The Planets. Andrew Cohen

theory is used to predict the development of large-scale events from a given starting point, as shown in this Henon mapping of a chaotic system.

      But 100 years ahead or behind us is nothing more than a proverbial blink in terms of the life of the Solar System, and over longer durations the clockwork becomes a lot less reliable. If there was only one planet orbiting one star – for example, if Mercury was the orphan child of the Solar System – we would be able to calculate precisely the gravitational force between Mercury and the Sun, and to plot Mercury’s orbit around the Sun with essentially infinite precision. But add one more planet into our rather vacant imaginary solar system – let’s say we make it Jupiter – so there is now a gravitational force between all three objects – the Sun, Mercury and Jupiter – and it’s no longer possible to calculate exactly where they’re all going to be in the future or where they were at some point in the past.

      ‘One possible theory is that Mercury didn’t form where it is today, but much closer to the other planets, maybe even outside of Venus, or Earth, or somewhere in between. Then because of interactions with Jupiter, Earth, Venus, and so on, it got put into a chaotic path that pushed it farther into the Sun.’

      Larry Nittler, cosmochemist, Messenger mission

      © NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

      The incredible shrinking planet

      The surface of Mercury is made up of just one continental plate covering the entire planet. Over the billions of years since its formation at the birth of the Solar System, the planet has slowly cooled, a process all planets undergo if they lack an internal source of heat renewal. As the liquid iron core solidifies, it cools, and the overall volume of Mercury shrinks.

      When NASA’s Mariner 10 mission circled the planet in the 1970s, it captured images of surface features created by the shrinkage. The contracting planet pushed the crust up and over itself, forming scarps that can extend miles below the planet’s surface. At the same time, the shrinking surface caused the crust to wrinkle up on itself, forming so-called ‘wrinkle ridges’.

      The scarps and wrinkle ridges identified by Mariner 10 allowed scientists to estimate that the planet had lost approximately 1 to 2 kilometres in global radius, a finding that contrasted with their understanding of the heat loss the planet suffered over time.

      © MARK GARLICK / SCIENCE PHOTO LIBRARY

      When there are more than two objects in play at any one time you have what physicists call a chaotic system. It means the planets can push and pull one another, moving entire orbits in ways we simply cannot predict. So the further we look back in time, the less certain we are of the position of any of the planets. Our mathematics fails, so instead we have to rely on circumstantial evidence to piece together a picture of the past. In the case of Mercury, it’s the evidence from Messenger detailing the levels of volatile elements like potassium and sulphur that enable us to begin to understand the early life of the planet and infer that Mercury must have begun life further out in the Solar System than it finds itself today. So what happened next? How did a planet that began its life in the sweet spot of the Solar System end up in the scorched interior?

      The answer lies in the other clue Messenger confirmed for us – Mercury’s massive iron core. Relative to its size, Mercury has the most massive core of any of the rocky planets: 75 per cent of its diameter and almost half of its mass is molten iron, compared to around just a fifth of the mass of the Earth. We’ve suspected the oddity of Mercury’s composition for well over 150 years, and that’s because of some brilliant deduction by a German astronomer called Johann Franz Encke, who determined the mass of Mercury by measuring the gravitational effect it had on a passing comet, a comet that we now call, unsurprisingly, Comet Encke. With an approximation of the planet’s mass we are able to calculate the density of the planet, and with that calculation approximate its composition.

      So we’ve known for some time that Mercury is odd, but only with the arrival of Messenger did we begin to reveal just how odd the smallest planet actually is. By accurately measuring Mercury’s magnetic field we’ve been able to confirm that far from being a geologically dead planet, Mercury has a dynamic magnetic field driven by an internal force, indicating that the core is at least partially liquid. This goes against the conventional thinking of planetary dynamics because we would expect a planet as small as Mercury to have lost its internal heat long ago. Just as Mars lost its heat because of its size (a story we will come to in the next chapter), we would have expected the core of Mercury to have cooled and solidified.

      But Messenger’s data proved otherwise. By combining precise measurements of Mercury’s gravity field with the extraordinary mapping of its surface, Messenger found that Mercury’s structure is unique in the Solar System. It appears to have a solid silicate crust and mantle above a solid layer of iron sulphide, which surrounds a deeper liquid core layer, possibly with a solid inner core at the centre of the planet. This challenges all the theories about its formation.

      © NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

      Messenger captured this image of Apollodorus crater, near the Caloris basin; the radiating troughs led scientists to give it the nickname ‘the spider’.

      © NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

      On 30 April 2015, NASA added its own crater to this region of Mercury. At 3.26pm EDT, Messenger impacted the planet’s surface, bringing the spacecraft’s mission to a dramatic end, but leaving its mark forever with a crater estimated to be over 15 metres wide.

      © redrawn from NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

      Four and a half billion years ago, we know that the inner Solar System was in turmoil. In the middle of it all, we think that the newly born Mercury found itself orbiting far out from today’s intimate proximity with the Sun, surrounded by rocky debris and scores of planetary embryos all jockeying for position. The young Solar System was still a place where planets could live or die. But it wasn’t just the rocky planets that found themselves disturbed; Jupiter, the largest and oldest of all the planets, was on the move, and when a planet of that size shifts its position there are almost always casualties. We’ll come back to the story of Jupiter’s grand tack and the havoc it spread throughout the Solar System in Chapter 3, but for now all we need to know is that the evidence suggests that the juvenile Mercury was kicked by the gravitational force of Jupiter on an inward trajectory, finding itself flung in towards the Sun and into the path of danger. In the crowded orbits of the early Solar System such a change of course was fraught with danger, and all of the evidence indicated that this was the most violent and defining of turns in Mercury’s history. As the planet swerved inwards it collided with another embryonic world and shattered.

      Today we see the evidence of this ferocious collision in the strange structure of this tiny planet. A giant core has been left behind, the exposed interior of a planet that had much of its outer layer, its crust mantle, stripped away and lost to space in the aftermath of the collision. This collision not only transformed the physical characteristics of the planet but also knocked Mercury further inwards on a lopsided trajectory that we see reflected in the most elliptical orbit of all the planets. Although we cannot be certain of these events, it’s a brilliant piece of scientific deduction to use the evidence we have to create a plausible scenario of events that happened unimaginably long ago. Events that drove the first rock from