Дава Собел – The Planets (страница 3)

All the while the planets were forming, projectiles flew through the young Solar System like avenging angels. Worlds collided. Icy bodies struck the Earth and disgorged a few oceans’ worth of water. Rocky bodies rained fire and destruction. In one such cataclysm 4.5 billion years ago, a hurtling Mars-sized object (roughly half as big as Earth) thrust itself into the Earth. The impact and upheaval blasted molten debris into near-Earth space, there to orbit as a disk before cooling and coalescing into the Moon.

The violence of the Solar System’s formative period ended shortly thereafter, about four billion years ago, in a final paroxysm descriptively termed ‘the late heavy bombardment’. In those ancient days, many still-wandering planetesimals crashed into existing planets, which incorporated them at once. Multitudes of other small bodies were forcibly ejected, by gravitational interactions with the giant planets, to a distant land of Nod in the outer Solar System.

The young Sun shone but faintly on the planets, growing gradually hotter and more luminous over its first two billion years, as it stored up helium in its core. At present, in middle age, the Sun continues to brighten while converting seven hundred million tons of hydrogen to helium every second. Even at this galloping rate of consumption, the Sun’s abundant hydrogen guarantees three to five billion more years of dependable light. But inevitably, as the Sun switches over to helium fusion, it will become so hot as to boil away Earth’s oceans and smite the life it spawned there. The ten-fold temperature increase required to burn helium will see the hotter Sun turn red, and swell in size until it swallows up the planets Mercury and Venus, and melts the surface of the Earth. One hundred million years later, when the Sun has reduced more helium to carbon ash, it will shrug off its outer layers and dispatch them past Pluto. A larger star could resort to carbon burning at that point, but our Sun, a relatively small star by the standards of the universe, will be unable to do so. Instead, it will smoulder as an ember, and shed a fading light on the charred cinder where God once walked among men. This dim future, however, lies so far ahead as to allow the descendants of Adam and Noah ample time to find another home.

The glorious Sun of our time, the planets’ progenitor and chief source of energy, embodies 99.9 per cent of the mass in the Solar System. Everything else – all the planets with their moons and rings, plus all the asteroids and comets – account for only .1 per cent. This gross inequity between the Sun and the sum of its companions defines their balance of power, for the universal law of gravity decrees that the massive shall have dominion over the less massive. The Sun’s gravity keeps the planets in orbit and also dictates their speeds: the closer they are to the Sun, the faster they go. The Sun, in turn, bends to the will of the concentrated mass of stars at the centre of our Milky Way Galaxy, around which it orbits once every 230 million years, carrying the planets along with it.

Just as they feel the Sun’s attraction more or less keenly, according to their distance, so too do the planets partake of the Sun’s light and heat. Solar energy diminishes in intensity as it radiates through interplanetary space. And so, while parts of Mercury bake at five hundred degrees, Uranus, Neptune and Pluto remain perpetually deep-frozen. Only in the Solar System’s milder middle section, called the habitable zone, have conditions supported the flourishing of ‘great whales, and every living creature that moveth, which the waters brought forth abundantly, after their kind, and every winged fowl after his kind: and … cattle, and creeping thing, and beast of the earth …’

The planets return the favour of the Sun’s light by reflecting its rays, and in this manner they pretend to shine, though they emit no light of their own. The Sun is the Solar System’s sole light-giving body; all the others glow by reflected glory. Even the full Moon that illumines so many lovely Earthly evenings owes its silvery light to Sunbeams bouncing off the dark lunar soil. The Earth shines just as beautifully when viewed from the Moon, and for the same reason.

The play of mirrored light from Venus, close to the Sun and also closest to Earth, makes that planet appear by far the brightest one to our eyes. Jupiter, though much larger, lies many millions of miles beyond, and therefore pales in comparison in our night sky. The even further worlds of Uranus and Neptune, immense as they are, catch and toss back so little light that Uranus can only occasionally be discerned (as a mere point of light) by the naked eye, Neptune never.

Although Pluto, too, is impossible for us to see without a telescope, other objects on the outskirts of the Solar System can and sometimes do flare into sudden visibility. When disturbed by a chance encounter, an ice-rock denizen from Pluto’s depths may be pushed Sunward and transformed from a dull lump into a spectacular comet. Basking in the Sun’s warmth, the frozen body heats up and throws out a trailing tail of castoff gas and icy dust that sparkles with the Sun’s reflection. The brilliance fades and disappears, however, after the comet rounds the Sun and returns to the outer Solar System.*

The visits of comets, long interpreted as signs and wonders, have recently sketched the true extent of the Sun’s domain. By tracing the visible parts of comet paths, and extrapolating the rest, astronomers have shown that numerous comets hail from out beyond Pluto’s neighbourhood, from a second comet reservoir, hundreds of times further away. Despite their unimaginable distance, these bodies still belong to the Sun, still heed the Sun’s gravity, still receive some glimmer of the Sun’s light.

Sunlight, which darts through space at the dazzling speed of 186,000 miles per second, takes ages to emerge from the dense interior of the Sun. Light advances only a few miles per year near the Sun’s core, where the crush of matter repeatedly absorbs it and impedes its escape. Radiating this way, light may journey for a million years before reaching the Sun’s convective zone, there to catch a quick ride up and out on roiling eddies of rising gas. As soon as these eddies release their cargoes of light, they sink back down, to soar again later with more.

The light-emitting, visible surface of the Sun – the photosphere – seethes as though boiling from the constant tumult of energy release. Gas bubbles bursting with light give the photosphere a grainy complexion, marred here and there by pairs of dark, irregularly shaped sunspots, with black centres and grey, graded shading around them like the penumbras of shadows. Sunspots designate areas of intense magnetic activity on the Sun, and their darkness bespeaks their relative coolness of about 4000°K, compared to neighbouring areas at nearly 6000.* The level of solar activity rises and falls in cycles averaging eleven years, and sunspots mingle, morph, and multiply according to this same schedule. Their number and distribution vary like famine and plenty, from no spots at ‘solar minimum’, or just a few spots dotting the Sun’s high latitudes, to ‘solar maximum’ five to six years later, when hundreds of them crowd closer to the equator. Although sunspots seem to gather and scud like clouds across the photosphere, really it is the Sun’s rotation that carries them around.

The Sun rotates on its axis approximately once a month, in a continuation of the spinning motion it was born in. Being an enormous ball of gas, the Sun spins complexly, in layers of various speeds. The core and its immediate surroundings turn at one rate, as a solid body. The overlying zone spins faster, and above that, the visible photosphere whirls around at several different rates, more quickly at the Sun’s equator than near its poles. These combined, contrary motions whip the Sun into a fury, with consequences felt clear across the Solar System.

The ‘solar wind’, a hot exhalation of charged particles (reminiscent of the ‘wind from God’), blows out from the turbulent Sun and keeps up a constant barrage on the planets. Were it not for the protective envelope of Earth’s magnetic field, which deflects most of the solar wind, humankind could not withstand the onslaught. From time to time, especially during solar maximum, the steady solar wind is augmented by sudden blasts of higher-energy particles from solar flares on the Sun’s surface, or by gargantuan blobs of ejected solar gas. Such outbursts can disable our communications satellites and disrupt power grids, causing blackouts. In milder doses, particles of solar wind trickle into the upper atmosphere near the North and South Poles, initiating cascades of electrical charge that draw curtains of coloured lights across the sky – the so-called Northern and Southern Lights. Other planets also sprout colourful auroras in response to the solar wind, which billows on past Pluto all the way to the heliopause – the undiscovered boundary where the Sun’s influence ends.