The last days of Earth
In the grand cosmic scale of the universe, our planet is doomed. All About Space explores what will occur during Earth’s final moments
In the grand cosmic scale of the universe, our planet is doomed.
All About Space finds out what will occur during our planet's final moments
Apollo 17 may have been the 20th century’s final crewed mission beyond Earth orbit, but its crew left us with one very public legacy – the iconic ‘Blue Marble’ image of planet Earth, which during the last 48 years has become one of the most reproduced images in human history. Admittedly that’s barely a pinprick when compared to the history of life on Earth as a whole: when you consider that the first ‘living’ molecules appeared some 3.8 billion years ago – and that by current estimates the last are likely to disappear from existence around 3 billion years from now – we’re still pretty much the new kids on the block.
Perhaps that explains our insecurity as a species and why we have spent so much of our recorded history predicting the end of the world as we know it – or at least the end of humanity’s time on Earth. Even now in 2020, cosmological events including predictable solar eclipses are taken by some people as evidence of an approaching Armageddon, and any new scientific advancements the rapture.
Yet very few scientists would suggest that our home will always be the beautiful ‘Blue Marble’ photographed by the Apollo 17 crew in 1972. Even assuming that we humans manage to hold out for significantly longer than most mammalian species on the planet have done so – say a few hundred million years – the small rock that we call home will inevitably become quite a different place during the next few billion years.
In the much-quoted ending to his poem The Hollow Men, T. S. Eliot suggested that the world will end “not with a bang but a whimper”. In truth it could be either, if by ‘bang’ you mean an unexpected, catastrophic event that comes
‘out of the blue’, contrasted with the ‘whimper’ of the passage of time and ongoing natural evolution. Either could cause existence-threatening devastation at a global level, but clearly these would take place over different time scales.
Certainly there’s evidence suggesting that numerous ‘bangs’ have already happened, even during the planet’s relatively recent geological history. Most famously there’s the Chicxulub impactor, the comet or asteroid now generally believed to have triggered the mass extinction of three-quarters of Earth’s plant and animal species – including non-avian dinosaurs – at the end of the Cretaceous Period.
The Chicxulub impactor is estimated to have been about ten kilometres (6.2 miles) in diameter. There are plenty of comparable objects in the Solar System, but while none are seen as an immediate threat to our planet, there’s still the chance that even ‘safe’ near-Earth objects will have their orbits dangerously deflected in the future. This isn’t just in the short term either; some 1.4 million years in the future, the star Gliese 710 will pass within 1.1 light years of the Sun. It’s been predicted that this will lead to a five per cent increase in the number of objects originating from the gravitationally disturbed Oort Cloud that are likely to hit Earth.
Indeed, the gravity of any massive object – a star, large planet or black hole – could prove catastrophic if it came sufficiently close to the Solar System.
Nor is it just asteroids; back in 2008, two computer simulations of long-term planetary motion in our own Solar System – one by Jacques Laskar of the Observatoire de Paris, the other by Konstantin Batygin and Gregory Laughlin of the University of California – suggested that Jupiter was still enough of a gravitational bully to potentially pull innermost planet Mercury out of its current elliptical orbit and throw it into a collision course with Earth. Other possible fates for the small planet were colliding with Venus, crashing into the Sun or being ejected from the Solar System altogether.
Before you get too worried, however, these models also suggested that there was only a one to two per cent chance of Mercury going for a wander before the Sun bloated in size sufficiently to swallow it up in a few billion years. However, given the global consequences of the Chicxulub impactor, it’s clear that nothing would survive the impact of a body some 4,879 kilometres (3,032 miles) in diameter. The last time anything on that scale happened – the hypothesised Mars-sized body astronomers call Theia smashing into the early
“There’s still the chance that even ‘safe’ near-Earth objects will have their orbits dangerously deflected in the future”
Earth some 4.5 billion years ago – the resulting debris was sufficient enough to form the Moon.
Scientists have also suggested a range of more distant cosmological events which could prove equally disastrous: from increases in cosmic dust impacting on comets and asteroids and leading to increased matter falling down on Earth to a statistically rare but decidedly possible gammaray burst from a nearby – at least in astronomical terms – supernova. Or, if that’s not big enough a threat, a super-luminous supernova – also known as a hypernova – which is ten or more times more powerful than the standard variety.
Some scientists have suggested that just such a hypernova within our own Milky Way Galaxy caused the second-largest extinction seen in
Earth’s history, the Ordovician-Silurian extinction events which led to the loss of about 85 per cent of all species between 443.8 million and 440.8 million years ago. The theory is that a sufficiently powerful and long burst of gamma rays hit Earth, stripping away at least half of the ozone from Earth’s atmosphere and exposing surface-dwelling life – including everything responsible for planetary photosynthesis – to dangerously high levels of ultraviolet radiation.
But what about the whimper? We know that Earth has undergone significant climate change during its existence, including numerous glacial ice ages and potentially at least one period around 650 million years ago when all the oceans were covered by ice – the so-called ‘snowball Earth’. A severe change in our existing environment – either hotter or colder – could yet undermine human civilisation. Yet even abrupt changes in the global climate regime are likely to pale in comparison to the natural evolution of our nearest star. The Sun which gives us life is as equally likely to end it.
James Lovelock is best known for the ‘Gaia hypothesis’, which he developed with support from microbiologist Lynn Margulis in the 1970s. This proposed that the evolution of our environment and all life on Earth is part of a large, generally selfregulating physiological system. This even included some form of climate control, which Lovelock suggested had ensured the maintenance of an “equable climate” since life had begun.
Yet as far back as 1982, in a scientific paper written alongside Michael Whitfield of the Marine Biological Association, Lovelock had considered ways in which levels of the greenhouse gas carbon dioxide had helped planet Earth to so far resist the warming tendency of the Sun. It was a system that Lovelock suggested might one day be in danger of breaking down.
Compared with when it first started to shine, the Sun we see in the sky today is thought to be about 30 per cent brighter thanks to 4.5 billion years worth of hydrogen in its core being converted into
“Earth has undergone significant climate change during its existence, including numerous glacial ice ages”