Solar System in Minutes

by Giles Sparrow

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  page 382

  Interior of Venus

  The challenge of placing instruments on the Venusian surface means that most of our information about the planet’s internal structure comes from comparison with Earth rather than direct measurement. Venus’s slightly smaller size and lower mass mean it is slightly less dense than Earth, but

  it probably has a similar elemental composition with a deep silicate crust and a nickel-iron core. The planet’s smaller size means it should have cooled slightly more rapidly than Earth, but it should still have a liquid outer core. However, the lack of a magnetic field around Venus suggests something is different in the core, which seems to lack the swirling currents of molten metal which should create an electromagnetic dynamo effect. Scientists suspect the difference lies in a lack of convection (bulk movement of hot liquid metal up or down within the core) – it seems that something has suppressed the ‘normal’ mechanisms of heat transfer familiar from Earth. This may be linked to the lack of tectonic activity allowing heat to escape at the surface (see page 86).

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  page 86

  Solid single-plate crust

  Silicate rock mantle

  Liquid nickel-iron outer core

  Solid nickel-iron inner core

  The Venusian atmosphere

  Venus has a uniquely hostile atmosphere, with a pressure more than 100 times higher than Earth’s, temperatures of around 460°C (860°F) and sulfuric acid rain that falls from high clouds but evaporates closer to the ground. The atmosphere is dominated by carbon dioxide, generating a runaway greenhouse effect that explains the planet’s soaring temperatures. Seen from space, Venusian clouds reflect more than three-quarters of the sunlight shining onto them, explaining the planet’s dazzling, featureless appearance in visible light. Ultraviolet cameras reveal vast chevron-shaped cloud patterns that circle the planet in about four days.

  Friction with the thick atmosphere is thought to be linked to Venus’s unique rotation. Over time, the planet’s spin has slowed to such an extent that it now rotates once every 243 Earth days, in the opposite direction to other planets. This means that the Venusian day is longer than its year; the Sun rises in the west and tracks slowly across the sky before setting in the east.

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  Venusian volcanoes

  Radar maps of Venus reveal a world shaped largely by volcanic activity. Vast lava plains stretch between huge conical and shieldlike volcanoes, and some volcano types are unique in the solar system (see page 90). Estimates from counts of impact craters (see page 88) suggest that a phase of major volcanic activity came to an end 500 million years ago. Some areas may still be active – a number of volcanic peaks appear hotter than their surroundings in infrared images, and descending probes recorded lightning storms over others, similar to those seen on Earth.

  Radar also shows that Venus’s crust is not broken into Earth-like tectonic plates, whose boundaries act as a focus for long-term volcanic activity. Many scientists think this lack of tectonics is the cause of Venus’s widespread volcanism. The theory is that Earth’s tectonics, lubricated by ocean water, allow the slow but steady release of heat from the interior. Venus’s solid crust, in contrast, acts like a pressure cooker, trapping heat for millions of years until it eventually escapes in a wave of planet-wide eruptions.

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  page 90

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  Venusian craters

  Craters on Venus are rare for two reasons. First, and most obviously, the planet’s dense atmosphere ensures that most incoming meteors burn up completely before they can reach the ground. Secondly, the Venusian landscape has been repeatedly resurfaced by widespread volcanism, ensuring that most traces of older impacts have been wiped away. Only a few highland areas were left unaffected by the last major resurfacing that came to an end about 500 million years ago, so craters found elsewhere must have formed since.

  Because incoming meteorites must be a certain size to reach the surface, even the smallest Venusian craters are at least 30 km (19 miles) across. Another notable effect of the atmosphere is to break up large objects during descent, resulting in crater clusters such as the Howe, Danilova and Aglaonice craters shown opposite. Dense air also limits the escape of the ejecta material thrown out during impact, creating compact ‘splashes’ beyond the crater walls.

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  Pancake domes, coronae and arachnoid domes

  Several volcanic structures are found nowhere else in the solar system. Pancake domes are flat-topped, steep-sided ‘pillows’ of volcanic rock tens of kilometres wide, formed as viscous lava erupted through fissures in the surface and solidified before it could spread over the surrounding terrain.

  Coronae (such as Aine Corona, opposite), in contrast, are slightly depressed circular regions, typically several hundred kilometres wide, surrounded by concentric rings of cracks. They seem to mark regions where underlying molten magma once pushed the surface upwards into a dome; when the magma pressure later withdrew (perhaps drained by eruptions elsewhere), the dome subsided to form a corona. Finally, arachnoids are marked by spiderweb-like networks of cracks in the surface. Like coronae, they are believed to mark regions forced upwards by magma from below, except in this case, it’s thought that the pressure grew so great that lava eventually erupted to the surface through a radial network of cracks.

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  Earth

  Our home planet is the third in order from the Sun. With a diameter of 12,756 km (7926 miles), it is the largest of all the solar system’s rocky worlds. When compared to the other rocky planets, Earth’s most distinctive features are the fact that its crust is split into slow-moving tectonic plates,

  the presence of abundant surface water and, of course, copious life.

  All three of these phenomena are due in part to our planet’s location in the solar system. Earth’s near-circular orbit sits neatly within the solar system’s so-called ‘Goldilocks zone’ – the region around the Sun where temperatures are neither so hot that surface water boils into the atmosphere and is lost to space, nor so cold that it freezes permanently. Deep oceans

  of water that collect in surface basins both lubricate the tectonic plates and allow for the evolution and survival of life. Interactions between tectonics, life, oceans and atmosphere give Earth the most complex environment in the solar system.

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  The early Earth

  Our planet is thought to have formed shortly after the birth

  of the Sun, around 4.54 billion years ago. Early in its history, a cataclysmic collision with a Mars-sized primordial world is thought to have given birth to Earth’s Moon (see page 126), and also added new material to Earth itself. As the crust began to solidify, Earth rapidly developed an early atmosphere rich in hydrogen and water vapour, likely through a mix of ‘outgassing’ from volcanic eruptions and bombardment with ice-laden comets and asteroids. As the Earth cooled, water began to condense into oceans.

  The cores of the continents are thought to have grown fairly rapidly – early in Earth’s history, the escape of heat from the interior would have created new crust and driven tectonic plates into one another at an accelerated rate. This powered head-on collisions that built up thick regions of crust, called ‘cratons’, over hundreds of millions of years. Traces of ‘biogenic’ chemicals linked to the activity of primitive life suggest that the first communities of microbes were established by at least 3.7 billion years ago.

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  page 126

  A simulated view models the effects of a large asteroid impact on the crust of the young Earth.

  Earth in its orbit

  Our planet orbits the Sun at an average distance of 149.6 mil
lion km (93 million miles), taking almost exactly 365.25 days to complete one orbit. As a result, from our point of view

  on Earth, the Sun appears to move eastwards around the sky relative to the more distant stars, returning to exactly the same position one year later. At the same time, Earth spins on its axis in about 23 hours and 56 minutes, causing the Sun and stars to apparently move westward across the heavens. Our 24-hour day, therefore, accounts for the extra few minutes it takes (on average) for the Sun to return to the same orientation in the sky, as seen from a particular location on Earth.

  However, Earth’s orbit is not a perfect circle – in reality, it

  is slightly elliptical, so the actual distance to the Sun varies between 147.1 and 152.1 million km (91.4 and 94.5 million miles). As a result, the amount of heat and light Earth receives from the Sun changes slightly through the year (Earth is at its closest to the Sun in January and at its most distant in July).

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  Equinox (20/21 March)

  Solstice (21/22

  June)

  152.1 million km

  (94.5 million miles)

  147.3 million km

  (91.5 million miles)

  Aphelion

  (4 July)

  Perihelion

  (3 January)

  Solstice (21/22 December)

  Equinox

  (22/23 September)

  This diagram shows the relationship between Earth’s position on its orbit and its seasons (see page 98).

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  The seasons

  Like most other planets, Earth does not sit exactly ‘upright’ in its orbit. Instead, our planet’s axis of rotation is tilted at an angle of 23.5 degrees. As Earth goes around the Sun during the course of a year, the axis remains pointing in the same absolute direction (roughly aligned with the direction of the famous ‘North Star’ Polaris). This means that for half the year, the northern hemisphere is, to some extent, angled towards the Sun, receiving the Sun’s light and heat for more than half of each 24-hour day. For the other half of the year, however, that hemisphere is angled away from the Sun and receives considerably less sunlight. This is the origin of Earth’s pattern of seasons – and, of course, when the northern hemisphere is tilted towards the Sun and experiencing summer, the southern hemisphere experiences winter and vice versa. The dates when Earth’s axis points directly towards or away from the Sun are known as summer and winter solstices, while the two occasions in Earth’s orbit where day and night are of equal length (and also the same in both hemispheres) are known as equinoxes.

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  Tilt of Earth’s axis at 23.5°

  September equinox

  June solstice: northern hemisphere tllted towards Sun

  December solstice: southern hemisphere tllted towards Sun

  March equinox: neither hemisphere is tilted towards Sun, day and night are equal.

  Earth’s interior

  As the largest rocky planet, Earth naturally has the hottest interior. Trapped heat left over from Earth’s formation is still slowly radiating away into space, and significant quantities of radioactive elements, such as uranium, release heat when they decay, helping to maintain the temperature.

  Beneath the relatively thin crust (see page 102) and a lubricating layer called the asthenosphere, Earth’s interior is mostly filled by its mantle – a deep layer of silicate rocks that are mobile over very long periods, slowly churning as hotter rocks push their way up from the regions around the core

  and cooler, denser ones slowly sink down. The transfer of heat through the mantle powers both surface volcanism and the slow motion of Earth’s tectonic plates. Earth’s core, meanwhile, occupies the central portion of the planet and seems to consist of two distinct regions – a liquid outer core of molten nickel and iron, and a solid inner core of the same elements that is slowly growing larger as the interior gradually cools.

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  page 102

  Crust fragmented into tectonic plates

  Silicate rock mantle

  Thin, mobile aesthenosphere

  Molten nickel-iron outer core

  Solid nickel-iron inner core

  Crustal geology

  Earth’s crust ranges in thickness from just a few kilometres beneath the ocean basins up to tens of kilometres deep beneath continents. Split into several dozen tectonic plates (eight major ones, ten minor ones and a host of ‘microplates’), the crust effectively ‘floats’ on top of the aesthenosphere and underlying mantle, moving around at rates of millimetres per year. Plates may contain two distinct types of crust – thin, but dense, oceanic crust dominated by iron-rich ‘mafic’ rocks, such as basalt, and much thicker but lighter continental crust rich in aluminium-based ‘felsic’ rocks. Oceanic crust tends to be relatively young – it is created in areas where plates are separating (typically beneath the oceans), and destroyed where it is driven against continental rocks and forced downwards to melt in the mantle. Continental crust, in contrast, tends to be much older and is a complex mix of igneous rocks (formed from solidified molten lava), sedimentary rocks (formed from compression of fine particles themselves eroded out of other rocks) and metamorphic rocks (formed where other rock types are compressed and heated beneath the surface).

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  Formation of igneous rocks from molten magma.

  Erosion and weathering wear down all rocks.

  Sedimentary rocks form from compression of eroded material.

  Metamorphic

  rocks are created by localized heating of sedimentary rocks.

  Crust is recycled where it descends into mantle.

  Earth’s rock cycle

  Atmosphere and oceans

  Earth’s atmospheric gases are dominated by nitrogen (78 per cent) and oxygen (21 per cent). Most of the remainder is inert argon, but a small amount of carbon dioxide traps heat close to the surface in a ‘greenhouse effect’ that is vital to keeping the planet warm, but rising dangerously due to human activity. Atmospheric circulation carries air from warm regions near the equator towards the colder poles, and is complicated by ‘coriolis’ forces created by Earth’s rotation.

  The oceans, meanwhile, account for 96.5 per cent of Earth’s water, and are saline due to the presence of dissolved minerals. Evaporation from their surfaces pumps water vapour into the atmosphere, which condenses to form clouds and returns to the surface as fresh water precipitation (rain or snow), creating a water cycle that plays a key role in regulating the climate, sustaining life and shaping the landscape through erosion. Although 3.5 per cent of Earth’s liquid water is fresh, most of this is locked into ice caps at the North and South Poles.

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  Life on Earth

  Earth’s abundant life makes it (so far as we know) unique in the solar system. Originating around 3.7 billion years ago, this life first took the form of simple ‘prokaryote’ microbes

  that reproduced themselves using instructions carried in the complex DNA molecule. Early microbes generated energy by processing or ‘metabolizing’ minerals and gases from the ancient atmosphere, gradually changing its composition over time. About 2.4 billion years ago, the first simple organisms to use the plantlike metabolic process of photosynthesis arose. The oxygen they produced cooled the planet, plunging it into a deep ice age, but opened the way for other metabolic processes. Around 1.6 billion years ago, the absorption of some of these innovative microbes into others allowed them to form symbiotic groups with specialized functions, known as ‘eukaryotes’. It took about another billion years for the first multicellular life forms to arise, in which different eukaryotic cells perform specific functions. Since then, they have flourished, giving rise to the enormous complexity of today’s plant, fungus and animal kingdoms.

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  This diagram offers a highly simplified view

  of how major types of life on Earth are related via common ancestry.

  Bacteria

  Animals

  Archaea

  Protists

  Fungi

  ‘Invertebrates’

  Sponges

  Jellyfi sh, anemones etc.

  Plants

  Annelids

  Molluscs

  Algae

  (Single-celled organisms)

  Arthropods

  Liverworts, mosses

  Ferns etc.

  Flowering plants

  Connifers etc.

  Vertebrates

  Echinoderms

  Cartilaginous fi sh

  Bony fi sh

  Amphibians

  Lizards and snakes

  Turtles and crocodiles

  Mammals

  Birds

  Earth’s climate

  Our planet’s climate is perhaps the most complex in the

  solar system, thanks to the intricate web of relationships between atmosphere, oceans and dry land. While the amount of heat we receive from the Sun varies slightly with solar activity, this has little effect on the overall climate (the average over longer periods of time), which is far more influenced by conditions on Earth. Overall, the climate system broadly conspires to maintain an environment suitable for water to move between solid, liquid and vapour forms in a ‘water cycle’ (see page 104). Several feedback mechanisms help keep the system in balance. One example is ‘chemical weathering’, in which falling rain takes