White-knuckle planet
By Kate Ravilious, New Scientist, 16 July 2005
Compared with being scorched on Mercury, suffocated on Venus or frozen on Pluto, our ride on planet Earth seems like a fairly safe option. But life here has been fraught with danger in the past. Close shaves with extraterrestrial hazards could explain some of the biggest catastrophes the Earth has seen, from the largest mass extinctions to the most widespread ice ages. Astronomers differ on what poses the greatest threat – whether it’s giant clouds of choking dust or magnetic neutron stars – but many agree that the alarming roller-coaster ride is far from over. Fasten your seat belts and take a deep breath as New Scientist reveals the five greatest astronomical threats to life on Earth. Kate Ravilious is our guide …
Once around the galaxy
Most serious cosmic hazards for life on Earth are a result of the sun’s regular passage through the Milky Way’s spiral arms. Our solar system completes an orbit of the galaxy every 250 million years. The spiral arms rotate too, but at a slower pace, because the clustering of stars in the arms tends to slow them down. “The spiral arms move slower than the stars and this creates a stellar traffic jam,” says Nir Shaviv of the Hebrew University of Jerusalem, who studies galactic hazards. About once every 150 million years, the sun catches up with a spiral arm and passes into it. For the few million years the solar system takes to traverse the arm, it’s at high risk from the first three dangers:
1: Supernovae
The Milky Way contains a hundred billion stars, many thousands of which are on their last legs at any given moment. Most die unremarkable deaths, but once every few decades, a particularly massive star runs out of fuel and explodes.
In less than one second the core of the star collapses. As matter is crushed together, repulsive quantum forces come into play that drive the surrounding material outwards in a cataclysmic explosion – a supernova. Radioactive isotopes and free electrons spew out in all directions, and their energy in turn generates X-rays and gamma rays.
This rain of high-energy particles, or “cosmic rays,” could have a powerful effect here on Earth – even posing a substantial threat to life if the explosion happened within 200 light years of us. No one knows exactly how likely such an encounter is in any given orbit of the galaxy, but we do know that it is much more likely to happen each time Earth passes through a spiral arm.
In 2003, Shaviv and colleague Jan Veizer from the University of Ottawa in Canada noticed that cold periods in Earth’s climatic history have tended to coincide with high levels of cosmic rays hitting the planet. They claimed that these could be coming from nearby supernovae.
To measure the ups and downs in the flux of cosmic rays, they turned to evidence from meteorites. When a meteoroid breaks away from its parent asteroid, its fresh surface is exposed to cosmic rays, which begin breaking down some of the meteoroid’s atoms to create lighter elements. So, the greater the proportion of light elements in a meteorite, the older it is. The ages of meteorites appear to be clustered around particular times, and Shaviv and Veizer argue that this is because there were peak times of cosmic ray intensity.
Shaviv and Veizer then compared cosmic ray levels with climate records that have been compiled by looking at ice cores and tree ring records. They say that periods of low cosmic ray activity match up with warm “greenhouse” times, while periods of high cosmic ray activity are linked to “ice-house” periods. “Approximately every 150 million years Earth has entered a spiral arm of the Milky Way and there has been a corresponding cold period with more ice at the poles and many ice ages,” Shaviv says.
How the cosmic rays could affect the climate in this way is not entirely clear. Shaviv thinks that the most likely mechanism involves increased cloud cover. When cosmic rays slam into Earth’s upper atmosphere they knock electrons off the atmospheric atoms that they hit, creating charged particles. “These extra charged particles could encourage cloud formation, resulting in more sunlight being reflected back into space and a cooler climate on Earth,” says Shaviv.
Other scientists have argued that the correlation between spiral-arm transits and low temperatures on Earth is not statistically compelling. Earth should pass through a spiral arm about once every 150 million years, but the cold snaps Shaviv and Veizer looked at were between 90 and 190 million years apart.
If the analysis is correct, however, Earth is heading for a relatively warm patch in just a few million years – but we may have to suffer a cold snap first. “In principle we should be witnessing a warmer climate now because Earth has left the Sagittarius-Carina arm, but in fact we also had to pass through the Orion arm, a temporary little spur next to the Sagittarius-Carina arm,” Shaviv explains. He and Veizer think that a short ice age is still a possibility, followed by a greenhouse period of around 50 to 60 million years, before we enter the Perseus spiral arm, and return to ice-house conditions. If the effect is as dramatic as Shaviv and Veizer think, there could be a global glaciation – the so-called “snowball Earth”.
RISK SUMMARY: supernovae pose their greatest threat to life on Earth whenever we pass through a galactic spiral arm, about once every 150 million years.
2: Giant molecular clouds
Clustering in the spiral arms of the Milky Way lurk more hazards for Earth: dense clouds of hydrogen gas, known as giant molecular clouds. Alex Pavlov of the University of Colorado, Boulder, and his colleagues believe that an encounter with such a cloud could lead to mass extinction and even a “snowball” glaciation, covering the entire planet in ice (New Scientist, 19 February, p.9).
Using an atmospheric climate model, Pavlov and his colleagues have calculated that the densest clouds are capable of filling Earth’s atmosphere with dust, choking out sunlight and sending the planet into an icy tailspin. Normally the atmosphere is protected from cosmic dust by the pressure of the solar wind – the stream of hot ions coming from the sun – but Pavlov reckons that a high-density cloud could flatten that wind. “The pressure from the hydrogen would overcome the pressure of the solar wind, exposing Earth to the interstellar dust,” he says. During the 200,000 years it would take our planet to travel through a cloud, the climate would rapidly fall into the grip of ice.
Pavlov estimates that around 1 in 30 clouds are dense enough to flood Earth with dust, and that the planet can expect to run into one of these high-density clouds approximately once every billion years. Geological evidence suggests that Earth has completely frozen over at least twice in its 4.6-billion-year history – around 600 million and 750 million years ago – but until now no one has come up with a convincing trigger for these big chills. Pavlov and his colleagues think that very dense clouds could be the answer. “Unlike other mechanisms a dense cloud would be intense, abrupt and long-lasting,” he says.
As if snowball glaciations weren’t bad enough, Pavlov thinks that moderately dense clouds could also be damaging to life on Earth. Fast-moving hydrogen nuclei could act as cosmic rays. “Even if the dust can’t get into the atmosphere, the cosmic rays from the cloud may be able to enter and start to break down ozone in the upper atmosphere,” he says. The sheer density of cosmic rays from the cloud could have a similar effect to a gamma-ray burst (see page 36), breaking nitrogen bonds and catalysing the decomposition of ozone.
Normally Earth is protected from cosmic rays by its magnetic field, but if a cloud coincided with one of the periodic reversals of the field, during which it is much weaker, the cosmic rays would pour in. “On average the magnetic field reverses every 200,000 years, and a collision with a cloud is likely to last around 1 million years, so it is plausible that most collisions would coincide with at least one magnetic reversal,” says Pavlov.
Calculations show that a combination of a cloud and a magnetic reversal would cause ozone levels to drop by at least 40 per cent – enough to allow significant amounts of ultraviolet radiation to reach the surface and trigger a mass extinction. And the probability of bumping into a moderately dense cloud is much higher: we probably hit eight every 250 million years.
So is there any evidence for past cloud collisions? John Lindsay from the Center for Advanced Space Studies in Houston, Texas, believes that soil samples from the moon back up the idea that Earth has passed through molecular clouds. Lunar soil is created by large meteoroids smashing into the moon and pulverising the surface rocks. However, this soil can become lumpy again if it gets bombarded by micrometeoroids and dust, which fuse the soil particles together again.
“In the cores that we drilled during the 1971 Apollo 15 lunar mission we found that there were variations in soil particle size going down through the stratigraphic column,” says Lindsay. It has proven extremely difficult to date this soil, but they have been able to spot regular peaks in the distribution of particle sizes, which they think were laid down around 250 million years apart – roughly the interval between major glacial periods on Earth.
“We speculated that these corresponded to the times when the moon and Earth were passing through the spiral arms of the galaxy,” Lindsay says. This fits with the fact that clouds are more common in the spiral arms and that the solar system is more likely to get bombarded with dust and cosmic rays when the sun is passing through those arms.
Back on Earth, Pavlov thinks that the influx of dust could have boosted levels of uranium-235 in rocks and he hopes that geologists might be able to spot this calling card today. “Several geologists are testing this hypothesis right now,” he says.
RISK SUMMARY: threat of global glaciation once every billion years, with more minor effects roughly every 30 million years
3: Comets and asteroids
Out beyond Pluto, at the edge of the solar system, orbits a spherical cloud of icy lumps known as the Oort cloud. Every so often the gravitational balance of the Oort cloud is disturbed, and one or more of these lumps begins falling as a comet towards the inner solar system, where it could hit Earth.
Closer to home, there are many small rocky asteroids in orbit around the sun, and if their orbits cross ours, they too can land on Earth with an almighty crunch. The age of the 65-million-year-old Chicxulub crater on the Yucatan peninsula in Mexico matches with the extinction of the dinosaurs. “If the Chicxulub impact happened again I have no doubt it would destroy our civilisation,” says Alan Hildebrand from the University of Calgary in Alberta, Canada, the scientist who discovered the Chicxulub crater.
So what are the chances of a similar event? “An impact of Chicxulub size is a 1-in-100 million-years event,” says Hildebrand. “But to cause the same degree of devastation as it did 65 million years ago, it must hit carbonate and sulphate rocks, which only cover around 2 per cent of the Earth.” The heat of impact would vaporise these rocks, pouring carbon and sulphur dioxide into the atmosphere. In contrast, the 100-kilometre-wide Popigai crater in Siberia is evidence for a major impact around 35 million years ago, and yet there was no major extinction event at that time. Location is the key.
At certain times across the ages, Earth is at greater risk of being bombarded. “During our voyage around the galaxy we also move up and down in the plane of our galaxy, with a period of around 30 million years. This perturbs the Oort cloud and increases the risk of cometary impact,” says Shaviv.
Meanwhile, traveling through a giant molecular cloud could also cause a violent redistribution of comets. That may be worth worrying about over the next few million years, but it’s the asteroids we need to watch for now. “Currently only 1 per cent of impactors are comets. The remaining 99 per cent are near-Earth asteroids,” says Don Yeomans at NASA’s Jet Propulsion Lab in California, who watches out for likely Earth impacts.
RISK SUMMARY: global threat once every 100 million years, with more minor impacts more frequently
4: Gamma-ray bursts
Not all the risks to Earth are due to our passage through spiral arms. In the late 1960s the US was operating military satellites equipped with gamma-ray detectors to keep a lookout for Soviet nuclear testing in violation of the atmospheric nuclear test ban treaty. But instead of getting signals from Earth, the detectors kept picking up short, intense signals from outer space.
Decades later, we are still not sure what causes these bursts, but astronomers’ best guess is that they betray a special kind of supernova from an especially massive star. Compared with an ordinary supernova, a gamma-ray burst is about loo times brighter, with tightly focused jets of gamma rays and cosmic rays pouring out of the rapidly collapsing star. They are also much shorterlived, usually lasting less than a minute.
Because gamma-ray bursts can be seen right up to the edge of the known universe, astronomers spot around one a day, compared with one supernova every few decades in our galaxy. Ordinary supernovae are way more common in the universe as a whole, but Brian Thomas from the University of Kansas in Lawrence and his colleagues think gamma-ray bursts could present a much greater threat to life on Earth. Using an atmospheric computer model they have calculated that if a burst happened within 6000 light years of Earth, it would strip away around 35 per cent of the planet’s ozone layer, frazzling life with three times the normal level of ultraviolet B.
“Gamma rays break nitrogen bonds in the upper atmosphere, creating nitrogen oxide compounds which act as a catalyst to break down ozone,” says Thomas. Although the burst would only last for a few seconds, its impact would be felt for many years afterwards.
“Our calculations showed that significant global ozone depletion would persist for over five years after the burst,” says Thomas. Such ozone loss could be devastating for life on Earth, the scientists believe. “We think that widespread extinctions would be likely because DNA is damaged by absorption at the UVB wavelength range,” explains Thomas.
And it is not just the gamma rays that are likely to be damaging. Charles Dermer from the US Naval Research Laboratory in Washington DC thinks that cosmic rays from the blast could also wreak havoc. “When the cosmic rays hit the upper atmosphere they make secondary particles such as muons, which would cause severe radiation damage at Earth’s surface,” he says.
However, Norman MacLeod from the Natural History Museum in London thinks that life is tougher than Thomas and Dermer suggest. “The ozone layer is largely a product of life on Earth and there have been variations in UV radiation throughout history. Just because life today would suffer with extra UV radiation doesn’t mean that life in the past – most of which was marine – couldn’t cope,” he says.
So has Earth ever been hit by a burst? There isn’t any direct evidence, but Thomas and his colleagues think that at least one of the major mass extinctions on Earth might have been caused by gamma rays. They are pointing the finger at the late Ordovician mass extinction, 443 million years ago, in which many animal and plant groups lost over half their species. One of the main pieces of evidence comes from the creatures that survived this event. “Recovering fauna were preferentially derived from either deep-water organisms or high-latitude dwellers, where the UVB flux would not have been so strong,” says Thomas.
Nonetheless, gamma-ray bursts certainly can’t account for all the mass extinction events Earth has seen, as close-range bursts simply aren’t frequent enough. “We would expect to have a gamma-ray burst close enough to the Earth to do some damage around once every billion years,” says Thomas.
RISK SUMMARY: once every billion years
5: Magnetars
Magnetars are some of the weirdest objects in the universe: a special kind of neutron star with a stronger magnetic field than anything else in the cosmos. They were discovered less than 10 years ago and are thought to be the remnants of supernovae. “Every so often they give out a flash of gamma and X-rays, lasting just half a second or so. This can be followed by a ringing signal that lasts for minutes,” says Neil Gehrels, an astrophysicist at NASA’s Goddard Space Flight Center in Maryland. When they flash, magnetars can give off as much energy in a single second as our sun does in an entire year.
Up until recently most astronomers looked upon magnetars simply as rather exotic curiosities, but on 27 December 2004, Gehrels and his colleagues observed a spectacular flare, 100 times brighter than any previously seen, coming from a magnetar named SGR1806-20, and it set them thinking about the kind of effect a magnetar could have on Earth. “SGR1806-20 is thought to be over 30,000 light years away from Earth, but if it was closer – say 10 light years or so – then the gamma rays would significantly deplete the Earth’s ozone layer,” says Gehrels. Just as with gamma-ray bursts, creatures and plants would sizzle under the extra dose of ultraviolet radiation.
Calculating our risk of getting caught in a magnetar flare is hard because magnetars are elusive and difficult to study. “It seems that they enter periods of activity where they signal every few days. This can last for years and then they seem to go dormant,” says Gehrels.
Magnetar flares are not as energetic as gamma-ray bursts, but Gehrels argues that they could present a greater threat to life on Earth. “Magnetar flares occur more frequently than gamma-ray bursts and they are more likely to happen close to the Earth,” he says. Astronomers have so far identified around a dozen magnetars, two of which are in our galaxy. With only this small sample to work on they can only speculate on the risk of one going off near us. “We are not sure how often giant magnetar flares occur, but over the last 500 million years there may have been one close to Earth, and it could have contributed to a great extinction event,” says Gehrels.
RISK SUMMARY: unknown
Linked from 16/9/2005 Journal