The Planets

by Dava Sobel

  By now some spacecraft have served such lengthy periods of active duty at Mars and relayed such steady streams of information as to enable Earthbound geologists and climatologists to monitor trends over time, notably the transient nature of the Martian polar caps. At the start of every autumn in the south, as much as one third of the atmosphere sifts like powdery snow from the salmon-colored sky in a white frost of carbon dioxide. The dry ice fluffs the south polar cap a yard thicker and coats the southern hemisphere halfway to the equator all through the winter, the south’s longest season. When spring comes, the white rime sublimes directly back into the atmosphere without pausing to melt. Soon it deserts the sky again, precipitating onto the north pole with the arrival of autumn there.

  In other studies, Mars-stationed spacecraft have tested the strength of my planet’s gravity field, measured the atmospheric content and pressure, clocked wind speeds, compared the heights of mountains to the depths of basins, listened at ground level for Marsquakes, and also detected an iron core, solidified now, and no longer capable of generating a magnetic field.

  Indeed, so many spacecraft currently share the Martian domain, returning so many thousands of images, that the picture of the planet grows constantly more refined and more complex to Earthly eyes, with new theories adduced accordingly, so that the controversy among planetary scientists escalates as missions proliferate.

  From a Martian perspective, the sum of all this scrutiny could be construed as a hostile invasion.* The Earth envoys, however, have found no entity sensitive to assault, and only the slightest, most equivocal suggestion of any biological activity. The reddish dirt of Mars, rich in iron peroxide and other oxidizing agents, routinely sterilizes itself and all new arrivals. Organic compounds carried to the Martian surface on meteorites or visiting spacecraft are destroyed at once by the highly reactive chemistry of the present era. Any organic material that survived the chemical attack would no doubt succumb to physical dismantling by the Sun’s ultraviolet radiation, since the Martian atmosphere provides no protection comparable to Earth’s layer of ozone.

  Astrobiologists insist that life on Mars, like the once-plentiful water on Mars, could simply have gone underground to avoid these dangers, and may yet be discovered, extant or extinct, through diligent pursuit. Astronomers agree, asserting that even if Mars ultimately proves void of life, its unique environment will continue to lure robotic and human explorers to its frozen shores.

  Some visionaries see in Mars a potential homestead on a high frontier, awaiting colonization.* Scientifically feasible programs for “terraforming” Mars to enhance its Earthly likeness propose the fabrication of suitable habitats by, for example, heating the Martian south pole with huge space-based mirrors that would focus and magnify the Sun’s light, forcing the residual polar cap of carbon dioxide to sublime like a geyser of greenhouse gas. In the ensuing warmth, pure drinking water might pour from the ice at the north pole, or be mined from the abundant buried permafrost or chemically extracted from select areas of the planet’s hardened crust.

  Planners say they can achieve the same effect another way, by preparing a safe environment for a few hardy strains of microbes, and releasing them into the Martian regolith, there to ingest available nutrients and excrete gases, including ammonia and methane, which would then thicken the atmosphere, enabling it to hold in more heat, thereby raising the ambient temperature to create a shirtsleeve environment.

  Proponents of interplanetary manifest destiny expect that whether or not Mars has ever been inhabited by sentient Martians, Earthlings will eventually become Martians.*

  I picture them on the pitiless surface, dressed in specially engineered Mars protection suits, living in domed modules, toiling under an artificially generated magnetic field that shields them from harmful cosmic rays as they harness the energy of the wind and convert local stores of heavy hydrogen to electric power. As they busy themselves in the desert, raising food crops in greenhouses and prospecting for troves of high-grade mineral ores, they continue their careful reconnaissance of the planet, traveling overland by tractor and on foot, scaling and spelunking, still half hoping, half fearing that they are trespassing.

  I suppose it is their condition of being alive, and their sense of living such short lives, that drives their obsession to seek other life in every possible redoubt. Even if they succeed in preparing the way for their compatriots to join them in founding a great Martian civilization, they will continue straining for traces of whatever might have scrabbled in the reddish dust before they arrived.

  *See, for example, Frank Herbert, Dune (1965), and Edgar Rice Burroughs, The Gods of Mars (1918).

  *See Percival Lowell, Mars (1895), Mars and its Canals (1906), and Mars As the Abode of Life (1908).

  *See S. Glasstone, The Book of Mars, NASA Special Publication 179 (1968).

  *See H. G. Wells, The War of the Worlds (1898).

  *See Arthur C. Clarke, The Sands of Mars (1951), Robert A. Heinlein, Red Planet (1949), and Kim Stanley Robinson, Red Mars (1993), Green Mars (1995), Blue Mars (1997).

  *See Ray Bradbury, The Martian Chronicles (1950).

  ASTROLOGY

  When Galileo, a Pisces with Leo rising, turned his spyglass to the dark over Padua in the winter of 1610, “guided,” he said, “by I know not what fate,” the planet Jupiter appeared to him, bearing four new moons no man had ever seen before.

  Galileo thanked God for granting him these sights, and praised his new spyglass as the means. But surely the alignment of the planets through those January nights had also favored his success. For Venus, along with Mercury, hid below the horizon. Saturn set early in the evening, and by the time Mars rose, three hours before dawn, cold and fatigue had long since forced Galileo indoors. Even the Moon, though almost full at the start of Galileo’s vigil, gradually withdrew, leaving bright Jupiter, aglow at opposition, to wander the stars alone.

  No sooner had Galileo discerned the planet’s four companions than he saw what they augured for his own future: He might gain a position at the Tuscan court by naming them for his most important patron, the young Florentine Prince Cosimo de’ Medici. Given Jupiter’s prominence in Cosimo’s horoscope, which Galileo had already cast, the four moons must represent the boy and his three younger brothers, and therefore should be known henceforth as the Medicean stars.

  “It was Jupiter, I say,” Galileo reminded Cosimo, “who at Your Highness’s birth, having already passed through the murky vapors of the horizon, and occupying the midheaven”—by which he meant Jupiter had risen to the dominant, most auspicious position in the sky according to Renaissance astrology—“and illuminating the eastern angle”—that is, affecting the ascendant sign—“from his royal house” (Jupiter being considered king of the planets), “looked down upon Your most fortunate birth from that sublime throne and poured out all his splendor and grandeur into the most pure air, so that with its first breath Your tender little body and Your soul, already decorated by God with noble ornaments, could drink in this universal power and authority.”

  Jupiter had thus conferred on Cosimo the expansive confidence and noble ethical concern that befitted a born leader. The positive effect of Jupiter, called “the greater benefic” by practitioners of the stellar art, was known to uplift a person from pettiness to greatness, as well as to promise health and sanity, levity, wisdom, optimism, and generosity.

  “Indeed,” noted Galileo, “it appears that the Maker of the Stars himself, by clear arguments, admonished me to call these new planets by the illustrious name of Your Highness before all others. For as these stars, like the offspring worthy of Jupiter, never depart from his side except for the smallest distance, so who does not know the clemency, the gentleness of spirit, the agreeableness of manners, the splendor of the royal blood, the majesty in actions, and the breadth of authority and rule over others, all of which qualities find a domicile and exaltation for themselves in Your Highness? Who, I say, does not know that all these emanate from the most benign star of J
upiter, after God the source of all good?”

  The uproar that followed Galileo’s announcement of his discoveries caused a few commentators to wonder aloud how the four new celestial bodies would affect astronomy, on the one hand, and astrology on the other.

  Soon the Medicean stars weighed in as astronomical evidence to support the unpopular heliocentric system of Copernicus. By showing they could circle Jupiter even as Jupiter continued his own heavenly rounds, the new satellites made plausible the idea of the Earth’s moving through space, together with its Moon, around the Sun.

  Astrology broke with astronomy at this point, forced by its focus on human experience to retain the geocentric outlook. Nor did astrologers see any need to assign a new sphere of influence to the Medicean stars. Rather, they continued to esteem only Earth’s Moon, which they regarded as the ancient, familiar, feminine controller of emotional responses and everyday patterns of activity.

  In Galileo’s own natal chart, for example, the Sun is in Pisces,* but the Moon lies in the sign of Aries at the mid-heaven, indicating a highly imaginative, self-reliant, independent, and inventive individual with a restless mind, someone who goes beyond existing boundaries as a pioneer, an adventurer, even a sky warrior. At the same time, the Moon occupies the ninth of the twelve mundane houses—the house ruled by Jupiter and traditionally associated with knowledge and understanding. The Moon in the ninth house signifies strongly held religious and philosophic beliefs, as well as an advanced education and a long-lived mother, all of which Galileo had. The ninth house also encompasses travel to foreign countries, and although Galileo never left Italy, it could be argued that his telescope carried him on the farthest possible journeys.

  The same Jupiter that swam as a small globe in the eyepiece of the spyglass resided, in Galileo’s horoscope, in the sign of Cancer—where astrologers say the planet is “exalted,” or most free to express itself through the individual’s experience—and also conjunct with Saturn in the twelfth house. Jupiter and Saturn aligned in the house of confinement spelled success for Galileo around the age of forty or fifty. (He was forty-seven when he published the astronomical findings that brought him instant fame.) Together, Jupiter and Saturn implied that Galileo would face ideological crises (such as his later clash with the Inquisition, perhaps) and live in seclusion and solitude (as he did under house arrest his last eight years). The ebullient increase and fertility of Jupiter is tempered, in Galileo’s nativity, by the sobering nearness of Saturn.

  Jupiter assumed its astrological mantle of benevolence and largesse in Babylonian times, around 1000 B.C.—long before Sir Isaac Newton (a Capricorn) grasped the planet’s true physical enormousness by watching it pull on Galileo’s moons. The ancients had no way to assess the sizes of the planets or the distances between them, so their association of Jupiter with grandeur poses a mystery for astronomy and astrology to share.

  As befits the planet of expansion, Jupiter more than doubles the mass of the other eight planets combined. Compared to the Earth alone, gaseous Jupiter is more than three hundred times more massive than the solid Earth. The size difference between them seems even more portentous when measured in terms of volume, for Jupiter’s volume exceeds the Earth’s by one thousand-fold.

  A world apart from the terrestrial planets, Jupiter mimics the Sun in both composition and attitude: It consists almost exclusively of hydrogen and helium, and reigns over its own replica solar system of at least sixty planet-like satellites—the four largest ones Galileo found, plus fifty-nine others discovered (so far) since the dawning of the Age of Aquarius.

  Although many of Jupiter’s moons are rocky bodies, the gas giant itself has no solid surface, no terrain of any kind. The face it presents to Earthly observers is an expanse of pure weather: Every identifiable feature resolves into a cloud bank, a cyclone, a jet stream, a thunderbolt, or a curtain of auroral lights. On Jupiter, a storm may continue for centuries, innocent of landfall. No seasonal changes disrupt the weather patterns either, since the planet stands erect on its axis, only three degrees atilt.

  Countervailing winds that tear east and west across Jupiter arrange its clouds in a canopy of horizontal stripes. The east-flowing jet streams alternate with the westward trade winds to form some dozen dark belts and bright zones, each one confined to its own latitude band, where it remains fixed over time. Generations of Jupiter watchers have marveled at the persistence of these neat divisions.

  Every band of wind hosts a meteorological drama within its bounds. In the South Equatorial Belt, for example, a stable, oval-shaped storm known as “the Great Red Spot” has been studied continuously since 1879. The Spot has faded from its once vivid vermilion to pale orange, and shrunk to half its former width (though it still exceeds the diameter of Earth) without ever changing lanes. When the Great Red Spot meets other clouds traveling faster or slower in the same direction in the same belt, it sweeps them up and keeps them circling its perimeter for weeks, until they either merge with it or whirl past. Small oval storms that form in the dangerous furrows between east- and west-rushing flows, however, quickly fall victim to shear forces and shred apart in a day or two, like downgraded hurricanes.

  Jupiter’s clouds take their red, white, brown, and blue colors from sulfur, phosphorous, and other impurities in the atmosphere. Winds marble the cloud colors, as though with an eye to beauty, and eddies feather the edges of designs. All the colors might well have blended and muddied by now, after eons of swirling, had not each pigment generally held fast to its own layer at a designated altitude in the atmosphere. The low-down cloud base of warm blues can be glimpsed only through breaks in the overlying browns and whites, which give way, a few hundred miles up, to the high-flying cold reds.

  A faint but detectable glow of infrared radiation leaks through gaps in the cloud cover. This is the lingering heat of the planet’s original accretion, rising slowly by convection from the core as Jupiter continues to cool and contract. Half a billion miles from the Sun, Jupiter releases more warmth than it receives. Most of the energy to drive the Jovian winds thus derives from within, augmented only slightly by faint Sunlight falling from afar. Jupiter’s radiance has earned it a reputation as a “failed star,” but its internal temperature, estimated at 17,000 degrees, falls very far short indeed of the fifteen-million-degree inferno that makes the Sun shine.

  The vast, variegated clouds, which are all anyone ever sees of Jupiter, constitute only a thin veneer surrounding the planet; they comprise less than 1 percent of its forty-five-thousand-mile radius. Underneath the clouds the atmosphere grows denser and hotter because of mounting pressure, and the weather stranger. Here the carbon content of methane and other trapped gases may be crushed to tiny diamonds in the sky. Gradually the gases cease to behave as gas, as they dissolve into a sea of liquid hydrogen.

  Some five thousand miles down into this milieu, where the pressure reaches at least a million times Earth’s norm, the liquid hydrogen turns opaque, metallic, molten, and electric. By far the greatest part of Jupiter consists of hydrogen compressed to this exotic phase.

  According to astrological lore, each planet corresponds to a specific metal, so that silver, for example, pairs with the Moon, gold with the Sun, and mercury with Mercury. Jupiter’s assigned metal has been tin, not hydrogen. But then, no medieval alchemist knew of hydrogen’s existence, let alone the bizarre concoction of liquid metallic hydrogen produced inside Jupiter.

  Modern scientists have fabricated only the minutest quantities of liquid metallic hydrogen, by means of reverberating shock waves inside laboratory apparatus, and each such painstakingly made sample lasts just one-millionth of a second. Nevertheless, theorists have gleaned the essence of the substance, and, by extrapolation, explained many aspects of Jupiter’s nature. Its magnetic field, for example, which is twenty thousand times the strength of Earth’s field, and extends all the way to the orbit of Saturn, arises from the liquid metallic hydrogen interior. A genuine Jovian dynamo is created deep inside the planet, where
warm currents of escaping heat stir a susceptible fluid shot through with electric currents generated by Jupiter’s rapid rotation.

  The whole mammoth bulk of Jupiter rotates in just under ten hours, faster than any other planet. Its massive body honors the memory of the Solar System’s earliest beginnings as a spinning disk, and none of Jupiter’s attendant moons can slow it down. As to the giant’s rate of revolution in orbit, however, its far remove from the Sun relaxes its pace and adds many miles to its annual travels.

  At five times the Earth-Sun distance, Jupiter takes a long year, the equivalent of twelve Earth-years (11 years and 315 days), to orbit the Sun. En route it spends about one Earth-year passing through each of the twelve zodiac constellations. In traditional Chinese astrology, Jupiter’s slow gait earned it the title of “Year Star” (Sui xing)—the determiner of the Chinese years of the rat, ox, tiger, rabbit, dragon, snake, horse, sheep, monkey, rooster, dog, and pig. The Chinese cycle of animals, however, bears only scant relation to the twelve signs of the western zodiac, which include a bull, a lion, and a crab, as well as half-human twins, a virgin, and a water-bearer.

  In western astrology, one or another planet “rules” the sign with which it shares a natural affinity. Jupiter, long regarded as the most fortunate planet, rules Sagittarius, the archer, the sign of people born in mid-November through mid-December, who are said to express themselves with open-minded vision and honesty. For many centuries Jupiter also ruled the sign of Pisces, the fish, whose February-March natives (including Galileo) are masters of memory and introspection. But then, after the discovery and naming of Neptune in 1846, the new planet became astrologically associated with water, and so took Pisces away from Jupiter.