Dark sunspots amid granulation patterns in the solar photosphere
Inside the Sun
The Sun’s interior is broadly divided into three layers. The hot dense core is where nuclear fusion (see page 56) releases energy that gradually escapes outwards. Energy leaves the core in the form of tiny packets, or photons, of gamma rays (the most energetic form of electromagnetic radiation). It then enters the ‘radiative zone’, where matter is so densely packed that the rays can only travel a tiny distance before encountering a particle and bouncing in a different direction. As a result, the photons take hundreds of thousands of years to move out across this zone, slowly losing energy to their surroundings and transforming into less energetic X-rays and ultraviolet forms as they do so. At the top of the radiative zone, changes to temperature and pressure render the
Sun’s matter opaque. Energy can no longer be transferred as radiation, so it is absorbed, heating huge masses of gas that naturally push upwards. At the top of this ‘convection zone’, the Sun’s gases become transparent once again, and energy can finally escape into space as visible light and other radiation.
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page 56
Photosphere - visible surface
Convection zone - gas rises and falls
Radiative zone - energy transported as radiation
Core generates energy through nuclear fusion
How the Sun shines
The Sun, like all stars, is powered by nuclear fusion – a process by which the tiny central nuclei of atoms are forced together to create new, heavier nuclei. Fusion can only take place under extreme temperatures and pressures that overcome the natural tendency of nuclei to repel each other; in the Sun’s case, temperatures of 15 million°C (27 million°F) and pressures 260 billion times greater than that of Earth’s atmosphere. Energy released by fusion helps to heat the core and maintain these conditions of extreme heat and pressure.
Fusion in the Sun mostly involves the so-called ‘proton-proton chain reaction’, a process that combines protons (the simple nuclei of hydrogen, the lightest and most abundant element in the Universe) to create helium, the next lightest element. Each fusion reaction converts a tiny amount of mass directly into energy (in accordance with Einstein’s equation E=mc2). Owing to the Sun’s huge size, however, this amounts to 4 million tonnes of material per second and an output of 3.8 x 1028 watts.
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Hydrogen nucleus (single proton)
Neutrino particle Energy
Nuclear fusion in the Sun happens mostly through a series of reactions that build a helium nucleus from multiple hydrogen nuclei.
Deuterium nucleus (1 proton + 1 neutron)
Helium-3 nucleus (2 protons + 1 neutron)
Energy
Proton
Proton
Helium-4 nucleus (2 protons + 2 neutrons)
Hydrogen nucleus (single proton)
The Sun’s atmosphere
Above the Sun’s brilliant photosphere lie several other layers that are sometimes collectively called the solar atmosphere. The innermost of these layers, known as the chromosphere, is around 5,000 km (3,100 miles) deep. It takes its name from its reddish colour (visible only during eclipses), and is the location of bright, hot plumes of gas known as prominences, and long, flame-like spicules that extend into the layers above. Temperatures in the chromosphere are significantly hotter than in the underlying photosphere, rising to around 35,000 °C (63,000°F) at the top. The source of energy that powers this heating is still poorly understood, but is probably associated with the Sun’s magnetic field. Above the chromosphere lies a thin transition region just a few hundred kilometres deep, where changes to sparse helium gas atoms cause them to absorb radiation. As a result, temperatures in this region soar to 1 million °C (1.8 million °F) or more. This heating effect continues into the Sun’s outer atmosphere or corona – a vast extended halo of thin gas that is shaped by the Sun’s magnetism and, ultimately, blends into the solar wind (see page 64).
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page 64
During a total solar eclipse, the Sun’s tenuous outer atmosphere can be seen with the naked eye.
Solar activity
While some of the Sun’s surface features, such as granulation and spicules, are permanent, others come and go. Most obvious of these are sunspots – dark patches in the photosphere that may be many thousands of kilometres across, and can persist for days or weeks. The spots appear dark because the material in them is cooler than their surroundings (though still
at temperatures around 3,000°C or 5,400°F). They form in
pairs, where loops of the Sun’s magnetic field push out through the photosphere and create areas of lower-density gas. These magnetic loops are also linked to other forms of solar activity. Gas flowing along them forms glowing loops called ‘prominences’, which can be seen above the Sun’s edge during solar eclipses. Furthermore, when the magnetic loops become overextended, they can ‘short circuit’, reconnecting closer to the surface and releasing a short-lived but huge burst of energy called a solar flare. The flare heats the surrounding solar atmosphere to millions of degrees and ejects energetic particles into space at speeds of up to 1,000 km (620 miles) per second.
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The solar cycle
Although the Sun’s overall energy output is fairly steady, it is not immune to short-term changes. Sunspot numbers and solar flares vary in number and intensity according to a solar cycle that repeats roughly every 11 years. The cycle begins with a relatively placid Sun and a few sunspots at high latitudes, then intensifies as sunspots increase and move towards the equator. Finally, the cycle subsides as sunspots nearing the equator disappear.
The solar cycle is driven by changes in the Sun’s magnetic field, which is generated not in the core, but by swirling gas in the convection zone. Because the Sun’s interior rotates more slowly at higher latitudes, the magnetic field becomes tangled over time, with loops erupting through the surface to create active regions of sunspots and flares, and eventually cancelling out as they start to make connections across the equator. The entire magnetic field regenerates with each solar cycle (with the magnetic poles ‘flipping’ each time), but the causes of deeper, decades-long variations in levels of activity are still poorly understood.
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1 Cycle begins with magnetic field aligned from pole to pole.
2 Over time, faster rotation of the Sun’s equator distorts the field.
3 Magnetic loops push out through photosphere, creating sunspots and prominences.
4 Loops emerge closer to equator as field is twisted further.
5 As loops approach the equator they begin to cancel out, weakening the magnetic field.
6 At the end of an
11-year cycle, the field regenerates with the opposite orientation.
Solar wind
High temperatures and strong magnetic fields in the Sun’s corona boost the energy of its electrically charged particles.
This allows them to overcome the Sun’s strong gravitational field and escape in a wind that is felt across the solar system. Solar wind particles are mostly fragments of the lightweight gaseous elements that dominate the Sun – atomic nuclei of hydrogen and helium, and electrons (the tiny particles that usually orbit the nucleus to complete an atom, but that are stripped away by intense coronal temperatures). Close to the Sun, the particles travel in a smooth flow at supersonic speeds, with faster wind typically emerging from the poles than the magnetically tangled equator. Solar flares and even larger events called coronal mass ejections (CMEs) – caused when loops of the Sun’s magnetic field ‘reconnect’ at lower levels and liberate huge amounts of energy – can eject vast clouds of material into the solar wind. As these clouds sweep past the planets in the following hours or days, they can distort p
lanetary magnetic fields and create brilliant light shows known as auroral storms (see page 110).
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page 110
The heliosphere
The flow of the solar wind defines a region where the Sun’s influence is dominant, known as the heliosphere.
Particles escape the Sun at supersonic speeds of hundreds of kilometres per second, blowing out a ‘bubble’ in the surrounding interstellar medium (the sparse but nevertheless measurable scattering of potentially star-forming gas and dust in the plane of our galaxy). However, the medium in its turn exerts a pressure that pushes back at the solar wind, especially in the direction of the solar system’s motion.
Between 75 and 90 AU from the Sun, at a boundary called the termination shock, the solar wind’s speed falls to less than
the speed of sound as it passes through (about 100 km/60 miles per second). Beyond this, in a region called the heliosheath, the wind’s hitherto smooth flow becomes turbulent. Finally, at the heliopause (crossed by NASA’s Voyager 1 spaceprobe in 2012 at about 121 AU from the Sun), the wind’s outward drift comes to a halt in the face of pressure from the interstellar medium.
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Bow shock created as solar wind slows motion
of interstellar medium
Sun
Heliopause
Inner solar system
Termination shock Heliosheath
Key features of the heliosphere
Mercury
The smallest major planet and the closest to the Sun, Mercury hurtles around its markedly elliptical orbit in just 88 days.
Around 40 per cent bigger than Earth’s Moon, Mercury is a similarly barren grey world, pockmarked with impact craters from the early days of the solar system, but also showing traces of ancient volcanic activity and an unusual internal structure. Atoms blasted out of surface rocks by fierce solar radiation form a sparse atmosphere, barely worthy of the name, that has to be constantly replenished as its fast-moving particles blow away into interplanetary space.
It’s predictable that Mercury has one of the hottest surfaces in the solar system, reaching temperatures of up to 425°C (800°F) at midday, but it is perhaps surprising that it’s dark side can be one of the coldest, plunging to –195°C (–319°F). This is because Mercury has a unique relationship between its day and its year that gives it some of the longest nights in the solar system (see page 70).
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page 70
Orbit of Mercury
Mercury’s orbit is the most elliptical of any major planet; its distance from the Sun ranges from 0.31 to 0.47 AU. Tides caused by gravity pulling the planet’s interior out of shape have long since forced Mercury to develop a ‘resonant’ rotation period that minimizes tidal forces. As a result, Mercury spins on its axis with a period of roughly 57 Earth days that is precisely two-thirds of its 88-day year. This means that the Sun moves very slowly across Mercury’s skies, with an average interval
of two Mercury years between successive sunrises. However, when Mercury is at its closest to the Sun and moving at its fastest speed along its orbit (about 57 km/35 miles per second), the effects of Mercury’s orbit can actually outpace its daily rotation, causing the Sun to move backwards on its path across the sky and, at the right locations, even rise, set and rise again in rapid succession. Tidal forces have also eliminated any trace of axial tilt, so Mercury orbits the Sun ‘bolt upright’. As a result, the Sun barely skims the horizon at its poles, allowing permanently shadowed craters to retain deep reservoirs of ice.
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Start –’midnight’ at this point
1
4
7
End of first orbit - ‘midday’ at this point End of second
orbit and third rotation - ‘midnight’ at this point
The complex relationship between Mercury’s rotation and orbital periods give rise to a unique pattern of day and night.
Direction of rotation
2
6
5
3
End of second rotation
End of first rotation
Interior of Mercury
As a rule of thumb, astronomers expect smaller solar system bodies to be less dense than larger ones made of similar materials – simply because the smaller worlds have less gravity to compress their interiors. It was somewhat unexpected, then, when the Mariner 10 flybys of 1974/75 revealed that Mercury is almost as dense as Earth.
The reason for Mercury’s high density is the unusually large metallic core that occupies 55 per cent of the planet’s interior. The core’s size is thought to be the result of a cataclysmic collision early in Mercury’s history; the theory is that Mercury was once significantly larger, before much of its mantle was blasted away into space by a glancing impact. The outsized core had a significant effect on Mercury’s later evolution –
it has retained enough heat to stay partially molten to the present day, and swirling electric currents within its liquid layer generate a magnetic field around the planet. Changes to the core have also affected surface features (see page 76).
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page 76
Fragmented crust
Large core with partially molten interior
Rocky silicate mantle
Caloris Basin
Mercury’s proximity to the Sun and its lack of atmosphere have seen it accumulate countless craters over its 4.5 billion years of history, which the planet’s limited geological activity has done little to erase. The largest craters are known as impact basins, and at 1,550 km (960 miles) across, the Caloris Basin is the largest known impact structure in the entire solar system.
Caloris formed when a large asteroid struck the planet about
3.8 billion years ago. Rimmed by a triple ring of mountains up to 2 km (1.2 miles) tall, its interior is filled with flat lava plains – the result of volcanic activity triggered by the impact that subsequently flooded the surface. A curious crater surrounded by radiating troughs (known appropriately as the Spider) may mark the origin of some of the volcanic lava. The Caloris impact was so large that it sent seismic shock waves rippling out through Mercury’s crust and straight through its core. Where they rejoined on the opposite side of the planet, they created a jumbled, chaotic region of so-called ‘weird terrain’.
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Scarps and rupes
Mercury’s most unusual and unique surface features are long, clifflike escarpments and troughs known as ‘rupes’ that
run for hundreds of kilometres across its surface, often straight through older features. For example some craters are split down the middle, with one half sitting on a raised plateau up to 1 km
(0.6 miles) or so higher than its counterpart. The overall impression is that Mercury’s crust has been split into a jigsaw that does not quite fit together, so some ‘crustal units’ are forced to sit higher than the others.
Astronomers attribute this unusual situation to Mercury’s oversized core, which occupies more than half of the planet’s interior. Models of the core’s thermal history suggest that shortly after Mercury’s formation, the core heated and swelled, causing the overlying crust to split apart. Since then, the core has cooled and shrunk in size. As the now-outsized units of crust have gradually fallen back, they have been jammed together with some forced to sit higher than others.
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The 600-km (370-mile) Beagle Rupes lifts the eastern side of Sveinsdóttir Crater a kilometre (0.6 miles) above the western half.
Ancient volcanoes
Although Mercury’s small size should have led it to cool much more rapidly than the larger rocky planets, its large core seems to have delayed that process, heating the mantle and powering geological activit
y for much of Mercury’s history. Mercury never developed the kind of plate tectonics found on Earth, but there is plenty of evidence for widespread volcanism. Smooth plains resemble the maria, or seas, of solidified lava seen on Earth’s Moon – many form a loose ring around the Caloris Basin. The fact that they overlap ejecta from that basin’s formation shows they formed more recently, perhaps
up to 3.5 billion years ago. Elsewhere, many impact craters have sunken pits on their floors and are covered with depositions
of distinctively coloured rocks. Scientists think these are probably the result of underground magma chambers giving way beneath the crater floor. Small bright patches of material, meanwhile, resemble those released by explosive volcanism on Earth – evidence suggests these could have continued until as recently as 1 billion years ago.
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Venus
After the Sun and Moon, Venus is usually the brightest object in Earth’s skies. It is thanks to its brilliance that it is named after the goddess of love and beauty in ancient Roman mythology. In terms of size, Venus is a near-twin to our own planet, just a little smaller (with a diameter of 12,104 km or 7520 miles) and orbiting a little closer to the Sun at 0.72 AU. This led some 19th- and early 20th-century astronomers to speculate that Venus might be a haven for alien life with a climate not too different from Earth’s tropics.
But Venus’s beautiful name is deceptive – in reality, the second planet from the Sun is a hellish furnace of a world blanketed by a choking, toxic atmosphere. Robot probes attempting to land on the surface (see page 382) fail after a few minutes at most, thanks to a combination of acid rains, searing temperatures and crushing pressures. It’s only thanks to orbiting probes equipped with cloud-piercing radar that we now have an understanding of Venus as a world shaped primarily by volcanic forces.
Solar System in Minutes Page 3
