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SYFY WIRE Bad Astronomy

Juno digs into Jupiter and sees the Great Red Spot goes way deeper than thought

Juno spacecraft sees deep beneath Jupiter's cloud tops.

By Phil Plait
Jupiter seen by Gemini (left) and Hubble (right)

New results from the Jupiter-divebombing spacecraft Juno reveal more about the depths of the gigantic planet including just how voluminous the Great Red Spot is, weird info about its stripes, and more about the even weirder and eerily regular cyclones swarming at its poles.

Juno is an armored tank of a space mission, designed to withstand the strong radiation generated by Jupiter's immense and powerful magnetic fields. Electrically charged ions blasted off its volcanic moon Io are swept up by Jupiter's magnetism and accelerated to extremely high speeds until they slam into its atmosphere at the poles. Any spacecraft that gets close to Jupiter can be fried by the environment.

And Juno swings in close: its extremely elliptical orbit takes it as far out as 2.7 million kilometers, but then it dives down to just 4,000 kilometers above its cloud tops, screaming past at 200,000 kilometers per hour.

Juno is equipped with several different detectors to probe Jupiter's interior (its main mission is to figure out what's going on in there). One of its main instruments is the Microwave Radiometer (or MWR), which can detect microwave radiation. This kind of long wavelength light is emitted by gas hundreds of kilometers deep in Jupiter's atmosphere, and which passes right through the upper layers. MWR can detect water and ammonia deep down in Jupiter in this way, and trace what it's doing.

Jupiter (and its moon Europa to the upper left) captured by Hubble in visible light on 25 August 2020. Credit: NASA, ESA, A. Simon (Goddard Space Flight Center), and M. H. Wong (University of California, Berkeley) and the OPAL team

Juno has passed over Jupiter 37 times since it arrived there in 2016, including several passes over the Great Red Spot, an anticyclonic (high-pressure) system that's existed for centuries at least. A few years back, MWR data showed that the Spot ran pretty deep, as much as 350 kilometers below the cloud tops.

But new results show it goes even deeper than that: at least 500 kilometers, which is amazing. Mind you, big storms on Earth are usually a few dozen kilometers top to bottom.

Jupiter does things big. Of course, it's 130,000 km across, ten times the diameter of Earth and a thousand times its volume. It has room for big.

Artwork depicting Earth compared to Jupiter's Great Red Spot.

What's more amazing is how this depth was determined. The atmosphere inside the Spot is a different density than the atmosphere around it, which means the amount of gas in it is different. This changes its mass, which means the gravity of Jupiter changes very subtly over the Spot, which in turn means Juno will move at a different velocity as it passes over. Scientists and engineers measured Juno's speed to an accuracy of 0.01 millimeters per second (!!) and could use that to see its velocity change. That was used to get the Spot's mass, which gave its volume, which gave its depth. Whoa.

Jupiter seen by Gemini (left) and Hubble (right)

Data from the MWR also yielded new results as well. Jupiter is striped, with visible light images showing bright zones and dark belts. These are like jet winds moving through Jupiter's atmosphere in opposite directions. In MWR data the brightness of the zones and belts switch relative to visible light: Air near the cloud tops in the belts is warmer, so they appear brighter, and zones are cooler so they appear darker.

But deeper down, new MWR data show that there's a transition where the gas suddenly becomes much cooler. Water in Earth's oceans undergo a similar transition, and it must have to do with the way heat is transported deep inside the planet. Further down (deeper than MWR can see) the gas must get warmer again, as pressure increases. They were also able to see ammonia in the belts and zones moving in a circulating pattern, similar to patterns on Earth called Ferrel cells, which transport moisture on our planet to mid-latitudes and affects weather. All these new data will help scientists understand how Jupiter's atmospheric motions work.

Juno thermal infrared data of Jupiter’s north pole (brighter = warmer) shows eight cyclones swirling around a central cyclone. Credit: NASA/JPL-Caltech/SwRI/ASI/INAF/JIRAM

Juno's orbit is different than any other mission before it: It passes directly over both poles, and from that vantage point see them more clearly than we can from Earth. Early results showed both poles were encircled by a line of huge storms, vortices well over a thousand kilometers across. The north pole has eight such vortices, arranged in two sets of four like two squares rotated slightly with respect to each other, while the south pole has five in a nearly perfect pentagon. It's one of the eeriest planetary features I've ever seen.

The new MWR results show that these vortices are remarkably stable. They move around a bit, jostling each other, but when they bump into each other they then move away again. This creates a mild oscillating motion that is slow and steady, implying, like with the Red Spot, that the storms run very deep into the atmosphere.

Juno thermal infrared data of Jupiter’s south pole has five huge cyclones surrounding a central polar cyclone. Credit: NASA/JPL-Caltech/SwRI/ASI/INAF/JIRAM

Jupiter is very different than Earth, and to be fair it's partly because it's so big: It's a gas giant, with an atmosphere that's mostly hydrogen and thousands of kilometers deep, merging into a liquid ocean of hydrogen, which then becomes metallic (acting in much the same way as metals, with free electrons able to conduct electricity). Before Juno we weren't even sure if it had a core; some models of how it formed predicted no core, while others showed it would have one. Juno observations showed it has one, kinda, but it's mushy and indistinct. A mashup of both ideas, in a sense.

Those are broad strokes, but the details of how Jupiter works on the inside are still being teased out. Some we can do from Earth, but to truly understand this immense and bizarre world there's nothing like being there. And, happily, Juno is right there.