Baby Got Back

After a short detour into prehistoric reptiles, it’s time to return to our tour of the Solar System. When we left off, we had just crossed the asteroid belt, which means we’ve finally reached the outer Solar System. This stage of the journey is especially exciting, because it brings us to Earth’s two giant siblings: Jupiter and Saturn. The voluptuous rump of the Solar System, if you will.

Every planet has something unique to offer, but none capture my imagination quite like these two. From Jupiter’s swirling Great Red Spot^{1: Go to reference 1 at the end of the page} to Saturn’s iconic rings^{2: Go to reference 2 at the end of the page}, they stand as symbols of beauty and mystery. More than any others, Jupiter and Saturn fuel a sense of wonder and invite us to picture what human life might look like if we ever colonised the wider Solar System.

Not the planets themselves, of course. Colonising a rocky neighbour like Mars feels at least faintly possible, but Jupiter and Saturn are gas giants,^{3: Go to reference 3 at the end of the page} immense spheres of hydrogen and helium with no solid ground to land on. Any spacecraft attempting to descend would be crushed by the overwhelming pressure long before approaching the core. The real promise lies not in the giants themselves but in their moons, which form a diverse set of targets, some of the most compelling in the entire Solar System.

Jupiter’s Europa,^{4: Go to reference 4 at the end of the page} with its hidden ocean beneath an icy crust, excites astrobiologists with the possibility of life. Its sister Ganymede,^{5: Go to reference 5 at the end of the page} the largest moon in the Solar System, is bigger than Mercury and almost a planet in its own right. Saturn’s Titan,^{6: Go to reference 6 at the end of the page} wrapped in a thick atmosphere and dotted with methane lakes, is strangely reminiscent of Earth in many ways. These moons may be hostile by human standards, but they hold the raw materials and scientific mysteries that make them some of the most compelling places we could ever hope to explore.

Jupiter^{7: Go to reference 7 at the end of the page} is the undisputed heavyweight of the Solar System, a planet so massive that it contains more than twice the material of all the other planets combined. Its immense gravity has shaped the history of the Solar System, shepherding asteroids, deflecting comets, and in many cases protecting the inner planets from catastrophic impacts.

Visually, Jupiter is a world of swirling storms and colourful cloud bands, dominated by the famous Great Red Spot, a raging anticyclone larger than Earth that has persisted for centuries. It has been observed for at least 350 years,^{8: Go to reference 8 at the end of the page} making it the longest continuously monitored storm in the Solar System. The first confirmed sighting is often credited to the Italian astronomer Giovanni Cassini^{9: Go to reference 9 at the end of the page} in 1665, who described a “permanent spot” on Jupiter’s surface. Since then, generations of astronomers have tracked its changing size and colour through telescopes, sketching and later photographing its turbulent form.

Despite centuries of observation, the storm remains something of a mystery. We know it is an anticyclonic vortex, with winds circling counterclockwise at speeds of up to 430 km/h. At its peak in the late 19th century, it stretched more than 40,000 km across, large enough to swallow three Earths side by side. In recent decades, however, the spot has been shrinking, now measuring just under 16,000 km across, and its hue has shifted from deep brick red to a paler orange.^{10: Go to reference 10 at the end of the page}

What drives the storm’s longevity is still not fully understood. Some models suggest that Jupiter’s deep atmosphere supplies energy that sustains it, while others propose that neighbouring jet streams help confine and reinforce the vortex. Spacecraft like Voyager, Galileo, and Juno have all contributed crucial data, yet the Great Red Spot continues to challenge our understanding of planetary weather.^{11: Go to reference 11 at the end of the page}

Jupiter also has a system of faint rings,^{26: Go to reference 26 at the end of the page} though they are nowhere near as prominent as Saturn’s. Composed mainly of tiny dust particles kicked up by impacts on its small inner moons, these rings are subtle and difficult to see from Earth, but they reveal yet another layer of complexity in the planet’s environment. They may be faint, but they are a reminder that even gas giants have structures beyond their swirling atmospheres and mighty magnetic fields.

In terms of sheer scale, Jupiter is a true behemoth. It has a mass roughly 318 times that of Earth and an equatorial diameter of about 143,000 kilometres, making it just over 11 times wider than our planet. Its enormous mass means its gravitational influence extends across the Solar System, helping to shape the orbits of other bodies and even shepherd asteroids, quite fitting for a planet often called the Solar System’s “king of planets”.^{27: Go to reference 27 at the end of the page}

One of the planet’s most striking features is its rapid rotation. Completing a full spin in just under ten hours, the planet experiences extreme centrifugal forces that cause it to bulge at the equator and flatten at the poles.^{28: Go to reference 28 at the end of the page} This fast rotation drives the planet’s complex atmospheric patterns, whipping its clouds into vivid stripes and powering storms that can last for centuries. The combination of speed and size gives Jupiter a level of energy and dynamism unmatched elsewhere in the Solar System, giving it a striking appearance when viewed through telescopes or spacecraft imaging.^{29: Go to reference 29 at the end of the page}

The planet’s atmosphere is a roiling cauldron of gases and clouds, layered in bands of varying colour and composition. Rising and falling gas currents create zones of high and low pressure, forming intricate patterns that shift continuously. Ammonia (NH₃), methane (CH₄), and water vapour (H₂O) interact under extreme temperatures and pressures, producing the reds, browns, and whites of the cloud bands. Lightning storms rage with astonishing intensity, far more powerful than anything on Earth, and massive cyclones appear and disappear over the course of weeks, adding to the planet’s turbulent beauty.^{30: Go to reference 30 at the end of the page}

Jupiter also emits more heat than it receives from the Sun, a legacy of its formation and ongoing gravitational contraction, along with internal differentiation, that provides a persistent internal energy source. This heat drives deep convective currents in the planet’s interior, which in turn power the winds and storms we observe at and above the cloud‑tops. Combined with Jupiter’s immense size and mass, this internal energy ensures the planet is never static, its appearance is constantly evolving in both subtle and dramatic ways. Even minor shifts in its cloud bands or storm colours are closely monitored by astronomers, because they offer valuable clues about the processes occurring far beneath the visible layers.^{33: Go to reference 33 at the end of the page}

The planet’s magnetic field is the strongest of any planet in the Solar System, extending millions of kilometers into space and shaping the environment around it. Charged particles trapped within this field form intense radiation belts, creating a hazardous region for spacecraft and electronics. These belts, along with the planet’s faint ring system, highlight Jupiter’s vast influence, demonstrating that it is not only a dominant gravitational presence but also a formidable electromagnetic force.^{34: Go to reference 34 at the end of the page}

Jupiter has almost 100 moons, but the four biggest, Io, Europa, Ganymede, and Callisto, are the ones most people talk about. They are called the Galilean moons because Galileo Galilei was the first to observe them in 1610, marking the first time anyone had seen moons orbiting another planet and forever changing our understanding of the Solar System.^{37: Go to reference 37 at the end of the page} These four moons are not just large, they are fascinating worlds in their own right. Each has a unique environment and set of features, from volcanic activity to hidden oceans, that make them compelling targets for scientific study and future exploration. While Jupiter itself is a gas giant with no solid surface, these satellites offer tangible landscapes and intriguing possibilities for understanding how moons form, how they interact with their parent planet, and even where life might exist beyond Earth.

Io is the innermost of Jupiter’s four largest moons, orbiting the planet at about 422,000 kilometres.^{38: Go to reference 38 at the end of the page} Its surface is covered with hundreds of active volcanoes, some erupting fountains of lava dozens of kilometres high, making it a place of constant change and violent beauty. 🤘

The intense volcanic activity is primarily driven by tidal heating. Jupiter’s immense gravity exerts strong tidal forces on the moon, flexing its interior as it orbits. These gravitational tugs generate enormous frictional heat, which melts rock into magma and fuels the continuous eruptions observed across its surface. In fact, the heat from this tidal flexing makes Io more volcanically active than any other known object in the Solar System, including Earth.

The moon’s surface bursts with a kaleidoscope of colors, from yellow and orange to red and green, created by deposits of sulfur and sulfur dioxide. Vast plains, mountains taller than those on Earth, and lava lakes give the moon a surreal, almost alien landscape. These features are constantly reshaped by volcanic eruptions, meaning the surface of Io is remarkably young compared to most other moons and planets. Craters are scarce because eruptions continuously cover older terrain with fresh lava and volcanic deposits.^{39: Go to reference 39 at the end of the page}

Europa is slightly smaller than Earth’s Moon, with an equatorial diameter of about 3,100 kilometers, making it approximately 90% the size of our Moon.^{44: Go to reference 44 at the end of the page} Despite its size, Europa has garnered significant attention due to the compelling evidence suggesting a vast ocean beneath its icy crust. Its surface is smooth and bright, crisscrossed with long fractures and streaks, indicating that the ice shell is constantly shifting and reshaping itself. These features give Europa a striking, almost frozen-network appearance, unlike any other moon in the Solar System. Europa orbits Jupiter at an average distance of about 670,900 kilometers, keeping it firmly within the giant planet’s powerful gravitational influence.^{45: Go to reference 45 at the end of the page}

Beneath the ice, scientists believe there is a global ocean of liquid water, kept warm by tidal heating caused by Jupiter’s gravitational pull. As the moon orbits the gas giant, the varying gravitational forces flex its interior, generating heat that prevents the ocean from freezing solid. This hidden ocean could be more than twice the volume of all Earth’s oceans combined, making this moon a prime candidate in the search for extraterrestrial life.^{46: Go to reference 46 at the end of the page}

The moon’s magnetic environment also adds to its intrigue. Europa has a weak induced magnetic field, which suggests that the subsurface ocean contains saltwater capable of conducting electricity. This magnetic interaction with Jupiter’s own enormous magnetic field may help create an energy source that could support simple life forms in the ocean below.^{48: Go to reference 48 at the end of the page}

For scientists, Europa represents a tantalizing mix of familiarity and alienness. Its icy shell and hidden ocean make it both a scientific laboratory and a potential haven for life, and missions such as NASA’s Europa Clipper aim to explore these mysteries in detail. Europa shows how a small moon can hold enormous secrets beneath a deceptively calm surface.^{49: Go to reference 49 at the end of the page}

Orbiting Jupiter at an average distance of about 1,070,000 kilometers, Ganymede stands out as the largest moon in the Solar System, surpassing even the planet Mercury in size. However, it is less dense (1.94 g/cm³ compared to Mercury’s 5.43 g/cm³) due to its composition of roughly equal parts rock and water ice.^{50: Go to reference 50 at the end of the page} Its sheer scale is remarkable, yet what makes it truly fascinating is the complexity of its surface, shaped over billions of years of geological change. Bright bands of ridges and grooves cut across darker, heavily cratered terrain, revealing a long history of tectonic activity and resurfacing that has continually reshaped the moon’s face.^{51: Go to reference 51 at the end of the page}

Unlike most moons, Ganymede possesses its own intrinsic magnetic field, a feature more commonly associated with planets. This magnetic field generates auroras at its poles and interacts with Jupiter’s immense magnetosphere, creating a unique and complex environment. Scientists believe the magnetic field is produced by a liquid iron-nickel core, similar to the mechanism that drives Earth’s magnetism.^{52: Go to reference 52 at the end of the page}

Beneath its icy exterior, Ganymede is also thought to harbor a subsurface ocean sandwiched between layers of ice. While not as accessible as Europa’s ocean, it could still contain more water than all of Earth’s oceans combined. This hidden ocean raises the possibility that Ganymede could support life in some form or at least provide clues about how large moons evolve and maintain liquid water over billions of years.^{53: Go to reference 53 at the end of the page}

The surface also shows evidence of past tectonic and possibly cryovolcanic activity. Grooved terrain indicates that the ice shell has been stretched and fractured, while older dark regions preserve the record of ancient impacts.^{54: Go to reference 54 at the end of the page} This geological diversity makes Ganymede an ideal target for understanding the processes that shape large icy moons.

Ganymede has been observed and studied by spacecraft including Voyager, Galileo, and most recently Juno, with ESA’s upcoming JUICE mission scheduled to enter orbit around the moon in the 2030s.^{55: Go to reference 55 at the end of the page} These missions aim to explore its magnetic field, ice shell, and potential subsurface ocean, offering a window into one of the most extraordinary moons in the Solar System.

Callisto is the most distant of Jupiter’s four Galilean moons, orbiting the planet at nearly 1.9 million kilometers.^{56: Go to reference 56 at the end of the page} Unlike its more geologically active siblings, Callisto boasts a heavily cratered and ancient surface, largely unchanged for billions of years. This makes it a frozen time capsule, preserving the history of impacts from the early Solar System and offering scientists a glimpse into the past.

The moon’s surface is a patchwork of bright and dark regions, riddled with craters of all sizes. The largest, Valhalla, spans over 3,800 kilometers, making it one of the most impressive impact structures in the Solar System.^{57: Go to reference 57 at the end of the page} Unlike Io or Europa, Callisto shows little evidence of tectonic or volcanic activity, suggesting that it has been geologically quiet for much of its history.

Beneath its icy crust, Callisto is thought to host a subsurface ocean, though it lies deeper and is less active than Europa’s or Ganymede’s. This ocean may still interact with the surface over long timescales, but the lack of tidal heating means the energy available for geological activity is minimal.^{58: Go to reference 58 at the end of the page} Despite this, the presence of liquid water makes Callisto a subject of interest for understanding icy moons and their potential for habitability.

Callisto also lacks a significant magnetic field, unlike Ganymede, but it does experience induced fields from Jupiter’s magnetosphere.^{59: Go to reference 59 at the end of the page} Its distance from the planet means it avoids the intense radiation that batters the inner moons, making it one of the safer destinations in the Jovian system for future spacecraft exploration.

For explorers and scientists alike, Callisto represents a quiet, ancient world. Its heavily cratered surface and frozen history contrast sharply with the fiery Io, the oceanic Europa, and the dynamic Ganymede, rounding out the diversity of Jupiter’s largest moons and highlighting the rich variety of worlds that orbit the giant planet.^{60: Go to reference 60 at the end of the page}

Saturn is one of the most recognisable planets in the Solar System, famed for its elegant rings that shimmer like a cosmic jewel.^{62: Go to reference 62 at the end of the page} Slightly smaller than Jupiter, Saturn is still a gas giant of immense proportions, composed predominantly of hydrogen and helium. Despite its size, Saturn has a remarkably low density, so low, that it would float in water (if a bath tub large enough existed). Like Jupiter, the planet’s rapid rotation, completing a spin in just over ten hours, causes it to bulge at the equator and flatten at the poles, giving it a distinctive oblate shape.

The planet’s rings are by far its most iconic feature.^{63: Go to reference 63 at the end of the page} Composed of countless ice and rock particles ranging from tiny grains to house-sized chunks, the rings form complex structures and divisions that reflect both Saturn’s gravity and the influence of its many moons. Aside from their striking appearance, the rings are scientifically fascinating, offering insights into the processes of planetary formation and the dynamics of orbiting particles. Even faint gaps, like the Cassini Division, tell stories of moon-ring interactions that are still being studied today.

Saturn’s magnetic field, though weaker than Jupiter’s, remains powerful and extends millions of kilometres into space. It traps charged particles in radiation belts and shapes a vast magnetosphere that interacts with the solar wind to produce bright auroras at the poles. These auroras, visible in ultraviolet light, are driven by both solar wind activity and interactions with charged particles from Saturn’s rings and moons.^{70: Go to reference 70 at the end of the page} This reveals the scale and complexity of the planet’s electromagnetic environment.

From a distance, Saturn’s calm, pale-yellow appearance conceals the turbulence of its atmosphere and the energy generated deep within. Its famous rings display intricate structure, while storms and high-speed winds continue to reshape the upper cloud layers. Beneath its tranquil exterior, Saturn remains a highly active world of gas, heat, and motion.

In 2004, the Cassini-Huygens mission marked a new era of Saturn exploration.^{72: Go to reference 72 at the end of the page} Cassini entered orbit around the planet, conducting a 13-year study of Saturn, its rings, and its moons. The Huygens probe.^{73: Go to reference 73 at the end of the page} landed on Titan in 2005, providing the first direct observations of its surface and liquid methane lakes. Cassini also investigated Enceladus, discovering active cryovolcanism and a global subsurface ocean, making it one of the most intriguing targets for astrobiology in the solar system. The mission’s extended observations of Saturn’s seasonal changes and ring dynamics dramatically expanded our understanding of this iconic gas giant.

Other missions, such as New Horizons in 2007, performed flybys of Saturn on their way to Pluto, collecting additional information on the planet’s atmosphere, rings, and magnetosphere.

Among these moons, Titan and Enceladus stand out for their potential habitability and unusual geologic activity. Titan, with its dense nitrogen-rich atmosphere and lakes of liquid methane, is a world almost eerily reminiscent of Earth.^{79: Go to reference 79 at the end of the page} Enceladus, while much smaller at just about 500 kilometres across, surprises with its icy plumes and hidden subsurface ocean.^{80: Go to reference 80 at the end of the page} Other mid-sized moons, like Rhea,^{81: Go to reference 81 at the end of the page} Dione,^{82: Go to reference 82 at the end of the page} and Iapetus,^{83: Go to reference 83 at the end of the page} are far larger than Enceladus but less active, each contributing in its own way to Saturn’s intricate system of gravitational interactions and ring dynamics.

Even beyond Titan and Enceladus, Saturn’s other moons play critical roles. They help maintain the structure and stability of the rings, influence each other’s orbits through gravitational interactions, and contribute charged particles to the planet’s magnetosphere. Together, the moons form a complex, interlinked system that is both beautiful and scientifically rich, setting the stage for a closer look at Saturn’s two most famous satellites.

Titan, the largest of Saturn’s moons, is a world unlike any other in the solar system. It boasts a dense, nitrogen-rich atmosphere thicker than Earth’s, making it the only moon with a substantial gaseous envelope. Methane and ethane form clouds, rain, and rivers on Titan, carving channels into its frozen surface and collecting in vast polar lakes.^{84: Go to reference 84 at the end of the page} Cassini’s radar images revealed sprawling dune fields of organic material, frozen plains, and possible cryovolcanoes, painting a picture of a dynamic, evolving landscape that mimics Earth’s hydrologic cycle, but with methane in place of water.^{85: Go to reference 85 at the end of the page}

Beneath this complex surface lies a hidden ocean of liquid water mixed with ammonia, extending hundreds of kilometres deep. This ocean, sandwiched between an icy crust and a rocky core, may provide a stable environment capable of supporting microbial life.^{86: Go to reference 86 at the end of the page} The interactions between Titan’s subsurface ocean, frozen surface, and thick atmosphere create a multi-layered system of weather, geology, and chemistry that has no equal elsewhere. These processes make Titan a key target for understanding how prebiotic chemistry might emerge in cold, organic-rich environments.

The upcoming Dragonfly mission, a rotorcraft drone set to launch in the 2030s, will explore Titan’s surface directly for the first time.^{87: Go to reference 87 at the end of the page} Built to fly through its thick atmosphere, Dragonfly will move between sites such as organic dunes and ancient impact craters, analysing surface materials, weather patterns, and chemical composition to study how complex molecules form in environments rich in hydrocarbons and water ice.^{88: Go to reference 88 at the end of the page}

By investigating these diverse landscapes, Dragonfly will help scientists understand how prebiotic chemistry might unfold on icy worlds. Titan’s blend of Earth-like weather, methane lakes, and organic compounds offers a glimpse into conditions that may have existed on early Earth, expanding our view of where and how life’s building blocks can emerge beyond our planet.^{89: Go to reference 89 at the end of the page}

Enceladus, though only about 500 kilometres across, is one of the most remarkable discoveries of modern planetary exploration. Right from the beginning its icy surface stood out, and scientists were struck by its extreme brightness, indicating fresh ice constantly resurfacing the moon. When the Cassini spacecraft first observed its surface, the real surprise came when it detected towering plumes of water vapour and ice particles erupting from fissures near Enceladus’s south pole, known as the “tiger stripes”. These fracture zones are where internal heat causes subsurface water to vent into space, feeding Saturn’s E ring and offering a glimpse into the moon’s hidden ocean beneath the crust.^{90: Go to reference 90 at the end of the page}

Detailed analysis of these plumes revealed a complex chemical mix, including water vapour, salts, silica, and organic molecules, ingredients essential for life as we know it. For example, scientists found phosphorus in salt-rich ice grains ejected from Enceladus, underscoring that this tiny moon has more of life’s building blocks than once thought.^{91: Go to reference 91 at the end of the page}

Evidence also suggests hydrothermal activity on the ocean floor, where seawater interacts with rock to release energy and nutrients, much like the deep-sea vents on Earth that host thriving ecosystems.^{92: Go to reference 92 at the end of the page} These findings have positioned Enceladus as one of the most promising locations in the search for extraterrestrial habitability within our solar system.

Cassini’s discoveries at Enceladus transformed our understanding of where habitable environments might exist beyond Earth. Before the mission, few scientists expected such a small, frozen moon to conceal a global ocean beneath its crust.^{93: Go to reference 93 at the end of the page} The detection of organic compounds and heat-driven plumes revealed that Enceladus is geologically active and chemically rich, proving that liquid water and energy sources can persist even in the outer solar system. These insights reshaped planetary science, highlighting that life’s essential ingredients may be more widespread than once imagined.^{94: Go to reference 94 at the end of the page}

Now, Enceladus stands at the forefront of astrobiological exploration. Its global ocean, internal heating, and continuous plumes make it a natural target for future missions seeking direct evidence of biological activity. Proposed orbiters and fly-through probes would sample the plumes to analyse their composition and search for complex organics or biosignatures.^{95: Go to reference 95 at the end of the page} With its mix of water, energy, and organic chemistry, Enceladus remains one of the most compelling places to investigate how life might arise beyond Earth.