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
Earth’s wild ride: Our voyage through the Milky Way
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.
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.
Although lunar soil is durable, over billions of years a constant rain of cosmic rays would obscure records of single events, even those as extreme as a nearby supernova. Crawford, together with Katherine Joy of the Lunar and Planetary Institute in Houston, Texas, and colleagues, thinks the trick will be to look for those relatively rare sites with a sequence of lava flows. When molten rock oozes out onto the surface and cools, it starts to collect traces of cosmic rays; if it is then covered over, it preserves a pristine record of the time it was exposed. Lava flows can be dated precisely by measuring the decay products of radioactive elements within them (Earth, Moon and Planets, vol 107, p 75).
Spacecraft have already spotted plenty of tempting lunar lava flows. So far they all date back more than a billion years, to a time when the moon was hotter and so more volcanically active. Crawford hopes to find smaller, more recent lava stacks, or layers of rock melted by large impacts. Buried within may be records of supernovae that we can compare with Earth’s fossil record to see if they match up with a mass extinction. Much more ancient rocks could tell us whether nearby supernovae were more frequent in the past - perhaps a sign that we once travelled through the denser, more eventful inner reaches of the galaxy.
And the moon may hold other memories…
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. Usually, the solar system’s interior is protected from harsh interstellar radiation by the solar wind, a stream of charged particles that flows deep into space, forming a huge electromagnetic shield called the heliosphere. When the interstellar gas gets denser, the solar wind can’t push as far, and the heliosphere shrinks. Above a density of around 1000 molecules per cubic centimetre, it will contract to within Earth’s orbit. That might happen every few hundred million years.
The accumulation of hydrogen in Earth’s high atmosphere would alter its chemistry, creating a reflective cloud layer, while dust could mimic the shading effect of sulphate aerosols from volcanic eruptions. Alex Pavlov of the University of Colorado, Boulder, says the dust alone could trigger a global ice age, or “snowball Earth” (Geophysical Research Letters, vol 32, p L03705).
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.
Then again, clouds may have had a happier influence on Earth. William Napierof the University of Buckingham in the UK has suggested that they could be staging posts for life, sheltering micro-organisms from cosmic rays and sprinkling them on to any receptive planet as it passes through (International Journal of Astrobiology, vol 6, p 223).
The moon could again tell us Earth’s tale. Up there, alien dust would have settled down to mix with the lunar soil. It would have a distinctive chemical signature, with high levels of uranium-235 and other isotopes that are generated in supernovae and scattered through space. Ideally, the dust would be entombed beneath a handy lava flow.
Getting to it won’t be easy. “We may need to sink a drill into an area known to have lots of lava flows,” says Joy. Setting up a drilling rig on the moon is beyond our present capabilities, but Joy points out that lava layers are exposed in some impact crater walls and long grooves on the lunar surface called rilles. A robotic probe could abseil down a crater wall and scoop out trapped soil from between the lava flows, Crawford suggests.
That soil could also hold mineral fragments that chronicle another chapter in Earth’s odyssey - a story of rocks and wreckage.
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.”
For the moment, we can only look at the craggy face of our old companion and wonder what stories it has to tell. If the world’s space agencies stick to their present plans, outlined in the 2011 Global Exploration Roadmap, “it ought to be possible to start accessing ancient deposits within a few decades,” says Crawford. Then, perhaps, we can start to write the definitive version of Earth’s epic odyssey.
