by Marcus Chown
Remember that the Earth’s gravity at the distance of the Moon is exactly that required to bend the trajectory of a body moving at the Moon’s speed into the closed orbit we see. So, if the Moon speeds up, it finds itself travelling too fast for its orbital distance and is effectively flung outwards. Outwards – that is, further from the Earth – is ‘up’, and, as we know, when an object such as a ball is thrown upwards, gravity slows it down. So, paradoxically, the Moon, which is speeded up by its tidal interaction with the Earth, ends up moving more slowly in an orbit further from the Earth. And this does indeed increase its angular momentum as required.27
This is not just theoretical. The manned American spacecraft, Apollo 11, 14 and 15, and the unmanned Russian rovers, Lunokhod 1 and 2, all left reflectors on the lunar surface. The fist-sized mirrors, known as ‘corner-cubes’, have the property that they reflect back light in exactly the direction it comes from. It is possible to fire a laser beam at the Moon, bounce it off a corner-cube reflector, and time how long it takes for the beam to return to Earth. Knowing the speed of light, it is a simple matter to deduce the distance of the Moon.28
The experiments reveal that every year the one-way journey of a laser beam gets longer by 3.8 centimetres.29 In other words, the Moon is receding from the Earth by a thumb’s length every 12 months. If you have made it to 70, during your lifetime the Moon will have receded by the length of a family car.
The visibility of total eclipses
That the Moon is moving away from the Earth at 3.8 centimetres a year obviously tells us it was closer in the past. And this has implications for the visibility of total eclipses, one of the most spectacular of natural phenomena.
As mentioned before, a total eclipse occurs when the Moon passes between the Earth and the Sun, blotting out the solar disc and bringing midnight in the middle of the day. Such an event is possible because, although the Sun is about 400 times bigger than the Moon, it is also about 400 times further away. This means the two bodies appear the same size in the sky. This is a very fortunate circumstance for us. Even though there are 170-odd moons in the Solar System, there is not another planetary surface from which such a perfect eclipse can be seen. But we are not simply lucky to be in the right place, we are also lucky to be alive at the right time.
Because the Moon is moving away from the Earth, in the past it appeared bigger and in the future it will appear smaller. It turns out that there were no total eclipses before about 150 million years ago and there will be no more after another 150 million years. Total eclipses have been possible for only a few per cent of the lifespan of the Earth. For most of the reign of the dinosaurs, for instance, there were no total eclipses.
That the Moon is moving away from the Earth and was closer in the past also ties in neatly with an idea about the birth of the Moon.
The planet that stalked the Earth
The Moon is unusually large compared to the Earth – about a quarter of its diameter. All other moons in the Solar System are tiny compared to their parent planets. Granted, Pluto has an even bigger moon relative to its size, but Pluto has not been considered a fully fledged planet since 2006.
The unusual size of the Moon is a hint that it had an unusual origin. In fact, it is believed that shortly after the birth of the Earth 4.55 billion years ago, the planet was struck by a body with a mass similar to Mars. The titanic collision with ‘Theia’ liquefied the exterior of the Earth, splashing mantle material into space to form a ring, not unlike the rings seen today around the gas giant planets of our Solar System. The material of the ring congealed quickly into the Moon – but orbiting about ten times closer than it does today. Thereafter, the Moon began moving away from the Earth.
The key evidence for this ‘Big Splash’ theory of the Moon’s origin came from NASA’s Apollo programme, which found that the Moon is made of material similar to the Earth’s exterior ‘mantle’. Its rocks are also drier than the driest terrestrial rocks, indicating that intense heat once drove out all their water. The problem has been that for a Mars-mass object to create the Moon and not shatter the Earth it must have made a glancing blow at a very low speed. But bodies orbiting the Sun, both inside and outside the Earth’s orbit, are moving far too fast.
The Big Splash theory can be made to work if Theia actually shared the same orbit as the Earth. This could have happened if it formed from rubble that accumulated at a stable ‘Lagrange point’, either 60 degrees behind or 60 degrees ahead of the Earth in its orbit around the Sun.30 Today, similar asteroidal rubble can be seen orbiting the Sun 60 degrees behind and 60 degrees ahead of Jupiter, becalmed in a kind of gravitational Sargasso Sea. According to this twist on the Big Splash theory, Theia stalked the Earth for millions of years before being nudged into a catastrophic colliding orbit.
While the gravity of a body weakens with the inverse-square of the distance from the body, the tidal force, which is due to differences in gravity, weakens as the inverse-cube. So, at the distance from the Earth at which the Moon formed – about ten times closer than it is today – the tidal force it exerted on the Earth was 103 = 1,000 times greater than today. The Earth at the time, being still molten from its fiery birth, is unlikely to have had any oceans. But say it had: twice daily the sea would have risen not by metres but by kilometres.
The new-born Moon did not only exert a tidal effect on the Earth, the Earth exerted a tidal effect on the Moon. And that effect too was 1,000 times bigger than today. In fact, the tidal braking of the Moon’s rotation was so huge that it is probable that its rotation became locked very early on – perhaps within only 10 million years of its violent birth. Since the first micro-organisms on Earth appeared much later, probably between 4 and 3.8 billion years ago, no living thing has ever looked up at the night sky and seen the far side of the Moon rotate into view.
The Moon was not always fleeing so fast
An interesting question is: Has the Moon always been receding from the Earth at 3.8 centimetres a year? In 2013, a team led by Matthew Huber of Purdue University in West Lafayette, Indiana, considered the situation 50 million years ago. They fed data on ocean depths and the contours of the continents that existed at the time into a computer model that simulates tides. They concluded that, 50 million years ago, the Moon was receding from the Earth at a slower rate, perhaps only half as fast.31
The key is the North Atlantic Ocean, which today is wide enough for water to create a large tidal bulge, which can pull on the Moon, causing it to recede relatively quickly. About 50 million years ago, the Atlantic had not grown to its present size so the tidal bulge created by the Moon in the Atlantic was smaller and its effect on the Moon’s recession was less marked. At the time, most of the Earth’s tidal effect on the Moon in fact came from the Pacific Ocean.
This is yet another illustration of the complexity of the ocean tides. How big they are and how much they brake the Earth’s rotation and the speed of the recession of the Moon depends on how easy it is for tidal bulges to move through the oceans. This in turn depends on the arrangement of the continents, which is continually changing over geological time because of continental drift, more correctly known as plate tectonics.
It is the long-term unpredictability of plate tectonics that makes it hard to predict how long it will take for the Earth’s spin to slow sufficiently that it perpetually presents one face to the Moon. It is possible to say only that this state, in which the Earth spins on its axis once every 47 present-day days and the Moon has receded to the point that it takes 47 days to orbit the Earth, will be achieved after more than 10 billion years. But, as pointed out, this is totally hypothetical since the Sun will have evolved into a monstrous red giant 10,000 times as luminous as it is today and destroyed or at least disrupted the Earth-Moon system.
Tides have a final twist. Every day as the sea surges up beaches around the shorelines of the continents, countless pebbles are tumbled and smashed together. Friction between them generates heat-energy, which ends up in the environment. In fact, it is t
he loss of energy in this way that is ultimately responsible for slowing down the rotation of the Earth.
The tidal heating of the Earth is modest. If you wade into the sea, the sand and pebbles are not likely to scorch your feet. But there is one place in the Solar System where tidal heating is not modest at all: Jupiter’s giant moon Io, discovered by Galileo in 1609.
Pizza moon
It is 8 March 1979. NASA’s Voyager 1 space probe has streaked through the Jupiter system faster than a speeding bullet. It is now heading towards a rendezvous with Saturn in autumn 1980. But, before the probe leaves the gas giant planet forever, the Voyager team orders its camera to point back the way the space probe has come and take a parting shot of Io. Navigation engineer Linda Morabito is the first to see the image after its 640-million-kilometre journey back to Mission Control. When she does, her heart misses a beat. Spouting from the tiny crescent moon, silhouetted against the starry backdrop of space, is a phosphorescent plume of gas.
Morabito is the first human in history to see the super-volcanoes of Io. Over the next days, as the Voyager team pore over image-enhanced photos and thermal data, they spot a total of eight gigantic plumes, pumping matter hundreds of kilometres into space. Io turns out to be the most geologically active body in the Solar System, with more than 400 volcanoes. The vents that pepper the orange and yellow and brown of its pizza-like surface are reminiscent of the geysers of Yellowstone Park. In fact, strictly speaking, that is what they are: geysers not volcanoes. Lava from the moon’s molten interior, instead of erupting directly, superheats liquid sulphur dioxide just beneath the surface, converting it into gas which bursts from the vents exactly like the pressurised steam of a terrestrial geyser.
Every year, Io pumps about 10,000 million tonnes of matter into space. As it falls back in the moon’s low gravity, it coats the surface with sulphur just like the deposits around a Yellowstone fumarole. This is the origin of the satellite’s pizza-like appearance. The lurid colours are simply the ‘phases’ sulphur exhibits at different temperatures.
Jupiter, a whopping 318 times as massive as the Earth, is obviously crucial to understanding Io’s super-volcanoes. Io orbits about as far from the giant planet as the Moon is from Earth. But the giant planet’s enormous gravity whirls the moon around not in 27 days like the Earth’s satellite but in only 1.7 days. That gravity, acting on the tidal bulges of Io, long ago braked Io’s rotation so that it orbits today with one face forever locked to its master. If, one day, people get to set foot on that face, what a view they will have, peering through the visors of their space-suits at Jupiter and its whirling, multicoloured cloud belts filling almost a quarter of the sky.
Because Io’s rotation is ‘locked’, the two bulges pulled in the moon by Jupiter point perpetually towards Jupiter and perpetually away from Jupiter. This means they do not travel through the rock of Io in the way that the tidal bulges on Earth move through the oceans. If such a process occurred on Io, it would stretch and squeeze the rock, over and over again, heating it by internal friction in much the same way that a rubber ball squeezed repeatedly is heated. Since such a process is not happening, it would appear there can be no tidal heating of Io by Jupiter.
But there is.
Critical to Io’s heating are two of the other ‘Galilean’ moons that orbit further out from Jupiter than Io – Europa and Ganymede. Ganymede is actually the largest moon in the Solar System, bigger even than the innermost planet Mercury. For every four circuits Io makes of Jupiter, Europa completes two and Ganymede one. Because of this, the two satellites line up periodically, reinforcing each other’s tug on Io. The effect is to yank Io, elongating its orbit. So Io swings in close to Jupiter and then flies back out again, repeatedly. And it is this, it turns out, that is behind the prodigious heating of Io.
Yes, the tidal bulges of Io perpetually point towards Jupiter and away from Jupiter. But, when Io is at its closest to the giant planet, the tidal bulge is bigger than when Io is at its furthest from Jupiter. Up and down, up and down, the rock is stretched and squeezed. The process is so effective at warming the moon that the body in the Solar System currently generating most heat, pound for pound, is not the Sun.32 It is Io.
The mystery of Pluto and Charon
The Jupiter—Io system is not the only one in the Solar System in which a tidal interaction has resulted in two bodies orbiting each other with their rotations locked so that they perpetually show the same face to each other. There is also Pluto and its giant moon Charon.
The most notable thing about Charon is that it has half the diameter of Pluto. For a while this made Pluto the planet in the Solar System with the biggest satellite relative to its size. But in 2006 it was stripped of its planetary status by the International Astronomical Union and demoted to the category of dwarf planet. It had been found to be merely one of the largest bodies in a swathe of tens of thousands of pieces of icy debris orbiting the Sun in the outer reaches of the Solar System.
The ‘Kuiper Belt’ is composed of icy builder’s rubble left over from the birth of the planets, which could never form a planet because it was spread too thinly. It is the outer Solar System’s analogue of the inner Solar System’s Asteroid Belt – rocky builder’s rubble left over from the formation of the planets, which was prevented from aggregating into a proper world by the gravity of Jupiter. The inner edge of the Kuiper Belt is near Neptune – about thirty times as far from the Sun as the Earth – whereas the outer edge is at about fifty times the Sun-Earth distance. Despite its name, the Kuiper Belt was actually predicted by a former Irish soldier and amateur astronomer called Kenneth Edgeworth in 1943, and, by rights, should be called the Edgeworth-Kuiper Belt.
Pluto fulfils the first two criteria of the IAU’s 2006 definition of a planet: it orbits the Sun and is round. But, because it is accompanied by a large number of Kuiper Belt Objects, Pluto does not meet the IAU’s third criterion that it should also have cleared its orbit of all other bodies.
On 14 July 2015, NASA’s New Horizons sped like an express train through the Pluto-Charon system, skimming just 14,000 kilometres above what had been a planet when the space-probe had been launched a decade earlier but was now a mere dwarf planet. The shock to those back at Mission Control on Earth was that a world they had fully expected to be dead and inert, suspended in the deep-freeze of the outer Solar System, was in fact alive with nitrogen glaciers and mountains of water-ice pushing up towards the thin swirling clouds. Most surprisingly, the pink, heart-shaped region christened ‘Tombaugh Regio’ after the discoverer of Pluto, Clyde Tombaugh, was devoid of any craters, indicating that ice had spilled across it relatively recently, erasing any sign of the craters that peppered the rest of Pluto.
Where does the energy to drive all this unexpected activity come from? The interior of the Earth is heated by the radioactivity of uranium, thorium and potassium but such heating is believed to be insufficient to warm the interior of Pluto. And tidal heating by Charon is also ruled out since it is impossible in a system in which a moon is in a circular orbit and its rotation is locked to its parent planet. But tidal heating is ruled out only if the capture of Charon took place at the birth of the Solar System, much like the capture of the Moon by the Earth. If, instead, Charon was captured relatively recently – within the past half a billion years – tidal heating could have occurred as the system gradually approached the locked state we see it in today. Nobody knows whether this happened. The jury remains out.
Ocean moons
Tidal heating also has implications for the prospects of life – not on Io perhaps, which seems too inhospitable, but on Europa. Europa is tidally heated by the tug of Jupiter, Io and Ganymede. But, instead of being made of rock like Io, Europa is predominantly made of ice. The unavoidable conclusion is that the interior of Europa must have melted. It must contain liquid water.
A body containing liquid spins differently from a solid body. And the evidence from the way Europa spins is that, beneath a surface layer of ice 10 kilometres t
hick, there lies a 100-kilometre-deep ocean – the biggest ocean in the Solar System.
From afar, Europa looks like a snooker cue ball, its super-smooth surface the largest ice-rink in the Solar System. But from close-up, giant cracks in the ice come into view. The crazy paving-like pattern of the surface is reminiscent of sea ice in the Arctic Ocean, which breaks up in summer, floats about, then refreezes in winter. This is yet further evidence of the existence of a sub-surface ocean.
A buried ocean, languishing in permanent sunless darkness, might not seem a likely place to find life. But a key discovery made back on Earth in 1977 has changed everything. Using the submersible ‘Alvin’, the American oceanographer Bob Ballard discovered ‘hydrothermal vents’. Kilometres down on the sea floor, they gush superheated minerals into the ocean and support a thriving ecosystem, all in total darkness. At the bottom of the food chain are bacteria which get their energy not from oxygen but from sulphur compounds. At the top are giant ‘tube worms’ the size of a forearm.
Given that Europa is tidally heated, there will almost certainly be hydrothermal vents on its sea floor. It makes Europa the most likely place to find life in our Solar System. Currently, NASA is planning to send a space probe to the moon. Ideally, of course, a lander should be dropped onto Europa capable of drilling down through 10 kilometres of ice to the ocean. But this is way beyond current technical capabilities. Nevertheless, Jupiter Icy Moon Explorer (JUICE), planned for launch in 2022, may be able to exploit a recent discovery.