“When you’re in love, you want to tell the world” – Carl Sagan
It’s time to write about science.
I did a Google search for the phrase “loveliest science story” and found no hits. This astounded me. If Carl Sagan was right, and I believe he was, then we write about science because we love its intricacies, its hidden connections, its poetry. What I’m about to tell you may not be the loveliest science story ever told, but at least now Google will have something to return.
Something less than one million years ago, while our human ancestors in Africa were only just beginning to raise their curious faces to the sky in wonder, a female green sea turtle, heavy with eggs, was blown off course by a prodigious storm. She washed up, quite by accident, on a dark, deserted, volcanic beach, lost and disoriented. Even so, a deep instinct stirred within her. The turtle crawled up the gentle slope of the beach, the ocean waves lapping at her domed shell and powerful flippers. She grunted and strained with the effort of moving her heavy body through the sand. Up until this moment, virtually her entire life she had been buoyed by ocean water in a world without weight. Finally, as she passed the high tide mark, the turtle chose a spot.
Those same flippers, which had demonstrated such power and fortitude in propelling the turtle up the beach, now became supple and delicate digging tools. The turtle deftly moved those flippers in and out of the sand to build a perfect nest, centered by a compacted, pear-shaped egg chamber just large enough to hold her clutch of around one hundred eggs. Soon those eggs came, soft and white, about the size and shape of ping pong balls. They passed from her body and into the nest, one after another, building slow layers below the sand.
As she worked, the mother turtle’s eyes filedl with salty tears. Her salt glands, hard at work protecting her from the salt of the sea, on the beach served a second purpose, keeping the turtle’s eyes clear of the stinging, abrasive sand that flew all around her as she worked. Covering her nest with that same sand, the sand that was once her own surrogate mother and now will cradle the babies she herself will never see, the mother turtle finally crawled back down the beach and disappeared into the surf, into the ocean of space and time that forever separates us from her.
Return to today. Off the coast of Brazil, on the shallow coastal banks covered in the green ocean grass that turtles so love to munch, a female green turtle pauses from her feeding. She swims upward the short distance to the surface, pokes her head above the warm, blue-green water, and takes a deep, filling breath. A scent in the air, the angle of the Sun, the taste of the water, all these things tell her that the time has come for her to return to the beach of her birth. It is time to return to Ascension.
For this turtle is a far-distant descendant of our million-year-old mother; her genes were among those ensconced in those ping-pong-sized eggs on a deserted beach so many generations ago. Like all members of her species, this female feels an inescapable urge to return to her home, to build her own nest, to perform her part in the powerful play to come. She begins swimming east.
Now flash back to four and a half billion years ago. Our planet is a hellish world, partially-molten rocks tortured by radioactive decay and pummeled by hurtling projectiles from the detritus of the newly-formed Solar System. The planet is so hot that solids cannot crystallize; the elements within the remain free to move and migrate, buffeted by chemistry and their own inherent mass. These heavy elements had all been born in the inner turmoil of enormous, dying stars and blown into this galactic backwater by explosions that had lit up the entire galaxy. Now they are about to play a role in a still more amazing story.
The iron and nickel, the most common of the truly dense elements, slowly sink into the young planet’s interior. There, spinning with their own internal motion, stirred by the internal heat of collapse and radioactive decay, ripped and rubbed into positive and negative electrical regions, over long stretches of time in processes that are still but poorly understood, our planet’s core forms a magnetic field that saturates our world to the surface and beyond.
A magnetic field is an extraordinary thing. The result of moving electric charge, it is in fact a manifestation of just how fast, and just how small, the atomic world can be. Any two charged particles feel either an attraction or a repulsion for one another. But when those charged particles move, an additional force arises – a force that comes directly out of Einstein’s special relativity. Electric charges moving relative to one another change time, change space, and result in a “new” force that shows up in our everyday world as magnetism. Every time you hang a note on the refrigerator with a magnet, you are making use of Special Relativity!
But magnets aren’t useful only on refrigerators. They also give creatures like sea turtles a sense that we humans lack. Lying within the head of our female green sea turtle are a few tiny flakes of magnetite, elemental iron in which the charged particles are aligned with one another, and with the ancient magnetic field of the Earth. This built-in compass will help our female turtle make one of the most amazing migrations in the animal world.
Ascension island lies in the center of the Atlantic Ocean, almost 1500 miles east of the Brazilian coast. It bubbled up just over a million years ago from the Mid-Ocean Ridge, the seam along the Earth’s crust where new ocean floor, and the occasional island, are born. Ascension’s very existence, at just over a million years still one of the youngest spots on Earth, shows how dynamic, how energetic, our planet remains even after four and a half billion years. Radioactive decay continues to keep our planet hot, active, churning just below a deceptively placid surface. We humans and turtles live on a young, vibrant, ever-changing world.
Using the cues of ocean currents, the Earth’s invisible magnetic field, and perhaps other clues we still don’t understand, both male and female green sea turtles make their yearly trek from their Brazilian feeding grounds to this tiny spot of land amid miles and miles of empty ocean. Once there, far from any food source or familiar landscape, they mate. The females, who make this trip perhaps once every three or four years, but build five or six nests with each journey, then repeat the drama played out by their distant ancestor, that first mother turtle, heavy with eggs, who crawled up this lonely beach to found a new green sea turtle society.
Blanketed under the nurturing sand, the young turtles grow and change. Everything the turtles need is provided for them within the egg – everything but the air that slowly passes back and forth through the thin, porous, but tough eggshell. As long as the nest remains dry and covered in sand, in about 8 weeks the babies will cut their way out of their shells with a special “tooth” (no turtles have true teeth – this projection is quickly reabsorbed after the babies hatch). For a few days the turtles gather under the sand; then, through some signal known only to the babies, the turtles break from the nest as a group in an event called a “boil.” Down the beach the babies go, doing their best to avoid divebombing gulls and ghost crabs waiting in ambuscade. Finally they reach the surf and the ocean that will be their home.
Many more dangers await; predatory fish, flipper-slashing sharks, and human fishing lines and nets all take their toll. Perhaps one in a thousand will survive to adulthood. The babies that survive the initial onslaught make their way to a unique ecosystem in the center of the North Atlantic Gyre, just north of Ascension.
There the young turtles hide in the seaweed of the Sargasso Sea, eating the tiny crabs and fish they find there. They grow. They learn. They begin to feel the tug of the western Atlantic. When they are large enough, these same turtles will leave their open ocean home and head for the coast of Brazil. There they find acres and acres of shallow water, the perfect environment for turtle grass, that green carpet that covers the ocean floor off Brazil’s north and east coasts. Here the greens take up their adult vegetarian lifestyle, essentially harvesting sunlight from the shallow ocean floor. Living and eating side-by-side with other Atlantic green turtles, these Ascension turtles remain a distinct population by breeding only in the waters surrounding their home island.
And then, when the time is right, these marvels of evolution feel the tug once more and begin their trek to faraway Ascension. It is an island that marks the time, formed by a restless Earth that through its interal churnings creates and maintains that very magnetic field by which the turtles accomplish their astounding migration.