The Planets (10 page)

Nothing could summon water from those dark seas of the Moon because they are, all of them, dry. Nor have the Moon’s so-called seas ever known the presence of water. Though the lunar maria hinted of a fluid interconnectedness to the first astronomers who eyed them and named them through telescopes, the first Moonwalkers to tread them retrieved the driest imaginable materials from their shores.

“Bone-dry,” the lunar samples were described, though they are much drier than bones, which form inside the Earth’s wet living systems, and retain the memory of water long after death.

Dry as dust, then? No, drier still. On Earth, even dust holds water.

Moon rocks set a new standard of dryness, distinguished by the
total
absence of water. Not a drop of water, not a bubble of water vapor lurks in the crystal lattice of any Moon rock among the lunar samples, and no ice ever so much as touched them. Comets, however, have probably tucked odd caches of imported water ice—perhaps ten million tons’ worth—in the shadows of unexplored craters near the lunar poles.

Lacking water as a potential ingredient limited the Moon’s creativity to a mere one hundred minerals, while the moist Earth has fabricated several thousand mineral varieties. The gems romantically or religiously associated with the Moon—pearl, quartz, opal, moonstone—could never have formed there, for each requires water in one way or another, and the Moon has none to offer.
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The primal lunar scenario currently favored by planetary scientists explains the Moon’s formation and dryness in a single blow: Early in the history of the Solar System, a rogue planet on a collision course struck the infant Earth. The impact, thought to have occurred 4.5 billion years ago, melted impactor and impact site alike, and shot hot debris into space. Swarms of dust and rock fragments, lofted into orbit around the stunned Earth, eventually reunited, 4.4 billion years ago, as the Moon. Having been ejected from one common cauldron, Moon rocks chemically resemble Earth rocks,
except that they have lost all their water and any other compounds capable of escape as vapors.

The furious pace of lunar assembly generated enough heat to melt the top layers of the new satellite into a global magma ocean, one hundred miles deep. Over time that ocean gradually cooled and hardened to stone. The errant rubble of the Solar System’s violent youth, still at large then, bombarded the Moon’s smooth new crust, blasting out vast impact basins and craters. Meanwhile radioactive heat trapped inside the young Moon drove more molten rock to the surface, to fill broad basins with black basalt—and paint the Moon’s facial features.

The all-encompassing ocean of magma attending the Moon’s birth was the first fluid to flow there. The rivers and pools of extruded lava were the last, and those froze up three billion years ago. At that time, the rate of cratering tapered off throughout the Solar System, and the Moon, having expended all its internal heat, solidified through and through, turning into a dry fossil generally considered “dead” by geological standards.

The parched Moon pulls at Earth’s seas as though jealous of them. Twice each day the ocean tides rise and fall to the call of lunar gravity.
The waters rise once when they pass beneath the Moon, which makes intuitive sense, but then they rise again after they have been twirled round to the other side of the world, where they face away from the Moon. There you might say they only appear to rise, when really the Earth is being pulled out from under them by the tug of the Moon. Looking at the whole world’s waters at once, the ocean directly under the Moon rises in response to the stronger tug of gravity there, while the ocean on Earth’s opposite side simultaneously rises as though relieved to feel so little force pulling it in the opposite direction.

Earthly tides answer to solar gravity as well as lunar, but not as much so, for the Sun’s greater distance, and its tendency to pull more equitably on all parts of the Earth at once, diminishes its effect on the tides. When the Sun and Moon align with the Earth in a straight line across the heavens, however, as they do at new and at full Moon, then the three bodies conspire to make tides rise higher. Such “spring tides,” which occur in every season, take their name from the rush of waters that may leap as much as twenty feet, twice in one day. Should the spring-tide lineup, or syzygy, occur with the
Moon close to Earth, at perigee, then so much the higher for the tides.

Some swear that the same strong tides of lunar attraction can hoist a person’s inner parts skyward, too. Why shouldn’t the human body, consisting mostly of water, heave in synch with Earth-Moon rhythms? Probably because it’s too small. Just as the small bodies of water in lakes and ponds fail to respond to the Moon in tides, so, too, do small living bodies of water bow out of interplanetary interactions. Therefore the Moonstruck sensation often evoked in the human breast is best explained as an emotional response to beauty, not a tide of bodily fluids. Likewise the match of the female menses with the lapse of the lunar month must be either a coincidence or a mystery.

Even as the Moon draws the oceans to and fro, the Earth drags down the Moon with the superior force of its greater mass. The uneasy power struggle between the bodies has slowed the rotation of the Moon to about ten miles per hour. Spinning this slowly, the Moon takes as long to turn once on its axis as it takes to complete its monthly 1.5-million-mile orbit. The Earth has thus coerced the Moon into a lock-step pattern of rotation
and revolution, called “Earth-lock,” that keeps the Moon’s same awestruck face trained Earthward at all times. No wonder the man in the Moon looks so familiar.

Compared to the Moon, the Earth spins like fury, rotating one hundred times faster. Yet the Earth, too, is decelerating, by a few millionths of a second annually, under the strain of tidal friction. For the Moon’s noticeable effect on ocean tides is accompanied by an insidious stretching of the Earth’s solid ground. The Moon pulls hardest on whatever part of Earth is nearest, actually raising a bulge. But no sooner has some expanse of Earth’s surface risen in response than the Earth’s rotation wrenches that region out from under the Moon, and rolls a neighboring area there instead. With some section of the planet always bulging and then subsiding, the constant friction stays the pace of rotation.

As the Earth slows, the Moon drifts an inch or so farther away each year, since the cascade of tidal effects gives a slight boost to the satellite. Eventually the Earth’s slowing down and the Moon’s slipping away will end in a standoff that stabilizes the Earth’s rotation and halts the Moon’s retreat. At that point the rotation of both bodies will be synchronized: Earth will eye the Moon with the same wary,
one-sided gaze the Moon now fixes on the Earth. Moon worshipers in that distant future will no doubt dwell on the half of Earth where the Moon hovers overhead all the time, while any remaining inhabitants of Earth’s other hemisphere, “the far side,” will need to journey as much as halfway around the world to get even a glimpse of the Moon.

For now, the almost imperceptible decrease in the Earth’s rotation amounts to a mere millisecond every fifty years. But this and other inconstancies have convinced official timekeepers to improve on the Sun, Moon, and stars as reliable standards, and occasionally to stitch an extra “leap second” into the worldwide calendar year. Unlike a leap year, which lasts a day longer than a typical year, a leap second measures the same fraction of time as any other second. But just like the leap year, the leap second sings the frustration of all recorded efforts to base a calendar of human affairs on the motions of the heavenly spheres.
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The Earth’s daily turning on its axis and annual revolution around the Sun refuse to mesh easily
with the Moon’s monthly orbit. Combining solar and lunar time cues has ever demanded elaborate formulae for alternating between twelve-month and thirteen-month years (which long ago made the number thirteen unlucky), or for legislating the duration of the months themselves. The mnemonic doggerel of “Thirty days hath September” quickly loses its rhyme and meter in the struggle to fit a requisite number of days into months that will tally with seasons through years to come.

Even though an atomic clock keeps better time than the dance of the planets, nevertheless it is the clock that must be readjusted accordingly, and yield to the authority of the imprecise orbs. For what good is the smug ability to judge the Earth a second short in her timing if spring comes when it will?

On the Moon, a single time span—our lunar month—serves for day and year alike. Over the course of this daily year, as the Moon turns on its axis and around the Earth, Sunlight and warmth spread first over one lunar hemisphere and then the other, granting each about two weeks of continuous daylight, to be followed by the frigid two-week night.

Many think of the far side of the Moon as the
dark side, on account of its being perpetually hidden from Earth, but it, too, goes through phases, which complement the fully or partially lit phases we observe on the near side. Just as the Sun’s light bathes half the Earth all the time, so, too, does it illuminate the sphere of the Moon.

Apollo astronauts who walked on the Moon landed on the near side in the early lunar morning, before the temperature rose to its noon high of 225 degrees Fahrenheit. Even the last two Apollo crews, who sojourned on the surface for an elapsed mission time of three days, came and went within half a morning on the Moon.

Not one of them set foot on the far side, though they all saw its strange terrain first-hand from lunar orbit, and remain the only humans ever to have done so. They could have exclaimed any expletive or sentiment in the private thrill of that revelation, since traveling behind the Moon cut off their radio contact with Houston and the rest of the world. Apollo command module pilots, who stayed in orbit while the landing parties worked the surface, experienced a profound solitude over the far side, out of touch with all civilization—including their teammates—for forty-eight minutes out of every two-hour loop around the Moon. The far side of
the Moon is the one place in the whole Solar System deaf to Earth’s radio noise.

Like the hidden half of any being, the lunar far side bears scant resemblance to the face the Moon shows the world. More craters abound there, overlapping in profusion, and one sees hardly any of the smooth dark expanses of pooled lava that characterize the near side. The thicker crust on the Moon’s back apparently checked the expulsion of lava from within.

All geologic ferment on the Moon ceased about three billennia ago, after the late heavy bombardment cleared the Solar System of most menacing massive projectiles. Today, a ton-mass meteorite strikes the Moon no more than once in three years, on average. The occasional Moonquake can be confidently dismissed as a weak reaction to tidal stress, not the stirrings of a living planet with a liquid core.

Only
micro
-meteorites continue to fall steadily on the dead Moon, thickening the dust by a millionth of a millimeter per year. This influx constitutes the major tectonic force now at work on the Moon. Selenologists call it “gardening,” because the new arrivals mix and turn over the sterile lunar “soil” as they insert themselves into it. The
gentle process barely disturbs the present still life on the Moon—the arrays of scientific instruments, the litter of spent rocket stages, the three parked rover vehicles.

Among the personal talismans intentionally left behind, a posed snapshot of an astronaut and his family calls attention to itself. Someone took care to wrap that photo in plastic for protection—as though anything could happen to it on the Moon’s arid, uneventful surface, where a bootprint enjoys a life expectancy of a million years, and every dust particle savors of immortality.

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Valued as exotica, a single carat of Moon rock sold at auction in 1993 for $442,500. Similarly, a site map of the Moon’s Descartes highlands, only slightly used by
Apollo 16
astronauts and bearing smudges of lunar dust, brought $94,000 at a sale in 2001.

*
A second, which once divided a mean solar day into 86,400 equal parts, is now defined as the time a confined cesium-133 atom takes to complete 9,192,631,770 natural vibrations. Since 1972, the International Earth Rotation Service has added twenty-four leap seconds, always inserted into the first moments of January or July.

C
all me “It,” or call me “Allan Hills 84001,” my given name—even “Thing from Mars” will suit. Although I am only a rock and cannot answer, allow me this conceit of conscious identity for the space of these few pages, that I may speak for Mars, whence I traveled via chance and the laws of physics.

Of the thirty-four Martian meteorites definitively identified to date, I am by far the most ancient, and the only one to show, under microscopic examination, internal shapes and residues similar to those formed by primitive terrestrial bacteria. These findings have made me the most studied rock of all time.

One might surmise I had been contaminated by Earth life during the thirteen thousand years I lay in the Antarctic ice fields before scientists collected me there in 1984. The scientists certainly assumed contamination, until they ruled out the possibility, concluding in near disbelief that it was more likely I had once sheltered small beings on my home planet—creatures perhaps already extinct when an asteroid impact flung me from the Martian surface sixteen million years ago.