THE

PLANETS

DAVA SOBEL

H A R P E R P E R E N N I A L

London, New York, Toronto Sydney

Dedicated with worldfuls of love to my big brothers,

Michael V. Sobel, MD,

who named our family cat Captain Marvel,

and Stephen Sobel, DDS,

who bunked with me at Space Camp.

At night I lie awake

in the ruthless Unspoken,

knowing that planets

come to life, bloom,

and die away,

like day-lilies opening

one after another

in every nook and cranny

of the Universe …

—Diane Ackerman,

from The Planets: A Cosmic Pastoral

In all the history of mankind, there will be only one generation that will be first to explore the Solar System, one generation for which, in childhood, the planets are distant and indistinct discs moving through the night sky, and for which, in old age, the planets are places, diverse new worlds in the course of exploration.

—Carl Sagan, from The Cosmic Connection: An Extra-terrestrial Perspective

CONTENTS

Cover

Title Page

Epigraph

1. Model Worlds (Overview)

2. Genesis (The Sun)

3. Mythology (Mercury)

4. Beauty (Venus)

5. Geography (Earth)

6. Lunacy (The Moon)

7. Sci-Fi (Mars)

8. Astrology (Jupiter)

9. Music of the Spheres (Saturn)

10. Discovery (Uranus and Neptune)

11. UFO (Pluto)

12. Planeteers (Conclusion)

Acknowledgments

Glossary

Details (Notes)

Bibliography

Praise

By the same author

About the Author

Copyright

About the Publisher

1 MODEL WORLDS

My planet fetish began, as best I can recall, in third grade, at age eight – right around the time I learned that Earth had siblings in space, just as I had older brothers in high school and college. The presence of the neighbouring worlds was a revelation at once specific and mysterious in 1955, for although each planet bore a name and held a place in the Sun’s family, very little was known about any of them. Pluto and Mercury, like Paris and Moscow, only better, beckoned a childish imagination to ultra-exotic utopias.

The few sure facts about the planets suggested fantastic aberrations, ranging from unbearable extremes of temperature to the warping of time. Since Mercury, for example, could circle the Sun in only eighty-eight days, compared to Earth’s 365, then a year on Mercury would whiz by in barely three months, much the way ‘dog years’ packed seven years of animal experience into the dog owner’s one, and thereby accounted for the regrettably short lives of pets.

Every planet opened its own realm of possibility, its own version of reality. Venus purportedly hid lush swamps under its perpetual cloud cover, where oceans of oil, or possibly soda water, bathed rain forests filled with yellow and orange plant life. And these opinions issued from serious scientists, not comic books or sensational fiction.

The limitless strangeness of the planets contrasted sharply with their small census. In fact, their nine-ness helped define them as a group. Ordinary entities came in pairs or dozens, or quantities ending in a five or a zero, but planets numbered nine and nine only. Nine, odd as outer space itself, could nevertheless be counted on the fingers. Compared to the chore of memorizing forty-eight state capitals or significant dates in the history of New York City, the planets promised mastery in an evening. Any child who committed the planets’ names to memory with the help of an appealing nonsense-sentence mnemonic – ‘My very educated mother just served us nine pies’ – simultaneously gained their proper progression outward from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto.

The manageable sum of planets made them seem collectable, and motivated me to arrange them in a shoe-box diorama for the science fair. I gathered marbles, jacks balls, ping-pong balls, and the pink rubber Spaldeens we girls bounced for hours on the sidewalk, then I painted them with tempera, and hung them on pipe cleaners and string. My model (more like a doll’s house than a scientific demonstration) failed to give any real sense of the planets’ relative sizes or the enormous distances between them. By rights I should have used a basketball for Jupiter, to show how it dwarfed all the others, and I should have mounted everything in a giant carton from a washing machine or a refrigerator, the better to approximate the Solar System’s grandiose dimensions.

Fortunately my crude diorama, produced with a complete lack of artistic skill, did not kill my beautiful visions of Saturn suspended in the perfect symmetry of its spinning rings, or the mutating patterns on the Martian landscape, which were attributed, in scientific reports of the 1950s, to seasonal cycles of vegetation.

After the science fair, my class staged a planets play. I got the part of ‘Lonely Star’ because the script called for that character to wear a red cape, and I had one, left over from a Hallowe’en costume. As Lonely Star, I soliloquized the Sun’s wish for companionship, which the planet-actors granted by joining up with me, each in a speech admitting his own peculiarities. The play’s most memorable performances were delivered by ‘Saturn’, who twirled two hula-hoops while reciting her lines, and ‘The Earth’, plump and self-conscious, yet forced to announce matter-of-factly, ‘I am twenty-four thousand miles around my middle.’ Thus was the statistic of the earth’s circumference indelibly impressed upon me. (Note that we always said ‘the earth’, in those days. ‘The earth’ did not become ‘Earth’ until after I came of age and the Moon changed from a nightlight to a destination.)

My role as Lonely Star helped me appreciate the Sun’s relationship to the planets as parent and guide. Not for nothing is our part of the universe called the ‘Solar System’, in which each planet’s individual makeup and traits are shaped in large measure by proximity to the Sun.

I had omitted the Sun from my diorama because I hadn’t understood its power, and besides, it would have posed an impossible problem of scale.* Another reason for leaving out the Sun, and likewise the Moon, was the bright familiarity of both objects, which seemed to render them regular components of Earth’s atmosphere, whereas the planets were glimpsed only occasionally (either before bedtime or in a still-dark, early-morning sky), and therefore more highly prized.

On our class trip to the Hayden Planetarium, we city kids saw an idealized night sky, liberated from the glare of traffic signals and neon lights. We watched the planets chase each other around the heavens of the dome. We tested the relative strength of gravity with trick scales that told how much we’d weigh on Jupiter (four hundred pounds and more for a normal-sized teacher) or Mars (featherweights all). And we gawked at the sight of the fifteen-ton meteorite that had fallen from out of the blue over Oregon’s Willamette Valley, posing a threat to human safety that few of us had thought to fear.

The Willamette meteorite (still on permanent display at what is now the Rose Center for Earth and Space) was said to be, incredibly, the iron-nickel core of an ancient planet once in orbit around the Sun. That world had shattered somehow, several billion years back, setting its fragments adrift in space. Chance had nudged this particular piece toward the Earth, where it hurtled down to the Oregon ground at tremendous speed, burning up from the heat of friction, and hitting the valley floor with the impact of an atom bomb. Later, as the meteorite lay still over aeons, the acidic rains of the Pacific northwest chewed large holes in its charred and rusted hulk.

Here was a primal scene to upset my innocent planet ideas. This dark, evil invader had no doubt consorted in space with hordes of other stray rocks and metal chunks that might strike Earth at any moment. My Solar System home, till that moment a paragon of clockwork regularity, had turned into a disorderly, dangerous place.

The launch of Sputnik in 1957, when I was ten, scared me to death. As a demonstration of foreign military strength, it gave new meaning to the school-wide air raid drills in which we crouched under our desks for pretended safety, our backs to the windows. Clearly we still had more to dread from angry fellow humans than from wayward space rocks.

All through my teens and twenties, as the country realized the young president’s dream of a rocket to the Moon, clandestine rockets in missile silos kept collective nightmares alive. But by the time the Apollo astronauts brought back their last batch of Moon rocks in December 1972, peaceful, hopeful spaceships had landed also on Venus and Mars, and another, the US Pioneer 10, was en route to a Jupiter flyby. Throughout the 1970s and 1980s, hardly a year passed without an unmanned excursion to another planet. Images radioed home to Earth by robot explorers painted detail upon detail on the planets’ long-blank faces. Whole new entities came to light, too, as spacecraft encountered literally dozens of new moons at Jupiter, Saturn, Uranus and Neptune, as well as multiple rings around all four of those planets.

Even though Pluto remained unexplored, deemed too distant and too difficult to visit, its own unexpected moon was discovered accidentally in 1978, through careful analysis of photographs taken by ground-based telescopes. Had my daughter, born in 1981, attempted her own diorama of the revised and expanded Solar System when she turned eight, she would have needed handfuls of jellybeans and jawbreakers to model the many recent additions. My son, three years her junior, might have opted to model his on our home computer.

Despite the increased population of the Solar System, its planets stayed stable at nine, at least until 1992. That year, a small, dark body, independent of Pluto, was detected on the Solar System’s periphery. Similar discoveries soon followed, until the total number of diminutive outliers grew to seven hundred over the ensuing decade. The abundance of mini-worlds made some astronomers wonder whether Pluto should continue to be regarded as a planet, or reclassified as the largest of the ‘trans-Neptunian objects’. (The Rose Center has already excluded Pluto from the planetary roll call.)

In 1995, only three years after the first of Pluto’s numerous neighbours was found, something even more remarkable came to light. It was a bona fide new planet – of another star. Astronomers had long suspected that stars other than the Sun might have their own planetary systems, and now the first one had surfaced at 51 Pegasi, in the constellation of the flying horse. Within months, other ‘exoplanets’- as the newly discovered extra-solar planets were quickly dubbed – turned up at stars such as Upsilon Andromedae, 70 Virginis b, and PSR 1257+12. At least 180 additional exoplanets have since been identified, and refinements in discovery techniques promise to uncover many more in the near future. Indeed the number of planets in our Milky Way Galaxy alone may far exceed its complement of one hundred billion stars.

My old familiar Solar System, once considered unique, now stands as merely the first known example of a popular genre.

As yet, no exoplanets have been imaged directly through a telescope, so their discoverers are left to imagine what they look like. Only their sizes and orbital dynamics are known. Most of them rival giant Jupiter in heft, because large planets are easier to find than small ones. Indeed, the existence of exoplanets is deduced from their effect on their parent star: Either the star wobbles as it yields to the gravitational attraction of unseen companions, or it periodically dims as its planets pass in front and impede its light. Small exoplanets, the size of Mars or Mercury, must also orbit distant suns, but, being too tiny to perturb a star, they elude detection from afar.

Already planetary scientists have appropriated the name ‘Jupiter’ as a generic term, so that ‘a jupiter’ means ‘a large exoplanet’, and the mass of an extremely large exoplanet may be quantified as ‘three jupiters’ or four. In the same fashion, ‘an earth’ has come to represent the most difficult, most desirable goal of today’s planet hunters, who are devising ways to probe the Galaxy for petite, fragile spheres in the favoured shades of blue and green that hint at water and life.

Whatever daily concerns dominate our minds at the dawn of the present century, the ongoing discovery of extra-solar planetary systems defines our moment in history. And our own Solar System, rather than shrink in importance as one among many, proves the template for comprehending a plethora of other worlds.

Even as the planets reveal themselves to scientific investigation, and repeat themselves across the universe, they retain the emotional weight of their long influence on our lives, and all that they have ever signified in Earth’s skies. Gods of old, and demons, too, they were once – they still are – the sources of an inspiring light, the wanderers of night, the far horizon of the landscape of home.

* In his ingenious pamphlet, ‘The Thousand-Yard Model, or, The Earth as a Peppercorn’, Guy Ottewell guides the construction of a scale model Solar System using a bowling ball for the Sun. The eight-thousand-mile-wide Earth, here reduced to a peppercorn, takes its rightful place seventy-eight feet (!) from the bowling ball.

2 GENESIS

‘In the beginning, God created the heaven and the earth,’ the first book of the Bible recounts. ‘And the earth was a formless void and darkness covered the face of the deep, while a wind from God swept over the face of the waters. Then God said, “Let there be light”; and there was light.’

The energy of God’s intent flooded the new heaven and earth with light on the very first day of Genesis. Light’s potent good thus pervaded the evenings and the mornings when the seas separated from the dry lands, and the earth brought forth grass and fruit trees – even before God set the sun, moon and stars in the firmament on the fourth day.

The scientific Creation scenario likewise unleashes the universe in a burst of energy from a void of timeless darkness. About thirteen billion years ago, scientists say, the hot light of the ‘Big Bang’ erupted, and separated itself instantly into matter and energy. The next three minutes of cooling precipitated all the atomic particles in the universe, in the unequal proportions of 75 per cent hydrogen to 25 per cent helium, plus minuscule traces of a few other elements. As the universe expanded exponentially in all directions and continued to cool, it shed no new light for at least a billion years – until it begat the stars, and the stars began to shine.

New stars lit up by pressuring the hydrogen atoms deep within themselves to fuse with one another, yielding helium and releasing energy. Energy fled the stars as light and heat, but helium accumulated inside them, until eventually it, too, became a fuel for nuclear fusion, and the stars melded atoms of helium into atoms of carbon. At later stages of their lives, stars also forged nitrogen, oxygen and even iron. Then, literally exhausted, they expired and exploded, spewing their bounty of new elements into space. The largest and brightest stars bequeathed to the universe the heaviest of elements, including gold and uranium. Thus the stars carried on the work of Creation, hammering out a wide range of raw materials for future use.

As the stars enriched the heavens that had borne them, the heavens gave rise to new generations of stars, and these descendants possessed enough material wealth to build attendant worlds, with salt seas and slime pits, with mountains and deserts and rivers of gold.

In its own beginning, some five billion years ago, the star that is our Sun arose from a vast cloud of cold hydrogen and old stardust in a sparsely populated region of the Milky Way. Some disturbance, such as the shock wave from a nearby stellar explosion, must have reverberated through that cloud and precipitated its collapse. Widely dispersed atoms gravitated into small clumps, which in turn lumped together, and kept on aggregating in an ever-quickening rush. The cloud’s sudden contraction raised its temperature and set it spinning. What had once been a diffuse, cool expanse of indeterminate shape was now a dense, hot, spherical ‘proto-solar nebula’ on the verge of starbirth.

The nebula flattened into a disk with a central bulge, and there in the heart of the disk the Sun came to light. At the moment the Sun commenced the self-consumptive fusion of hydrogen in the multi-million-degree inferno of its core, the outward push of energy halted the inward gravitational collapse. Over the ensuing few million years, the rest of the Solar System formed from the leftover gas and dust surrounding the infant Sun.

The Book of Genesis tells how the dust of the ground, moulded and exalted by the breath of life, became the first man. The ubiquitous dust of the early Solar System – flecks of carbon, specks of silicon, molecules of ammonia, crystals of ice – united bit by bit into ‘planetesimals’, which were the seeds, or first stages, of planets.

Even as they assembled themselves, the planets asserted their individuality, for each one amassed the substances peculiar to its location in the nebula. At the hottest part, flanking the Sun, Mercury materialized from mostly metallic dust, while Venus and Earth matured where rocky dust as well as metal proliferated. Just past Mars, tens of thousands of rocky planetesimals availed themselves of plentiful carbon supplies, but failed to amalgamate into a major planet. These herds of unfinished worlds, called ‘asteroids’, still roam the broad zone between Mars and Jupiter, and their territory, the ‘Asteroid Belt’, marks the Solar System’s great divide: on its near-Sun side lie the terrestrial planets; on the far side, the frigid gaseous giants grew.

The planetesimals at greater distances from the Sun, at lower temperatures, assimilated stores of frozen water and other hydrogen-containing compounds. The first one to reach appreciable proportions then attracted and held on to great quantities of hydrogen gas, and thus grew into Jupiter, the mammoth planet whose mass doubles that of all the others combined. Saturn, too, aggrandized itself with gas. Farther out from the Sun, where dust proved even colder and scarcer, planetesimals took longer to develop. By the time Uranus and Neptune had achieved sufficient mass to pad themselves with hydrogen, the bulk of that gas had already dissipated. At Pluto’s remove, only rock shards and ices could be had.

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.

From Earth, we see the Sun as a blazing circle in the sky, brighter but no bigger than the circumference of the full Moon. The ‘two great lights’, as the Sun and Moon are described in Genesis, make a matched pair. For although the Moon measures only one four-hundredth the Sun’s million-mile diameter, it nevertheless lies four hundred times closer to Earth. This uncanny coincidence of size and distance enables the puny Moon to block out the Sun whenever the two bodies converge on their shared path across Earth’s sky.

Approximately once every two years, some narrow swath of Earth – as often as not a godforsaken, all but inaccessible place – is blessed with a total solar eclipse. There, dusk falls and dawn breaks twice on the same day, and the stars come out with the Sun still overhead. Temperatures may drop ten or fifteen degrees at a stroke, allowing even the most jaded observer to sense the bizarre disorientation that birds and animals share as they hasten to their nests or burrows through the sudden midday darkness.

No total eclipse can last much longer than seven minutes, because of Earth’s persistent turning on its axis and the Moon’s unwavering march along its orbit. But totality of the briefest duration affords sufficient reason for scientific expeditions and curious individuals to travel halfway around the world, even if they have seen one or more eclipses before.

At totality, when the Moon is a pool of soot hiding the bright solar sphere, and the sky deepens to a crepuscular blue, the Sun’s magnificent corona, normally invisible, flashes into view. Pearl and platinum-coloured streamers of coronal gas surround the vanished Sun like a jagged halo. Long red ribbons of electrified hydrogen leap from behind the black Moon and dance in the shimmering corona. All these rare, incredible sights offer themselves to the naked eye, as totality provides the only safe time to gaze at the omnipotent Sun without fear of requital in blindness.

Moments later, the shadow of the Moon passes and the natural world order is restored by the ordinary grace of the Sun’s familiar light. But visions of the eclipse persist among viewers, as though a miracle had been witnessed. Is it an accident that the Solar System’s lone inhabited planet possesses the only satellite precisely sized to create the spectacle of a total solar eclipse? Or is this startling manifestation of the Sun’s hidden splendour part of a divine design?

* Discarded comet dust litters interplanetary space, and when the Earth trundles into a patch of it, the particles that fall through the atmosphere are incinerated, appearing as isolated ‘shooting stars’ or whole showers of meteors.

* Degrees K, for Kelvin, are the same size as degrees Celsius (or centigrade) – almost double the value of Fahrenheit degrees. However the Kelvin scale starts lower, at −273 °C, or ‘absolute zero’, the point at which all motion ceases, and has no upper limit, which makes it useful for describing the temperatures of stars.

3 MYTHOLOGY

The planets speak an ancient dialect of myth. Their names recall all that happened before history, before science, when Prometheus hung shackled to that cliff in the Caucasus for stealing fire from the sky, and Europe was not yet a continent but still a girl, beloved by a god, who beguiled her disguised as a bull.

In those days Hermes – or Mercury, as the Romans renamed the Greek messenger god – flew fleet as thought on divine errands that earned him more mentions in the annals of mythology than any other Olympian: after the goddess of the harvest lost her only daughter to the god of the underworld, Mercury was sent to negotiate the victim’s rescue, and drove her home in a golden cart pulled by black horses. When Cupid got his wish, making Psyche immortal and therefore fit to marry him, it was Mercury who led the bride into the palace of the gods.

The planet Mercury appeared to the ancients, as it appears to the naked eye today, only on the horizon, where it coursed the twilight limbo between day and night. Swift Mercury either heralded the Sun at dawn, or chased after it through dusk. Other planets – Mars, Jupiter, Saturn – could be seen shining high in the sky all night for months on end. But Mercury always fled the darkness for the light, or vice versa, and hastened from view within an hour’s time. Likewise the god Mercury served as a go-between, traversing the realms of the living and the dead, conducting the souls of the deceased down to their final abode in Hades.

Myth may have conferred the god’s name on the planet, because it mirrored his attributes, or perhaps the observed behaviour of the planet gave rise to legends of the god. Either way, the union of planet Mercury with divine Mercury – and with Hermes, and the Babylonian deity Nabû the Wise before him – was sealed by the fifth century BC.

The persistent image of Mercury, lean and hell-bent as a marathon runner, personifies dispatch. Wings on his sandals urge him on, spurred faster by the wings on his cap, and the magic powers of his winged wand. Although speed tops the panoply of his powers, Mercury also gained fame as a giant-killer (after he slew thousand-eyed Argus) and as the god of music (because he invented the lyre, and his son, Pan, fashioned the shepherd’s pipe of reeds), god of commerce and protector of traders (for which he is remembered in words like ‘merchant’ and ‘mercantile’), of cheats and thieves (since he stole herds from his half-brother Apollo on the very first day of his life), of eloquence (having given Pandora the gift of language), as well as of cunning, knowledge, luck, roads, travellers, young men in general, and herdsmen in particular. His snake-entwined wand, the caduceus, has invoked fertility or healing or wisdom over the ages.

Mercury and his fellow travellers called attention to themselves by moving among the fixed stars, which earned them the name ‘planetai’, meaning ‘wanderers’ in Greek. The orderliness of their motions brought ‘cosmos’ out of ‘chaos’ in the same language, and inspired an entire lexicon for describing planetary positions. Just as the gods’ names still cling to the planets, Greek terms such as ‘apogee’, ‘perigee’, ‘eccentricity’ and ‘ephemeris’ endure in astronomical discussions. The first observers to coin such words fill a roster of ancient heroes, from Thales of Miletus (624–546 BC), the founding Greek scientist who predicted a solar eclipse and questioned the substance of the universe, to Plato (427–347 BC), who envisioned the planets mounted on seven spheres of invisible crystal, nested one within the other, spinning inside the eighth sphere of the fixed stars, all centred on the solid Earth.* Aristotle (384–322 BC) later raised the number of celestial spheres to fifty-four, the better to account for the planets’ observed deviations from circular paths, and by the time Ptolemy codified astronomy in the second century AD, the major spheres had been augmented further by ingenious smaller circles, called ‘epicycles’ and ‘deferents’, required to offset the admitted complexities of planetary motion.

‘I know that I am mortal by nature, and ephemeral,’ says an epigraph opening Ptolemy’s great astronomical treatise, the Almagest, ‘but when I trace at my pleasure the windings to and fro of the heavenly bodies I no longer touch earth with my feet: I stand in the presence of Zeus himself and take my fill of ambrosia, food of the gods.’

In Ptolemy’s model, Mercury orbited the stationary Earth just beyond the sphere of the Moon. The impetus for motion came from a divine force exterior to the network of spheres. More than a millennium later, however, when Copernicus rearranged the planets in 1543, he argued that the mighty Sun, ‘as though seated on a royal throne’, actually ‘governs the family of planets’. Without specifying the force by which the Sun ruled, Copernicus ringed the planets round it in order of their speed, and set Mercury closest to the Sun’s hearth because it travelled the fastest.

Indeed, Mercury’s proximity to the Sun dominates every condition of the planet’s existence – not just its tantivy progress through space, which is all that can be easily gleaned from Earth, but also its internal conflict, its heat, heaviness, and the catastrophic history that left it so small (only one-third Earth’s width).

The pull of the nearby Sun rushes Mercury around its orbit at an average velocity of thirty miles per second. At that rate, almost double the Earth’s pace, Mercury takes only eighty-eight Earth-days to complete its orbital journey. The same Procrustean gravity that accelerates Mercury’s revolution, however, brakes the planet’s rotation about its own axis. Because the planet forges ahead so much faster than it spins, any given locale waits half a Mercurian year (about six Earth-weeks) after sunrise for the full light of high noon. Dusk finally descends at year’s end. And once the long night commences, another Mercurian year must pass before the Sun rises again. Thus the years hurry by, while the days drag on for ever.

Mercury most likely spun more rapidly on its axis when the Solar System was young. Then each of its days might have numbered as few as eight hours, and even a quick Mercurian year could have contained hundreds such. But tides raised by the Sun in the planet’s molten middle gradually damped Mercury’s rotation down to its present slow gait.

Day breaks over Mercury in a white heat. The planet has no mitigating atmosphere to bend early morning’s light into the rosy-fingered dawn of Homer’s song. The nearby Sun lurches into the black sky and looms enormous there, nearly triple the diameter of the familiar orb we see from Earth. Absent any aegis of air to spread out and hold in solar heat, some regions of Mercury get hot enough to melt metals in daylight, then chill to hundreds of degrees below freezing at night. Although the planet Venus actually grows hotter overall, because of its thick blanket of atmospheric gases, and Pluto stays altogether colder on account of its distance from the Sun, no greater extremes of temperature coexist anywhere in the Solar System.

The drastic contrasts between day and night make up for the lack of seasonal changes on Mercury. The planet experiences no real seasons, since it stands erect instead of leaning on a tilted axis the way Earth does. Light and heat always hit Mercury’s equator dead on, while the north and south poles, which receive no direct sunlight, remain relatively frigid at all times. In fact, the polar regions probably harbour reservoirs of ice inside craters, where water delivered by comets has been preserved in perpetual shadow.

Mercury usually eludes observation from Earth by hiding in the Sun’s glare. The planet becomes visible to the unaided eye only when its orbit carries it far to the east or west of the Sun in Earth’s skies. During such ‘elongations’, Mercury may hover on the horizon every morning or evening for days or weeks. It remains difficult to see, however, because the sky is relatively bright at those times, and the planet so small and so far away. Even as Mercury draws closest to Earth, fifty million miles still separate it from us, which is quite remote compared to the Moon’s average distance of only a quarter of a million miles. Moreover, the illuminated portion of Mercury thins to a mere crescent as the planet approaches Earth. Only the most diligent observers can spot it, and only with good fortune. Copernicus, caught between the miserable weather in northern Poland and the reclusive nature of Mercury, fared worse than his earliest predecessors. As he grumbled in De Revolutionibus, ‘The ancients had the advantage of a clearer sky; the Nile – so they say – does not exhale such misty vapours as those we get from the Vistula.’

Copernicus further complained of Mercury, ‘The planet has tortured us with its many riddles and with the painstaking labour involved as we explored its wanderings.’ When he aligned the planets in the Sun-centred universe of his imagination, he used observations made by other astronomers, both ancient and contemporary. None of those individuals, however, had sighted Mercury often enough or precisely enough to help Copernicus establish its orbit as he had hoped.

The Danish perfectionist Tycho Brahe, born in 1546, just three years after Copernicus’s death, amassed a great number of Mercury observations – at least eighty-five – from his astronomical castle on the island of Hven, where he used instruments of his own design to measure the positions of each planet at accurately noted times. Inheriting this trove of information, Brahe’s German associate Johannes Kepler determined the correct orbits of all the wanderers in 1609 – ‘even Mercury itself.’

It later occurred to Kepler that although Mercury remained hard to see at the horizon, he might catch it high overhead on one of those special occasions, called a ‘transit’, when the planet must cross directly in front of the Sun. Then, by projecting the Sun’s image through a telescope onto a sheet of paper, where he could view it safely, he would track Mercury’s dark form as it travelled from one edge of the Sun’s disk to the other over a period of several hours. In 1629 Kepler predicted such a ‘transit of Mercury’ for November 7, 1631, but he died the year before the event took place. Astronomer Pierre Gassendi in Paris, primed by Kepler’s prediction, prepared to watch the transit, then erupted into an extended metaphor of mythological allusions when the event unfolded more or less on schedule and he alone witnessed it through intermittent clouds.

‘That sly Cyllenius,’ wrote Gassendi, calling Mercury a name derived from the Arcadian mountain Cyllene, where the god was born,

introduced a fog to cover the earth and then appeared sooner and smaller than expected so that he could pass by either undetected or unrecognized. But accustomed to the tricks he played even in his infancy [i.e., Mercury’s early theft of Apollo’s herds], Apollo favoured us and arranged it so that, though he could escape notice in his approach, he could not depart utterly undetected. It was permitted me to restrain a bit his winged sandals even as they fled. […] I am more fortunate than so many of those Hermes-watchers who looked for the transit in vain, and I saw him where no one else has seen him so far, as it were, ‘in Phoebus’ throne, glittering with brilliant emeralds.’*

Gassendi’s surprise at Mercury’s early arrival – around 9 a.m., compared to the published prediction of midday – cast no aspersions on Kepler, who had cautiously advised astronomers to begin searching for the transit the day before, on November 6, in case he had erred in his calculations, and by the same token to continue their vigil on the 8th if nothing happened on the 7th. Gassendi’s comment about the small size of Mercury, however, generated big surprise. His formal report stressed his astonishment at the planet’s smallness, explaining how he at first dismissed the black dot as a sunspot, but presently realized it was moving far too quickly to be anything but the winged messenger himself. Gassendi had expected Mercury’s diameter to be one-fifteenth that of the Sun, as estimated by Ptolemy fifteen hundred years before. Instead, the transit revealed Mercury to be only a fraction of that dimension – less than one-hundredth the Sun’s apparent width. The aid of the telescope, coupled with Gassendi’s sighting Mercury silhouetted against the Sun, had stripped the planet of the blurred, aggrandizing glow it typically wore on the horizon.

Over the next several decades, precise measuring devices mounted on improved telescopes helped astronomers pare Mercury close to its acknowledged current size of 3,050 miles across, or less than one three-hundredth the actual diameter of the Sun.

By the end of the seventeenth century, mystic and magnetic attractions among the Sun and planets had been replaced with the force of gravity, introduced by Sir Isaac Newton in 1687 in his book Principia Mathematica. Newton’s calculus and the universal law of gravitation seemed to give astronomers control over the very heavens. The position of any celestial body could now be computed correctly for any hour of any day, and if observed motions differed from predicted motions, then the heavens might be coerced to yield up a new planet to account for the discrepancy. This is how Neptune came to be ‘discovered’ with paper and pencil in 1845, a full year before anyone located the distant body through a telescope.

The same astronomer who successfully predicted Neptune’s presence at the outer margin of the Solar System later turned his attention inward to Mercury. In September of 1859, Urbain J. J. Leverrier of the Paris Observatory announced with some alarm that the perihelion point of Mercury’s orbit was shifting ever so slightly over time, instead of recurring at the same point in each orbit, as Newtonian mechanics required. Leverrier suspected the cause to be the pull of another planet, or even a swarm of small bodies, interposed between Mercury and the Sun. Returning to mythology for an appropriate name, Leverrier called his unseen world Vulcan, after the god of fire and the forge.

Although the immortal Vulcan had been born lame and ever walked with a limp, Leverrier insisted his Vulcan would hasten around its orbit at quadruple Mercury’s speed, and transit the Sun at least twice a year. But all attempts to observe those predicted transits failed.

Astronomers next sought Vulcan in the darkened daytime skies around the Sun during the total solar eclipse of July 1860, and again at the August 1869 eclipse. Enough scepticism had developed by then, after ten fruitless years of hunting, to make astronomer Christian Peters in America scoff, ‘I will not bother to search for Leverrier’s mythical birds.’

‘Mercury was the god of thieves,’ quipped French observer Camille Flammarion. ‘His companion steals away like an anonymous assassin.’ Nevertheless the quest for Vulcan continued through the turn of the century, and some astronomers were still pondering the whereabouts of Vulcan in 1915, the year Albert Einstein told the Prussian Academy of Sciences that Newton’s mechanics would break down where gravity exerted its greatest power. In the Sun’s immediate vicinity, Einstein explained, space itself was warped by an intense gravitational field, and every time Mercury ventured there, it sped up more than Newton had allowed.

‘Can you imagine my joy,’ Einstein asked a colleague in a letter, ‘that the equations of the perihelion movement of Mercury prove correct? I was speechless for several days with excitement.’

Vulcan fell from the sky like Icarus in the wake of Einstein’s pronouncements, while Mercury gained new fame from the role it had played in furthering cosmic understanding.

Still Mercury frustrated observers who wanted to know what it looked like. One German astronomer postulated a dense cloud layer completely shrouding Mercury’s surface. In Italy, Giovanni Schiaparelli of Milan decided to track the planet overhead in daylight, despite the Sun’s glare, in the hope of getting clearer views of its surface. By pointing his telescope upward into the midday sky, instead of horizontally during dawn or dusk, Schiaparelli avoided the turbulent air on Earth’s horizon, and also succeeded in keeping Mercury in his sights for hours at a time. Beginning in 1881, avoiding coffee and whisky lest they dull his vision, and forswearing tobacco to the same end, he observed the planet on high at its every elongation. But the pallor of Mercury against the daytime sky confounded his efforts to perceive surface features. After eight years at this Herculean task, Schiaparelli could report nothing but ‘extremely faint streaks, which can be made out only with greatest effort and attention’. He sketched these streaks, including one that took the shape of the number five, on a rough map of Mercury he issued in 1889.

A more detailed map followed in 1934, drawn as the culmination of a decade-long study by Eugène Antoniadi at the Meudon Observatory outside Paris. By his own admission, Antoniadi saw little more than Schiaparelli, but, being an excellent draughtsman and having a bigger telescope, he rendered his faint markings with better shading, and named them for Mercury’s classical associations: Cyllene (for the god’s natal mountain), Apollonia (for his half-brother), Caduceata (for his magic wand), and Solitudo Hermae Trismegisti – the Wilderness of Thrice-Great Hermes. Although these suggestions have disappeared from modern maps, two prominent ridges discovered on Mercury by spacecraft imaging are now named ‘Schiaparelli’ and ‘Antoniadi’.

Both Schiaparelli and Antoniadi assumed, given the persistence of the features they discerned over long hours of observation, that only one side of Mercury ever came into view. They thought the Sun had locked the little planet into a pattern that flooded one of its hemispheres with heat and light while leaving the other in permanent darkness. Likewise many of their contemporaries and most of their followers up to the mid-1960s believed that Mercury maintained eternal ‘day’ on one side and ‘night’ on the other. But the Sun constrains the rotation and revolution of Mercury according to a different formula: the planet spins around its axis once every 58.6 days – a rate rhythmically related to its orbital period, so that Mercury completes three turns on its axis for every two journeys around the Sun.

The 3:2 pattern affects observers on Earth by repeatedly offering them the same side of Mercury six or seven apparitions in a row. Schiaparelli and Antoniadi indeed beheld an unchanging face of Mercury throughout their studies, and must be forgiven for reaching the wrong conclusion about its rotation, since the planet’s behaviour indulged them in their error.

Throughout the twentieth and into the twenty-first century, Mercury has continued to be a difficult target. Even the Hubble Space Telescope, orbiting above the Earth’s atmosphere, avoided looking at Mercury, for fear of pointing its delicate optics so dangerously close to the Sun, and only one spacecraft has so far braved the hostile heat and radiation of the near-Mercury environment.

Mariner 10, Earth’s emissary to Mercury, flew by the planet twice in 1974 and once more in 1975. It relayed thousands of pictures and measurements of a landscape riddled with crater holes, from small bowls to giant basins. Light or dark trails of debris marked the places where newer assaults had overturned the rubble of the old. Lava that flowed among the impact scars had smoothed over some of the depressions, but overall poor battered Mercury preserved a clear record of the era, ended nearly four billion years ago, when leftover fragments of the Solar System’s creation menaced the fledgling planets.

The most violent attack on Mercury inflicted a wound eight hundred miles wide, which has been named Caloris Basin (‘the Basin of Heat’). The mile-high mountains on Caloris’s rim must have sprung up in response to the massive impact explosion that excavated the Basin, and all around the mountains, further signs of disturbance lay in ridges and rough ground rippling out for hundreds of miles. The collision at Caloris also sent shock waves clear through Mercury’s dense, metallic interior, to set off quakes that lifted the crust on the far side of the planet and cut it to pieces.

Mariner 10 photo montages, which captured less than half of Mercury’s surface, revealed a network of scarps and fault lines that indicate the whole planet must have shrunk to its present dimensions from some larger beginning. When Mercury’s interior contracted, the global crust readjusted itself to fit the suddenly smaller world – like some furtive trick of the god Mercury, disguising himself.

After a thirty-year hiatus in exploration, a new mission called MESSENGER (an acronym for MErcury Surface, Space ENvironment, GEochemistry and Ranging) is now en route to Mercury. Launched in August 2004, but unable to fly as quickly or directly as its namesake, the craft will not reach Mercury’s vicinity until January 2008. At first sight of the planet, MESSENGER will start a detailed mapping effort requiring three flybys of Mercury over the following three years, while the spacecraft orbits the Sun, protected under a sunshade made of ceramic cloth. Then, in March 2011, MESSENGER will manoeuvre into orbit around Mercury itself, for a year-long odyssey (as measured in Earth time) to monitor the planet through two of its long days. Circling Mercury rapidly and repeatedly every twelve hours, MESSENGER will function as a new oracle, streaming answers to the questions posed by anxious truth-seekers on Earth.

* The ancients recognized seven planets: Sun, Moon, Mercury, Venus, Mars, Jupiter and Saturn.

* Gassendi quotes here from Ovid, referring to the Sun god Apollo by his other name, Phoebus.

4 BEAUTY

For a breeze of morning moves,

And the planet of Love is on high,

Beginning to faint in the light that she loves

On a bed of daffodil sky,

To faint in the light of the sun she loves,

To faint in his light, and to die.

Alfred, Lord Tennyson, ‘Maud’

Now ‘morning star’, now ‘evening star’, the bright ornament of the planet Venus plays prelude to the rising Sun, or postscript to the sunset.

For months at a time Venus will vault the eastern horizon before dawn and linger there through daybreak, the last of night’s beacons to fade. She begins these morning apparitions close to the Sun in time and space, so that she arrives in a lightening sky. But as the days and nights go by, she comes up sooner and ventures further from the Sun, rising while Dawn is still a distant idea. At length she reaches the end of her tether, and the Sun calls her back, making her rise a little bit later each night, till she again verges on the day. Then Venus vanishes altogether for the time it takes her to pass behind the Sun.

After fifty days, on average, she reappears at the Sun’s other hand, in the evening sky, to be hailed as evening star for months to come. Shimmering into view as the Sun goes down, Venus hangs alone in the twilight. The first few sunsets find her bathed in the afterglow colours of the western horizon, but at length Venus comes to light already high overhead, where she dominates night’s onset. Who knows how many childhood wishes are squandered on that planet before the gathering darkness brings out the stars?

Thou fair-haired angel of the evening,

Now, whilst the sun rests on the mountains, light

Thy bright torch of love; thy radiant crown

Put on, and smile upon our evening bed!

Smile on our loves, and, while thou drawest the

Blue curtains of the sky, scatter thy silver dew

On every flower that shuts its sweet eyes

In timely sleep. Let thy west wind sleep on

The lake; speak silence with thy glimmering eyes,

And wash the dusk with silver.

William Blake, ‘To The Evening Star’

Hours into the night, Venus still outshines every other light, unless the Moon intrudes to best her. The Moon appears bigger and brighter, by virtue of lying about one hundred times closer to us, though Venus is the larger and the fairer by far. Venus’s shroud of yellow-white cloud reflects light much more effectively than the dun-coloured, dust-covered surface of the Moon. Virtually 80 per cent of the Sunlight lavished on Venus just skitters off her cloud tops and spills back into space, while the Moon beams back a mere 8 per cent.

The remarkable brightness of Venus gains lustre from her nearness to Earth. Venus comes within twenty-four million miles of Earth at closest approach – closer than any other planet. (Mars, Earth’s second nearest neighbour, always stays at least thirty-five million miles away.) Even when Venus and Earth recede as far from each other as possible, separated by more than one hundred and fifty million miles, Venus retains her superlative brilliance for Earthbound observers. On the scale of ‘apparent magnitude’ astronomers use to compare the relative brightness of heavenly bodies, Venus far exceeds the most luminous stars.*

What strong allurement draws, what spirit guides,

Thee, Vesper! brightening still, as if the nearer

Thou com’st to man’s abode the spot grew dearer

Night after night?

William Wordsworth, ‘To the Planet Venus’

The nearer Venus draws to Earth, the brighter she appears, naturally enough. Yet as her glow crescendos, the globe of Venus actually diminishes from full to gibbous through quarter and then crescent phase. Like the Moon, Venus appears to change shape as she moves along her orbit, and by the time she reaches her closest, most vivid aspect in our skies, only about one-sixth of her visible disk remains illuminated. But proximity stretches this little sliver to a great length, allowing the perceived brightness of Venus to increase even as she thins and wanes away.

Watching Venus through a telescope or binoculars every evening over a period of months shows how she gains in height and brightness as her disk shrinks, and vice versa. Little else becomes apparent, however, since none of Venus’s surface features can ever be discerned by sight through her cloud deck. Thus the very clouds that account for her blatant visibility also act to veil her.

Those who know just where to look can sometimes pick out the steady white light of Venus against the light blue background of a fully daylit sky. Napoleon spotted Venus that way while giving a noon address from the palace balcony at Luxembourg, and interpreted her daytime venue as the promise (later fulfilled) of victory in Italy.

On Moonless nights when Venus is nigh, her strong light throws soft, unexpected shadows onto pale walls or patches of ground. The faint silhouette of a Venus shadow, which evades detection by the colour-sensitive inquiry of a direct gaze, often answers to sidelong glances that favour the black-and-white acuity of peripheral vision. But no matter how avidly you hunt the elusive Venus shadow with eyes averted and downcast, your search may still prove vain, while overhead, as though to mock you, the planet’s dazzle mimics the landing beam of an oncoming aeroplane, even triggers police reports of unidentified flying objects.

I stopped to compliment you on this star

You get the beauty of from where you are.

To see it so, the bright and only one

In sunset light, you’d think it was the sun

That hadn’t sunk the way it should have sunk,

But right in heaven was slowly being shrunk

So small as to be virtually gone,

Yet there to watch the darkness coming on-

Like someone dead permitted to exist

Enough to see if he was greatly missed.

I didn’t see the sun set. Did it set?

Will anybody swear that isn’t it? …

Robert Frost, ‘The Literate Farmer and the Planet Venus’

Ancient legends celebrated the beauty of planet Venus by declaring her not only divine but also womanly – perhaps because her visitations generally lasted a significant nine months. Although Venus orbits the Sun in just 224 Earth-days, the Earth’s own orbital motions help govern Venus’s observed behaviour. As seen from the moving Earth, Venus averages 260 days as either morning star or evening star, coinciding with the human gestation period of 255 to 266 days.

The Chaldeans called the planet Ishtar, the love goddess ascending the heavens, and to the Semitic Sumerians she was Nin-si-anna, ‘the Lady of the Defences of Heaven’. Her Persian name, Anahita, associated her with fruitfulness. The dual (dawn and dusk) nature of Venus cast her by turns as virgin or vamp to her worshippers.

Ishtar metamorphosed into Aphrodite, the Greek incarnation of love and beauty. She became the Venus of the Romans, revered by the historian Pliny for spreading a vital dew to excite the sexuality of earthly creatures. In China, Venus blended male and female genders in a married couple consisting of the husband evening star, Tai-po, and his wife, the morning star, Nu Chien.

Only the Mayas and the Aztecs of Central America seem to have seen Venus as consistently male, the twin brother of the Sun. The rhythmic association between Venus and the Sun inspired meticulous astronomical observations and complex calendar reckoning in those cultures, as well as blood rituals to recognize the planet’s descent into the underworld and subsequent resurrection.

In North America, among the Skidi Pawnee, the veneration of Venus involved human sacrifice to ensure her return. The last teenage girl known to have died in such devotions was kidnapped and ceremonially killed on 22 April 1838.

As a symbol of loveliness, Venus figures in three paintings by Vincent Van Gogh. His Starry Night of June 1889, the best-known example, depicts Venus as the bright orb low to the east of the village of Saint-Rémy, during the time the artist’s dementia confined him to an asylum there. Art historians and astronomers have also definitively identified Venus in Road with Cypress and Star, which Van Gogh completed in mid-May 1890, the day before he left Saint-Rémy. A few weeks later, in Auverssur-Oise, near Paris, where he created eighty works in the two months before his suicide, Van Gogh depicted Venus for the last time, inside a scintillating halo, hovering above the west chimney of White House at Night.

Venus voyages … but my voice falters;

Rude rime-making wrongs her beauty,

Whose breasts and brow, and her breath’s sweetness

Bewitch the worlds.

C. S. Lewis, ‘The Planets’

If ever two worlds invited comparison, the twin sisters Earth and Venus lay such a claim, for these planets are almost identical in size, and orbit the Sun at similar distances. Early discoveries about Venus from afar – especially the detection of her atmosphere by Russian astronomer and poet Mikhail Lomonosov in 1761 – fanned widespread fantasies of a lush abode of Earth-like life.

Recent research, however, has exposed only the most glaring contrasts between the two planets. Although at an earlier epoch Venus probably possessed many of the same attributes as Earth, including once-abundant seas, her water has all boiled away. Now Venus parches and bakes under an obscuring sky that blocks light but traps heat, and bears down upon her surface with heavy pressure.

The ten Russian Venera and Vega spacecraft that successfully landed on Venus between 1970 and 1984 barely had time to take a few pictures and measurements, or quickly sample the surroundings, before succumbing to the harsh conditions. Within an hour or so of arrival, each vehicle either melted in the heat or crumpled under atmospheric pressure comparable to that found underwater on Earth, nearly three thousand feet below sea level.

Discoveries of the drastic differences between Earth and Venus evoked surprise sometimes expressed in moral terms, as though one sister had chosen the right course while the other veered down an errant path. Nevertheless Venus, the wayward sister, preaches an important cautionary tale to careless humans, for her hostile environment proves how even small atmospheric effects can conspire over time to convert an earthly paradise into a hellfire cauldron. Indeed, much current study of Venus aims to save humanity from itself by verifying, for example, the destruction that chlorine compounds wreak in high-altitude clouds.

And art thou, then, a world like ours,

Flung from the orb that whirled our own

A molten pebble from its zone?

How must the burning sands absorb

The fire-waves of the blazing orb,

Thy chain so short, thy path so near

Thy flame-defying creatures hear

The maelstroms of the photosphere!

Oliver Wendell Holmes, ‘The Flâneur’*

Differences between Earth and Venus doubtless began in their youth, with the Sun beating hotter on the closer of the two sisters. The Sun warmed the waters of Venus until they rose in steam, until water vapour and the hot breath of volcanic eruptions enveloped the planet. These gases then did the work of greenhouse glass: they allowed the Sun’s heat to reach the surface of Venus, but refused to let heat escape. Instead of dissipating into space, the heat rebounded back down to ground level and made the surface hundreds of degrees hotter still.

High over Venus, Sunlight split the water vapour into its components, hydrogen and oxygen, and the lighter hydrogen escaped the planet’s hold. Oxygen remained behind; it recombined with the surface rocks on Venus, and with gases vented by volcanoes, to create an atmosphere consisting almost entirely (97 per cent) of carbon dioxide, the most efficient and pernicious of all greenhouse gases. Today, although only a trickle of solar energy penetrates Venus’s cloud cover and arrives at the surface, the greenhouse effect keeps temperatures above eight hundred degrees Fahrenheit all around the planet, day side and night side, even at the poles. Ice on Venus? Liquid water? Impossible, although traces of water vapour do lace the sky.

The abundant carbon dioxide weighs on Venus’s hot terrain with ninety times the pressure of Earth’s atmosphere. On and just above the surface, where the Russian robot explorers conducted their brief surveys, the Venusian air is thick but transparent, enabling the spacecraft’s cameras to see clear to the horizon in the dim available light. All the light was red. Since only the long red wavelengths of light survive the journey down through the cloud canopy, the landscape presents itself as a monochrome in the sepia tones of old photographs. When night takes even this low-level light away, the vista glows in the dark. Its red-hot rocks, cooked halfway to their melting point by the ambient heat and pressure, resemble the embers of a fire.

Some twenty miles above the surface, the clouds set in, in layers fifteen miles thick, admitting no breaks in their coverage. They bar the Sun from ever showing itself at all during the whole course of the long Venusian day. The planet turns so slowly that a single day takes what would be reckoned as two months on Earth just to get from Sunrise to Sunset. Diffuse signs of the Sun’s light spread slowly from horizon to horizon as the hours pass, but even the brightest hours of the day stay as dimly lit as vespertide. At night, no stars or other planets ever appear through the perpetual overcast.

Venusian clouds comprise large and small droplets of real vitriol – sulphuric acid along with caustic compounds of chlorine and fluorine. They precipitate a constant acid rain, called virga, that evaporates in Venus’s hot, arid air before it has a chance to strike the ground.

Scientists suspect that every several hundred million years the clouds may be remade by a fresh injection of sulphur from global tectonic upheaval on Venus, but failing that, they probably never part.

At their topmost layer, the Venusian clouds display dark swirls when imaged in ultraviolet light. These markings change rapidly, revealing the high velocity at which the clouds roll by – about 220 miles per hour – circling Venus every four Earth-days on fierce winds. Lower down in the atmosphere the winds slacken gradually until they reach the surface, where they don’t so much blow as creep across the planet at two to four miles an hour.

Fast or slow, the winds head ever westerly, the same way Venus turns. In contrast to all the other planets, Venus rotates to the west, even as she revolves eastward with them around the Sun. If you could see the Sun rise on Venus, it would come up in the west and set in the east. Astronomers attribute the backward spin to some violent collision that overturned Venus early in her history. The same presumed impact could explain Venus’s very slow rotation rate, or perhaps it is the Sun that impedes the planet’s spin by raising tides in the vast ocean of Venusian air.

Deep within that

libidinous albedo

temperatures are hot enough

to boil lead,

pressures

90 times more unyielding

than Earth’s.

And though layered cloud-decks

and haze strata

seem to breathe

like a giant bellows,

heaving and sighing

every 4 days,

the Venerean cocoon

is no cheery chrysalis

brewing a damselfly

or coaxing life

into a reticent grub,

but a sniffling atmosphere

40 miles thick

of sulphuric, hydrochloric,

and hydrofluoric acids

all sweating

like a global terrarium,

cutthroat, tart, and self-absorbed.

Diane Ackerman, ‘Venus’

After hiding for an eternity beneath her seething atmosphere, Venus’s surface has surrendered to radar examination by Earth-based telescopes and a series of orbiting spacecraft. The finest of these envoys, Magellan, circumnavigated Venus eight times a day for four years beginning in 1990.* Magellan resolved the planet’s vague face into distinct features, most of which turned out to be volcanoes of every variety on plains paved with lava.

Magellan’s sudden identification of millions of land forms fomented a crisis in nomenclature. The International Astronomical Union responded with an all-female naming scheme that evoked a goddess or giantess from every heritage and era, along with heroines real or invented. Thus the Venusian highlands, the counterparts to Earth’s continents, took the names of love goddesses – Aphrodite Terra, Ishtar Terra, Lada Terra, with hundreds of their hills and dales christened for fertility goddesses and sea goddesses. Large craters commemorate notable women (including American astronomer Maria Mitchell, who photographed the 1882 transit of Venus from the Vassar College Observatory), while small craters bear common first names for girls. Venus’s scarps hail seven goddesses of the hearth, small hills the goddesses of the sea, ridges the goddesses of the sky, and so on across low plains named from myth and legend for the likes of Helen and Guinevere, down canyons called after Moon goddesses and huntresses.

The only male name on the map of Venus – the great mountain range Maxwell Montes – belongs to Scottish physicist James Clerk Maxwell, who performed pioneering work on electromagnetic radiation during the nineteenth century. When the five-mile-high peaks were detected in the 1960s via Earth-based radar studies made possible by Maxwell’s insights, it seemed fitting to attach his name to them. For several decades after discovery, Maxwell Montes stood as the sole eponymous feature on the planet, while the low regions on either side of the mountains were designated simply as Alpha Regio and Beta Regio (‘A’ region and ‘B’ region). When Magellan arrived thirty years later, and its revelations gave rise to names derived from women’s history, no one wished to evict Maxwell from his rightful place on Venus.

Yes, the faces in the crowd,

And the wakened echoes, glancing

From the mountain, rocky browed,

And the lights in water dancing –

Each my wandering sense entrancing,

Tells me back my thoughts aloud,

All the joys of Truth enhancing

Crushing all that makes me proud.

James Clerk Maxwell,

‘Reflex Musings: Reflections from Various Surfaces’*

Magellan’s radar images look like night-time aerial reconnaissance photos, except that instead of providing a visual record, their blacks and whites reflect the varying textures of Venus’s exposed beauty: hundreds of thousands of small Venusian volcanoes pop out as bright (rough) bumps against the dark (smooth) background of the plains. On the flanks of giant volcanoes, bright (new) layers of lava drape themselves over the dark (old) flows. Mountainsides glittering in radar brightness seem to boast slopes coated with a veneer of reflective metal, perhaps fool’s gold, that adheres to Venusian rock at the cooler temperatures a few thousand feet up.

Etched in these images, Venus reveals her unique oddities, such as overlapping ‘pancake dome’ volcanoes that rise from surprisingly round bases to flat or softly mounded tops, and her numerous ‘coronae’, or sets of concentric rings that ornately surround so many of her domes, depressions and crowds of small volcanoes. Rushing streams of lava dug the long riverine channels that wind across her ample plains. On her high plateaux, tectonic folding and faulting have decorated several thousand square miles to look like crazy-tiled floors, now called ‘tesserae’. Evocative patterns in Venus’s extruded lava and cracked ground that reminded scientists of sea anemones and spider webs have become ‘anemone volcanoes’ and ‘arachnoids’.

After amassing their gallery of radar portraits, Venus specialists enhanced many of the images with colour for improved resolution. They chose a fire-and-brimstone palette, beginning with the russet hue of the first photos taken by the Russian Venera spacecraft, continuing the theme in ochre, umber, sienna, copper, pumpkin and gold. The vibrant colours suit the seared scenery, the rock that spewed forth as lava and still retains its near-plastic consistency, the massifs ascending to altitude without ever hardening harder than toffee. Bright shades befit the youthful visage of a planet that only recently (within the last half billion years) repaved itself in veritable floods of lava, which welled up and covered over almost every vestige (about 85 per cent) of her ancient past.

Relatively few craters mar the new face of Venus, since the rate of cratering over these past 500,000 years is much reduced from the Solar System’s earliest days. Many small would-be intruders are vaporized on their way through the thick atmosphere, never to touch down, so that only the very largest impactors reach the surface intact. These collisions eject copious debris, yet all the rubble hugs close around the crater margins in neat festoons, as though contained there by the heavy air. The atmosphere likewise may have soothed the fury of Venusian volcanoes, compelling their expelled lava to seep and pour rather than erupt with explosive force.

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