Juno's Last Secrets: What NASA's Most Resilient Probe Revealed About Jupiter — and Why None of It Was Expected

A Tank Going to Space

On August 5, 2011, a rocket carrying one of the most unusual spacecraft NASA had ever built lifted off from Cape Canaveral. Juno was enormous — 20 meters across when its solar panels were fully extended — and yet it would generate just over 400 watts of electricity at Jupiter, where sunlight is only 4 percent as intense as it is at Earth. Every other deep-space probe had relied on radioisotope thermoelectric generators. Juno would run on sunshine, barely.

The journey to Jupiter took five years and covered 2.8 billion kilometers. Along the way, mission planners executed a maneuver that sounds almost too clever: Juno flew away from Jupiter first, looped back toward Earth, and used our planet's gravity to slingshot itself forward at an additional 14,000 kilometers per hour. It was more fuel-efficient than simply burning engines the whole way. It was also elegant.

When Juno finally entered polar orbit around Jupiter on July 4, 2016, no one was entirely sure how long it would survive. Jupiter's radiation belts — charged particle fields powerful enough to destroy most electronics — were expected to degrade Juno's instruments within months. Engineers had encased its most critical systems in a titanium vault a centimeter thick. They called it, without irony, a tank going to space.

"The radiation Juno encountered was ten times lower than expected — a stroke of luck that would eventually extend the mission by years and transform it into something no one had planned."

The Poles of Jupiter, Seen for the First Time

Before Juno, no spacecraft had ever given us a clear view of Jupiter's poles. The images that came back in 2016 left planetary scientists nearly speechless.

What they expected: the familiar banded structure of Jupiter's equatorial regions, stretched and distorted but recognizable. What they got: a chaotic, swirling ocean of cyclones unlike anything in the solar system. At the south pole, a cluster of enormous storms — each one larger than Earth — arranged in a geometric pattern around a central vortex. At the north pole, eight of them. The contrast with Saturn's famous polar hexagon couldn't have been more stark.

Juno's microwave radiometer — an instrument built specifically for this mission — could peer deeper into Jupiter's atmosphere than any previous sensor, to a depth of roughly 350 kilometers below the cloud tops. What it found there surprised everyone: a massive band of ammonia gas hugging the equator, plunging downward in a column that the instrument couldn't even reach the bottom of. Scientists had expected ammonia to distribute itself relatively uniformly across the planet. It doesn't.

Jupiter's Great Red Spot, that storm larger than Earth that has been raging for at least 400 years, turned out to extend far deeper into the atmosphere than anyone predicted — and to be colder than its surroundings near the top while dramatically warmer further down, suggesting the heat below may be driving the storm's extraordinary persistence.

What Lives at the Heart of Jupiter

The deepest and most consequential discovery Juno made was one that couldn't be seen directly at all.

For decades, two competing theories had tried to explain Jupiter's interior. The first held that Jupiter formed from the direct gravitational collapse of a gas cloud and had no solid core at all — just churning gas all the way down. The second argued for a dense, solid core of heavy elements at the center, around which hydrogen and helium had accumulated over millions of years. The second theory, known as core accretion, was the dominant model. It predicted a sharp boundary between the compact core and the surrounding gas layers.

Juno mapped Jupiter's gravitational field with a precision roughly 100 times better than any previous measurement. The method was almost absurdly sensitive: as Juno passed over regions of varying density, its velocity changed by amounts as small as 0.01 millimeters per second, detectable through the Doppler shift of the radio signal it continuously beamed back to Earth.

What the gravity map revealed was that both theories were wrong.

Jupiter has a core — but it isn't solid. It isn't absent. It is diffuse: a region where heavy elements gradually blend into the surrounding hydrogen layers with no clear boundary, extending outward to cover perhaps half the planet's total radius. It is enormous and it is fuzzy, and its existence overturned decades of planetary science.

"Both theories about Jupiter's core were wrong. A third possibility — one no one had seriously considered — was what Juno actually found: a vast, blurry region where the boundary between 'core' and 'planet' simply doesn't exist."

The most popular explanation that emerged was that Jupiter once had a solid core, which was destroyed by a catastrophic collision early in the solar system's history. But when researchers tried to model that collision on supercomputers, simulating every plausible variation of size, angle, and velocity, they kept getting the same result: after the initial impact, the heavy material settled back toward the center. The diffuse core wouldn't form. It didn't fit.

Saturn's Confession

In 2021, researchers studying wave patterns in Saturn's rings discovered that the planet's interior was also partially resistant to convection. In a fully convective planet, heat escapes by circulating fluids, continuously mixing the interior. But Saturn's interior showed evidence of compositional gradients — heavier elements concentrated toward the center, fading outward — exactly what you'd expect from a diffuse core.

If both Jupiter and Saturn have diffuse cores, the explanation can't be a unique collision. It has to be something fundamental about how giant planets form.

Scientists are now working toward a new theory of giant planet formation in which diffuse cores aren't accidents — they're the expected outcome. The old model of a solid seed that captures gas may be incomplete. The boundaries we drew between planetary interiors and envelopes may have always been too neat. The implications reach beyond our solar system: every giant exoplanet we've discovered and modeled has been interpreted through the lens of the old theory. That lens may need to be replaced.

Lightning, Mushballs, and Things That Don't Exist on Earth

When Voyager flew past Jupiter in 1979, it detected mysterious flashes in the atmosphere that looked like lightning. Juno solved a 40-year mystery — and replaced it with something stranger.

Juno's star reference unit camera — an instrument designed for navigation, not science — spotted flashes of light at altitudes far higher than any lightning should be able to occur. At those heights, temperatures drop below minus 88 degrees Celsius. Normal water-based storms can't exist there.

What Juno found was a form of precipitation that has no equivalent on Earth: storms powered by a mixture of water and ammonia that remains liquid well below water's normal freezing point. Ammonia acts as an antifreeze. These storms produce what scientists have started calling "mushballs" — a slurry of ammonia and water ice that falls through Jupiter's atmosphere and may be responsible for the planet's unusual ammonia distribution. The term is whimsical. The phenomenon is not.

Io's Fury, Europa's Secrets, and Ganymede's Surprise

When Juno's original mission ended in 2021 with the spacecraft still functioning far beyond expectations, NASA extended the mission and turned the probe outward, toward Jupiter's moons.

Ganymede received Juno's closest attention first. In June 2021, the probe flew within 1,000 kilometers of the surface and returned images of unprecedented detail. Astronomers expected simultaneous Hubble ultraviolet observations to confirm oxygen in Ganymede's thin atmosphere. Instead, they found mostly water vapor — a discovery that forced a revision of the atmospheric model and raised new questions about the prevalence of water vapor on icy bodies throughout the outer solar system.

Europa's flyby in September 2022 revealed a surface of fractured ice ridges so complex they challenged the simple model of a deep ocean beneath a uniform shell. A dark, irregularly shaped region near the equator is interpreted as evidence of cryovolcanic activity — eruptions pushing material through more than 10 kilometers of ice. A 2023 paper proposed that the cause may not be pressure from the deep ocean at all, but from pockets of highly saline liquid trapped within the ice shell itself, expanding until they burst.

Io was last — and most dramatic. In 2023 and 2024, Juno made multiple close passes of the most volcanically active body in the solar system, approaching within 1,500 kilometers of a surface covered by hundreds of active volcanoes, lava lakes, and plumes rising tens of kilometers above the horizon.

On December 27, 2024, Juno witnessed something that had never been recorded anywhere in the solar system: a simultaneous eruption across a region of 65,000 square kilometers, involving hundreds of volcanoes firing in apparent coordination. The energy released was the most intense volcanic event ever observed beyond Earth. The synchronization strongly suggests these volcanoes are connected by shared subsurface lava channels — a geological network operating at scales that dwarf anything on our planet.

The Probe That Wouldn't Die

Juno was originally scheduled for decommissioning in 2018. Its extended mission was supposed to end in September 2025. Under NASA's planetary protection protocols, the plan was always to deorbit the spacecraft into Jupiter's atmosphere, ensuring it could never contaminate the potentially habitable moons below.

Then the United States government entered a period of budget uncertainty. NASA faced significant funding pressure. And Juno — which should have been gone — simply kept transmitting.

NASA's communications about the mission's status became notably sparse. Then, shortly before this article was prepared, the agency published new research using Juno's data to estimate the thickness of Europa's ice shell. The paper concluded with a brief, almost casual mention: Juno would perform its 81st Jupiter flyby on February 25.

Juno was sent to see through Jupiter's clouds. What it found was that the clouds were only the beginning — and that the planet beneath them was stranger, more complex, and more poorly understood than anyone had admitted.