JUPITER

The king of the planets.

Beyond the small terrestrial worlds huddled closely around the Sun, just past the expansive fields of asteroids encircling them, looms a colossus. A monstrous world with unrelenting storms of planetary scale, crushing depths, and the gravitational might to hurl humongous rocks across space. Nothing in the solar system casts a larger shadow than the celestial behemoth, Jupiter. Being named after the Roman counterpart of Zeus, the king of the gods, is no overstatement. Jupiter is the largest and most massive planet in the solar system. Known to ancient civilizations long before the invention of the telescope, Jupiter can easily be seen with the naked eye. It is outshined only by the Sun, Moon and Venus (and occasionally Mars). This is certainly due to the planet’s immense size. From Earth, Jupiter can appear as large as 50 arcseconds across, about 1/36th the size that our Moon appears.

In reality, Jupiter’s diameter spans about 11 Earths across at the equator. You could fit more than 1,300 Earths inside its entire volume. That isn’t for show, either. Jupiter has a lot packed into that size. Truly a super heavyweight, Jupiter has 318 times the mass of Earth! Make no mistake, even Jupiter can’t hold a candle to the Sun, which is ten times wider and over a thousand times more massive. The Sun contains 99.85% of the entire solar system’s mass while Jupiter only accounts for 0.1%. However, take away the Sun, and Jupiter shoots up to more than 70% of the remaining mass. That’s 2.5 times the mass of all the other planets combined! It is the only planet whose shared center of mass with the Sun, their common barycenter, lies outside the Sun’s surface.

Fun fact: If Earth was the size of a marble, Jupiter would be the size of a basketball, and the Sun would reach the height of a basketball hoop!

Fun fact: If you drove a car non-stop around the circumference of Jupiter’s equator at highway speeds, it would take you over six months to do a full lap around the planet! For comparison, it would take less than two and a half weeks on Earth.

You’d think hauling all that weight around would make the planet sluggish but Jupiter in fact has the fastest spin in the solar system, making a full rotation in just 10 hours. This is so fast that the centrifugal force actually causes Jupiter to bulge at the equator. When you were a kid, did you ever try to ride a fast-spinning merry-go-round? Your body got pulled outward and you had to hold on, right? It’s the same idea. Jupiter is nearly 10,000 km (6,200 mi) wider at its equator than it is from pole to pole, giving it an oblate shape that’s noticeable with even a backyard telescope! It sounds counterintuitive but this rapid rotation is actually a product of the planet’s size. Astronomers highly suspect that Jupiter formed first among the planets, beginning to accumulate material from the protoplanetary disk of gas and dust surrounding our young Sun. As this swirling material fell inward, the conservation of angular momentum caused it to spin faster and faster. Naturally, the largest concentration of mass amongst the planets would generate gravity to pull material with the greatest force.

Jupiter orbits more than five times farther from the Sun than Earth, taking nearly 12 Earth years to complete one revolution. This is almost six times longer than Mars, but less than half as long as Saturn. Imagine Earth and Jupiter as hands on a clock, with both moving counterclockwise. While Earth completes a full revolution from midnight back to midnight, Jupiter advances only from midnight to about 11 o’clock. As a result, Jupiter takes roughly a year to cross each zodiac constellation along the ecliptic. After each opposition, Earth needs about 13 months to catch up with Jupiter at the same clock position again. This shifts Jupiter’s opposition, and its best viewing season, backward by roughly one month each year. A summer opposition, for example, becomes a winter opposition about six years later.

Video: The orbit of Jupiter sped up to one month per second. Credit: NASA’s Eyes on the Solar System

It is suspected that Jupiter may not have formed at this distance but migrated through the solar system over time before settling there. This would have played a significant role in the evolution of the solar system. One of the more popular proposals, but one that has not yet been confirmed, is the grand tack hypothesis. It proposes that Jupiter initially formed closer to the Sun, then migrated even farther inward before reversing course. As Jupiter formed, it cleared out a gap in the protoplanetary disk, creating an inner disk and an outer disk farther from the Sun that orbited slower (top image). Gravitational interactions with both sides resulted in Jupiter losing angular momentum and it spiraled inward toward the Sun. The planet settled in around where Mars orbits today. Saturn soon followed, and when the two planets became synced into an orbital resonance and opened a shared gap in the disk, the balance of gravitational torques changed (middle image). Jupiter and Saturn reversed direction, resembling a sailboat’s tack maneuver. Together, the two planets migrated outward, eventually reaching their present orbits (bottom image). Throughout this migration, Jupiter’s immense gravity disrupted the developing solar system, scattering rocky material and influencing the orbits of the other planets. This may help explain why Mars remained relatively small and why the asteroid belt contains a mixture of rocky and icy bodies.

Jupiter is a gas giant. The classification is fairly self-explanatory: it is a giant planet made up entirely of gas. Well, kind of. Unlike the terrestrial planets, Jupiter does not have a solid surface you could walk on. Instead, what we see as its “surface” is actually the cloud tops of its immense atmosphere, which extends deep into the planet (we’ll explore what lies beneath those cloud tops shortly).

Being a gas giant does involve a bit more. As a gas giant, Jupiter is composed primarily of hydrogen and helium, the same two elements that make up the vast majority of the Sun. Even the proportions are similar. Hydrogen makes up about three-quarters of Jupiter’s mass while helium accounts for most of the remaining quarter. In this respect, Jupiter resembles a star in its composition. If Jupiter formed first, it does make sense that it comes from the same raw materials that formed the Sun. That being said, Jupiter is partly composed of trace amounts of compounds like methane, ammonia and water. While they are a very small proportion of the planet's total makeup, they play an important role in how the atmosphere behaves. This composition distinguishes Jupiter from the other class of giant planets, known as ice giants. Uranus and Neptune belong to this category because their interiors contain substantially greater proportions of these heavier compounds, which are classified as “icy volatiles.”

Closer look: There is a commonly shared sentiment that Jupiter is a ‘failed star,’ but this isn’t really the case. Jupiter’s mass would need to be multiplied by at least 75-80 times to trigger thermonuclear fusion. It’s more accurate to say that Jupiter is a really successful planet. Actual failed stars would be brown dwarfs.

Jupiter retains primordial heat from its formation, and its interior continuously releases energy as it slowly compresses under the planet’s crushing gravity. In fact, Jupiter actually radiates more energy than it receives from the Sun. This escaping heat drives hot, buoyant gas upward towards the atmosphere's “surface” while denser, cooler gas sinks down to replace it. This cyclical process is called convection and drives much of Jupiter’s atmospheric motions.

So why do zones and belts look different?

As we said earlier, much of Jupiter’s interior is a massive circulation of convecting gas, with hotter gas rising and cooler gas sinking. Rising gas from deep within the atmosphere expands and cools as the surrounding pressure decreases. At high enough altitudes, ammonia in the gas can condense into tiny ice crystals. These crystals form bright reflective high-altitude clouds that obscure the deeper layer and produce the pale appearance of Jupiter’s zones.

Meanwhile, gas within belts is sinking. Increasing pressure compresses and warms the gas. The opaque ammonia clouds evaporate, revealing the deeper atmosphere below. The reason the lower altitude gas is darker in color is not entirely clear, but scientists suspect it has to do with its chemistry. They have proposed the presence of certain sulfur, phosphorous and carbon-based compounds that could absorb blue and violet wavelengths of sunlight and reflect red and yellow wavelengths.

Closer look: When we view Jupiter through infrared sensors, we find that zones are dark and belts glow bright, indicative of their colder and hotter temperatures, respectively.

Jupiter’s atmosphere is home to hundreds of vortices: spiraling storms of gas driven by the planet’s rotation. While most are difficult to observe with small backyard telescopes, space telescopes and orbiters have captured them in stunning detail. Vortices are broken down into two categories: cyclones and anticyclones. In the northern hemisphere, cyclones rotate in the same counterclockwise direction as Jupiter around centers of low pressure. Gas from the surrounding atmosphere is pushed into the storm and forced up toward the cloud tops. These tend to be more chaotic and short-lived. Meanwhile, anticyclones rotate against Jupiter’s rotation around centers of high pressure. Gas near the top of the storm is forced down and thrown outward from the storm. These tend to be more stable, uniform structures. In the southern hemisphere, they work the same way, but their rotations are reversed, with cyclones rotating clockwise and anticyclones rotating counterclockwise. Unlike most terrestrial storms, Jovian vortices can persist for extraordinary lengths of time, ranging from days to hundreds of years! This longevity is aided by Jupiter’s enormous size, rapid rotation, and gaseous composition (no friction from a solid surface), which contribute to an exceptionally powerful Coriolis effect.

Pictured: Cloud top images captured by the Juno spacecraft orbiting Jupiter. The beige-colored clouds of the North North Temperate Belt were imaged from 7,000 km above, while the blue-colored clouds of the North Polar Region were imaged from 19,000 km above.

We, of course, can’t talk about Jupiter’s storms without talking about the one pretty much everyone knows, the Great Red Spot. This is easily one of Jupiter’s most recognizable features. It is a gargantuan high-pressure tempest, large enough to comfortably fit the entire Earth inside of it. At its widest, the storm’s elliptical shape spans 16,000 km (10,000 mi). This is ten times the diameter of the largest storms on Earth, making it the largest storm in the solar system.

There is some ambiguity surrounding the discovery of Jupiter’s Great Red Spot. The earliest recorded sighting is often attributed to English astronomer Robert Hooke, who observed a spot on Jupiter in 1664. However, his spot appeared in the North Equatorial Belt, whereas the Great Red Spot lies in the southern edge of the South Equatorial Belt. Some researchers suggest Hooke may instead have mistaken the shadow of a transiting moon for a feature in Jupiter’s atmosphere. The following year, Italian astronomer Giovanni Cassini observed a persistent spot at the correct latitude, tracking its position as Jupiter rotated. Yet it remains uncertain whether this was the same storm we see today or an entirely different one that eventually faded away. The mystery stems from a peculiar 48-year gap in the historical record: Cassini’s Permanent Spot was last observed in 1713, and no comparable feature was recorded again until 1831, when the modern Great Red Spot was first identified. It has been consistently monitored ever since. Due to the historical gaps, scientists are unable to say with great confidence how old this storm has been raging. It is at least 200 years old, but if it’s been the same one observed by Robert Hooke, it is likely older than 360 years.

The Great Red Spot is an anticyclonic storm straddling the boundary between the South Equatorial Belt and the South Tropical Zone. It rotates counterclockwise roughly once every 11 Jovian days (4.5 Earth days), with peripheral winds exceeding 400 km/h (250 mph), influenced in part by the powerful jet streams surrounding it. Like much of Jupiter’s atmosphere, the storm is highly dynamic, constantly changing in size, shape, position, and color. It can appear brick red, pale salmon, or even white. Scientists aren’t really sure what produces its varying colors, but they are likely influenced by things like chemistry, sunlight, and altitude. Its shape fluctuates between more circular and elongated forms, but the long-term trend has been toward a smaller storm with slower winds. In the late 19th century, it was estimated to span nearly 40,000 km (25,000 mi), more than three times Earth’s diameter!

Fun fact: Gravity data from the Juno spacecraft suggests the Great Red Spot extends 500 km (310 mi) below the cloud tops.

In 2000, three white oval storms along Jupiter’s South Temperate Belt merged into a single, larger storm. Officially named Oval BA, but affectionately nicknamed “Red Spot Jr.,” it periodically shifts between different sides of the Great Red Spot and fluctuates in color between white and red.

Earlier, while talking about Jupiter being a gas giant, you might have caught that it’s made entirely of gas “kind of.” Indeed, if you were to pass into Jupiter’s thick gaseous mass, you won’t hit a solid surface. So, what is actually going on underneath those clouds? Instead of starting at the lowest layer and working our way up like we did with the terrestrial worlds, let's start at the outermost layer of Jupiter’s atmosphere and make our descent through the depths to the planet's core.

Jupiter’s outermost atmospheric layer, the exosphere, is so tenuous that it is virtually indistinguishable from interplanetary space. Sparse hydrogen and helium atoms travel vast distances without colliding, and the atmosphere gradually fades into space without a clear boundary. As we descend toward the planet to around 1,000 km (620 mi) above the cloud tops, we reach the thermosphere, where temperatures rise sharply as the atmosphere absorbs solar radiation. Though the exosphere can reach even higher temperatures, the thermosphere marks the steepest temperature increase with altitude. Despite this extreme heat, the atmosphere is so sparse, at roughly one-millionth Earth’s surface atmospheric density, that it would transfer very little heat to a passing object. The intense heating is especially curious given Jupiter’s distance from the Sun.

We descend to 320 km (200 mi) above Jupiter’s cloud tops, entering its stratosphere. The scene is breathtakingly serene. Below, beige and cream-colored clouds twist together like marbled paint, interwoven with immense, knotted storms occasionally flickering with lightning. Toward the horizon, the clouds fade into a soft, hazy edge, blurring the boundary between the planet and the darkness of space. Here, Jupiter’s gravity is 2.5 times Earth’s. If you started to drop from this altitude, you’d accelerate downward at approximately 25 m/s² while the tenuous atmosphere offers little resistance. As you fall, the darkness of space gradually gives way to a pale haze, and the cloud tops below acquire an increasing sense of depth.

Passing into the clouds marks your entry into Jupiter’s troposphere, the planet’s so-called “surface.” From within, Jupiter is no longer a flat tapestry of colored bands, but a vast, three-dimensional world. Towering cloud banks rise around you, while gaps offer fleeting glimpses of deeper layers. Sunlight filters through the clouds, scattering into a diffuse, milky haze, until thick clouds engulf everything in an opaque veil. This is a realm of violent weather, where powerful jet streams, ferocious winds, and terrifying lightning storms drive the atmosphere. The troposphere contains three main cloud decks: an upper layer of white ammonia ice clouds associated with Jupiter’s zones; an intermediate layer of ammonium hydrosulfide clouds associated with its reddish-brown belts; and a deeper layer of water clouds, which play an important role in the planet’s atmospheric dynamics. Somewhere within these cloud decks, the atmosphere reaches a density on par with the surface of Earth.

As you descend below the clouds, you enter Jupiter’s true interior: a continuous, dense layer of hydrogen and helium gas. Sunlight can no longer reach you, leaving you to plunge through pitch-black darkness, punctuated only by flashes of lightning that briefly illuminate your hypersonic descent. The deeper you go, the denser and hotter the gas becomes. In 1995, NASA’s Galileo orbiter plunged into Jupiter’s atmosphere, surviving just 58 minutes before contact was lost. It reached 132 km (82 mi) below the cloud tops, where its final measurements recorded a temperature of 152°C (307°F) and a pressure 22 times that of Earth’s sea-level atmosphere. By 250 km (155 mi) down, the pressure rivals that at Venus’s surface. Farther below, the hydrogen becomes increasingly compressed, gradually transitioning toward a liquid state. At 1,000 km (620 mi) beneath the clouds, you find yourself in a scorching ocean of hydrogen darkness, under pressures comparable to the lowest depths of the Pacific Ocean and temperatures hot enough to melt steel. A car teleported here would instantly crumble like paper. Its metal would heat to a red glow, then yellow, before melting into a blob of molten metal that would eventually disperse into the surrounding hydrogen.

Around 25,000 km below Jupiter’s cloud tops, roughly two Earth diameters down, something bizarre starts to occur. The pressure here is 2 million times that of Earth at sea level and the temperature has reached 8,000°C (14,400°F), hotter than the surface of the Sun. The atoms here are being squeezed so tightly that their electrons break away and travel freely through the liquid, generating an electric current just like a metal. Because this vast region makes up roughly two-thirds of Jupiter’s mass, the flowing current of this metallic fluid can generate Jupiter’s powerful magnetic field (more on that shortly). Scientists are pretty sure about the physics and role that this region plays but are less sure what it would actually be like here.

Closer look: Deep inside Jupiter, immense pressures may cause helium and neon to separate from liquid metallic hydrogen and sink as discreet droplets, sometimes called helium and neon rain. Scientists have also proposed that carbon may crystallize into raining diamonds.

Finally, we reach Jupiter’s core. Even today, it’s difficult for astronomers to make a lot of confident statements about this central region because it’s incredibly difficult for scientific instruments to penetrate Jupiter’s immense mass. It wasn’t until gravity and magnetic data from the Juno orbiter gave us any idea. What we found was perplexing. Instead of a distinct solid sphere, like many astronomers were expecting, we found a core that was larger and more dilute. Many astronomers have come to call it “fuzzy” in that it seems to gradually blend into the surrounding liquid metallic hydrogen layer. It might be composed of a mix of light elements, like hydrogen and helium, and heavy elements, like silicon and iron. The truth is that we honestly don’t know what the core is made of, how it’s structured or if it’s even solid. The only thing we are sure of is that it is unbelievably dense and hot. Pressure could be 45 million times that of Earth at sea level and temperature could be 20,000°C (36,000°F), more than triple that of the Sun’s surface. Jupiter’s core is easily one of the most mysterious and exotic environments in the solar system.

Jupiter has the strongest magnetic field of any planet in the solar system. A magnetic field is an invisible region of space that produces an attractive or repulsive force between moving electric charges. Magnetic fields are often illustrated using a series of invisible lines that extend from one side of a magnet and curve around to meet at the opposite side. A magnet could be a simple bar magnet you’d find in a science classroom or celestial objects, such as planets and stars. On Earth, the magnetic field is generated by the motion of molten iron and nickel in its core; in the Sun, by the motion of ionized plasma; and in Jupiter, by the motion of liquid metallic hydrogen. These various processes are known as dynamos.

Video: An animation of Jupiter’s magnetosphere as measured by the Juno orbiter in 2017. Credits: NASA/Scientific Visualization Studio/Tom Bridgman

Jupiter’s magnetic field extends millions of kilometers into interplanetary space, reaching about 7 million km (4.3 million mi) toward the Sun. Here, it forms a protective magnetic bubble called the magnetosphere, which deflects much of the solar wind, a constant stream of charged particles flowing from the Sun. On the opposite side, the solar wind stretches Jupiter’s magnetic field hundreds of millions of kilometers, reaching roughly as far as Saturn’s orbit! The charged particles from the solar wind, as well as volcanic material from Jupiter’s moon Io, get trapped by the magnetic field and travel around the planet in a series of radiation belts. These are analogous to Earth’s famous Van Allen belts. Charged particles also get funneled down the magnetic poles where they slam into Jupiter’s atmosphere. They become ionized and glow, producing Jupiter’s aurorae.

Pictured: Image captured by the Hubble Space Telescope in 2016 of Jupiter’s aurora.

Jupiter’s incredibly strong gravity exerts its influence across the solar system. For planets, this results in gravitational nudges in their orbits, known as perturbations. On the other hand, Jupiter has the ability to hurl asteroids way off course. A regularly repeated factoid about Jupiter is that it acts as a steadfast guardian protecting Earth from rogue asteroids. While the general sentiment might be true, we don’t actually have a confirmed example of an asteroid that we are sure would have hit us were it not for Jupiter intercepting it. In fact, this phenomenon goes both ways. Jupiter is just as likely to divert an asteroid's trajectory towards our orbit. It’s a common refrain by astronomers to put suspicion on Jupiter whenever we find an asteroid lurking where we didn’t expect.

To illustrate Jupiter’s significant influence on asteroids, consider the Lagrange points of the Sun-Jupiter system, one of the solar system’s most powerful gravitational partnerships. Lagrange points are five regions where the gravitational forces of two orbiting bodies and a smaller object’s orbital motion combine to allow the smaller object to maintain a relatively stable position. The first three points (L1-L3) lie along a straight line through the Sun and Jupiter and are unstable, meaning an object can remain near them only with careful positioning or orbital adjustments. The last two (L4-L5) lie 60° ahead of and behind Jupiter in its orbit. These are stable because the gravitational and orbital forces allow objects to maintain a triangular configuration with the Sun and Jupiter. Think of a telescope: which is less likely to tip over, one balanced on two legs or one balanced on three? These stable points have trapped large populations of asteroids known as trojans. The first, 588 Achilles, was discovered at L4 in 1906 by German astronomer Max Wolf. Since then, more than 15,300 Jupiter trojans have been confirmed, though the actual population may approach one million.

Fun fact: L4 trojans generally receive Greek names, while L5 trojans receive Trojan names. Two early exceptions are 624 Hektor, named after a Trojan warrior, and 617 Patroclus, named after a Greek warrior.

In 1977, NASA launched the twin Voyager probes which would tour the giant planets of the outer solar system. About a year and a half later, Voyager 1 would reach Jupiter, the third spacecraft to ever make the visit. During its brief flyby, it would spot numerous details not previously caught by the Pioneer 10 & 11 probes. Voyager glimpsed the turbulence of Jupiter’s weather, including lightning storms, aurorae and all manner of spiraling storms. Possibly the biggest surprise happened when Voyager passed into Jupiter’s shadow, allowing the planet to eclipse the Sun. Shimmering against the solar backlight, the probe captured the first images of Jupiter’s ring system. While they are narrow and faint compared to Saturn and Uranus, Voyager 1’s unique vantage point turned these dusty rings radiant.

Pictured: The Voyager 1 spacecraft’s first image of Jupiter’s rings in 1979.

Pictured: The Galileo spacecraft reveals the full span of Jupiter’s rings in 1996.

In 1996, the Galileo orbiter confirmed that Jupiter’s rings formed from the dust blasted out by meteor impacts on small nearby moons. These moons play a fundamental role in the shaping and maintaining of this structure. Astronomers have identified four main components to the ring system:

• A thick, donut-shaped inner ring known as the halo ring. It is vertically the thickest ring and is thought to be composed of very fine dust grains. Unlike the other rings, which appear reddish, the halo ring is slightly bluish. It is thought that dust in the halo ring migrates inward from the main ring.

• A relatively bright but thin ring known as the main ring. It is the brightest and most populated ring of the four. It is positioned just outside the halo ring. It is shepherded and continuously replenished by the small moons, Adrastea and Metis, which both orbit inside the ring’s outer edge.

• Two wide, thick and faint outer rings known as the gossamer rings.

  • The inner ring is associated with dust from Amalthea.

  • The outer ring is associated with dust from Thebe.

Pictured: An infrared image captured by the James Webb Space Telescope that reveals Jupiter’s rings. The bright object to the left is Amalthea while the pinpoint object at the tip of the ring is Adrastea.

JOVIAN MOONS

The terrestrial planets of the inner solar system have a combined total of three moons, two of those planets don’t have any and only Earth’s moon is massive enough to pull itself into a sphere. It’s a much different story with the giant planets out past the asteroid belt. Jupiter currently has 115 known moons caught up in its immense gravitational influence. That’s a huge rock collection! What’s wild is Jupiter doesn’t even have the most. This population of rocks and ice are diverse in their form, orbit and origin. They come in many different shapes and sizes, some orbit backwards, and many weren’t always moons.

Jupiter’s moons roughly break down into three main groups:

• The four inner moons

• The four Galilean moons

• The numerous and irregular outer moons

INNER MOONS

The first group is the four closest moons to Jupiter. These often get overshadowed by the much larger quartet further out but, as we learned earlier, they play a fundamental role in shaping Jupiter’s rings. They are relatively small and are unable to shape themselves into spheres, looking lumpier and potato shaped. They generally orbit in-line with Jupiter’s equator with little to no incline between them.

Pictured: Jupiter’s inner moons from left to right (innermost to outermost): Metis, Adrastea, Amalthea, Thebe

Metis is Jupiter’s closest moon, which inspired it to be named after the first wife of Zeus. It travels in a nearly circular orbit about 56,000 km away from Jupiter’s surface, inside of the main ring. lt is a smaller moon, though its elongated shape still makes it three times wider than Adrastea.

Adrastea orbits along the outer edge of Jupiter’s main ring and is thought to be its greatest contributor of dust. It’s named after the daughter of Zeus and is the first natural satellite to be discovered by an interplanetary spacecraft, Voyager 2. This is largely due to its size as Adrastea is the smallest of the inner moons. It is just 20 km across at its widest, about the size of Manhattan. We still don’t know much else about its physical characteristics as we don’t have any high-resolution images.

Fun fact: Metis and Adrastea are the only moons whose orbits outpace Jupiter’s rotation speed.

Amalthea is the largest of Jupiter’s inner moons and the fifth largest overall, spanning 252 km at its widest. On Earth, it would reach from Baltimore to New York City. It was the last natural satellite discovered through direct observation, rather than photography. Named after Zeus’s foster mother, Amalthea orbits the outer edge of the Amalthea gossamer ring. Imaging has revealed it to have the reddest surface in the solar system, likely caused by sulfur ejected from Io’s volcanic eruptions.

Thebe is the most distant of the inner moons, orbiting within the outer edge of the Thebe gossamer ring. It is 116 km across at its widest, roughly the distance between Baltimore and Harrisburg.

Pictured: Io, Europa and Ganymede share a 1:2:4 orbital resonance with Jupiter. This means that Io can orbit Jupiter twice for every one Europa orbit and four times for every Ganymede orbit.

IO orbits the closest to Jupiter, completing one revolution in 1.5 Earth days. It’s only a little bigger than Earth’s moon but is the densest of Jupiter’s moons. When the Voyager 1 space probe passed Io in 1979, it revealed a bizarre surface that was yellow, orange, red and black and didn’t seem to have any obvious impact craters.

It was later discovered that Io is the most volcanic object in the solar system, with over 400 active volcanoes. The erupted material, rich in sulfur, changes the surface over time and gives the moon its odd color scheme. Io’s volcanic activity is attributed to the tidal forces from Jupiter and other passing moons pulling and stretching it which heats up its interior via friction. A lot of that sulfur ends up as a thin atmosphere and even gets picked up by Jupiter’s magnetic field, creating a tremendous donut-shaped radiation belt between the two objects.