on carbon dioxide from the atmosphere, and reacts with rocks to form carbonate minerals; a rise in carbon dioxide and a small increase in temperature should lead to increased rainfall and weathering that removes the gas from the atmosphere. However, such mechanisms typically work on timescales of thousands
of years, and are less suited to coping with relatively sudden stresses such as human industrial emissions.
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Increased rainfall leads to more weathering of rocks, absorbing excess CO2.
Warmer, wetter atmosphere creates unpredictable weather and heavier rainfall.
Increased CO2 warms atmosphere, causing more evaporation from oceans.
Earth’s weathering feedback mechanism
The magnetosphere and aurorae
The space around our planet is dominated by a powerful magnetic field known as the magnetosphere. Its shape resembles the field around a familiar bar magnet, emerging from one magnetic pole, looping around the planet and re-entering at the other. The field originates in Earth’s outer core, where it is generated by currents carried in the turbulent molten metal. Its orientation roughly matches Earth’s axis of rotation, but the currents that create it periodically reverse, causing the field to flip its direction every few hundred thousand years.
The magnetosphere affects susceptible, electrically charged particles that pass through it, such as solar wind particles and high-energy ‘cosmic rays’ from the wider Universe. Most are deflected by the field, but others are swept up and energized to form doughnut-shaped radiation belts around the Earth. Particles channelled downwards around the magnetic poles collide with gases in the upper atmosphere to form the glowing ‘aurorae’, or northern and southern lights.
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Earth’s aurora australis (southern lights), photographed by an astronaut aboard the International Space Station
Meteors
As Earth moves along its orbit through space at an impressive 29.8 kilometres per second (66,600 miles per hour), it inevitably encounters other objects travelling at similar speeds. Most of this interplanetary matter is little more than dust or ice left behind in the wake of passing comets or asteroids. As they enter Earth’s upper atmosphere, these tiny particles are heated by collisions with the sparse air molecules, burning up in short-lived streaks of light called ‘meteors’ or ‘shooting stars’. The brightest, known as fireballs, can reach the lower atmosphere and outshine any star before they, too, are destroyed.
Most meteors enter the atmosphere from random directions, but some, still clinging to the orbits of their parent comets, form predictable meteor showers. They occur at the same time each year as Earth crosses their orbit and, thanks to perspective, they appear to radiate from a specific point in the sky. When Earth occasionally passes through a particularly dense region of such a comet trail, the result can be a spectacular meteor storm.
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Meteorites
When a large or robust fragment of interplanetary rock enters Earth’s atmosphere, it may survive its fiery journey and reach the ground partially intact, becoming a meteorite. About 500 such space rocks hit Earth each year – the last stages of their descent through the dense lower atmosphere are usually marked by a brilliant, slow-moving meteor called a ‘bolide’. Meteorites have huge scientific value as samples of material from elsewhere in the solar system. Scientists divide them into several classes depending on how much geological ‘processing’ they have been through. Some are clearly fragments of core
or mantle material from broken-up asteroids, while a few rare ones can even be traced to the Moon or Mars. The majority, however, known as ‘chondrites’, comprise tiny mineral spheres – raw material left unchanged from when the planets formed. Most are made from silicate minerals fused together by heat, but rare ‘carbonaceous’ chondrites seem to have avoided heating, allowing them to retain primordial water and delicate carbon-based chemicals from the ancient solar nebula.
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The stony-iron Esquel meteorite is probably a fragment from an asteroid that began to develop a core and mantle before being shattered in an ancient collision.
Impacts from space
Although large impacts from space might seem rare in the brief span of human history, they are inevitable on geological timescales. If an object measuring tens of metres or larger strikes Earth’s land surface, the energy unleashed can carve out a deep bowl-shaped crater many times bigger, spraying debris known as ejecta across an even wider area.
Earth’s geological activity, atmosphere and abundant life can soften and disguise craters in a matter of decades, but the advent of satellite photography has revealed many hidden scars from ancient impacts. Some of the largest impacts have had a lasting effect on the history of life (see page 118), but even smaller ones can have potentially devastating consequences – the object that exploded in the air above Siberia in 1908 did not even leave a crater, but still carried enough energy to flatten a city-sized area of forest. For this reason, various projects are underway to map the distribution of potentially threatening objects in Earth-crossing orbits.
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Arizona’s famous ‘Meteor Crater’, some 1,200 metres (3,900 ft) across, was formed by the impact of a
50-metre (160-ft) meteorite about 50,000 years ago.
Chicxulub Basin
Earth’s most significant surviving impact crater has long since been buried beneath the surface by geological activity. The 180-km-wide (112 miles) Chicxulub crater lies hidden beneath the modern Gulf of Mexico and was only discovered by accident during petroleum exploration in the 1980s. Chicxulub’s huge scale suggests that it was formed
by an asteroid or comet some 10–15 km (6–9 miles) across, and geological clues suggest that the impact sent clouds of pulverized rock high into the atmosphere, from where it slowly settled across the planet a little less than 66 million years ago.
Chicxulub owes its fame to a key event in the history of life
on Earth – it coincides precisely with the mass extinction
that saw the disappearance of the large reptilian dinosaurs and countless other species, paving the way for the rise of mammals. Although the precise chain of events is still disputed, it seems that widespread climate cooling played a key role, triggered by dust flung into the atmosphere during the impact.
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A map shows anomalies in Earth’s gravitational field beneath the coast of Mexico, caused by compressed rock from the Chicxulub impact.
Earth’s Moon
Our planet’s Moon (typically given a capital ‘M’ to distinguish it from the satellites of other planets) is the most prominent object in the night sky. Orbiting Earth in 27.3 days at an average distance of 384,400 km (238,850 miles), tidal forces (see page 122) have long since slowed its rotation to the same period, so that one hemisphere is permanently turned towards Earth. The amount of the Earth-facing side illuminated by sunlight, however, is continually changing, giving rise to a familiar cycle of phases that repeats every 29.5 days (as the Moon comes back into the same alignment with the Sun). With a diameter of 3,474 km (2,158 miles), the Moon is just over one- quarter the size of Earth, and has 1.2 per cent of its mass. This makes it by far the solar system’s largest satellite relative to the size of a major planet, and gives it considerable influence over Earth. However, lunar gravity (at about one-sixth the strength of Earth’s) is far too weak to hold onto an atmosphere; as a result, the Moon is an airless bal
l of rock on which temperatures range between –170°C and 220°C (–274°F and 428°F).
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The Earth-Moon system
Earth and Moon are so similar in size and exert so much influence on each other that some astronomers have argued they should really be considered as a ‘double planet’. The most obvious effect is in the form of tides – forces caused by ‘fall-off’ in the strength of each world’s gravity as felt between the near and far sides of the other body. On Earth, tides create a ‘bulge’ several metres high in the oceans lying closest to the Moon, and a slightly smaller bulge in oceans on the opposite side of the planet. The bulges stay in roughly the same position as Earth rotates beneath them – hence, there are two high tides per day. The situation is further complicated by weaker tidal bulges caused by the Sun. Tides experienced by the Moon were once much stronger, though less obvious because of its solid rocky composition. Over billions of years, forces acting on the Moon’s small but noticeable tidal bulges were reduced as its rotation slowed to match its orbital period. Today, the Moon suffers small tides as its distance from Earth varies around its monthly orbit, giving rise to occasional ‘moonquakes’.
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Seas and highlands
The Moon’s surface is divided into two main types of terrain – bright, heavily cratered highlands and darker, smoother ‘seas’ or ‘maria’. Satellite images and robot landers have shown that the rough highland terrain is saturated with craters down to microscopic scales, and which date back to the earliest lunar history. With no water or wind and little geological activity, the main factor eroding ancient craters is the arrival of later impacts on top of them. The lunar maria,
in contrast, are vast plains of solidified lava, formed between 3 and 4 billion years ago, in a period when low-lying regions of the Moon were vulnerable to volcanic eruptions that flooded the deepest impact basins. This activity obliterated heavy cratering from the period of the Late Heavy Bombardment (see page 36) and left a clean slate onto which more recent craters have accumulated at a much-reduced rate. Nevertheless, bombardments have pounded the upper few metres of their surfaces into a jumble of rocks and dust of different sizes, known as ‘regolith’.
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page 36
Origin of the Moon
The Moon’s origin has been a subject of fierce debate for almost two centuries; astronomers have found it hard to explain why such a large satellite would form around a relatively small planet. Early theories, such as the ejection of the Moon from a fast-spinning ancient Earth, contradicted basic laws
of physics, while the once-popular idea of the Moon as a small rogue planet captured from elsewhere in the solar system has faltered in the face of evidence that the rocks of Earth and Moon are almost identical. The most credible modern theory, known as the ‘Big Splash’ hypothesis, appears to solve most of these problems. It proposes that, shortly after its formation, Earth was involved in a collision with a rogue Mars-sized planet (sometimes called Theia). The collision saw Earth and Theia merge together, but in the process ejected vast amounts of debris from both worlds into orbit, where it rapidly coalesced to form the Moon. However, the theory is not without its problems – particularly when it comes to delivering a satellite with the Moon’s own unique geology.
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Mare Crisium
One of the most distinctive features of the lunar nearside, the Mare Crisium is a relatively small ‘sea’ some 555 km
(345 miles) in diameter. Lying in the Moon’s northeast quadrant, its appearance is significantly foreshortened by the Moon’s curvature, making it look oval, rather than circular in shape.
The sea’s surface is mostly flat, but ‘wrinkle ridges’ around
the edges show how solidifying lava once piled up as it lapped against the surrounding highland shores. The impact basin in which the mare lies is about 3.9 billion years old, and analysis of rock samples returned by the unmanned Soviet Luna 24 probe in the 1970s suggests that the basaltic lavas within it formed in three episodes between about 3.5 and 2.5 billion years ago.
Like several other lunar seas, the Mare Crisium coincides with an area of unusually high density known as a mascon (mass concentration). Thanks to their higher gravity, these poorly understood phenomena present a significant hazard to satellites and spacecraft orbiting the Moon.
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Montes Apenninus
The Moon’s most notable mountain range, the Montes Apenninus or Lunar Apennines, forms a curving chain that stretches across some 600 km (370 miles) in mid-northern latitudes of the lunar nearside. Rather than being the result of uplift from Earth-like tectonic forces, the range is simply the broken-down rim of the huge Imbrium impact basin, formed about 3.9 billion years ago and subsequently disguised by eruptions of mare basalt lavas that flowed between its peaks.
The range’s highest point, Mons Huygens, lies towards the range’s southern end and rises some 5.5 km (3.4 miles) above the nearby lava plains. At the northeast end, meanwhile, stands Mons Hadley Delta, a 3.6-km (2.2-mile) triangular peak close to the 1971 landing site of the Apollo 15 mission. Another major feature of this region is Hadley Rille, a serpentine,
round-bottomed valley some 135 km (84 miles) long and up to a kilometre (0.6 miles) across, formed by the collapse of an abandoned subterranean lava channel.
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Tycho crater
One of the Moon’s most prominent craters, Tycho lies in the southern highlands and draws attention to itself at the centre of a system of bright ‘rays’ (material sprayed out during formation that travelled for huge distances in the weak gravity before settling back to the surface) that spread for up to 1,500 km (930 miles) across the lunar surface. Tycho presents a particularly perfect example of a lunar crater, with a diameter of 85 km (53 miles) and a depth of 4.8 km (3 miles). Terraced walls and a distinctive central mountain were both created in the
last stages of the crater’s formation, when the rim of an initially bowl-like depression became too steep and slumped downwards, pushing up the middle of the crater floor in the process.
Tycho is one of the youngest large craters on the Moon – it has remained in pristine condition for some 108 million years, as have its surrounding rays. Tycho is also surrounded by a series of smaller satellite craters, formed as more substantial chunks of ejecta fell back closer to the impact site.
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The lunar far side
Unseen by astronomers until the Soviet Luna 3 probe sent back the first grainy images in October 1959, the far side of the Moon appears distinctly different from the near side, thanks to an almost complete lack of dark lunar seas. The 180-km (112-mile) crater Tsiolkovskiy is one of the few obvious dark features, while the huge South Pole-Aitken Basin, some 2,500 km (1,550 miles) wide and 13 km (8 miles) deep, remains entirely unfilled.
Scientists believe the difference in general appearance between near and far sides is due to the same tidal forces that long ago slowed the Moon’s rotation to match its orbital period. Earth’s gravity, which tugs more strongly
on the nearer side of the Moon, seems to have pulled the Moon’s molten core a few kilometres closer to its Earth-facing side, causing greater volcanic activity on that side. Quite
why Tsiolkovskiy should be an exception to this general pattern remains unclear.
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Mars
The outermost of the solar system’s rocky planets, Mars is also the second smallest, with a diameter of 6,779 km (4,212 miles) – just over half that of Earth. The planet’s 687-day orbit
is markedly elliptical, ranging between 1.38 and 1.67 AU from the Sun, but in other respects Mars is remarkably Earth-like; it spins on its axis in 24 hours and 37 minutes and has an axial tilt of 25.19 degrees that produces a familiar pattern of seasons.
The Martian atmosphere, however, is very different from Earth’s – composed almost entirely of carbon dioxide, it is so thin that
it exerts just one per cent of Earth’s atmospheric pressure. Nevertheless, it acts as a surprisingly effective insulator, so while temperatures can fall as low as –143°C (–225°F) at the poles
in winter, they can also rise as high as 35°C (95°F) around the equator in summer. When Mars is at its closest to the Sun during the southern summer, heat in the atmosphere can lift huge amounts of fine red dust into the air, engulfing the entire planet in a global dust storm that may take months to subside.
Solar System in Minutes Page 5
