Meteorite enthusiast Greg Hupé holds a small meteorite he found from the April fireball in southern Wisconsin. Hupe has been fascinated with meteorites for 14 years, calling his hobby "an obsession". Credit: Jack Schrader and Michael Johnson
Two years ago Greg Hupé of Lake Placid, Florida, was in Morroco pouring over a meteorite he was considering buying from a contact there. As he turned the 7 1/2-lb. rock in his hands examining it with a loupe, he thought it looked different from the others in the lot. "It was familiar, but very unfamiliar," said Hupé. "It glittered."
He took a gamble and purchased the rock, brought it back home and sent it out to a lab at the University of Washington for testing and classification. While a long-time meteorite collector and hunter like Hupé can make an educated guess as to what kind of space rock he has in his hand, it takes chemical analysis to determine the exact type.
The main mass of the possible Earth-related meteorite NWA 5400 brought to light by Greg Hupé. It weighs about 7 1/2 lbs. Credit: Greg Hupe
Scientists break down meteorites into three broad groups: irons, stony-irons and stones. Irons, the most familiar of the groups, derive from the cores of asteroids that were large enough to melt like the Earth. Heavy elements like iron and nickel drained into the core, while lighter materials floated to the top to form the mantle and crust. Later, collisions between asteroids cracked some of them wide open, liberated iron fragments from the core as well as rocky materials at and below the surface. Stony-iron meteorites are derived from a zone between core and mantle as well through impact-related melting between colliding asteroids. Solar system bodies that have melted and separated into separate zones of crust, mantle and core are referred to as being differentiated. Earth, all the planets, the larger moons and asteroids are all differentiated.
A small, part slice of NWA 5400 shows an interesting sparkly mix of metal and interlocking crystals of olivine. Photo: Bob King
From there, a meteorite is further classified according to the amount and kinds of minerals and metals it contains. Hupé’s rock was classified broadly as an achondrite (AY-kon-drite), a stony meteorite from beneath an asteroid’s crust that suffered heating, melting and crystallization. Its primary ingredient is olivine, a lovely green crystalline rock found in abundance in Earth’s mantle. Another name for olivine is peridot, often sold in jewelry stores as the "birthstone" for the month of August.
This is a very, very thin slice of NWA 5400 — called a thin section — viewed under a special lighting arrangement through a microscope. Many of the strikingly colorful crystals are composed of olivine. Credit: Greg Hupe
Achondrites are rare enough to put a smile on any collector’s lucky enough to hold 7 1/2 lbs. of it, but this one turned out to be much more unusual. The lab contacted him after their analysis, and it would be safe to say that Hupe was blown away by what he heard. The scientists discovered that his stone was closely related to both moon rocks and Earth rocks but was neither of lunar origin nor from the current Earth. Based on detailed analysis of oxygen isotopes present in the sample, the researchers offered the possibility that the sample, now officially named Northwest Africa 5400 or NWA 5400, could be a terrene meteorite or one derived from the ancient Earth around the time of its formation 4.5 billion years ago. Other possibilities include another differentiated, Earth-like body that formed in the Earth-moon neighborhood or a fragment of the material that coalesced to form the moon.
"I was cautious when I heard the news, but wow, are you sure?", he asked the scientists. They told him that NWA 5400 needed further study and sent it off to another lab.
Let’s see, what might be in Earth’s neighborhood over four billion years ago? Ever hear of the Giant Impact Hypothesis? Moon rocks are very similar in composition to rocks found in Earth’s mantle but very deficient in metals like iron and nickel. Both also have the same amounts of three different oxygen isotopes. Almost all the oxygen atoms plants produce and we breathe have eight protons and eight neutrons in their nuclei. Scientists add them up and call the most common form of O2 oxygen-16. Other varieties or isotopes exist like oxygen-17 and oxygen-18. These have the same number of protons but additional neutrons in their nuclei.
In this artist’s impression, we see Mars-sized Theia (left) delivering a glancing blow to Earth approximately 4.4 billion years ago when our planet was in the latter process of formation. Most of the ejected material came together through gravity to form the moon. Scientists call the scenario the Giant Impact Hypothesis. Credit: NASA/Joe Tucciarone
Planets and asteroids have their own individual mix or signature of these three forms of oxygen. One of the main reasons we believe the moon derives from the Earth is because both Earth and moon rocks have identical proportions of O-16, O-17 and O-18. To explain this as well as other particulars about the moon, scientists hypothesize that a Mars-sized body named Theia (mother of the moon goddess Selene) struck Earth with a glancing blow some 50 million years after our planet formed. The impact blasted millions of tons of molten rock from both Earth’s and Theia’s crust and mantle into space. The remains of Theia were absorbed by our planet while much of the jettisoned material gathered into a ring around Earth and soon coalesced into the moon. Because our planet’s metal core was spared by the imapct, the moon received very little iron. This accounts for what we know today of moon’s composition.
See where we’re going with this? Greg’s meteorite could be the very first remnant found from the impact event that created the moon. "It’s important for what it means to our solar system. How cool is that!" said Hupé, adding, "I feel smiled upon by the meteorite gods."
Hupé, who’s been a guest observer for the Catalina Sky Survey and helped co-discover several Earth-approaching asteroids, has long been fascinated with where meteorites come from. He’s 47 years old and retired from a job as a partner in a computer corporation. When asked what he likes to do with his time, Hupe is direct: "I go find rocks. I’m always after something new at the risk of great loss, but it’s all worth it when a meteorite like NWA 5400 comes along."
More tests are in the works for NWA 5400 especially relating to its precise age. In the meantime, we might envision this scenario. Theia forms in the Earth’s vicinity and collides with our planet shortly after its formation. Most of the blasted material gathers together into the moon, but some is ejected out to the asteroid belt. More recent collisions in the asteroid belt break off chunks of what would become NWA 5400 and hurl them in the Earth’s direction where a piece eventually lands in the Sahara Desert in northwest Africa. A chunk of Earth returned home after a four billion year journey? Maybe. Time and testing will tell.
I caution that this is just one possibility and though plausible, more science needs to be done. For now we can sit back and enjoy the tale unfold as a bit more of our planet’s history comes to life thanks to the efforts of a dedicated hunter and the skills of some of the best scientists in the world.