuring the sizes and densities of the concentric shells of its interior, from its core all the way out to its atmosphere. I'm a solid-planet geologist. Beyond the moment of inertia stuff -- how much denser a planet is in the middle -- I think of gravity anomalies in terms of permanent features of solid worlds. Things like holes in the ground (impact craters) or mountains or dense volcanic rocks intruding light continental rocks. On a gassy giant planet, how much excess or missing mass can there be? If you somehow managed to dig a hole in an atmosphere, wouldn't gas just flow to fill it?

This is why Jupiter's winds are important. Where winds are blowing east, their motion adds speed to the planet's already high rotational speed. It's like that bit of Jupiter is spinning faster than the rest of the planet. Spinning faster means that around the latitude of the east-blowing winds, centrifugal force wants to make the planet wider. And conversely, where winds are blowing west, it's like slowing the spin of that part of Jupiter, and that part of Jupiter wants to be closer to spherical. So the winds are going to have an effect on the gravity field.

I feel like it's been too much text without an image, so here, have a pretty picture of Jupiter.