IN JULY 1969, THE CREW OF THE APOLLO 11
BROUGHT BACK NEARLY 48 POUNDS (22 KILOGRAMS) OF PRISTINE LUNAR ROCKS FROM THE MOST INCREDIBLE — AND EXPENSIVE — ROCK-COLLECTING EXPEDITION IN HISTORY. NASA HAD STATE-OF-THE-ART CLEAN LABORATORIES AND EQUIPMENT READY TO ANALYZE THESE SAMPLES IN UNBELIEVABLE DETAIL. IN THE SAME YEAR, HOWEVER, NATURE ALSO PROVIDED SEVERAL TONS OF COSMIC DEBRIS FOR FREE.
In February 1969, a massive meteorite rained a couple of tons of stones on the Mexican town of Allende, not far from the Texas border. And in September, over 200 pounds (90 kg) of cosmic material fell near the town of Murchison in Victoria, Australia, about 100 miles (160 kilometers) north of Melbourne.
The timing of these events was perfect. Geologists, chemists, and other scientists were better prepared to coax secrets from these otherworldly rocks than at any other time in history. For most of human history, the origin of these stones was an enigma. But by the mid-20th century, there was no doubt that these rocks had a cosmic origin. The bits of detritus that found their way to Earth in 1969 marked a milestone in our quest to unlock their mysteries.
EARLY VISITORS
Humans have seen rocks falling from the sky for thousands of years. One of the earliest potential recorded accounts dates to 1478 b.c., when, according to the Parian Chronicle, a “thunderstone” fell on the island of Crete. In 465 b.c., the Greek poet Pindar saw a meteorite land not far from the hill where he was sitting. And in 1492, a stone fell from the sky just outside the city of Ensisheim, France, becoming a marvel in Europe for centuries. It was widely believed that these stones formed in clouds and, when heavy enough, simply fell to Earth. Where else could these ordinary-looking rocks have originated?
But at the start of the 19th century, a number of events came together that changed the way people understood and studied these objects. On April 26, 1803, the villagers of L’Aigle, France, saw and heard an amazing fall. Over 3,000 stones were recovered, making the event impossible to ignore. Just two years earlier, the astronomer Giuseppe Piazzi had discovered the asteroid Ceres, clearly showing that there were objects other than planets circling the Sun. Geologists and chemists also were making great strides in understanding terrestrial rocks and developing techniques to reveal their structure.
Around the year 1800, the British chemist Edward Charles Howard acquired several suspected meteorites, including examples of each of the three main meteorite types recognized today: stony, iron, and stony-iron. Howard was the first to dissect and subject these extraterrestrial stones to chemical analysis. In 1802, he reported that all three types of meteorites had a high level of nickel, a composition unlike anything seen before in terrestrial rocks.
Two years later, a British mineralogist, William Thomson, tried polishing an iron meteorite with nitric acid, revealing a striking crystalline pattern. These became known as Widmanstätten lines after Count Alois von Beckh Widmanstätten, who made a similar discovery in 1808. No such pattern is seen in iron mined on Earth. These two men had discovered the ancient frozen crystal structure of iron meteorites, unchanged for billions of years. Leaping forward to the 20th and 21st century, meteorite research progressed thanks to new techniques and equipment used to study these cosmic visitors. These investigations included,
unexpectedly, an archaeological mystery. In 1911, British archaeologist Gerald Avery Wainwright discovered necklace beads made of iron in a 5,500-year-old Egyptian cemetery in Gerzeh, about 44 miles (70 km) south of modern Cairo. And when the British archaeologist Howard Carter opened the tomb of the pharaoh Tutankhamun in 1922, he found — among many beautiful artifacts — a magnificent ceremonial dagger with a gold handle and an iron blade.
The presence of these iron artifacts was conspicuous, since during Tutankhamun’s life 3,300 years earlier, Egyptians had not yet mastered the art of smelting iron and were still using bronze for their weapons. Chemical tests indicated a high level of nickel in the Gerzeh beads and Tutankhamun’s blade, pointing to an extraterrestrial origin.
However, in the 1980s, some archaeometallurgists suggested that nickel-rich iron ores found on Earth could have been the source of these artifacts.
Finally, in 2016, researchers reported in Meteoritics and Planetary Science a noninvasive examination of King Tutankhamun’s iron dagger that confirmed its meteoritic origins. The team used a portable X-ray fluorescence spectrometer, which looks at the wavelengths of fluorescing elements to determine their abundance. The researchers found the dagger was nearly 11 percent nickel and around 0.6 percent cobalt — whereas terrestrial iron produced before the 19th century rarely exceeds 4 percent nickel. They then compared this to iron meteorites found within a 1,200-mile (1,930 km) radius of Tutankhamun’s tomb and found a possible match — the Kharga meteorite, found in 2000 near the city of Marsa Matruh, Egypt. Using similar tests, the Gerzeh beads were shown in 2013 to be from an iron meteorite.
DIGGING IN
One of the primary devices used to study meteorites is the mass spectrometer. This instrument can detect atoms of specific elements and measure the abundance of their isotopes — atoms of the same element with the same number of protons but with differing numbers of neutrons. Measuring the abundances of various isotopes can be used to date samples. For example, carbon-14 is widely used to determine the age of organic material. Isotope analysis can also be used to dissect and study the atomic components of meteorites.
There is a downside: The sample is destroyed in the process. To probe a sample with a mass spectrometer, a small piece of meteorite is placed in a chamber, where it is heated until it vaporizes. The gas is then ionized, and the resulting ions are accelerated with an electric or magnetic field. Since different isotopes have different masses, they are deflected by differing amounts, indicating their relative abundance. This technique can be used to reveal some of the secrets locked up in meteorites.
The massive fireball that exploded over Allende in February 1969 provided plenty of material, scattering thousands of stones over a huge area. Over 2 tons were recovered, giving researchers — already primed by the impending Apollo missions — an abundance of material to investigate. As a result, the Allende meteorite has become one of the most studied meteorites in history.
The Allende meteorite is a rare primitive meteorite known as a carbonaceous chondrite. It is rich in carbon in the form of graphite, organic compounds, water, and amino acids. When sliced open, its interior is black and filled with beautiful white, snowflakelike inclusions. When analyzed with a mass spectrometer, these white specks were found to be the oldest known minerals in the solar system, an estimated 4.567 billion years. The calciumaluminum materials had to form out of material from the nebula that birthed the Sun, and at extremely high temperatures that could only be found in the early solar system. Studies using mass spectrometers also showed oxygen isotopes similar to those found in the Sun. The Allende meteorite is now considered one of the oldest objects ever found on Earth.
When the Murchison meteorite fell just two months after Apollo 11 returned from the Moon, researchers had yet another unusual carbonaceous