The Earth seems to have dodged the most deadly bullets so far. On another interpretation, Someone is perhaps just pushing us out of the way of them.
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.
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.
…….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 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.
Planetary scientist Ian Crawford of Birkbeck, University of London, suggests we can look to the moon to find clear evidence of such astro-catastrophes. “The moon is a giant sponge soaking up everything thrown at it as we go around the galaxy,” he says. Cosmic rays from a supernova will plough into the moon, leaving trails of damage in surface minerals that will be visible under a microscope and knocking atoms about to create exotic isotopes such as krypton-83 and xenon-126.
The darkness is coming. It starts with just a small patch of starless black, but slowly grows until it blots out the sky. For a half a million years, the sun is the only visible star. As alien dust and gas rains down and pervades our atmosphere, Earth is swathed in white cloud and gripped with ice; a pale mirror to the dark cosmic cloud bank above.
Interstellar gas permeates the Milky Way, but not evenly. The solar system happens now to inhabit an unusually empty patch of space, the local bubble, with only one hydrogen atom per five cubic centimetres of space. In the past we must have drifted through much denser gas clouds, including some more than 100 light years across in whose cold and dark interiors hydrogen forms itself into molecules.
In such nebulae, Earth may have caught a cold. …
We know Earth has suffered such episodes, including big chills some 650 and 700 million years ago. Their cause remains obscure. It could have been the weathering of mountains that pulled carbon dioxide from the air, or volcanic eruptions, or changes to Earth’s orbit around the sun – or a black cloud in space.
The faint red star seems harmless at first, a barely perceptible speck outshone by 10,000 other points of light. But it grows. In only a few thousand years, it waxes to become the brightest star in the sky. Out in the Oort cloud far beyond Pluto, giant balls of ice and rock begin to deviate from their delicately balanced orbits and move in towards the sun. Soon the skies teem with comets – ill omens for Earth.
The moon’s pitted surface records aeons of bombardment. Apollo astronauts found many samples of ancient melted rock, revealing that around 4 billion years ago the inner solar system was being pelted with massive bodies.
This “late heavy bombardment” is thought to have been caused by movements of the outer planets Uranus and Neptune disturbing asteroids in the Kuiper belt, where Pluto resides. Incidents in our galactic odyssey would have unleashed other storms of comets and asteroids. Passing stars or dust clouds might have triggered a one-off spike in the bombardment. A more regular pattern of new crater formation could reflect a repeated encounter on our path around the galaxy – passing through a particularly dense and unchanging spiral arm, for example.
To find out we would need to visit a variety of surfaces, taking small rock samples to determine their ages, and then making a careful census of craters to see how the impact rate has fluctuated. Buried soils could help, says Joy. “We might find fragments that would tell us what type of asteroids or comets were hitting the moon.”