by Neil Shubin
Pierre-Simon Laplace (1749–1827) was one of the greatest mathematicians of all time, called by some the French Newton. Laplace’s name peppers the fields of mathematics and statistics. There are, for example, the Laplace equation, the Laplacian operator, and the Laplace transform, tools to understand electricity, magnetism, and the motion of bodies in space. His real passion was to uncover the order in the heavens, the shape of the planets and the orbits of celestial bodies. With this intellectual goal, he converted the philosophical ideas of Swedenborg and Kant to the precise language of mathematics.
If a dust cloud in space gets to the right size, Laplace conjectured, particles inside will interact such that gravity will pull them together as other forces act to separate them. This push-pull means that a relatively amorphous cloud of dust can, under conditions where the pull wins out, develop into a swirling disk of debris. Over time, the gravitational attraction of the particles of dust in the disk break it into separate concentric rings—imagine a striped Frisbee. If the mass of dust in the rings is large enough, the particles could then condense to form the various planets of the solar system. These big events would happen not overnight but over timescales of millions of years.
Laplace’s mathematical reformulation of Swedenborg’s and Kant’s ideas served as midwife for their transformation from interesting concepts to testable predictions. But the problem was that the technology to make the necessary measurements did not exist in the late eighteenth and early nineteenth centuries. Consequently, our understanding of the formation of the solar system stagnated for over a hundred years.
Enter big science. In 1983, scientists from the Netherlands, Britain, and the United States developed a satellite that could map the stars from an orbit around Earth. This predecessor to the Hubble Space Telescope was designed to perform one kind of observation really well: measure the infrared spectrum of the entire sky to assess how much heat is emanating from different stars. Through the course of their lives, stars emit everything from visible light to infrared, ultraviolet, and gamma rays. Our eyes sense only a small fraction of the light stars create, so astronomers use a wide range of telescopes, each tuned to different wavelengths of light, to capture a more complete view.
Because infrared signals from deep space are often weak, every source of interference needs to be removed from the sensors, even those made by vibrating atoms. To still the atoms, the device was cooled by liquid helium to a temperature of -452 degrees Fahrenheit. With room on board for only one year’s supply of the coolant, the whole project became a race against time. It did its job, and the satellite, now defunct, continues to orbit the sky. In the years since, a small community of scientists has proposed a mission to give the satellite a helium recharge to put the sensors back in business. Limited budgets and the development of better technology have kept the satellite switched off.
Despite the short life span of the satellite’s detectors, the mission was a huge success. In less than a year it charted almost 96 percent of the sky. The satellite mapped new asteroids and comets until, in early 1984, it captured a glimpse of a star radiating far too much heat for its size and type. We have a good idea about how much heat different kinds of stars should produce, and something was clearly different about this star. The source of that extra radiation became clear upon closer inspection of the images. The star was encircled by a vast cloud of dust and debris that held heat. This system, Beta Pictoris, became the first example of a solar system caught in the act of being born. A prediction born as intuition and converted to mathematics was confirmed after two hundred years.
Beta Pictoris. One of the first images of a distant solar system being born. (Illustration Credit 3.1)
Soon after its formation, our solar system would have looked like Beta Pictoris. This moment of our history was chaotic; rocky debris fragments of different sizes collided with one another as they swirled around the sun. The gravitational pull of the sun meant that heavier material would orbit closer to it, while lighter particles and gas orbited farther away. To some extent, this state of affairs remains in effect today, with the solar system composed of rocky inner planets, Mercury, Venus, Earth, and Mars, and gaseous outer ones, Jupiter, Saturn, Uranus, and Neptune.
Whether the object of a search is Easter eggs, fossil bones, or a new kind of solar system, one discovery typically leads to the next. What once was rare turns up everywhere, often right under our noses. The years since the recognition of the dust surrounding Beta Pictoris have witnessed the launch of new satellites, the construction of ever-bigger telescopes, and the use of powerful computers to crunch all the data returning to Earth. This technology has changed our view of the heavens. Far from being a lonely solar system, ours is only one of many in the galaxy. The sky is filled with other worlds at different stages of their development surrounded by planets of almost every description.
Powerful technology and great ideas have transformed our notions of the heavens. But do not discount the impact of pure luck.
In the wee hours of the morning on February 8, 1969, a massive fireball woke residents of the Mexican state of Chihuahua. A visitor from space had arrived: a large meteorite that broke apart in the atmosphere. After learning of the event, scientists and collectors poured into the area in droves. Given the size of the boom, the collectors had expected a bonanza, but they had no idea of the extent until they looked carefully inside the rock. Tiny white patches interrupted the dull gray body of the rock itself. Meteorites with these specks were known before, but they were incredibly rare. Laboratory work on the few other meteorites with inclusions like these revealed grains that hint at the chemical signature of primordial rocks of the solar system.
The meteorite exploded into fragments that spread over about twenty-five square miles of desert. Two to three tons of fragments have been collected in the years since the impact. Even today, more than forty years later, pieces are occasionally found.
The impact could not have occurred at a more opportune time. In 1969, Project Apollo was in high gear. With Apollo 8 having circled the moon just two months before the meteor strike and another as-yet-undetermined Apollo mission set to land on it, labs across the country were gearing up to investigate the chemistry of moon rocks. Now, at no expense to the taxpayer, special rocks from space had arrived right on our doorstep. Not only that, but the meteor was so huge prior to breaking up that there were a large number of fragments to share among the different chemistry laboratories capable of making sense of them.
Scientists performed the routine analysis of the atoms inside the rocks. Some of the mineral grains are so similar to those of Earth rocks that they point to a shared history of the bodies of our solar system, just as Swedenborg, Kant, and Laplace predicted. Other minerals can be dated using the decay of the atoms inside as a kind of clock. When a mineral forms, the atoms come together as a crystal structure. Once born as a crystal, some of the atoms, such as uranium and lead, change at a regular pace as defined by the laws of physics and chemistry. If you know the relative abundances of the different forms of the atom inside the mineral, and the rates at which they convert to one another, then you can calculate the time since the mineral formed (see Further Reading and Notes for more details). Uranium 238 converts into lead 206 very slowly; it takes 4.47 billion years for half the original amount to decay in this way. This slow rate of atomic change makes uranium and lead ideal atoms to measure the age of very ancient crystals. The uranium and lead concentrations of the Mexican meteorite point to an age for when the solar system got its start: 4.67 billion years ago.
But what was happening on Earth during these early moments? Direct evidence is hard to come by. In the ideal world, we would have a rock that formed at the moment Earth’s crust cooled and has lain undisturbed for the billions of years since. The easiest geological conditions to study are those in which one layer of rock lies on top of the next, much like a birthday cake. The deepest layers would be the object of the hunt because typically those would be the most ancient. Yo
u could drill a deep core for them, but this is way too expensive for the typical geology budget. What’s more, the drilling would be a bit of a shot in the dark: it would be hard to know where to look miles under the surface of Earth. You’d be better served to find places where the ancient rock layers are poking out at the surface of the crust. The challenge with finding these places is that the surface of the planet is continually being reworked. Mountains and oceans rise and fall. Under the action of such a dynamic planet, rock layers are buried, heated, and then eroded by water and wind. If the ideal geological conditions are regular cakelike layers, imagine a cake pulled asunder, crushed, and then superheated. Now throw 99.99999 percent of that dessert away. The hunger you’d experience trying to eat that cake would be similar to that of geologists who seek to find artifacts of the planet’s formation.
Some places just feel primordial, almost like an ancient landscape frozen in time. At the Jack Hills in the arid desert of Western Australia, lowland scrub pokes out from orange and yellow bluffs of rock. Aboriginal art lies etched on boulders, the artists having died tens of thousands of years ago. The region’s climate is so hot and dry that the nearby bays and inlets of Shark Bay are home to odd doorknob-shaped mats of microbes. These microbial communities are some of the most ancient living relics on Earth, with their closest relatives fossils that are over 2 billion years old. Fittingly, the bluffs of rock that jut to the surface match like jigsaw puzzle pieces to ancient ones buried deeply elsewhere. These are old-looking rocks too; heavily transformed by heat and pressure over time, they carry their history like wrinkles on a face. These rocks from the geological basement have been witness to most of the entire history of our planet.
Befitting such survivors, these layers have experienced eons of torment; from formation inside hot volcanic fluids to great pressures as they lay buried underground, finally to the stresses and strains that came when the layers were wrenched to the surface. Moment after moment is recorded in these layers; the trick, as always, comes from learning how to see the history inside.
Every rock in the ground is an artifact that, when you know how to interpret it, becomes a time capsule, a thermostat, even a barometer of the health of our planet. To wrest these details from stones, we have to zoom from a bird’s-eye view of rock layers all the way down to a microscopic one. The smallest components of rocks—the individual grains of sand or minerals inside—often tell the biggest stories. One of these grains, zircon, has unique properties. It is virtually indestructible, and it can survive superheating, high pressure, erosion, and virtually every other torture that the planet can throw at it.
Large, clear crystals of zircon make great fake diamonds. To those interested in the formation of the planet, zircons are far more valuable than gems, because zircon’s durability makes it an ideal window into the ancient Earth. The rocks that contain zircons can come and go, but zircons are (nearly) forever. The clocks of uranium and lead from the Jack Hills produce a range of ages from 4.0 to 4.4 billion years.
The chemistry of zircons tells us more than the age of Earth. It holds a true surprise. The abundances of the various forms of oxygen inside the crystal can only have come from rock that interacted with liquid water as it formed.
Tiny grains tell the story of the solar system: it got its start over 4.6 billion years ago, and by at least 4.1 billion years ago liquid water, so essential for life, was already on Earth.
MAKING A SPLASH
We may live on the “blue planet”—unique in the known universe for its abundance of liquid water—but our bodies’ ocean lies on the inside. Adult humans are about 57 percent water by weight. Our body dries out with every passing year; newborns are about 75 percent water, not much different from an average potato. Most of the body’s water is not in the fluid of our blood, but remains locked inside the cells of our muscles, brains, and hearts. Metabolism of food and oxygen depends on water, as do the growth and communication of our cells. Even reproduction, with the motility of sperm and egg, is based in a fluid medium. Virtually every chemical reaction in our bodies depends in some way on the presence of water.
We are tied to water for more than our present lives: our bodies contain the history of water itself. The first 2.7 billion years of our history was entirely in water, and the imprint is in every organ system of our bodies. The fundamental organization of our head is based on a series of swellings that develop into the bones of our jaws, ears, and throats as well as the muscles, nerves, and arteries that supply all of them. Equivalent structures are seen in everything with a head, including fish and sharks. In these creatures, the bones develop into the structures that support and supply the gills. In a sense, the muscles, nerves, and bones that we use to talk, chew, and hear correspond to the gill bones of our fishy ancestors. This deep tie to gills is also seen in fossils, where we can follow the transformation of gill bones to structures deep within our own heads, including our ear bones.
While most of our past lay inside the water, the most recent 300 million years has been defined by our separation from it. Our kidneys have developed specializations to help balance the water and salts inside the body in the face of life on dry land. Our reproduction doesn’t depend as much on water as it did for our ancestors: sperm and egg are fertilized inside the body, and the developing fetus is shielded from the outside world by membranes and vessels that protect it and attach it to the mother. Our hands and legs, structures so adept at supporting life on land, are modified fish fins. Our terrestrial existence comes about through repurposed organs that fish use to live in water.
The human kidney, like that of other mammals, is a magnificent adaptation to life on land; kidneys help kangaroo rats and antelope live in dry deserts, surviving only on the water locked inside the molecules of their food. Yet deep within this most unique of terrestrial organs lie roots of its aquatic origins. All jawless fish—ones we share a common ancestor with over 500 million years ago—have a very primitive kind of kidney: tissues that run the length of the body, take fluid wastes from the bloodstream, and dump them directly into the body cavity, ultimately allowing for excretion from an opening at the tail. Bony fish, which share a common ancestor with us 450 million years ago, have a more specialized arrangement in which these clumps of tissue connect to a plumbing system that carries wastes outside the body. The most recent of these kidneys, the system that mammals use, doesn’t run the length of the body but sits at the level of the lower back.
During our time in the womb, we form three different kinds of kidneys, one after the other. The first kidneys are clumps of tissue that line the body and open to the body cavity, much like those seen in jawless fish. The second, like those of bony fish, run the length of the back to a common plumbing system. The adult kidney, which appears at the end of the first trimester, replaces both of these. In our first three months, we track our fishy past.
Life’s connection with water is no accident; the water molecule itself has special properties. With one oxygen and two hydrogen atoms, it looks something like a Mickey Mouse head: small hydrogen atoms form the ears atop a head made by a large oxygen atom. This whole molecule is polarized, with a negative charge at one end, where the oxygen resides, and positive charges at the opposite end, corresponding to the hydrogens. This arrangement makes water the ideal medium in which to dissolve a large variety of substances. Salts, proteins, amino acids—so many compounds can be incorporated into water that it provides the matrix for the chemical reactions on which life depends. No longer dependent on the vagaries of the water outside our bodies for our metabolic processes, we maintain that stable watery environment inside us.
Water has another property, one seen in a kitchen: it can exist as a liquid, solid, and gas within a relatively narrow range of temperatures and pressures. We have so many different kinds of interactions with water because it occurs on the planet as solid ice, gas in the air, and the fluids that are the substrate for living processes. Over 97 percent of the planet’s water lies in the oceans, with the r
est in the clouds, ice, and freshwater, and each of these forms is vital to our existence and that of the planet.
Where’s the water? The relative abundance of water on the planet.
Just as water is the matrix for the chemical processes that run inside our bodies, so too is it for the metabolism of the planet. Water raining from the sky and from melting ice erodes rock on land and, as it flows from high to low elevations, returns minerals to the sea. This gradual weathering provides the counterpoint to the uplift of mountains and plateaus over geological time. Molecules in the air, many of which impact climate and atmosphere, are continually recycled between rock and sea by the action of water. Water provides the links that define a livable Earth.
The water inside our bodies and in the oceans also tells of its origins. Being two parts hydrogen and one part oxygen, water can be thought of as a two-to-one ratio of atomic nuclei derived from the big bang to those derived from fusion reactions inside stars. While their constituent atoms have a history that extends across the universe, water molecules themselves are linked to the solar system. The chemistry of the water in Earth’s oceans, particularly the mix between different kinds of hydrogen atoms, is distinctive and can be compared to the ice in comets, asteroids, and other planets. Probes sampling water in the ice of the comet Hale-Bopp, which passed by Earth in 1997, revealed differences between Hale-Bopp’s water and that of Earth. This discovery was a huge disappointment to many because the reigning dogma in the 1990s was that comets were the likely source of Earth’s water. Fans of the cometary hypothesis were in for a treat in 2011, when newer probes sent to other comets, such as Hartley 2, revealed water with very oceanlike proportions of atoms. The story of water is more complex than simply comets, because the more we look across the solar system, the more water we find. With powerful telescopes and ever-newer satellites, we have seen water turn up on the moon and within asteroids. There are even hints of water in the most unlikely places imaginable. Mercury is the closest planet to the sun; its surface reaches temperatures of 800 degrees Fahrenheit, hot enough to melt lead. NASA’s MESSENGER satellite, sent to Mercury in 2004, captured photos of structures that have the distinctive reflective properties of ice deep within craters at the poles of the planet. Water may survive there because the craters of Mercury, shielded from the sun and on a planet with no atmosphere, are likely very cold. With so much water across the solar system, it seems likely that some water arrived here from space, but it is also possible that some came from the rocks of the forming Earth itself. When rock is superheated, as was the likely condition 4.5 billion years ago, it can vaporize and release water molecules trapped inside its molecular structure. Whether originating from the ice of comets or vaporized from the rocky debris of the early solar system, or both, each glass of water we drink is derived from sources at least as old as the solar system itself. And, as zircons tell us, water has been here on Earth in liquid form since at least 4 billion years ago.
