By Dr. Robert Thorson

Meteorite

By Tim Gregory

Basic, 299 pages, $30

Struck with cosmic awe on the rocky, windswept summit of Maine’s Mount Katahdin, Henry David Thoreau asked in 1846: “Who are we? Where are we?”

Nearly two centuries later, British geologist Tim Gregory offers the best scientific answer I’ve yet found in his meticulously researched and eminently readable “Meteorite: How Stones from Outer Space Made Our World.” We are infinitesimally small “pieces of the Universe,” he writes, a “nebula come to life” whose “molecules evolved into minds” that miraculously “discovered the natural history of our Solar System and of ourselves.”

Since the dawn of Homo sapiens about 200,000 years ago, we’ve been addressing fundamental enigmas about our origins with myth, just-so stories and received wisdom. Only in the last 500 years, or so, though, have we been able to transcend our paleolithic mindsets to replace mystery with fact. The tool we use is science, a stepwise cognitive algorithm for making new information: observing, questioning, testing, concluding, communicating and repeating.

The ancient Greeks found answers in myth. The demigods Deucalion and Pyrrha, they said, created humans by throwing stones over their shoulders. In the modern scientific version, the atoms within our bodies fell to Earth in stones tossed over the sun’s shoulder. We call them meteorites, the rocky remains of asteroids that survive being burned up in the Earth’s atmosphere as part of the exploding streaks of light we call meteors.

Mr. Gregory, a specialist in the “wonder- ful and often bewildering field” of cosmochemistry at the University of Bristol, creates a gallery of science by hanging one gorgeous hypothesis after another on the walls. He writes with aplomb, his sentences backed by hard evidence and served with clarity, erudition and occasionally humor. “Mercury the scorched sphere, Venus the morning star, Earth the blue marble, and Mars the red planet. . . . Jupiter the giant, Saturn the graceful, Uranus the ice giant, and Neptune the bitterly cold.” Some pithy phrases, short and sweet, could be strung together as a poem to sum up the entire book:

From gas to dust.

From dust to worlds.

Creation; destruction; renewal.

All that is now Earth was once sky.

Two narrative strands are woven throughout: the “elegance of the scientific method and the storytelling power of meteorites.” Together they reveal a true story of continuous creation that is part certainty, and part chance. “Nature is systematic and predictable,” Mr. Gregory reminds us, governed by the fundamental laws of chemistry, physics and mathematics. But, simultaneously, the cosmology of the solar system — as with organic evolution and Earth history — is a contingent narrative of unexpected complexities and oddly differentiated outcomes — two million large asteroids, uncounted comets, eight planets and hundreds of moons — from a common time and place.

Earth is “just another rock,” in the grand scheme of things, “albeit a large one with a few quirks.” Humanity is one of those quirks, a small twig on a side branch of the mammalian family tree grown too big for its britches. The 7.8 billion souls presently occupying Earth, and their numberless ancestors, have so disrupted the planetary surface that the geological calendar warrants a new epoch, the Anthropocene. On the upside, we may be “the only way . . . the Universe can appreciate its own beauty,” reason enough to bounce out of bed in the morning.

An essential chart provides the opener: a family tree of meteorite types divided by source and material. In a prologue, Mr. Gregory draws a clever parallel between the histories of writing and cosmology, followed by two chapters that unfold a story of human discovery: a review of the archaeology, historical accounts, and early understanding of meteorites, beginning with a sun god who fell to Earth in the form of an iron meteorite about 4,000 years ago at the Field of the Sky, the Campo del Cielo, in the Chaco region of Argentina.

“Meteorite” continues with the larger story of scientific discovery: a review of cosmic history from the big bang 13.9 billion years ago to modernity. Within the glorified dirtballs that are chondrite meteorites, for example, scientists have discovered “bona fide stardust,” nano-diamonds with exotic chemistries and ages indicating that they formed in a star system beyond our own. Mr. Gregory describes how the stardust survived: explosive ejection from a parental star; ingathering within our nebula; heating in our protoplanetary disk; accretion into the first rocky bodies, the “planetesimals”; orbiting for 4.6 billion years; cataclysmic collisions; a fiery plunge through Earth’s atmosphere; and the destructive processing within human laboratories. Even reading this as a physical scientist, the complexity, beauty and improbability of the saga fully warrants his use of “miraculously” in descriptions; his dazzled reverence may explain why he consistently capitalizes Universe, Solar System and Nature, rather than following standard publishing styles.

The final three chapters are topical. “Star-tar” refers to the surprising abundance of organic compounds — the carbon-hydrogen molecules on which living systems depend — in the solar system. They are present in every carbonaceous chondrite; they coat the surface of some asteroids and moons. Disappointingly, the author concludes, “there is no convincing evidence within any meteorite that it once contained anything alive.” The penultimate chapter, “Pieces of the Red Planet,” covers interplanetary meteorite exchanges between Earth, the moon and Mars. Finally, “Calamitous Tales” describes the devastating disruptions caused by asteroid impacts on Earth in history and prehistory. Most famous is the 20-km-diameter chondrite that ended the reign of the dinosaurs 66 million years ago, opening the evolutionary door for our mammalian antecedents to gain dominance. I’m as thankful for that lucky Chicxulub event as I am anxious about a repeat performance.

“Meteorite” is a treasure for those who enjoy the stepwise narrative of scientific discovery. Mr. Gregory walks readers toward important conclusions, from initial observations to final proofs, like a detective sequencing clues in a mystery novel. This occasionally requires that he gets down to the nitty gritty: On a single page describing isotope mass spectrometry, he mentions the element oxygen 43 times, 32 using the symbolic letter O superscripted with a mass number, and the rest spelled out. Some passages prove heady: “Likewise, the fragments of orthopyroxene comprising the diogenites are identical to the orthopyroxene fragments found in the howardites.” But even the denser scientific language becomes comprehensible in context, and it contributes to a fully robust learning experience. Readers who are less science-minded can easily skip over such passages without losing the magnificence of the story.

Thoreau got his cosmological answers a decade after his sublime experience on Mount Katahdin. Having learned from personal astronomical observations that Earth is effectively one big meteorite, he asked that we reverence the stones of our home planet as much as we do those that fall from heaven.

“Meteorite” can help you do just that.