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Orbits of the Trojan asteroids
Direction of orbits
‘Trailing’ Trojans
Jupiter
60°
60°
Earth
Sun
‘Leading’ Trojans
617 Patroclus and 624 Hektor
The two largest Trojan asteroids, Hektor and Patroclus, were discovered within months of each other in 1906 and 1907.
Neither world has yet been visited by a space probe and they are hard to resolve through even powerful telescopes, but variations in their light have revealed a few details. In 2001, astronomers discovered that Patroclus is actually a binary system – a pair of oval asteroids orbiting each other at a distance of about 665
km (413 miles). The ‘true’ Patroclus has an average diameter
of 113 km (70 miles), while the slightly smaller world, 104 km (65 miles) across, is now know as Menoetius. Their dark surfaces
and relatively low masses suggest that they may be comet-like bodies from the outer solar system. Hektor, in contrast, is a single object with dimensions around 403 x 201 km (250 x 125 miles). Its two-lobed shape suggests that it may be a ‘contact binary’ – a pair of asteroids that collided and stuck together at some point in their past. Hektor is orbited by a 12-km-diameter (7.5 mile) moon, Skamandrios, and its dark-reddish surface matches a small number of other asteroids in the ‘Hektor’ family.
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An artist’s impression of Patroclus and its sibling world Menoetius
2015 BZ509
In late 2017, astronomers announced the discovery of an elongated object called ‘Oumuamua, retreating from a close encounter with the Sun. Its orbit proved it was not a true member of the solar system, but a fleeting visitor, an asteroid kicked out of orbit around another star millions of years ago. A few months later, scientists found evidence for another stray in the solar system – and this one is a permanent resident. The recently discovered asteroid 2015 BZ509 has a rare ‘retrograde’ orbit around the Sun, moving in the opposite direction to the planets in a Jupiter-crossing orbit. While
the giant planet’s gravity changes its orbit in the short term, the asteroid follows a long-term cycle that has been stable throughout the history of the solar system. Computer models suggest that 2015 BZ509 could not have achieved such an orbit starting from a ‘normal’ path – instead, the Sun must have swept it up (probably alongside several others) around 4.5 billion years ago, in a close encounter with another nascent solar system within its star-forming nebula.
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Asteroid’s orbit is tilted at 163 degrees to plane of solar system.
Asteroid orbits the Sun in a retrograde (clockwise) direction.
Orbit of 2015 BZ509
Orbit of Mars
Orbit of Jupiter
Jupiter
The solar system’s largest planet by far, Jupiter is big enough to engulf all the other worlds with room to spare. Orbiting the Sun every 11.9 years at an average distance of 5.2 AU, it is a gas giant world – a vast ball of lightweight hydrogen in various forms, surrounding a hypothetical solid core with apparently ‘blurred’ edges. Everything about the planet is on an enormous scale – its average diameter of 139,822 km (86,881 miles) is 11 times that of Earth, and its mass 318 times greater. Unsurprisingly, therefore, this monster planet wields a huge influence over the rest of the solar system.
Jupiter’s predominant features are its colourful weather systems, including multicoloured cloud bands that run parallel to the equator and turbulent, swirling storms of varying colours. As the fastest-rotating planet in the solar system (spinning on its axis in just 9 hours 55 minutes), even Jupiter’s powerful gravity has difficulty holding onto fast-moving material at its equator, producing a pronounced equatorial bulge.
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Inside Jupiter
Jupiter’s colourful cloud bands and storms form the uppermost layer of a deep atmosphere that comprises the vast majority of the planet, with perhaps just a small solid core at the centre. Lightweight hydrogen is the dominant element. In the outer few thousand kilometres (Jupiter’s true ‘atmosphere’) it takes the familiar form of gaseous molecules (H2). However, deeper inside the planet it changes state as pressure increases, condensing at first into liquid hydrogen, and ultimately breaking down into a fluid metallic form whose swirling motions generate a powerful magnetic field (see page 206).
Although Jupiter’s overall diameter has not changed much since its formation, its inner layers are in a state of gradual contraction, with denser materials sinking towards its centre. Similar processes are thought to take place inside all the giant planets, generating heat that helps to drive their weather systems – in Jupiter’s case, this means that the planet emits more energy than it receives from the Sun.
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Outer gaseous atmosphere Mantle dominated by liquid molecular hydrogen
Possibly ‘fuzzy’ core created by internal contraction
Metallic hydrogen ocean
Cloud belts and zones
Jupiter’s contrasting cloud bands wrap around the planet, staying more or less parallel to the equator due to Jupiter’s rapid 10-hour rotation. Bright, cream-coloured bands are called ‘zones’, while darker brown and blue stripes are known as ‘belts’. Individual bands are identified according to their latitude.
Varying wind speeds in the atmosphere mean the belts and zones appear to move in opposite directions around the planet. The belt-and-zone terminology derives from the early assumption that the belts are overlaid on top of the zones, but space-probe flybys revealed that the real distribution of Jupiter’s cloud patterns is exactly the opposite. Zones mark low-pressure regions within which clouds (in particular, those containing creamy-white ammonia crystals) condense at higher, colder altitudes. Belts, meanwhile, are regions where high-pressure forms ‘clearings’ that offer a view into the deeper layers of Jupiter’s atmosphere, where the clouds are formed of more complex and colourful chemicals.
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The Great Red Spot and other storms
Jupiter’s most famous feature is a vast oval storm known
as the Great Red Spot (GRS). Large enough to swallow the Earth, it swirls anticlockwise in tropical latitudes 22 degrees south of the equator, rotating once every six days. Space-probe images have confirmed that the GRS towers up to 8 km (5 miles) above the surrounding cloud levels, and it is usually interpreted as a vast high-pressure region that dredges up complex chemical material from deeper within the atmosphere. The spot has been observed continuously since 1830, and may well be much older than that, but it has been shrinking for much of the past century and may eventually disappear completely. Since the 1990s, astronomers have observed two other red spots in Jupiter’s atmosphere. Oval BA or ‘Red Spot Junior’ formed from the merger of three smaller white-storm ovals
in the late 1990s. It began to turn red in 2005 and has since continued to grow in size. In 2008, a so-called ‘Baby Red Spot’ appeared in Jupiter’s southern cloud bands, but this was swiftly torn apart and cannibalized in a close encounter with the GRS.
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Jupiter’s magnetosphere
The vast, rapidly spinning sea of electrically
charged, liquid metallic hydrogen that occupies much of Jupiter’s interior creates a powerful magnetic field around the giant planet that is ten times stronger than that of Earth at its surface. However, this magnetosphere’s strength and range are modified by an unusual interaction with the moon Io (see page 212). Active volcanoes on Io’s surface eject large amounts of sulfur dioxide into a doughnut-shaped ‘torus’ that surrounds the moon’s orbit. Solar radiation breaks the sulfur dioxide down into electrically charged sulfur and oxygen ions, creating a ring of electrically charged ‘plasma’ that effectively amplifies the magnetic field. The overall effect is to extend and flatten the magnetosphere, allowing Jupiter’s magnetic influence to reach as far as the orbit of Saturn. Meanwhile, many of the trapped oxygen and sulfur particles rain down at high speeds into Jupiter’s upper atmosphere above its poles, giving rise to intense aurorae (northern and southern lights) that emit radio waves, ultraviolet radiation and high-energy X-rays, as well as visible light.
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page 212
Rings of Jupiter
Like all of the solar system’s giant planets, Jupiter is surrounded by a system of rings. However, for such an impressive planet, the rings are perhaps one disappointing aspect – they amount to little more than a thin disc of dust particles in orbit around Jupiter. Divided into four distinct regions, they begin at around 20,000 km (12,500 miles) above the cloud tops and stretch as far as the orbit of the small satellite Thebe, some 154,500 km (96,000 miles) above the planet’s equatorial cloudtops.
The rings are only visible when seen from Jupiter’s night side and backlit by the Sun – as a result, they were only discovered when the retreating Voyager 1 photographed the system after its 1977 flyby. Two outer ‘gossamer’ rings and a relatively dense main ring each consist of dust ejected when micrometeorites strike Jupiter’s small inner moons (Thebe and Amalthea for the gossamer rings, and both Metis and Adrastea for the main ring). The innermost ‘halo’ ring is thicker and consists of microscopic particles spread out by interactions with Jupiter’s magnetic field.
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Comet crashes
One of Jupiter’s most important and under-appreciated roles is as the protector of the inner planets. Comets crossing its orbit on their way towards the Sun are prone to having their paths disrupted, and are often flung back into the outer reaches of the solar system (or even out of the Sun’s grasp completely). Sometimes their orbits become circularized, so that encounters with Jupiter are more frequent and disruptive. In 1993, astronomers discovered that a close encounter with Jupiter had broken comet Shoemaker-Levy 9 into a string of smaller fragments doomed to collide with
the gas giant the following year (see page 330). While seizing the opportunity to study the impact’s effect on Jupiter’s atmosphere, most regarded it as an infrequent chance event. However, the sighting of a ‘scar’ in Jupiter’s clouds formed by another impact in 2009, and direct observations of further comet impacts since, show that objects hit Jupiter with surprising frequency, reducing the chances of them making damaging impacts on the far more vulnerable rocky planets.
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page 330
Io
Jupiter’s vast family of satellites (69 at the latest count) is dominated by just four ‘Galilean’ moons – worlds that were first reported by Galileo Galilei when he turned his primitive telescope towards the giant planet in 1610. The innermost Galilean, Io, is a little larger than Earth’s Moon and orbits Jupiter in just 42.5 hours – so close, that it is bombarded by radiation trapped in the planet’s magnetosphere. Forced into a slightly elliptical orbit by the influence of its outer neighbours, Io suffers from tremendous tidal forces. Changes to the strength of Jupiter’s gravity pull its interior in various directions and generate huge amounts of heat as Io’s sulfur-rich rocks
grind past each other. The rocks melt easily at relatively low temperatures, generating reservoirs of underground magma that make Io the most volcanically active world in the solar system. Its surface is constantly being changed and reshaped by new eruptions, but a survey by the Galileo space probe in
the late 1990s identified more than 200 volcanic craters or calderas more than 20 km (12.5 miles) across.
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Pele volcano
Io’s volcanic activity takes various forms. Plumes of sulfur dioxide above the planet were first discovered during the Voyager 1 space-probe flyby of 1979. These enormous fountains of molten material resemble Earth’s geysers; as they escape into Io’s airless skies, they rapidly boil. The first of eight plumes to be found in Voyager 1 images was traced to a region that was subsequently named Pele, after the Hawaiian volcano goddess.
Pele itself is a volcanic crater or ‘patera’ with dimensions of 30 x 20 km (19 x 12 miles). The crater floor has several flat plains
at varying levels formed in different phases of activity, and a lava lake at one end (the origin of the intermittent plume). Temperature measurements by space probes suggest that the lava reaches temperatures around 1,250°C (2,280°F), indicating that it is molten silicate rock similar to Earth’s lava, rather
than cooler sulfurous material. Crystals of sulfur, meanwhile, condensing out of the plume and settling back on the surface, form a reddish-brown ring around the patera.
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Europa
The second of Jupiter’s major ‘Galilean’ moons, Europa is also the smallest, with a diameter of 3,122 km (1,940 miles)
– slightly less than Earth’s Moon. Its smooth, brilliant-white surface seems to have little in common with the colourful volcanic landscape of Io, but the moons are more alike than appearances suggest. Being in orbit between Jupiter and Io on one side and the much larger moon Ganymede on the other renders Europa vulnerable to the same tidal heating that shapes Io. In Europa’s case, however, the presence of large volumes of water ice means that the heat has a different effect, warming a deep ocean of liquid water that is hidden from view (and protected from boiling into airless space) by a thick icy crust. Enhanced-colour images reveal that this crust is criss-crossed with pale, reddish-brown streaks, while some areas resemble the pack ice seen at Earth’s polar caps. Most planetary scientists agree that these features are caused by the crust slowly rearranging itself over time, as mineral-laden water and warmer ice well up from beneath.
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Pwyll crater
Only a handful of impact craters have been found on Europa, among which Pwyll is one of the youngest and best defined.
Its central crater is about 26 km (16 miles) in diameter, while a surrounding spray of ejecta material extends across hundreds of kilometres, overlaying everything else within range. Pwyll’s dark crater floor is probably formed from impure, mineral-laden ice that welled up from the ocean below. Its ejecta, in contrast, is pure water ice, vaporized out of the moon’s icy crust as the crater formed, only to condense rapidly back into snow as it fell back to the surface.
Craters on Europa seem to disappear slowly with age, losing their definition on a relatively short timescale (perhaps thousands of years) as the icy material of the crust slumps and flattens out, until only a ghostly remnant survives. As a result of slow flowing of the surface, Europa’s crust is so smooth that it is often compared to a pool ball; if this moon were the size of Earth,
it would still have no hills higher than 200 m (660 ft).
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Europa’s ocean
The presence of an ocean beneath Europa’s icy crust was confirmed in the 1990s, when the Galileo space probe discovered the magnetic field it produces through interaction with Jupiter’s own magnetosphere. Europa’s water layer is thought to be about 100 km (60 miles) deep, with the upper 10–30 km (6–19 miles) composed of various forms of ice. The upper crust is frozen rock hard by surface temperatures around –160°C (–256°F), but underlying ‘warm’ ice that pushes its way towards the surface brings clues to the nature of the ocean beneath. For example, long surface streaks called ‘lineae’ are stained by colourful chemicals, such as magnesium sulfate, that probably originated from volcanic vents on the sea floor (similar to those that belch minerals into Earth’s oceans). This raises the intriguing possibility that, like Earth’s own ‘black smokers’, Europa’s sea-floor vents could give rise to their own uniquely adapted forms of life. The discovery of water plumes above Europa, similar to those ejected by Saturn’s moon Enceladus, may offer future space probes a way to investigate the ocean’s chemistry without a technically challenging drilling mission.
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Ice crystals venting at surface
Mobile ice layer
Deep ocean layer
Ganymede
The third and largest of Jupiter’s Galilean moons, Ganymede is the biggest moon in the solar system. With a diameter of 5,269 km (3,274 miles), it’s significantly larger than the planet Mercury. Despite its size, Ganymede has no significant atmosphere, but a substantial magnetic field suggests the presence of a core that still has molten iron. Other magnetic effects, meanwhile, seem to reveal a saltwater ocean layer around 200 km (124 miles) below the surface.
Solar System in Minutes Page 8