Jupiter: The Large Adult Son of the Solar System
“We are really learning about a brand new Jupiter in many ways.”
Everything about Jupiter is large. The planet’s diameter is 11 times Earth’s size, and it is more massive than all other planets, dwarf planets, asteroids, comets, and moons put together. Jupiter’s Great Red Spot is a storm bigger than Earth that has lasted for as long as we humans have built telescopes to see it.
Jupiter’s mysteries are also large. NASA’s Juno mission currently in orbit around the planet is teaching us that we really don’t understand the solar system’s large adult son. Four new papers published in Nature on Wednesday outline the strangeness of Jupiter’s atmosphere, and how none of our current theories about how planets work are adequate to explain that weirdness.
“What most scientists didn’t expect is that almost everything you were going to learn was going to change fundamentally how you pictured the way Jupiter worked,” Scott Bolton of the Southwest Research Institute (SWRI) told The Daily Beast. “We are really learning about a brand new Jupiter in many ways.”
The first thing most people notice when they look at Jupiter is how colorful it is. In addition to the Great Red Spot, Jupiter is distinguished by alternating blue-white and red-brown stripes, known as “zones” and “belts.” By measuring small fluctuations in Jupiter’s gravitational field, scientists with the Juno mission discovered that these stripes extend 3000 kilometers (1900 miles) below the tops of the clouds. (Earth’s diameter is about 12,700 km. Remember: Jupiter is huge.)
For comparison, that would be like if Earth’s weather extended far below the lowest point on the ocean floor. However, as a “gas giant” planet, Jupiter doesn’t have a solid surface like Earth, Mars, and other rocky worlds. Instead, the planet is made mostly of hydrogen and helium, the stuff the sun is made of, but not nearly as hot as a star. Deep inside Jupiter, the gases are compressed into a weird fluid like nothing on Earth, which we still don’t understand very well.
On the other hand, according to what we know from Earth and other planets, Jupiter’s outer layers should still behave meteorologically—like a kind of weather.
“The zones and belts and the flows of the bands are assumed to be meteorological, and of course meteorological stuff is often driven by sunlight,” Bolton, who is the principal investigator of the Juno mission, said. “The structure of the zones and belts appears to be penetrating down to 3000 km depth into Jupiter, which is well below the sunlight. What’s creating those flows at that depth? It’s hard to understand how sunlight could do that.”
The Juno spacecraft orbits Jupiter in wide elliptical loops, which means its distance to the planet changes as it flies. Additionally, the probe doesn’t stick to one plane of orbit, like the Moon or Earth satellites do: it flies over different latitudes, including over the planet’s poles.
Jupiter’s gravity makes Juno speed up and slow down during various parts of its orbit. Sensitive Doppler radio measurement—like those used to measure the speed of clouds by meteorologists—between Earth and Juno allow scientists to determine how fast the probe is moving with extreme accuracy. That speed measurement in turn lets researchers calculate the exact gravitational strength of Jupiter, and how it varies across the planet.
Gravity is determined by mass: how much stuff there is in a place. Earth’s gravity also varies a little bit from place to place, depending on what kind of rock and how much there is in various places. It also changes in time as ice forms and melts, and slightly with the tides and atmospheric density.
Since Jupiter doesn’t have any rock, fluctuations in its gravity all come from the hydrogen and helium gases that swirl in its outer layers. More gravity at a point in Juno’s orbit means more gas on the planet below to tug on it. Juno’s data shows that gravity varies with the pattern of stripes we see on Jupiter’s exterior. The only way for gravity to vary that much is if the stripes extend 3000 km into the planet’s interior.
That’s a very weird discovery, not least because Jupiter is asymmetrical.
“I just cut the planet in half at the equator, the north and south [hemispheres] look different,” Bolton said. “The bands are not organized perfectly symmetrically, and one of the key features of the asymmetry is the south has the Great Red Spot, which obviously the north doesn’t have.”
As a result, Jupiter’s gravity is measurably different in the north than in the south, and varies by latitude.
“That was unexpected, because Jupiter’s rotating quite fast: It spins around in 10 hours,” Bolton said. “Most scientists assumed that at that speed for that size of a planet spinning that fast, that you wouldn’t have a north-south asymmetry except at the very top of the atmosphere.”
Unlike a rocky planet like Earth, Jupiter’s fluid nature and short “day” means its interior should mix up fairly well, instead of keeping its stripey nature 3000 kilometers down. But Bolton isn’t alarmed by these entirely unexpected results.
“When you stumble into something where you realize all your ideas were wrong, or need to be modified, then you’re really learning brand new things about nature,” he told The Daily Beast. “That’s one of the main purposes of exploring, so it’s very exciting to scientists as well as the public. You’re at the edge of what we know and understand.”