Or what if our home planet had two moons, or none, or was a moon itself?
IN THE quirky H. G. Wells tale The Man Who Could Work Miracles, a character called George Fotheringay discovers that he has supernatural powers. Egged on by the local vicar, Fotheringay uses his gift to miraculously improve his village by night, mending buildings and reforming drunks. Then he realises there is a way to buy more time for good deeds before sunrise: simply order the Earth to stop spinning.
The moment Fotheringay gives the command, all hell breaks loose. “When Mr Fotheringay had arrested the rotation of the solid globe, he had made no stipulation concerning the trifling movables upon its surface,” Wells wrote. “Every human being, every living creature, every house and every tree – all the world as we know it – had been so jerked and smashed and utterly destroyed.”
Advertisement
Wells loved to play God with the planets in his fiction, but such fantasies are good for more than entertainment. Implausible scenarios of the sort Wells cooked up are worth exploring because they give clarity to what makes the real world tick, says Neil Comins, an astrophysicist at the University of Maine in Orono. In his new book What if the Earth Had Two Moons? Comins gives the Earth some other-worldly makeovers to see how different our environment could have been. “Thinking through what could happen or might have happened gives a much better perspective on how things actually are,” he says.
So what strange scenarios would drastically alter our world, and what would be the upshot?
Toying with the moon is a good place to start. The moon has had an enormous influence on our planet, not least in the cataclysmic events that led to its formation. The consensus is that a Mars-sized body, often called Theia, slammed into the Earth around 4.5 billion years ago, spraying debris into orbit. This eventually clumped together to form the moon at about a tenth of its current distance, roughly the same altitude as most of today’s communications satellites. Around this time, the Earth would have rotated on its axis once every 8 hours or so, but gravitational interactions between the Earth and moon, including effects to do with the tides, have since slowed that to the familiar 24-hour day.
So what if the moon had not formed? The only significant tidal forces on Earth would come from the sun, which would have increased the length of a day from 8 to about 12 hours. You would weigh less too, since about 10 per cent of the mass of the Earth is thought to come from the remnants of Theia that it absorbed, so gravity would be that much weaker.
Unique life forms
Without the moon, life might not have taken hold as quickly as it did. The newborn moon was so close to the Earth that it would have raised tides 1000 times higher than today’s. Those vast tides probably caused the oceans to scour the continents, enriching the sea with minerals and helping to create the “primordial soup” that gave rise to life. Comins suspects life would eventually have emerged even without the moon, but says there would not have been animals adapted to live in tidal pools, or to hunt or navigate by moonlight. What’s more, with no lunar gravity to stabilise the Earth’s rotation, our planet could have ended up spinning on its side, like Uranus. Over the course of a year, sunlight would drift from one pole to the other, then back again. “Virtually every living thing would have to migrate on such a world,” says Comins. “Life would have to follow the sunlight.”
The impact of Theia could, in theory, have created more than one moon. Would that have made a difference? Probably not. Even if the debris had formed two lumps, gravitational effects would have made them collide long before complex life forms appeared on Earth some 600 million years ago.
The only way the Earth could support a second moon today – let’s call it Moon2 – would be if it had been captured recently from a pair of bodies that wandered into its vicinity. This encounter could have left Moon2 settled into a stable Earth orbit, as long as a large chunk of its kinetic energy was transferred to its companion, which would have zoomed off into space. The gravity of Moon2 and its companion would have caused havoc as they approached Earth, triggering mammoth tidal waves and volcanism. The skies would have been dark with dust, and there would almost certainly have been a mass extinction of life. Things would have calmed down eventually – probably within a few years of the departure of Moon2’s companion.
Suppose that Moon2 was the same size as the original moon and that its orbit was in the same plane and direction, but half as far away from Earth. Any surviving land dwellers would be treated to a spectacular second moon twice as wide and four times as bright as the first, circling Earth once every 10 days. When both moons were full, it would be easy to read a book at midnight.
Not that it’s all good news: Moon2 would be spitting lava. The ever-changing gravitational forces due to the Earth and our original moon would massage Moon2’s interior, keeping it molten and making lava spew out through volcanoes and cracks in the surface. “How spectacular that would be,” says Comins. “You’d also see glowing rivers of lava on this moon.” Some would be ejected fast enough to escape and fall to Earth, making clear nights a glittering show of shooting stars.
The two moons would be destined to collide. While tidal interactions make our moon recede from Earth by 3.8 centimetres each year, Moon2 would recede faster, catching up with the original roughly 1.5 billion years after capture. Their catastrophic collision would send debris raining down on Earth, likely causing another mass extinction.
Another of Comins’s scenarios has a moon orbiting Earth the “wrong” way, rather than in the same direction as the Earth rotates. Given the manner of our moon’s birth, that would be impossible: if Theia had carried enough momentum to splatter a moon into orbit in the opposite direction, it would almost certainly have destroyed Earth in the process, Comins argues.
So a moon orbiting in the opposite direction – called Noom, say – could only exist if it had been captured from a pair of passing bodies. According to Comins, that is unlikely, but not impossible. Let’s say Noom has the same mass as our moon and orbits at the same distance with the same period, albeit in the opposite direction. The previously moonless Earth would now have a faster rotation rate, turning once in 12 hours.
The complex gravitational interplay between the two worlds would lead to Noom gradually spiralling towards Earth, its orbit becoming ever faster and possibly more elliptical. Meanwhile, Earth’s rotation rate would slow to zero, before eventually spinning in the opposite direction.
As the Earth’s rotation ground to a halt, the days would become as long as the planet’s year, causing extreme heating on the daylight side and extreme cooling elsewhere. But the slowing would occur over billions of years, so animals would have time to evolve migratory patterns to follow comfortable climates. “There could easily be life on the boundary, where the sun is on the horizon,” says Comins.
After that, the sun would rise in the west and set in the east as Earth began to spin in the opposite direction. As Noom got closer, shorelines would be ravaged by tides reaching 3 kilometres high. Eventually, Noom would get so near that it would be torn apart by gravitational tidal forces, disintegrating into a ring of boulders around 4500 kilometres above the Earth. Some of these would give the planet a good pummelling – perhaps severe enough to cause a mass extinction.
So much for monkeying around with the moon. But what if the Earth was not a planet but a moon akin to the Earth-like moon Pandora in the film Avatar? Imagine the Earth in orbit above the equator of a clone of Neptune – Neptune2 – and that both bodies rotate about an axis perpendicular to the plane of the solar system. For this satellite Earth to be warm and habitable, the system would have to orbit the sun at roughly the same distance as the real Earth does now.
Planet shine
Over a few billion years, the satellite Earth’s rotation would become synchronised with its orbit, so that one face would be permanently turned towards Neptune2. If the satellite Earth were orbiting around 300,000 kilometres from the centre of the planet, it would have an orbital period and a day lasting just over 100 hours. The view of Neptune2 from Earth would be spectacular, spanning 9 degrees of sky, or 18 times the angular size of the full moon.
If you lived in the centre of the Neptune2-facing side, the planet would be directly overhead, and half of it would be lit up when the sun rose. It would shrink to a crescent before eclipsing the sun for about 2 hours around noon, the stars appearing in an inky black sky. Then Neptune2 would gradually grow through another crescent phase as night fell to a “full Neptune2” around midnight, shining about 2800 times brighter than our moon ever gets. Midnight on this side of Earth would be far brighter than noon. “Effectively, there would be two periods of daylight,” says Comins. Animals living on the Neptune2 side and the far side would experience different day-night cycles and would have different body clocks.
The long days and nights on the satellite Earth would create daily temperature swings roughly twice as big as those on our Earth so life would have to adapt. Worse, Neptune2’s gravity would make it a magnet for asteroids and comets and the captive Earth would be at risk from cosmic crossfire. “Neptune2 would pull debris onto it, and that is also going to potentially threaten the Earth,” Comins says.
Now imagine drastically altering conditions on Earth, not by tinkering with the moon or having the Earth orbit another planet, but simply by giving the Earth a thicker crust. The average thickness of the Earth’s continental crust is about 40 kilometres, while the oceanic crust is 7 kilometres. What would the world be like if the crust was, say, about 100 kilometres thick on average? That could have come about if the young Earth had been very dry.
Most of the water on Earth is thought to have been delivered by icy comets and asteroids. This water makes the Earth’s crust and upper mantle – its lithosphere – flexible enough to be pushed aside when blobs of magma carry heat up from the Earth’s interior. “Water provides lubrication for the movement of the crust,” says Comins.
Had comets brought water to Earth much later, its crust would have become much thicker. That’s because, over time, magma blobs would have stacked up and congealed beneath the lithosphere. Heat would build up in the Earth’s interior until, eventually, something would give, causing parts of the lithosphere to melt all the way to the surface every few tens of millions of years. As these tracts of land or seabed turned molten, they would release heat to space over a few hundred years before solidifying again.
These meltdowns would release a cocktail of toxic gases and erase all features in the vicinity. “Perhaps thousands of square kilometres are going to become totally uninhabitable,” says Comins, who based his model on the parched crust of Venus, which lacks tectonic plates. “Venus has seen these melts, which is why the planet has so few craters – it has been resurfaced.” He adds that any successful species on Earth would need the ability to sense when the land under its feet was about to melt, and flee: “Otherwise it’s just going to be plain wiped out.”
Finally, back to the miracle worker in H. G. Wells’s tale: what would happen if the Earth suddenly stopped spinning? Certainly everything on the surface would continue moving at up to 1667 kilometres per hour, the rotation speed at the equator. “Anything on the surface that is not held down with incredible strength is going to fly off parallel to the surface,” Comins says. He calculates that people outdoors would be flung outwards to an altitude of about 11 kilometres, then fall and hit the ground at more than 1000 kilometres per hour. Buildings would be ripped from their foundations, while the oceans would engulf the land. Such a catastrophe could extinguish all life on Earth.
Life would fare better if the Earth stopped spinning over a longer timescale, say two or three decades. There would be a profound effect on the oceans, however. The centrifugal effect of the Earth’s rotation makes the solid Earth bulge outwards at the equator, and creates equatorial ocean bulges 8 kilometres high. Were the Earth to stop spinning, the oceans would migrate to the poles, where surface gravity is slightly stronger because the land is closer to the Earth’s centre.
Witold Fraczek of the Environmental Systems Research Institute in Redlands, California, has simulated this scenario and shown that once the Earth had lost half its angular momentum, , one at each pole, with shorelines at roughly 30 degrees north and south. In between, a mega-continent would emerge, its mountains peaking at up to 10 kilometres above the new sea level (see diagram). The northern ocean would drown most of Canada, Europe and Russia.
Whether people could survive on this new world is unclear. Much of the agricultural land would be lost, and the atmosphere would become too thin above most of the equator for people to survive there. Humans would separate into two populations, living along the shores of the northern and southern oceans, kept apart by rugged terrain in between, Fraczek says.
Add to that the challenge of scorching days and frigid nights, each lasting six months, spring accompanying sunrise, sunset signalling autumn. People might live in the twilight zones, migrating to keep pace with the gradual shift of light around the globe.
While there’s no way the Earth could, in reality, stop spinning over just a couple of decades, its rotation is gradually slowing. Many billion years into the future, it’s possible that the Earth’s day could become as long as its year. Wells pictures a perpetual sunset on the ageing Earth in his classic book The Time Machine, but beyond fiction, that future is just too distant to foretell.
- What if the Earth Had Two Moons? by Neil F. Comins (St Martin’s Press, 2010)
