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SYFY WIRE Bad Astronomy

We knew Jupiter was weird. Now we're finding out HOW weird.

By Phil Plait

If there’s one thing that shouldn’t surprise astronomers, it’s being surprised. The trend is pretty clear: Every single time we look at the Universe in a new way —bigger telescopes, different wavelengths (colors) of light, space probes equipped with better detectors— we find stuff that is massively unexpected. Being surprised is in no way surprising.

Yet here we are, surprised once again, standing in awe before the mightiest of the planets: Jupiter.

The Juno spacecraft entered Jupiter orbit on July 4, 2016, and is on a looping 53-day trajectory that takes it 8 million kilometers out from the planet, then drops it screaming in to just 4200 kilometers above the planet’s north pole, traveling at a terrifying 200,000 kilometers per hour (125,000 mph). It swings down the planet, over the south pole, and is flung out once again. The purpose of the mission is to help scientists understand how Jupiter formed and how it changed over time, to see how this affected its internal structure and in turn figure out how that affects what we see closer to the surface.

Juno just finished its sixth orbit, but scientists have published the results they found after the first couple of orbits (in two main papers and dozens of others). Even after that short period of time Juno has sent back a vast amount of data, enough to — say it with me now — surprise scientists.

One of the biggest questions scientists hope Juno will answer actually sounds pretty simple: Does Jupiter have a core? The Earth, for example, has a dense nickel/iron core, formed as those heavy metals fell to the center of our planet as it cooled. Jupiter, though, may not have one at all!

I always assumed it had one, but when I was researching Jupiter for my Crash Course Astronomy episode on it I found that may not be the case. It depends in part on how Jupiter itself formed. In the early solar system, a lot of material starting clumping together to form bigger and bigger objects, going from grains of sand to rubble to boulders to things which were starting to look like planets, called protoplanets. If a bunch of those smashed together to form Jupiter — creating it from the bottom up, so to speak — then yeah, it should have a dense rocky/metal core, probably more massive than our own planet.

It’s also possible Jupiter collapsed directly from the disk of gas and dust surrounding the Sun — from the top down. If that’s the case then it won’t have a core. I’ll note that it’s possible it could have started with a core, but it got eaten away by currents of hot metallic hydrogen deep inside the planet as well.

The presence of a core or lack thereof will change the way Jupiter’s gravitational field is shaped, and this in turn will affect Juno’s orbit. By carefully measuring the spacecraft’s trajectory, this jovian riddle can be solved!

Or maybe. Maybe not. The result scientists found after that first orbit is that Jupiter may have a core, but it’s ... fuzzy. Dilute. It may be bigger than first thought, too, containing 7-25 times the mass of Earth (Jupiter’s total mass is 318 times Earth’s). I had to laugh when I read that; I can imagine groups of scientists on either side of this issue arguing for years over whether Jupiter has a core or not, and then finding out that, in a way, they may both be right.

Mind you, though, that’s just after two orbits. There are a lot more to come. More information will hopefully equal more refined understanding.

Jupiter's rings and Orion

And still, that’s just the start of the weirdness.

When you look at Jupiter through a telescope, the most obvious features are its stripes. These are weather patterns whipped completely around the planet, and signify areas where the atmosphere is rising or falling (like convection cells on Earth). But what happens near the poles? That’s hard to tell from Earth, because we’re close to being in Jupiter’s equatorial plane, so the poles are distorted and blurred due to perspective of the curving planet.

Jupiter's south pole

Juno travels directly over the poles, giving us a sharper view of them than ever before. And what it found is interesting: The familiar stripes break down at latitudes within 30° of the poles. Instead, the polar atmosphere is dominated by huge numbers of cyclonic storms moving around in a much darker background than at lower latitudes. But they’re different at the different poles: At the north pole, they range in size from 1400 km across down to Juno’s camera’s resolution of 50 km. In the south they’re more limited in size, from 200 – 1000 km. The storms’ distribution is different at the two poles as well. It’s not at all clear why the poles are so different.

The poles are also different than Saturn’s. There’s no large organized wave pattern like Saturn’s eerie hexagon (though a mild wave was detected, it’s nowhere near as obvious). Also, there’s no small, well-organized vortex like Saturn has at its north pole. Clearly, the forces operating at Jupiter’s poles are very different than Saturn’s. This is a new development, and I’m sure the planetary atmospheric scientists are working feverishly on the new data coming back from Juno to figure this out.

swirling storms on Jupiter

At the equator, another mystery literally arises. A main constituent of Jupiter’s atmosphere is ammonia. It forms white clouds in rising, cooling air and scientists assumed that below the clouds it was mixed in with everything else. What they found in the Juno data is this isn’t the case. There’s a plume of ammonia right a the equator from deep inside the atmosphere, from a depth where the pressure is about 60 times the Earth’s atmospheric pressure at sea level. This was completely unexpected, and means that the models of how Jupiter’s atmosphere works need to be looked at again.

map of ammonia in Jupiter

The giant planet’s magnetic field is different than expected, too. It’s far more powerful than Earth’s (we already knew that!) but it varies spatially in strength more than expected. Like Earth’s magnetism, Jupiter’s is created deep under the surface, so the composition and structure inside Jupiter is different than expected (which at least jibes with what is seen in the gravitational studies looking for the core). Juno data implies that Jupiter’s magnetic field is generated not only in the core but may be influenced by material above the core, which is very different than here on (well, under) Earth. That was unexpected.

And if I had to pick one more weirdness out of everything, it’s what’s happening in Jupiter’s aurorae. On Earth, these are created when subatomic particles from the Sun’s solar wind are captured by Earth’s magnetic field and funneled down into our atmosphere at the poles. The particles slam into the air, which strips off electrons from the atoms and molecules. When the electrons recombine, they emit light, causing the aurorae.

On Jupiter, Juno showed that this happens as well, but also electrons are stripped off the atmosphere of the planet and sent up into space above the poles. That was unexpected, and must have to do with Jupiter’s more intense magnetic field strength, but the detailed mechanism is, for now, unknown.

Now look, I can see where it might seem like this is all very esoteric, but it has an interesting implication. We learned most of what we know about the way planets work by studying our own and then comparing and contrasting what we think we understand with what we see happening on other planets. But if we see things that don’t work there as they do here, does that mean conditions really are different there, or that some of our assumptions about Earth need to be updated? We really do understand a lot about the Earth, but there could be some pieces missing that we need to figure out. We can only do that by observing these other worlds.

Only by venturing away from our home do we come to understand it, and perhaps more importantly come to understand what we don’t understand. We must explore space so that we can explore our own world. Looking outward is looking inward.

And the good news is that there’s a lot more to come. There are a dozen or more orbits still ahead, so we’ll get better data as time goes on. Sadly, the ridiculously harsh radiation environment so close to Jupiter will take its toll, and the JunoCam, which has provided the incredibly rich and detailed images we’ve seen, will eventually succumb. But the images, spectacular as they are, do not provide the main science Juno is doing; the other instruments are better protected inside the spacecraft. It’ll be tough when we stop getting these phenomenal images, but the scientists will continue to get more data as the mission itself continues.

Jupiter is a tough neighborhood, but we’re finally starting to get a good map of it. And it’ll only get better.