The Second Kind of Impossible

by Paul Steinhardt

  A year earlier, I had questioned the wisdom of using express mail, after Glenn had used it to send me some rare samples from the Smithsonian. But he just laughed, and told me everyone in geology uses some form of express mail, even when dealing with the most valuable minerals. I got the same answer from John Eiler at Caltech. Everyone convinced me that I was being paranoid.

  But then, the Air Express disaster happened. And suddenly, nobody was laughing anymore.

  From that point on, I refused to entrust any delivery service with our Khatyrka samples. Nothing would ever be sent by express mail again, not even international packages to Luca in Italy. I insisted that everything be delivered by hand, if not by me, then by a student or a colleague who happened to be traveling to and from Italy, California, Washington, D.C., or Princeton.

  Unfortunately, the lost samples were also the last samples we had left that had not been epoxied, so we were never able to perform an X-ray tomography test, the 3-D imaging experiment that might have opened up a whole new dimension in our study. It was, and remains, a major disappointment. But we are still considering using the technique to sort through additional meteorites in search of more metallic aluminum alloys and quasicrystals.

  QUASICRYSTALS UNDER PRESSURE

  We had to accept the fact that two—TWO!—of our most valuable samples were lost. We tried to move on as best we could, and turned our focus to finding new ways to determine how Khatyrka and its natural quasicrystals formed.

  Evidence from Grain #125, along with earlier studies, indicated that the Khatyrka meteorite had experienced a high velocity collision in space, the impact of which created ultra-high pressures. That raised an important question: Could we ever expect a quasicrystal that was buried in a meteorite, specifically icosahedrite, to survive extreme pressures of more than 50,000 times the atmospheric pressure at the surface of the Earth?

  If not, we would know that icosahedrite could never have been part of Khatyrka during the birth of the solar system, the theory Glenn favored, because it would not have survived the high velocity impact Khatyrka later incurred while traveling through space. Instead, we would know that it must have been created sometime after the last big impact experienced by Khatyrka when the pressure was much lower, as Lincoln believed.

  That issue struck at the core of our research. The stability of quasicrystals and the interatomic forces that hold their atoms together are questions of fundamental importance for condensed matter physicists and materials scientists. Stability tests had already been performed at lower pressures or temperatures, but no one had ever performed tests at the combination of high pressures and temperatures relevant to Khatyrka. Decades earlier, however, Dov Levine, Josh Socolar, and I had constructed cardboard and plastic models suggesting that there could, in principle, be interatomic forces to ensure stability under extreme conditions.

  This time, there was no need to put any of our actual samples at risk. The test could be performed with man-made icosahedrite quasicrystals. The fact that synthetic quasicrystals had become so easily obtainable was a reminder of how long I had been fascinated by the material. It was astonishing to think that quasicrystals were now so commonplace that man-made versions could be purchased inexpensively from a chemical company.

  Arranging for the high-pressure, high-temperature stability experiment itself was much more challenging. Very few labs are capable of performing such delicate tests with reliable accuracy. Luca identified Vincenzo Stagno and his colleagues Ho-Kwang Mao and Yingwei Fei at the Carnegie Institution for Science in Washington, D.C.

  The setup required three components: a tiny tungsten carbide “anvil” cell measuring less than an inch across for producing the pressure; a particle accelerator nearly three miles in circumference that can accelerate electrons to speeds that are 99.9999998 percent of the speed of light and bend them in circles that cause them to emit high-intensity X-rays; and advanced magnets and detectors that can aim the X-rays very precisely at material within the diamond cell and measure the X-ray diffraction pattern that is produced.

  Accelerators and detectors like this only exist at five places in the world. The Carnegie Institution has a dedicated high-intensity X-ray beamline at Argonne National Laboratories outside Chicago, which was where the trial experiments were performed. The final measurements were done at a similar facility called SPring-8 (Super Photon ring-8 GeV) in Hyogo Prefecture about 250 miles southwest of Tokyo.

  Our plan called for surrounding synthetic samples of icosahedrite, the same type of quasicrystal we had discovered in Khatyrka, with a graphite heating device and placing it in the tungsten carbide anvil cell, a box whose walls could be squeezed together by a press to crush whatever was inside. As the pressure and temperature gradually increased, the X-rays emitted by the electron beam were aimed at the quasicrystal so that any changes in the diffraction pattern could be continuously tracked. The exquisite measurements took one and a half years to plan and execute, and the results were well worth the effort.

  The findings were conclusive and indisputable. Icosahedrite did not transform, not even under the extreme conditions of pressure and temperature that Khatyrka experienced during a high-velocity impact.

  This meant that, in principle, icosahedrite could have been part of Khatyrka since its inception over 4.5 billion years ago, as Glenn had proposed, and subsequently survived all the impacts the meteorite experienced in space. Even so, the findings were not enough to prove Glenn’s theory was correct. Lincoln’s alternate explanation was still a possibility, as well. It was conceivable that the crystal metal alloys and the icosahedrite could have formed as a direct result of an intense impact in space. Icosahedrite might still turn out to be the direct result of an impact.

  NOBLE GASES

  We knew that parts of the Khatyrka meteorite dated back 4.5 billion years and that sometime after that there was an intense collision in space between Khatyrka and another meteor. But when?

  To address that issue, we needed to perform another extraordinarily difficult experiment with yet another group of highly trained specialists. Luca brought tiny bits of silicate from the Khatyrka meteorite to Henner Busemann, Matthias Meier, and Rainer Wieler at the Swiss Federal Institute of Technology in Zurich, shown left to right in the photo on the next page. Wieler had specifically designed the laboratory to measure rare helium and neon isotopes in meteorites. Matthias and Henner, his protégés, performed most of the experiments. Matthias was particularly captivated by the project and volunteered to lead the test.

  Helium and neon are known as noble gases, two of the six elements in the right-most column of the periodic table that are odorless, colorless, and have very low chemical reactivity.

  As they travel through space, meteoroids are bombarded by cosmic rays, energetic subatomic particles traveling at nearly the speed of light. The cosmic rays strike atomic nuclei in the rock, creating helium and neon isotopes with different numbers of neutrons than the helium and neon nuclei typically found on Earth. By measuring the percentage of atypical nuclei, they can gauge how long a meteoroid had been exposed to cosmic rays in space.

  If the Khatyrka meteorite had experienced a strong impact in space, all of its accumulated helium and neon would be lost as a result of the elevated pressures and temperatures created by the collision. If it then continued its journey through space, cosmic ray bombardment would resume and create a new population of atypical helium and neon isotopes. That process would continue for as long as Khatyrka remained in space. Once Khatyrka reached its ultimate destination and landed on Earth as a meteorite, the Earth’s atmosphere would protect it from any further bombardment.

  Matthias would begin by destroying the sample to extract the isotopes. What made the experiment so difficult was that he would then have to trap and isolate each and every helium and neon atom that emerged. Next, he would measure the concentration of those isotopes.

  When I visited the Swiss laboratory, it occurred to me that the sophisticated equipment, with its maze of criss
crossing pipes and tubes, looked a lot like a plumber’s nightmare. Once the sample was vaporized, the equipment would capture the gas that was created and transport it through a series of twists and turns, which were specially designed to ensure that only the helium and neon would survive the labyrinth. The microscopic survivors would be counted and categorized by the detector at the far end of the tubing.

  It took several years to set up, perform, and analyze the results of the highly delicate procedure. It was a calculated risk, because the sample would have to be destroyed in order to extract the isotopes. Fortunately, the gamble paid off magnificently. The Zurich test revealed nuanced information about Khatyrka’s history that we could never have obtained otherwise, and helped us create a timetable for its journey through space.

  Caltech’s NanoSIMs test had already established that some of the minerals in Khatyrka dated back to the birth of the solar system, about 4.5 billion years ago.

  Then, sometime between a few hundred million and a billion years ago, according to Zurich’s isotope test, Khatyrka was part of a large parent asteroid that underwent a powerful collision. The impact was sufficiently violent to kick out all of the helium and neon isotopes that had been created by cosmic rays up to that time. There may have been earlier major collisions, but this was the most recent one, based on isotope measurements recovered from the tiny sample.

  For the first time, we could estimate the date of the collision that had probably created the stishovite and the ladder of ahrensite and silica we had observed in our samples.

  The results also showed that the Khatyrka fragments were part of a meter-sized chunk that had broken off from its parent asteroid between two and four million years ago. Some event, perhaps a gentle collision with another asteroid orbiting the sun, caused it to detach and begin its slow and meandering path toward the Earth. Based on Chris Andronicos’s earlier evaluation and carbon dating, we knew that the chunk entered the Earth’s atmosphere about seven thousand years ago.

  The windfall of information was staggering. The results proved that the meteorite’s impact on Earth could not have been responsible for the stishovite and ahrensite. That collision was simply not powerful enough. If it had been, there would have been no rare helium or neon isotopes whatsoever detected in our sample.

  These results were independent confirmation of what we had claimed all along. If the impact on Earth was too gentle to get rid of the helium and neon isotopes, and therefore not strong enough to create the stishovite and ahrensite found in our sample, it could not have been powerful enough to create the aluminum alloys we observed in Grain #125. The only logical possibility that remained was that the metallic alloys were already part of Khatyrka before it had entered the Earth’s atmosphere. They were made in outer space and melted at some point during Khatyrka’s earlier travels through the solar system.

  This was one of those cases where Leave no stone unturned really paid off. When Luca and I first considered trying these difficult noble gas isotope experiments, we were concerned about having to sacrifice bits of our rare samples on a risky test that might yield nothing. But sticking to our philosophy, we had charged ahead despite the long odds and were rewarded with more information about Khatyrka’s history than we could have ever imagined.

  A NAMESAKE

  I was already impressed with everything we had learned about Khatyrka, but then came a new series of miracles provided by our L’Uomo dei Miracoli, Luca Bindi.

  By now, we had given up fighting with Air Express and had accepted the permanent loss of Grain #124 and Grain #126. But Luca had been keeping a secret from me. Small chips from Grain #126, each about the thickness of a fingernail, had broken off while he was packing the samples for shipment. The main piece had gone to its demise at the hands of Air Express. But Luca had recovered the small chips and stored them in a tube in his lab.

  When he finally had a chance to take a look at the leftover chips, Luca found something unusual. Most of the other grains included metallic minerals of aluminum and copper, but Grain #126 also had metallic minerals containing aluminum and nickel. Luca soon discovered a crystalline mineral containing roughly an equal mix of aluminum, nickel, and iron that had never been seen before in nature.

  As with all of our other new mineral discoveries, Luca meticulously prepared a proposal for the International Mineralogical Association. This time, however, he chose to hide everything from me. Luca had privately decided to name the new mineral “steinhardtite” in my honor. He consulted with other members of the expedition team, who secretly approved and agreed to coauthor the application. Not even my son Will, who joined the conspiracy, told me what was going on. Luca submitted the paperwork to the International Mineralogical Association and soon thereafter, steinhardtite was officially approved.

  I was deeply touched when Luca told me the news. Such a thing is a rare occurrence and a true honor, particularly for a theoretical physicist. It was especially meaningful for me that the whole thing was engineered by my teammates. Thanks to them, I am forever mineralized.

  The amount of natural steinhardtite available today is microscopic. The holotype sample, the tiny grain shown suspended on a thread in the image above, is now permanently housed at Luca’s Natural History Museum in Florence. A similar sample sits in a treasured box on my desk at Princeton.

  A SECOND QUASICRYSTAL?

  And then, L’Uomo dei Miracoli did it again. While trying to recover more steinhardtite from the microscopic chips of Grain #126, Luca discovered something even better—a second kind of natural quasicrystal. If someone had not known the story up to this point, they would no doubt say that finding two different kinds of natural quasicrystals in a single sample was impossible. But by now, we were used to the fact that virtually everything we were achieving was impossible.

  The second quasicrystal was different both chemically and geometrically from the first natural quasicrystal, icosahedrite. Chemically, the new quasicrystal was a mix of metallic aluminum, nickel, and iron, similar to steinhardtite, but with different percentages of the three elements.

  What was absolutely stunning about the new quasicrystal was its symmetry. Just as there can be crystals with different symmetries, we knew that there could, at least in principle, be natural quasicrystals with different symmetries. But none of us had ever expected to see a natural quasicrystal with a different symmetry in the same meteorite. Khatyrka was turning out to be an absolute marvel.

  The first-ever natural quasicrystal discovered several years earlier, icosahedrite, has six different directions along which one can observe the famously forbidden five-fold symmetry. The second natural quasicrystal, though, had only one direction with forbidden symmetry. And it was forbidden ten-fold symmetry.

  As shown on the top panel to the right, the structure is full of little rings of atoms that form decagons. The diffraction pattern on the bottom left panel of the previous page confirms the ten-fold symmetry along one direction. But other directions are periodic, like an ordinary crystal, as proven by the regularly spaced rows of diffraction spots on the bottom right panel.

  Finding a completely different type of quasicrystal was far, far beyond anything Luca and I could have ever imagined. Over Skype, we cheered our good fortune.

  Once again, Luca submitted the evidence to the International Mineralogical Association with a proposal for a new mineral. They rapidly voted in favor and accepted our proposed name of decagonite.

  Decagonite is a new mineral, but a familiar substance to quasicrystal experts. A quasicrystal with the same composition and symmetry had been synthesized by An-Pang Tsai and his collaborators in 1989, two years after they had created the world’s first bona fide example of a synthetic quasicrystal.

  No one had ever anticipated finding a decagonal quasicrystal in nature. But that was the feat Luca accomplished, all from a tiny, leftover chip from the long-lost Grain #126. Imagine what my talented colleague might have discovered if Air Express had not been so careless with the rest of the sample
.

  THE AMAZING GRAIN #126A

  Incredibly enough, Luca managed to squeeze a third discovery out of the remnants of Grain #126. One of those chips turned out to be so important that it was given its own designation. We called it Grain #126A, and it was chock-full of new evidence about the Khatyrka meteorite.

  Since the onset of our investigation, we had been looking for a sample in which metallic aluminum was in direct contact with, and chemically reacting with, the silicates normally found in carbonaceous chondritic meteorites. The best example we had managed to find up to this point was in Grain #125, which Chaney and Lincoln had been studying. Unfortunately, the grain’s mineral contacts had been broken during the epoxy process.

  We got an unexpected surprise with Grain #126A, shown in the image below.

  At first sight, it appears to be another example of a dog’s breakfast, the memorable phrase Glenn had once derisively used to describe the messy images we had recovered from the broken remains of Luca’s computer hard drive.

  Here, too, the image looks like a jumble. But at the microscopic level there is incredibly informative detail. The investigation of this single little scrap of a dog’s breakfast managed to occupy our team—Chaney, Lincoln, Luca, and me—for more than two years. At key moments, we reached out to colleagues from our expedition, Chris Andronicos and Glenn MacPherson, for guidance. We ultimately recruited even more specialized experts for the team from Caltech.

  In the sample, one can immediately identify many examples of the metallic minerals, which are the whitish and light gray materials. The silicate and oxide minerals are represented by the dark gray materials. Most importantly, we could tell by this image that the two materials had chemically reacted with each other.