The weather on Jupiter’s surface plays similar tricks with elements. This shouldn’t be surprising on a planet that can support the giant red eye—a hurricane three times wider than the earth that hasn’t dissipated after centuries of furious storming. The meteorology deep inside Jupiter is possibly even more spectacular. Because the stellar wind blew only the lightest, most common elements as far out as Jupiter, it should have the same basic elemental composition as real stars—90 percent hydrogen, 10 percent helium, and predictable traces of other elements, including neon. But recent satellite observations showed that a quarter of the helium is missing from the outer atmosphere, as is 90 percent of the neon. Not coincidentally, there is an abundance of those elements deeper down. Something apparently had pumped helium and neon from one spot to the other, and scientists soon realized a weather map could tell them what.
In a real star, all the mini–nuclear booms in the core counterbalance the constant inward tug of gravity. In Jupiter, because it lacks a nuclear furnace, little can stop the heavier helium or neon in the outer, gaseous layers from falling inward. About a quarter of the way into Jupiter, those gases draw close to the liquid metallic hydrogen layer, and the intense atmospheric pressure there crushes the dissolved gas atoms together into liquids. They quickly precipitate out.
Now, everyone has seen helium and neon burning bright colors in tubes of glass—so-called neon lights. The friction from skydiving above Jupiter would have excited falling droplets of those elements in the same way, energizing them like meteors. So if big enough droplets fell far enough fast enough, someone floating right near the metallic hydrogen layer inside Jupiter maybe, just maybe, could have looked up into its cream and orange sky and seen the most spectacular light show ever—fireworks lighting up the Jovian night with a trillion streaks of brilliant crimson, what scientists call neon rain.
* * *
The history of our solar system’s rocky planets (Mercury, Venus, Earth, and Mars) is different, their drama subtler. When the solar system began to coalesce, the gas giants formed first, in as little as a million years, while the heavy elements congregated in a celestial belt roughly centered on the earth’s orbit and stayed quiet for millions of years more. When the earth and its neighbors were finally spun into molten globes, those elements were blended more or less uniformly inside them. Pace William Blake, you could have scooped up a handful of soil and held the whole universe, the whole periodic table, in your palm. But as the elements churned around, atoms began tagging up with their twins and chemical cousins, and, after billions of passes up and down, healthy-sized deposits of each element formed. Dense iron sank to the core inside each planet, for instance, where it rests today. (Not to be outdone by Jupiter, Mercury’s liquid core sometimes releases iron “snowflakes” shaped not like our planet’s familiar, water-based hexagons, but like microscopic cubes.*) The earth might have ended up as nothing but huge floes of uranium and aluminium and other elements, except that something else happened: the planet cooled and solidified enough to make churning difficult. So we’re left today with clusters of elements, but enough clusters spread far enough apart that—except in a few notorious cases—no one country monopolizes their supply.
Compared to planets around other stars, our system’s four rocky planets have different abundances of each type of element. Most solar systems probably formed from supernovae, and each system’s exact elemental ratios depend on the supernova energy available beforehand to fuse elements and also what was present (like space dust) to mix with the ejecta. As a result, each solar system has a unique elemental signature. From high school chemistry you probably remember seeing a number below each element on the periodic table to indicate its atomic weight—the number of protons plus the number of neutrons. Carbon weighs 12.011 units, for example. In reality, that’s just an average. Most carbon atoms weigh exactly 12 units, and the 0.011 gets tacked on to account for the scattered carbons that weigh 13 or 14 units. In a different galaxy, however, carbon’s average could stray slightly higher or lower. Furthermore, supernovae produce many radioactive elements, which start decaying immediately after the explosion. It’s highly unlikely two systems would have the same ratio of radioactive to nonradioactive elements unless both systems were born at once.
Given the variability among solar systems, and given that their formation took place incomprehensibly long ago, reasonable people might ask how scientists have the foggiest idea how the earth was formed. Basically, scientists analyzed the amount and placement of common and rare elements in the earth’s crust and deduced how they could have gotten where they are. For instance, the common elements lead and uranium fixed the birth date of the planet through a series of almost insanely meticulous experiments done by a graduate student in Chicago in the 1950s.
The heaviest elements are radioactive, and almost all—most notably uranium—break down into steady lead. Since Clair Patterson came up professionally after the Manhattan Project, he knew the precise rate at which uranium breaks down. He also knew that three kinds of lead exist on earth. Each type, or isotope, has a different atomic weight—204, 206, or 207. Some lead of all three types has existed since our supernova birth, but some has been created fresh by uranium. The catch is that uranium breaks down into only two of those types, 206 and 207. The amount of 204 is fixed, since no element breaks down into it. The key insight was that the ratio of 206 and 207 to the fixed 204 isotope has increased at a predictable rate, because uranium keeps making more of the former two. If Patterson could figure out how much higher that ratio was now than originally, he could use the rate of uranium decay to extrapolate backward to year zero.
The spit in the punch bowl was that no one was around to record the original lead ratios, so Patterson didn’t know when to stop tracing backward. But he found a way around that. Not all the space dust around the earth coagulated into planets, of course. Meteors, asteroids, and comets formed, too. Because they formed from the same dust and have floated around in cryogenic space since then, those objects are preserved hunks of primordial earth. What’s more, because iron sits at the apex of the stellar nucleosynthesis pyramid, the universe contains a disproportionate amount. Meteors are solid iron. The good news is that, chemically, iron and uranium don’t mix, but iron and lead do, so meteors contain lead in the same original ratios as the earth did, because no uranium was around to add new lead atoms. Patterson excitedly got hold of meteor bits from Canyon Diablo in Arizona and got to work.
Only to be derailed by a bigger, more pervasive problem: industrialization. Humans have used soft, pliable lead since ancient times for projects like municipal water pipes. (Lead’s symbol on the periodic table, Pb, derives from the same Latin word that gave us “plumber.”) And since the advent of lead paint and leaded “anti-knock” gasoline in the late nineteenth and early twentieth centuries, ambient lead levels had been rising the way carbon dioxide levels are rising now. This pervasiveness ruined Patterson’s early attempts to analyze meteors, and he had to devise ever more drastic measures—such as boiling equipment in concentrated sulfuric acid—to keep vaporized human lead out of his pristine space rocks. As he later told an interviewer, “The lead from your hair, when you walk into a super-clean laboratory like mine, will contaminate the whole damn laboratory.”
This scrupulousness soon morphed into obsession. When reading the Sunday comics, Patterson began to see Pig-Pen, the dust-choked Peanuts character, as a metaphor for humanity, in that Pig-Pen’s perpetual cloud was our airborne lead. But Patterson’s lead fixation did lead to two important results. First, when he’d cleaned up his lab enough, he came up with what’s still the best estimate of the earth’s age, 4.55 billion years. Second, his horror over lead contamination turned him into an activist, and he’s the largest reason future children will never eat lead paint chips and gas stations no longer bother to advertise “unleaded” on their pumps. Thanks to Patterson’s crusade, it’s common sense today that lead paint should be banned and cars shouldn’t vaporize lead for us
to breathe in and get in our hair.
Patterson may have pinned down the earth’s origin, but knowing when the earth was formed isn’t everything. Venus, Mercury, and Mars were formed simultaneously, but except for superficial details, they barely resemble Earth. To piece together the fine details of our history, scientists had to explore some obscure corridors of the periodic table.
In 1977, a father-son physicist-geologist team, Luis and Walter Alvarez, were studying limestone deposits in Italy from about the time the dinosaurs died out. The layers of limestone looked uniform, but a fine, unaccountable layer of red clay dusted the deposits from around the date of extinction, sixty-five million years ago. Strangely, too, the clay contained six hundred times the normal level of the element iridium. Iridium is a siderophile, or iron-loving, element,* and as a result most of it is tied up in the earth’s molten iron core. The only common source of iridium is iron-rich meteors, asteroids, and comets—which got the Alvarezes thinking.
Bodies like the moon bear crater scars from ancient bombardments, and there’s no reason to think the earth escaped such bombardments. If a huge something the size of a metropolis struck the earth sixty-five million years ago, it would have sent up a Pig-Pen-esque layer of iridium-rich dust worldwide. This cloud would have blotted out the sun and choked off plant life, which all in all seemed a tidy explanation for why not just dinosaurs but 75 percent of all species and 99 percent of all living beings died out around that time. It took a lot of work to convince some scientists, but the Alvarezes soon determined that the iridium layer extended around the world, and they ruled out the competing possibility that the dust deposits had come from a nearby supernova. When other geologists (working for an oil company) discovered a crater more than one hundred miles wide, twelve miles deep, and sixty-five million years old on the Yucatán Peninsula in Mexico, the asteroid-iridium-extinction theory seemed proved.
Except there was still a tiny doubt, a snag on people’s scientific conscience. Maybe the asteroid had blackened the sky and caused acid rain and mile-high tsunamis, but in that case the earth would have settled down within decades at most. The trouble was, according to the fossil record, the dinosaurs died out over hundreds of thousands of years. Many geologists today believe that massive volcanoes, in India, which were coincidentally erupting before and after the Yucatán impact, helped kill off the dinosaurs. And in 1984, some paleontologists began arguing that the dinosaur die-off was part of a larger pattern: every twenty-six million years or so, the earth seems to have undergone mass extinctions. Was it just a coincidence that an asteroid fell when the dinosaurs were due?
Geologists also began unearthing other thin layers of iridium-rich clay—which seemed to coincide geologically with other extinctions. Following the Alvarezes’ lead, a few people concluded that asteroids or comets had caused all the major wipeouts in the earth’s history. Luis Alvarez, the father in the father-son team, found this idea dubious, especially since no one could explain the most important and most radically implausible part of the theory—the cause of the consistency. Fittingly, what reversed Alvarez’s opinion was another nondescript element, rhenium.
As Alvarez’s colleague Richard Muller recalled in the book Nemesis, Alvarez burst into Muller’s office one day in the 1980s waving a “ridiculous” and speculative paper on periodic extinctions that he was supposed to peer-review. Alvarez already appeared to be in a froth, but Muller decided to goad Alvarez anyway. The two began arguing like spouses, complete with quivering lips. The crux of the matter, as Muller summarized it, was this: “In the vastness of space, even the Earth is a very small target. An asteroid passing close to the sun has only slightly better than one chance in a billion of hitting our planet. The impacts that do occur should be randomly spaced, not evenly strung out in time. What could make them hit on a regular schedule?”
Even though he had no clue, Muller defended the possibility that something could cause periodic bombardments. Finally, Alvarez had had enough with conjectures and called Muller out, demanding to know what that something was. Muller, in what he described as an adrenaline-fueled moment of improvised genius, reached down and blurted out that maybe the sun had a roaming companion star, around which the earth circled too slowly for us to notice—and, and, and whose gravity yanked asteroids toward the earth as it approached us. Take that!
Muller might have meant the companion star, later dubbed Nemesis* (after the Greek goddess of retribution), only half-seriously. Nevertheless, the idea stopped Alvarez short, because it explained a tantalizing detail about rhenium. Remember that all solar systems have a signature, a unique ratio of isotopes. Traces of rhenium had been found blended in the layers of iridium clay, and based on the ratio of two types of rhenium (one radioactive, one not), Alvarez knew that any purported asteroids of doom had to have come from our home solar system, since that ratio was the same as on earth. If Nemesis really did swing on by every twenty-six million years and sling space rocks at us, those rocks would also have the same ratio of rhenium. Best of all, Nemesis could explain why the dinosaurs died out so slowly. The Mexican crater might have been only the biggest blow in a pummeling that lasted many thousands of years, as long as Nemesis was in the neighborhood. It might not have been one massive wound but thousands or millions of small stings that ended the famous age of the terrible lizards.
That day in Muller’s office, Alvarez’s temper—easy come, easy go—evaporated as soon as he realized that periodic asteroids were at least possible. Satisfied, he left Muller alone. But Muller couldn’t let go of the serendipitous idea, and the more he pondered it, the more he grew convinced. Why couldn’t Nemesis exist? He started talking to other astronomers and publishing papers on Nemesis. He gathered evidence and momentum and wrote his book. For a few glorious years in the mid-1980s, it seemed that even if Jupiter didn’t have enough mass to fire itself up as a star, maybe the sun had a celestial companion after all.
Unfortunately, the noncircumstantial evidence for Nemesis was never strong, and it soon looked even scantier. If the original single-impact theory had drawn fire from critics, the Nemesis theory had them lined up to volley bullets like redcoats in the Revolutionary War. It’s unlikely that astronomers had simply missed a heavenly body in thousands of years of scanning the sky, even if Nemesis was at its farthest point away. Especially not since the nearest known star, Alpha Centauri, is four light-years away, while Nemesis would have had to inch within half a light-year to inflict its retribution. There are holdouts and romantics still scouring our cosmic zip code for Nemesis, but every year without a sighting makes Nemesis more unlikely.
Still, never underestimate the power of getting people thinking. Given three facts—the seemingly regular extinctions; the iridium, which implies impacts; and the rhenium, which implies projectiles from our solar system—scientists felt they were onto something, even if Nemesis wasn’t the mechanism. They hunted for other cycles that could wreak havoc, and they soon found a candidate in the motion of the sun.
Many people assume that the Copernican revolution tacked the sun to a fixed spot in space-time, but really, the sun is dragged along in the tides of our local spiral galaxy and bobs up and down like a carousel as it drifts.* Some scientists think this bobbing brings it close enough to tug on an enormous drifting cloud of comets and space debris that surround our solar system, the Oort cloud. Oort cloud objects all originated with our supernova birth, and whenever the sun climbs to a peak or sinks to a trough every twenty-some million years, it might attract small, unfriendly bodies and send them screaming toward earth. Most would get deflected by the gravity of the sun (or Jupiter, which took the Shoemaker-Levy bullet for us), but enough would slip through that earth would get pummeled. This theory is far from proved, but if it ever is, we’re on one long, deadly carousel ride through the universe. At least we can thank iridium and rhenium for letting us know that, perhaps soon, we’d better duck.
In one sense, the periodic table is actually irrelevant to studying the astrohistory of
the elements. Every star consists of virtually nothing but hydrogen and helium, as do gas giant planets. But however important cosmologically, the hydrogen-helium cycle doesn’t exactly fire the imagination. To extract the most interesting details of existence, such as supernova explosions and carboniferous life, we need the periodic table. As philosopher-historian Eric Scerri writes, “All the elements other than hydrogen and helium make up just 0.04 percent of the universe. Seen from this perspective, the periodic system appears to be rather insignificant. But the fact remains that we live on the earth… where the relative abundance of elements is quite different.”
True enough, though the late astrophysicist Carl Sagan said it more poetically. Without the nuclear furnaces described in B2FH to forge elements like carbon, oxygen, and nitrogen, and without supernova explosions to seed hospitable places like earth, life could never form. As Sagan affectionately put it, “We are all star stuff.”
Unfortunately, one sad truth of astrohistory is that Sagan’s “star stuff” didn’t grace every part of our planet equally. Despite supernovae exploding elements in all directions, and despite the best efforts of the churning, molten earth, some lands ended up with higher concentrations of rare minerals. Sometimes, as in Ytterby, Sweden, this inspires scientific genius. Too often it inspires greed and rapaciousness—especially when those obscure elements find use in commerce, war, or, worst of all, both at once.
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Elements in Times of War
Sam Kean Page 7
