by Dava Sobel
Mercury and his fellow travelers 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 B.C.), the founding Greek scientist who predicted a solar eclipse and questioned the substance of the universe, to Plato (427–347 B.C.), 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 centered on the solid Earth.* Aristotle (384–322 B.C.) 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 A.D., 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 traveled 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 sunup 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 forever.
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 harbor 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 one-quarter 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 vapors 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 labor involved as we explored its wanderings.” When he aligned the planets in the Sun-centered 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 traveled 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 favored 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 three thousand fifty 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 skepticism 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 whiskey 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,” he said, “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 draftsman 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 behavior 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 wi
de, 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 earthquakes that lifted the crust on the far side of this world 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 maneuver 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.
