The King of Planets
Jupiter is a planet of superlatives. It is, by a wide margin, the largest and most massive planet in our solar system. Its mass is more than double that of all the other planets, moons, and asteroids combined. With an equatorial width of about 143,000 kilometers (nearly 89,000 miles), it is so vast that all of its planetary neighbors could fit comfortably inside it; more than 1,300 Earths could be contained within its volume. This colossal world is the fifth planet from the Sun, orbiting at an average distance of about 778 million kilometers (484 million miles). This distance is so great that sunlight, traveling at the fastest speed possible, takes 43 minutes to complete the journey.
The planet’s orbital and rotational characteristics are a study in contrasts and are fundamental to understanding its nature. A Jovian year, the time it takes to complete one long, elliptical journey around the Sun, is equivalent to nearly 12 Earth years. Yet, for all its unhurried travel through the solar system, Jupiter spins on its axis faster than any other planet. A day on Jupiter lasts just under 10 hours. This rapid rotation has consequences. It causes the planet to bulge noticeably at its equator, giving it the shape of a slightly squashed sphere, known as an oblate spheroid. This spin is also the engine behind the planet’s powerful weather systems and its immense magnetic field. Because its axis is tilted by only about 3 degrees relative to its orbit, Jupiter spins nearly upright and does not experience the kind of distinct seasons that occur on Earth.
In its composition, Jupiter more closely resembles a star than a rocky planet like Earth. It is made almost entirely of hydrogen and helium, the same light elements that fuel the Sun. This has led to its common description as a “failed star.” Had the primordial cloud of gas and dust from which it formed been about 80 times more massive, the pressure and temperature at its core would have ignited nuclear fusion, and our solar system would have had a second, albeit much smaller, star. Even without fusion, Jupiter’s interior is incredibly hot. The planet radiates more energy into space than it receives from the distant Sun, a product of residual heat left over from its violent formation 4.5 billion years ago and a process of slow, ongoing gravitational contraction.
Known since ancient times, Jupiter is named for the king of the Roman gods. It is usually the second-brightest planet in the night sky, outshone only by Venus, making it a familiar and brilliant point of light for observers on Earth.
The Turbulent Atmosphere
Jupiter’s atmosphere is the largest in the solar system and a place of extreme dynamics and complex chemistry. It doesn’t have a solid surface like Earth; instead, the gaseous atmosphere simply gets denser with depth, gradually transitioning into a liquid interior under crushing pressure. The composition is overwhelmingly simple, made of about 90% hydrogen and 10% helium by volume, with trace amounts of other compounds like methane, ammonia, and water vapor.
This atmosphere is structured in layers defined by temperature. The outermost layers are the exosphere and the thermosphere, where temperatures can rise to over 725 °C (1,340 °F). Below this is the stratosphere, and then the troposphere, which is where all of Jupiter’s visible weather occurs. The “surface” of Jupiter is conventionally defined as the level in the troposphere where the atmospheric pressure is equal to one bar, the same as the average pressure at sea level on Earth.
Cloud Structure
The visible face of Jupiter is not a single surface but a complex, multi-tiered system of clouds that, taken together, span a depth of about 71 kilometers (44 miles). These clouds are arranged in three primary layers, each formed by a different chemical compound condensing at a specific temperature and altitude.
The highest and coldest cloud deck, visible as bright white wisps, is made of ammonia ice. This layer forms at temperatures around -150 °C (-240 °F). Beneath this ammonia haze, at a warmer level, lies a second layer thought to be composed of ammonium hydrosulfide crystals. Deeper still is the third and lowest observable cloud layer, which is likely made of water ice and vapor. The pressure and temperature increase dramatically with depth, preventing other compounds like methane from condensing into clouds.
Belts and Zones
The planet’s most recognizable features are its colorful stripes, which are organized into light-colored bands called “zones” and darker bands called “belts.” This pattern is a direct visual representation of a gigantic, planet-spanning heat engine. Unlike Earth, where weather is driven primarily by energy from the Sun, Jupiter’s weather is fueled by its own powerful internal heat.
This internal heat drives convection on a massive scale. Warmer gas from deep within the planet rises, and as it reaches higher altitudes, it cools. These columns of rising, cooling gas form the zones. Because they are higher in the atmosphere, the zones are colder at their tops and their white color comes from the high-altitude ammonia ice clouds that condense there. Conversely, the belts are regions where cooler, denser gas from the upper atmosphere sinks and warms. This downward motion clears away the high ammonia clouds, allowing a view of the deeper, more colorful cloud layers below.
This planet-wide system of rising and sinking gas is organized into the distinct horizontal stripes by Jupiter’s rapid 10-hour rotation. The spin creates powerful and remarkably stable jet streams, or zonal winds, that flow in opposite directions in adjacent bands at speeds exceeding 539 kilometers per hour (335 mph). These winds shear the convective cells into the planet-girdling belts and zones we observe.
The Mystery of Color
While the white of the zones is explained by ammonia ice, the source of the rich reds, yellows, and browns seen in the belts and storms remains an area of active research. The colors are likely produced by “chromophores,” or color-bearing molecules, but their exact identity is unknown.
One leading hypothesis suggests that plumes of sulfur and phosphorus-containing gases rise from the planet’s warmer interior. When these chemicals reach the upper atmosphere, they undergo chemical reactions, perhaps triggered by sunlight or lightning, to form colorful compounds. Another possibility is that the reddish-brown colors are due to ammonium hydrosulfide or ammonium polysulfides, which are expected to form in the second cloud layer.
More recent research points to the possibility of a “universal chromophore” created by the interaction of ammonia with acetylene, a hydrocarbon formed when sunlight breaks down methane in the upper stratosphere. When subjected to ultraviolet radiation, these chemicals can form complex organic molecules that could produce the range of colors seen across the planet. The true answer may be a combination of these processes, with different chemicals contributing to the palette at different depths and locations within the turbulent atmosphere.
A World of Storms
Jupiter’s atmosphere is a chaotic realm of swirling vortices and colossal storms, some of which have raged for centuries. These weather systems are born from the planet’s internal heat and organized by its rapid rotation, and they offer windows into the complex physics operating at different depths and latitudes.
The Great Red Spot
The most famous of all Jovian features is the Great Red Spot, a gigantic storm system located in the planet’s southern hemisphere. It is an anticyclone, a high-pressure region that rotates counter-clockwise, and it is the largest and most powerful vortex known in the solar system.
The storm’s history and age are a subject of debate. It is often said to be over 300 years old, which assumes it is the same “Permanent Spot” observed by astronomer Giovanni Cassini between 1665 and 1713. However, a 118-year gap in observations, coupled with differences in the spot’s appearance and movement, has led many scientists to conclude that the modern Great Red Spot is a separate feature. The first reliable and continuous observations of the current storm began in 1831, making it at least 190 years old.
In the late 19th century, the Great Red Spot was a vast oval, measuring some 40,000 kilometers (25,000 miles) across, wide enough to swallow three Earths. Over the past century and a half, it has been steadily shrinking and becoming more circular. Today, it is about 1.3 times the diameter of our own planet. As it has contracted horizontally, observations suggest it has been forced to stretch vertically, growing taller.
The winds at the storm’s periphery are ferocious, with speeds exceeding 432 kilometers per hour (268 mph) and potentially reaching up to 680 kilometers per hour (425 mph). Data from the Hubble Space Telescope indicates that these outer wind speeds have actually increased in recent years. The storm is not a shallow phenomenon. Data from NASA‘s Juno spacecraft revealed that the Great Red Spot’s roots penetrate deep into the atmosphere, extending at least 300 to 500 kilometers (about 200 to 310 miles) below the cloud tops. Its incredible longevity is a result of Jupiter’s unique environment; with no solid surface to provide friction and a constant supply of energy from the planet’s internal heat, the storm can persist for centuries.
Polar Cyclones
One of the most stunning discoveries of the Juno mission was the existence of massive cyclones at Jupiter’s poles, arranged in stable, geometric patterns. At the north pole, a central cyclone is surrounded by eight others in a near-perfect octagon. At the south pole, a central vortex is encircled by five others in a pentagon.
These storms are enormous, each thousands of kilometers across. On Earth, polar storms tend to drift and dissipate, but Jupiter’s have remained in their geometric configurations for years. This remarkable stability is thought to be maintained by “anticyclonic rings”—rings of wind spinning in the opposite direction around each cyclone. These rings effectively create a shield that causes the storms to repel each other, preventing them from merging into one giant polar vortex. The cyclones are observed to slowly drift toward the pole, but as they get closer, they begin to interact and “bounce” off one another, stabilizing the entire configuration.
The Great Blue Spot
Unlike the Great Red Spot, the Great Blue Spot is not a weather storm. It is an intense and isolated patch of magnetic field located near Jupiter’s equator. Its name does not come from a visual color but from the blue used to represent southern magnetic polarity on field maps. This spot is a significant anomaly in Jupiter’s magnetic field, which is otherwise mostly concentrated near the poles.
The Great Blue Spot provides a direct link between the planet’s deep interior and its atmosphere. Juno’s observations have revealed that this magnetic feature is being distorted by Jupiter’s powerful east-west jet streams, which extend thousands of kilometers deep. The winds are shearing the spot, stretching it and pulling it apart. This shows that atmospheric dynamics reach deep enough to physically influence the region where the planet’s magnetic field is generated. Recent data even suggests the jet stream associated with the spot fluctuates in a wave-like pattern over a four-year period, hinting at complex processes occurring far below the visible clouds.
Together, these three types of “spots” paint a picture of a planet with distinct dynamic regimes. The Great Red Spot is a feature of the weather layer, the polar cyclones are a product of high-latitude fluid dynamics, and the Great Blue Spot is a window into the deep dynamo region, all interconnected in a complex, multi-layered system.
Journey to the Center of Jupiter
Venturing from the cloud tops toward Jupiter’s center would be a journey into a realm of unimaginable pressure and temperature, where the very nature of matter is transformed. Since Jupiter has no solid surface, the gaseous atmosphere seamlessly transitions into a liquid interior.
As one descends, the immense weight of the overlying atmosphere compresses the hydrogen gas until it becomes a liquid. This gives Jupiter the largest ocean in the solar system—a planet-spanning ocean made not of water, but of liquid molecular hydrogen.
About a third of the way to the center, roughly 20,000 kilometers below the clouds, the conditions become truly extreme. The pressure rises to millions of times that of Earth’s atmosphere, and the temperature soars to thousands of degrees Celsius. Under this incredible compression, the hydrogen itself changes. Electrons are squeezed free from their atoms, creating a dense, swirling fluid of protons and free electrons. This exotic state of matter is known as liquid metallic hydrogen. As its name implies, it conducts heat and electricity like a metal. This vast, rotating ocean of metallic hydrogen, which may account for 75% of the planet’s mass, acts as a colossal dynamo. The rapid spin of the planet drives powerful electrical currents within this conductive fluid, generating Jupiter’s immense magnetic field.
The “Fuzzy” Core
For many years, the standard model of Jupiter’s formation predicted that at its very center lay a small, dense core of rock, ice, and heavy elements, perhaps 10 to 15 times the mass of Earth. This solid core was thought to have formed first, its gravity then pulling in the vast envelope of hydrogen and helium gas that makes up the rest of the planet.
However, precise gravity measurements from the Juno spacecraft have upended this tidy picture. The data revealed that Jupiter’s core is not the compact, solid ball that was expected. Instead, it appears to be a large, “dilute” or “fuzzy” core. This means the heavy elements are not neatly separated at the center but are mixed and partially dissolved into the metallic hydrogen layer above it, with no clear boundary between them. The core region is much larger than predicted, perhaps extending out to nearly half of Jupiter’s diameter.
This discovery fundamentally changes our understanding of Jupiter’s history. A neat, compact core is the expected outcome of a relatively quiet formation process. A large, fuzzy core, on the other hand, suggests a more violent past. One leading hypothesis is that the primordial Jupiter was struck by another massive planetary embryo—a protoplanet perhaps ten times the mass of Earth—early in its history. Such a cataclysmic impact would have shattered the original solid core, scattering its rocky and icy material and mixing it into the deep interior.
This finding has implications for the entire solar system. If its largest planet experienced such a violent collision, it suggests the early solar system was a far more chaotic and dynamic place than previously thought. It lends support to theories like the “Grand Tack,” in which Jupiter migrated significantly from its original orbit, disrupting the formation of other planets like Mars and shaping the structure of the asteroid belt. The state of Jupiter’s core today is a fossilized clue, hinting at a turbulent youth that had consequences for all its planetary neighbors.
The Jovian Magnetosphere
Jupiter is surrounded by a magnetic field of immense power and scale, a direct consequence of the planet’s rapid rotation and its unique interior structure. This field creates a vast bubble in space known as the magnetosphere, which dominates its local environment and interacts with its moons in complex ways.
Origin and Strength
The magnetic field is generated deep within the planet by a dynamo effect. The combination of Jupiter’s fast 10-hour rotation and the swirling motions within its huge internal ocean of electrically conductive liquid metallic hydrogen creates powerful electrical currents. These currents produce the most formidable planetary magnetic field in the solar system.
Its magnetic moment—a measure of the field’s intrinsic strength—is roughly 18,000 to 20,000 times greater than Earth’s. At the cloud tops, the field is about 10 to 20 times stronger than the field at Earth’s surface. Interestingly, its polarity is opposite to that of Earth; the north magnetic pole is in Jupiter’s southern hemisphere, so a terrestrial compass would point south.
Structure and Scale
Jupiter’s magnetosphere is the largest and most powerful of any planet, and by volume, it is the largest known continuous structure in the solar system, second only to the Sun’s own heliosphere. It creates a cavity in the solar wind, the stream of charged particles flowing from the Sun. On the side facing the Sun, the magnetosphere extends 1 to 3 million kilometers (600,000 to 2 million miles). On the night side, it is drawn out by the solar wind into a colossal, windsock-shaped magnetotail that stretches over 1 billion kilometers (600 million miles)—a distance so vast it reaches past the orbit of Saturn. If this structure were visible to the naked eye from Earth, it would appear several times larger than the full Moon in the night sky.
A Moon-Fueled System
While Earth’s magnetosphere is primarily shaped and fueled by its interaction with the solar wind, Jupiter’s is a fundamentally different, hybrid system. It is largely driven from within, powered by the planet’s own rotation and, crucially, by its innermost large moon, Io.
Io is the most volcanically active body in the solar system, and its volcanoes continuously spew about a ton of material—mostly sulfur and oxygen—into space every second. This material is stripped from the moon and becomes ionized (electrically charged) by sunlight and collisions. Jupiter’s rapidly rotating magnetic field sweeps up this plasma, accelerating it and trapping it in a donut-shaped cloud that encircles the planet along Io’s orbit. This structure is known as the Io Plasma Torus.
This torus acts as a massive, constantly replenished reservoir of charged particles that feeds the entire Jovian magnetosphere. This internal source of plasma, driven by the geological activity of a moon, makes Jupiter’s magnetosphere a self-sustaining ecosystem, fundamentally different from the more passive, solar-wind-driven magnetosphere of Earth.
Auroras and Radiation
Jupiter’s powerful magnetic field funnels charged particles from the magnetosphere—both from the solar wind and the Io torus—down toward the planet’s north and south poles. As these high-energy particles slam into the gases of Jupiter’s upper atmosphere, they create brilliant and permanent light shows known as auroras. These Jovian auroras are hundreds of times more energetic than Earth’s northern and southern lights and are visible across the electromagnetic spectrum, from infrared to X-rays. Recent observations have shown that they are incredibly dynamic, with features that can fizz, pop, and flicker on timescales of just a few seconds.
The magnetosphere also traps particles in intense radiation belts, similar to Earth’s Van Allen belts but thousands of times stronger. This harsh radiation environment poses a significant hazard to any spacecraft venturing close to the planet and bombards the surfaces of the inner moons.
The Faint Rings
While Saturn is famous for its magnificent and bright rings, Jupiter also possesses a ring system, though it is much more subtle. Jupiter’s rings are so faint and tenuous that they remained undiscovered until NASA‘s Voyager 1 spacecraft flew past the planet in 1979. They are composed primarily of tiny, dark dust particles, not the bright, reflective ice that makes up Saturn’s rings. This composition makes them very difficult to see from Earth, except when they are backlit by the Sun, which causes the fine dust to scatter light forward and become visible.
The ring system is not a static, primordial feature but rather a dynamic and constantly evolving one. Its existence is a direct consequence of the planet’s small inner moons. The rings are continuously replenished by dust kicked up when interplanetary micrometeoroids collide with the surfaces of four small moons that orbit within or near the rings: Metis, Adrastea, Amalthea, and Thebe. The small dust particles are eventually removed from the system by Jupiter’s powerful gravity and magnetic field, meaning the rings we see today are relatively young, perhaps less than a million years old. They are essentially a visible record of the ongoing bombardment of these tiny moons.
The Jovian ring system is divided into four main components:
- The Halo Ring: The innermost part of the system is a thick, faint, donut-shaped torus of dust particles.
- The Main Ring: Extending outward from the halo is the brightest and thinnest part of the system. This relatively narrow ring is primarily sourced from dust ejected from the moons Adrastea and Metis.
- The Gossamer Rings: Beyond the main ring lie two much wider, fainter, and thicker rings. These are known as the gossamer rings because of their transparency. They are composed of microscopic debris from the moons Amalthea and Thebe and are named accordingly. The Amalthea gossamer ring is the inner of the two, and the Thebe gossamer ring is the outermost component of the system.
A Miniature Solar System: The Moons of Jupiter
Jupiter is the anchor of its own planetary system, orbited by a large and diverse family of at least 95 known moons. These range from tiny, captured asteroids a few kilometers across to four giant worlds large enough to be considered planets in their own right. These four—Io, Europa, Ganymede, and Callisto—were the first celestial objects discovered to be orbiting another planet. Their observation by Galileo Galilei in 1610 was a landmark moment in the history of science, providing powerful evidence that Earth was not the center of all motion in the universe.
These four “Galilean” moons are fascinating worlds, each with a unique character shaped by its distance from Jupiter. They form a gradient of geological activity driven by the immense gravitational forces of their parent planet, creating a perfect natural laboratory for studying how planetary bodies evolve.
Io: The Volcanic World
Io is the innermost of the Galilean moons and the most geologically active body in the solar system. Its surface is a tormented landscape of constant change, dotted with hundreds of active volcanoes. Some of these volcanoes erupt with such force that they send plumes of sulfur and sulfur dioxide dozens, or even hundreds, of kilometers high. This relentless volcanic activity continuously paves over the surface with fresh lava and colorful sulfur compounds, which is why Io is completely devoid of impact craters. Its vibrant, mottled appearance of yellow, orange, red, and black has earned it the nickname “the pizza moon.”
Composed primarily of silicate rock around a molten iron or iron-sulfide core, Io is the driest known object in the solar system. Its intense volcanism is not driven by internal radioactive decay, as on Earth, but by tidal heating. Io is caught in a gravitational tug-of-war between the immense pull of Jupiter and the rhythmic tugs of its neighboring moons, Europa and Ganymede. This constant flexing and squeezing of Io’s interior generates tremendous frictional heat, keeping its mantle partially molten and powering its spectacular volcanoes.
Europa: The Ocean World
Europa, the next moon out from Jupiter, presents a starkly different face. Its surface is one of the smoothest of any solid body in the solar system, a bright shell of water ice that is crisscrossed by a complex network of long, linear cracks and ridges. The near-total lack of large impact craters suggests that its surface is geologically young and is constantly being renewed.
The most compelling aspect of Europa lies beneath this icy shell. Multiple lines of evidence, particularly from measurements of its interaction with Jupiter’s magnetic field, strongly indicate the presence of a global ocean of liquid saltwater. This ocean is hidden beneath an ice crust estimated to be 15 to 25 kilometers (10 to 15 miles) thick. The ocean itself could be 60 to 150 kilometers (40 to 100 miles) deep. Though Europa is only about the size of Earth’s Moon, its hidden ocean may contain more than twice the amount of water found in all of Earth’s oceans combined.
The energy needed to keep this ocean liquid comes from the same tidal heating that drives Io’s volcanoes, though to a lesser extent. The combination of liquid water, a potential energy source, and the necessary chemical elements leached from its rocky seafloor makes Europa one of the most promising places in the solar system to search for present-day life.
Ganymede: The Giant Moon
Ganymede is not only Jupiter’s largest moon, but the largest moon in the entire solar system—it is bigger than the planet Mercury. It is a world of rock and ice in roughly equal measure and is a fully differentiated body, with a layered structure consisting of a liquid, iron-rich core, a rocky mantle, and a thick outer shell of ice.
Ganymede is unique among all moons in the solar system because it generates its own intrinsic magnetic field. This field, produced by convection within its liquid metallic core, is strong enough to carve out its own small magnetosphere within Jupiter’s larger one.
Its surface is a study in contrasts, composed of two distinct types of terrain. About a third of the surface consists of ancient, dark, heavily cratered regions, which record a history of bombardment from the early solar system. The other two-thirds are covered by younger, lighter-colored terrain that is crosscut by intricate systems of grooves and ridges. This grooved terrain is evidence of a period of intense tectonic activity in Ganymede’s past, when its crust was stretched and fractured. Like Europa, Ganymede is also thought to harbor a subsurface saltwater ocean, though it is likely sandwiched between layers of ice rather than in direct contact with a rocky seafloor.
Callisto: The Cratered World
Callisto, the outermost of the Galilean moons, is a primitive and ancient world. Its surface is the most heavily cratered in the solar system, a testament to a 4-billion-year history of impacts with little to no geological activity to erase the scars. It has no large mountains, volcanoes, or tectonic features.
Unlike its three siblings, Callisto appears to be only partially differentiated. Its interior is thought to be a cold, stiff mixture of rock and ice that becomes gradually denser toward the center, rather than having separated into a distinct core and mantle. This suggests it never experienced enough tidal heating to melt and fully separate its components. Its great distance from Jupiter means it is largely spared from the intense tidal forces that shape the inner moons.
Despite its seemingly inert nature, magnetic field data suggests that even Callisto may hide a salty liquid ocean deep beneath its thick, ancient, icy crust.
A History of Exploration
Our understanding of Jupiter has been built over centuries of observation, culminating in a series of robotic missions that have peeled back the layers of this complex world. Each mission built upon the knowledge of its predecessors, answering old questions while uncovering new, more mysteries. This progression is a clear demonstration of the scientific method in action, evolving from initial reconnaissance to highly specialized investigation.
The Pioneer Flybys (1973–1974)
NASA‘s Pioneer 10 and 11 were the true trailblazers. As the first spacecraft to traverse the asteroid belt and journey to the outer solar system, they served as scouts for all future missions. Pioneer 10 flew past Jupiter in December 1973, followed by Pioneer 11 in December 1974. They returned the first-ever close-up images of the planet and made fundamental discoveries. They found that Jupiter is predominantly a liquid planet and made the first measurements of its massive magnetic field. Crucially, they discovered that the radiation belts encircling Jupiter were thousands of times more intense than Earth’s, a critical finding that forced engineers to redesign subsequent spacecraft to survive the harsh environment.
The Voyager Flybys (1979)
The twin Voyager 1 and 2 spacecraft flew past Jupiter in 1979 and completely revolutionized our view of the Jovian system. Armed with more sophisticated cameras and instruments, they unveiled a world of stunning complexity and activity. Their landmark discoveries included:
- Active Volcanoes on Io: The most surprising discovery was active volcanism on Io, the first time such activity had been seen anywhere beyond Earth.
- Jupiter’s Rings: The Voyagers discovered Jupiter’s faint, dusty ring system, a feature that had been completely invisible to telescopes on Earth.
- Diverse Moons: They provided the first detailed look at the Galilean moons, revealing the smooth, cracked ice shell of Europa, the complex grooved terrain of Ganymede, and the ancient, cratered surface of Callisto.
- New Moons: They discovered three small inner moons: Metis, Adrastea, and Thebe.
The Galileo Orbiter (1995–2003)
While the Pioneer and Voyager missions were brief flybys, NASA‘s Galileo was the first spacecraft to go into orbit around Jupiter, allowing for nearly eight years of sustained, in-depth study. Galileo’s mission included an atmospheric probe that it released into Jupiter’s clouds—the first and only probe to sample the atmosphere of a gas giant directly. The probe found the atmosphere to be much hotter, drier, and windier than expected.
The Galileo orbiter’s long-term observations led to more paradigm-shifting discoveries. It gathered the data that provided the strongest evidence for the existence of deep, saltwater oceans beneath the icy crusts of Europa, Ganymede, and Callisto. It discovered that Ganymede generates its own magnetic field, a first for any moon. And it mapped Io’s extensive volcanism in great detail, showing its activity to be far greater than Earth’s. To protect the potentially habitable ocean of Europa from contamination, the mission was concluded by deliberately plunging the spacecraft into Jupiter’s atmosphere.
The Juno Orbiter (2016–Present)
NASA‘s Juno mission was designed specifically to answer the fundamental questions left by Galileo about Jupiter’s deep interior. Arriving in 2016, Juno entered a unique polar orbit, flying closer to the planet’s cloud tops than any previous spacecraft and repeatedly passing through its intense radiation belts. Its primary goals were to map Jupiter’s gravitational and magnetic fields, probe the deep atmosphere, and understand the planet’s origin.
Juno’s discoveries have once again rewritten textbooks. It provided the first views of Jupiter’s poles, revealing the bizarre geometric arrangements of giant cyclones. Its microwave instrument has peered deep beneath the clouds, showing that the belts, zones, and even the Great Red Spot are not shallow weather phenomena but have roots that extend hundreds of kilometers down. Its gravity measurements led to the discovery of the “fuzzy,” dilute core, challenging long-held theories of planet formation. And by mapping the magnetic field with unprecedented precision, Juno has documented its lumpy, irregular nature and detected changes over time, linking them directly to the planet’s deep winds. In its extended mission, Juno has also performed close flybys of the Galilean moons, adding to our knowledge of these diverse worlds.
Summary
Jupiter is a world of extremes, a gas giant whose scale and influence have shaped the solar system. It is a “failed star” composed of the same primordial hydrogen and helium as the Sun, and its immense gravity holds a miniature solar system of its own, with dozens of moons and a faint, dusty ring system. The planet’s character is defined by a deep internal heat source and the fastest rotation of any planet, a combination that drives its turbulent atmosphere into the familiar pattern of colorful bands and spawns storms like the Great Red Spot that can last for centuries.
Deep within, unimaginable pressures transform hydrogen into an electrically conductive liquid metal, whose motion generates the most powerful magnetic field among the planets. This field creates a colossal magnetosphere, an invisible structure larger than the Sun, that is fueled not just by the solar wind but by the constant volcanic eruptions of its moon Io. This interaction produces brilliant, permanent auroras at Jupiter’s poles.
Exploration has revealed that Jupiter’s interior is as complex as its atmosphere. The long-held idea of a small, solid core has been replaced by the discovery of a large, “fuzzy” core that is partially dissolved into the metallic hydrogen above it, a finding that suggests a more violent and chaotic history for the planet and for the early solar system.
The four large Galilean moons present a tableau of planetary evolution. They range from the fiery, volcanic furnace of Io and the ice-shelled ocean world of Europa—a prime candidate in the search for life—to the giant, magnetized Ganymede and the ancient, cratered Callisto. Their diversity is a direct result of their distance from Jupiter and the powerful tidal forces it exerts. Decades of exploration by robotic spacecraft have transformed our view of Jupiter from a distant point of light into a complex and dynamic system, yet each new discovery has only deepened the sense of mystery, ensuring that the king of planets will remain a compelling target of scientific inquiry for generations to come.
