Just as ancient historians loved to draw up lists of the Seven Wonders of the World, deep in our modern age of interplanetary exploration, astronomers love to argue over the finest sights our Solar System has to offer.
From the cracked crust of Mercury to the frozen mountains of Pluto, there’s a wealth of wonders to consider, so how do you even begin to make a list? Sticking to the traditional limit of seven wonders, our own selection aims to be as diverse as possible, with everything from ancient and unchanging geology to short-lived but beautiful atmospheric phenomena.
Some of these wonders have been known and admired since the ancient times, some since the invention of the telescope, and still others have been unveiled far more recently by visiting space probes.
The worlds that play host to these wonders range from bloated gas giant planets to tiny rock ice moons, with some phenomena right on our doorstep, and others in the distant depths of the outer Solar System. And even now, only a few of the many worlds in orbit around our Sun have been explored in any detail, so we can be certain there are plenty more wonders just waiting to be found.
Saturn’s astonishing ring system makes it the most beautiful planet in the Solar System – the brightest regions of the rings stretch to three times the diameter of the planet, while fainter clouds of material extend for thousands of kilometres into space. Each major ring is made up of narrow ringlets, and each ringlet is a stream of individual particles following near perfect circular orbits around Saturn.
The brightness of the rings varies depending on the material within them: the prominent A and B Rings are dominated by densely packed, house-sized chunks of water ice, while particles in the C and D rings are smaller and far more scattered. Between and within the rings lie several apparent gaps, most prominent of which is the Cassini Division between the A and B Rings, and the Encke Gap within the A ring.
Despite initial appearances, these gaps are not entirely empty – the Cassini Division contains material with the same density as that of the C Ring. Astronomers have understood the nature of the rings since 1859, when James Clerk Maxwell showed that any solid structures orbiting that close to Saturn would be torn apart by gravity.
Instead, the disc-like structure is a result of ‘jostling’ between particles – collisions between objects moving in slightly different orbits averages out their motion and herds them onto
circular orbits directly above Saturn’s.
Iapetus’ Walnut Wall
The strange wall that runs along the equator of Saturn’s moon Iapetus
remains a tantalising mystery. Discovered by NASA’s Cassini probe in 2004, the wall (or ‘equatorial ridge’) is 20 kilometres (12.4 miles) high in places, with some of the tallest mountains in the Solar System.
Iapetus’ other claim to fame is the stark division between the dark hemisphere that faces forward in its orbit, and its far brighter trailing hemisphere – the ridge structure is only present on the moon’s darker side.
Seen from space, the wall gives Iapetus a walnut-like appearance, and as with most curiosities of planetary geology, some have suggested that it might be an artificial structure. All the evidence, however, points to the ridge being a natural feature and many theories have tried to explain its origin and location on the equator.
One theory is that the ridge is a ‘fossil’ remnant from shortly after the planet formed, when Iapetus span much faster, giving it a pronounced equatorial bulge. Others suggest that upwelling of material from the moon’s interior caused the ridge. But the most fascinating theory is that the ridge came not from below, but from above.
Iapetus is far enough from Saturn’s gravitational pull that it could have formed with a ring system of its own, aligned with the moon’s equator. If this became unstable, material could have rained down onto the surface, creating the ridge that we see today.
The largest mountain in the Solar System, Olympus Mons is a towering volcano of an almost unimaginable scale, creating a vast blister on the face of the Red Planet. The only disappointment for observers is that it seems inactive – but that might change.
With a height of 21 kilometres (13 miles) above the Martian ‘surface datum’ (the Red Planet’s average surface level), but 25 kilometres (16 miles) above surrounding lowland plains, Olympus Mons is almost three times higher than Mount Everest and 2.5 times the height of Mauna Kea in Hawaii (Earth’s tallest volcano if measured from its seabed base to its peak). Little wonder then, that astronomers named the monstrous mountain after the home of the ancient Greek gods, Olympus.
In contrast to the traditional image of a volcano as a conical mountain with a lava-filled crater at its peak, Olympus Mons is a shield volcano. Such structures (typical of the largest volcanoes on Earth and Mars) have a similar profile to a shield or shallow dome – they form when lava emerges along weak fissures in the crust and flows out across the landscape before solidifying.
A shield’s growth is often fuelled by eruptions around its flanks rather than its peak, and the overall structure may be tens or hundreds of kilometres across. In the case of Olympus Mons, steep cliffs have formed where parts of the shield were unable to support their own weight.
The huge central crater, or caldera, is a complex series of overlapping pits over 80 kilometres (50 miles) across and hemmed by walls up to 3,000 metres (9,842 foot) deep. It was never a lavafilled lake but instead formed through subsidence as the underlying reservoir of magma (molten subterranean rock) beneath the volcano diminished and withdrew in the relatively recent past.
Planetary scientists can estimate the age of Olympus’ last eruptions from the number of impact craters on its lava solidified flows, and the results are tantalising – while most of the volcano is thought to have built up 3 billion years ago, some parts of the northwestern flank appear to have formed as recently as 2 million years ago. That’s the blink of an eye in geological terms, and suggests that Olympus Mons is probably still active to this day.
Earth’s Life Show
Earth’s northern and southern lights, the Aurora Borealis and Aurora Australis, are a wonder of the Solar System that come close to home. These colourful displays of glowing light result from the interaction between our planet’s magnetic field and atmosphere, and the stream of particles constantly flowing out from the Sun.
Aurorae are created when electrically charged particles from the solar wind are drawn towards Earth and interact with scattered atoms and molecules of gases in the atmosphere. Collisions with the gas particles cause changes in their internal structure and give them a short-term energy boost, but this is rapidly dissipated through emission of electromagnetic radiation.
The structure of Earth’s magnetic field creates funnels that channel and concentrate solar wind particles in ‘auroral ovals’ surrounding each magnetic pole, which is why displays are usually only seen at high northern or southern latitudes. Here, particles enter the atmosphere and create shifting curtains of eerie light. The level of auroral activity depends on thequantity and energy of particles in the solar wind – factors that are controlled by the Sun’s magnetic cycle, waxing and waning in intensity over a period of roughly 11 years.
Violent events called solar flares and coronal mass ejections unleash huge amounts of high-speed material that buffets the Earth in a geomagnetic storm. This can cause intense aurorae and affect the flow of Earth’s own magnetic field. The colour of aurorae depends on the altitude at which they form and the gases involved – the more active the Sun, the further particles penetrate the atmosphere and the more intense the display.
Red aurorae are caused by interactions with atomic oxygen at altitudes of 200 250 kilometres (124- 155 miles) and are the most common but the hardest to see, as gas atoms are sparsely scattered and human eyesight is poorly attuned to red light. Green colours form in denser oxygen at 100 150 kilometres (62 93 miles) and are the most common aurorae that are actually seen, while blue and deep red emissions, created at lower altitudes by excitation of nitrogen, are only produced in the most intense auroral displays.
The most volcanic world in the Solar System, Jupiter’s tortured satellite Io is home to a shifting, multicoloured landscape unlike anything else we’ve seen in our interplanetary explorations. Io is the innermost of four ‘Galilean’ moons – giant satellites of the Solar System’s largest planet, named after the Italian astronomer Galileo Galilei who discovered them using one of the first telescopes in 1610.
While its outer neighbours (like most moons in the outer Solar System) are dominated by ice, Io is pure rock, with a startling terrain of red, yellow, brown, green and white blotches that make it look similar to a burnt pizza. Io owes its strange appearance to tidal forces far more brutal than those that cause Earth’s seas to rise and fall.
Like most other satellites, including our Moon, Io has long since settled into a synchronous rotation period that matches its orbit and keeps one side permanently facing towards its parent planet. In theory, this keeps tidal forces, created by the changing strength of Jupiter’s gravity from one side of Io to the other, at a minimum, but Io’s orbit is prevented from being 5 Io’s volcanoes perfectly circular by the pull of the other Galilean moons. This means that the strength of Jupiter’s powerful gravity changes significantly from one side of its orbit to the other. As a result, the tidal bulge on the side of Io facing Jupiter rises and falls by as much as 100 metres (328 foot) around its orbit, creating friction in the moon’s rocks that’s hot enough to melt both silicate rocks and the abundant sulphur compounds on Io’s surface.
Sulphur is famous for taking a wide variety of different solid forms known as allotropes, and its compounds can be equally colourful – together they are largely responsible for Io’s fantastic appearance.
Interaction between hot silicate magmas and easily melted sulphur triggers powerful eruptions that send plumes of sulphur high into the sky, while lava oozes across the landscape and reshapes it on time scales measured in years rather than centuries – when NASA’s Galileo probe entered orbit around Jupiter in the late 1990s, it found Io’s terrain had altered substantially from that photographed by the Voyager probes around two decades before.
Enceladus is an icy moon of Saturn with a diameter of just 500 kilometres (310 miles), yet it is unusually bright for its size. In the 1980s, the Voyager space probe images revealed that many of its craters were blotted out by what looked like a blanket of snow. This made the moon an obvious target for NASA’s Cassini probe when it arrived at Saturn in 2004.
But what no one expected was that on its first approach to Enceladus, Cassini flew straight through a plume of icy particles arcing hundreds of kilometres above the moon’s south pole. Backlit images soon revealed the presence of several geyser-like jets erupting into the sky: much of the material falls back to dust Enceladus with pristine snow, but some escapes the moon’s weak gravity altogether and ends up in orbit around Saturn, creating the planet’s tenuous, doughnut-shaped E Ring.
Geyser activity is created when an underground reservoir of liquidis heated above boiling point and eventually boils away explosively at a weak point in the overlying crust. On Enceladus, the temperatures needed to do this are lower than on Earth, as there’s no substantial atmosphere, but the ice must still be warm enough to melt – such a tiny moon should have frozen solid long ago.
Enceladus’ unusual activity is thought to be due to tidal heating – the moon is in a tug of war between Saturn and the larger satellite Dione, which orbits further out, and this generates heat through friction as its overall shape flexes through each orbit. This, it seems, is enough to create an ocean of liquid water beneath the surface, which forces its way out along weak spots in the southern hemisphere known as ‘tiger stripes’.
The geysers are beautiful but also offer a tantalising insight into Enceladus’ interior chemistry– during its flights through the icy plumes, Cassini’s particle ‘sniffers’ detected not just water ice, but complex carbon-based or ‘organic’ chemicals.
Jupiter’s Monstrous Storm
The most famous storm in the Solar System, the Great Red Spot (GRS) dominates the southern hemisphere of Jupiter, forming a baleful whirlpool in its turbulent atmosphere. The storm has been observed since the 1830s but was likely seen much earlier, by Jean Dominique Cassini and others from the 1660s to 1713. And its disappearance for over a century may be because the spot periodically changes colour and size.
At its peak, however, the GRS is a huge eye-like oval with twice the diameter of Earth. Studies of Jovian weather show that it is a vast anticyclone (rotating counterclockwise). As this is the opposite direction to the way storms rotate on Earth, it indicates the GRS is an area of high pressure.
What’s more, infrared images show the visible red cloud tops are colder than Jupiter’s other clouds and are eight kilometres (five miles) higher up in the sky. The origin of the storm’s colour is still unclear – its uppermost clouds are thought to be richer in ammonium hydrosulphide, but this chemical is not red so something else must be
One theory is that the chemistry of the clouds changes as they are hit by cosmic rays (high energy particles from the Sun and space), while another says the colour comes from chemicals deep inside Jupiter that are drawn to the surface. But the GRS may not last forever – over the past century or more it has steadily shrunk and become more circular, while another southern hemisphere storm, Oval BA, has grown in size and turned from white to red.