Our planet has faced many dangers on its epic journey around the galaxy. The evidence of our turbulent history might lie buried on the moon
FOR billions of years, Earth has been on a perilous journey through space. As our planet whirls around the sun, the whole solar system undertakes a far grander voyage, circling our island universe every 200 million years. Weaving our way through the disc of the Milky Way, we have drifted through brilliant spiral arms, braved the Stygian darkness of dense nebulae, and witnessed the spectacular death of giant stars.
Many of these marvels may well have been deadly, raining lethal radiation onto Earth's surface or hurling huge missiles into our path. Some may have wiped out swathes of life, smashed up continents or turned the planet to ice. Others may have been more benign, perhaps even sowing the seeds of life.
As yet, this is guesswork. We cannot retrace our path through the galaxy's gravitational melee, still less calculate what incidents befell us where and when. Earth itself, its rocks constantly recycled by plate tectonics and remodelled by erosion, is remarkably forgetful of past assaults from space.
But a repository of our cosmic memories might be close at hand. The moon's soil and rocks endure undisturbed for aeons. Deep under the lunar surface there could lie an archive of our planet's voyage. What Earth forgets, the moon remembers.
A long time ago, in this galaxy but far, far away... the sky is packed with bright stars and glowing nebulae, far denser than today's tame heavens. But this scene is not to last. A great curving wave of stars picks up the solar system like a scrap of flotsam, sweeping it out into the empty galactic fringes, far from its forgotten homeland.
Today, the solar system travels a near-circular path around our galaxy, keeping a constant 30,000 light years between us and the seething galactic core. We once assumed most stars stayed in such quiet orbits for their entire lives. Our ride may have been more exciting. The characteristic spiral arms of a galaxy such as the Milky Way are waves of higher density, regions where stars and gas are a little closer together than elsewhere in our galaxy's disc. Their additional gravity is normally too weak to alter a star's path by much, but if the star's orbital speed happens to match the speed at which the spiral arm is itself rotating, then the extra force has more time to take effect (Monthly Notices of the Royal Astronomical Society, vol 336, p 785). "It's like surfers on the ocean - if they're paddling too slow or too fast they don't get anywhere. They have to match the speed just right, then they get pushed along," says Rok Roskar of the University of Zurich, Switzerland.
Roskar's simulations show that a lucky star can ride the wave for 10,000 light years or more. Our sun may be such a surfer. Some measurements imply the sun is richer in heavy elements than the average star in our neighbourhood, suggesting it was born in the busy central zone of the galaxy, where stellar winds and exploding stars enrich the cosmic brew more than in the galactic suburbs. The gravitational buffeting the solar system received then might also explain why Sedna, a large iceball in the extremities of the solar system, travels on a puzzling, enormously elongated orbit (arxiv.org/abs/1108.1570).
This is mere circumstantial evidence. But we might find more direct traces of disturbing incidents from the distant past...
The sky blossoms with brilliant, blue-white young stars, some still cocooned in a gauze of the gas from which they formed. The brightest shines with the light of 20,000 suns, but its brilliance is a warning sign. Soon the star will explode, banishing the night for several weeks. Unlike the life-giving warmth of the sun, this light will bring death.
In a nearby spiral arm of the Milky Way, more than 1000 light years away from our solar system's present position, lies the Orion nebula, a birthplace of giant stars. Our solar system must at times have drifted much closer to such stellar nurseries. To do so is to flirt with disaster. A massive star burns its fuel rapidly, and in a few million years its core can collapse, unleashing the vast energy of a supernova.
X-rays from a supernova just tens of light years away could deplete or destroy Earth's ozone layer, letting in harmful ultraviolet rays from the sun. High-energy protons, or cosmic rays, would continue to bombard Earth for decades, depleting ozone, damaging living tissue and possibly seeding clouds to spark climate change. Such convulsions might have triggered some of the mass extinctions that so cruelly punctuate the history of life on Earth - perhaps even hastening the demise of the dinosaurs 65 million years ago, according to a theory formulated in the 1990s.
Evidence for past supernovae is thin on the ground, although in 1999 German researchers found traces of iron-60 in south Pacific sediments (Physical Review Letters, vol 83, p 18). This isotope, with a half-life of 2.6 million years, is not made in significant quantities by any process on Earth, but is expelled by supernovae. The interpretation is disputed, but if iron-60 is a supernova's dirty footprint, it suggests a star exploded only a few million years ago within about 100 light years of us.
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