Mercury is a planet of extremes: the closest to the Sun, travelling fastest, and both very hot and very cold. Mercury is extreme in another way, too: it is by far the most neglected of the inner planets. Several spacecraft have flown past distant Jupiter and Saturn, while Mercury, one of our closest neighbours, has entertained only one visitor from Earth. Admittedly, the planet is its own worst enemy. Even to the most ardent space enthusiast, Mercury’s climate is not exactly inviting. Worse still, the planet’s surface looks just like our boring old Moon, where only a crater counter could find any sort of happiness.
But Mercury is important. Its nondescript surface hides a strange, iron-filled interior that resembles Earth more than the Moon. Mercury’s high levels of iron challenge astronomers to explain how the planet originated, testing theories of Solar System formation to the breaking point. Indeed, some of Mer-cury’s attributes are so unusual that they sound impossible. To find out more, astronomers say they need a new and unusual space mission – to put a spacecraft in orbit around Mercury.
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The one spacecraft that has visited Mercury, Mariner 10, photographed only 45 per cent of the planet’s surface, and that was nearly 20 years ago. But the Mariner results were a vast improvement over what was available before, when Mercury was as mysterious as Uranus, Neptune and Pluto (see Box for a round-up). Before the mission in 1974, scientists knew that Mercury was a small, dense planet that must contain a lot of iron. But they did not know what its surface looked like. Despite its nearness to us, Mercury was tantalisingly elusive, and no one knew what a visitor might see. Mariner 10 changed that. Launched on 3 November 1973, the Mercury-bound craft flew past Venus on 5 February 1974 and arrived at Mercury on 29 March that year. Mariner then passed Mercury twice more, once on 21 September and again on 16 March 1975.
The three encounters have provided most of what we now know about Mercury. The first thing the spacecraft revealed was Mercury’s surface, which looks like the Moon – grey and heavily cratered, from impacts early in the history of the Solar System. Mariner 10 also discovered that Mercury has cliffs, which look like wrinkles, some 3 kilometres high and stretching for hundreds of kilometres. The cliffs are believed to mark fault lines, formed when the planet cooled and shrank soon after its formation.
But the ordinary surface hides a strange interior, far more like the Earth than the Moon. Mercury is full of iron, more so than any other planet in the Solar System. The five worlds of the inner Solar System – Mercury, Venus, Earth, the Moon and Mars – are made of two basic ingredients: iron and rock. Planet Earth, for example, consists of an iron core surrounded by a mantle and crust of silicate minerals. Iron is more than twice as dense as the average rock. ¿ìè¶ÌÊÓƵs can therefore tell how much iron and how much rock an inner world has from its density: the denser the planet, the more iron it has.
Earth is the densest planet, at 5.52 grams per cubic centimetre, so it must have a lot of iron. ¿ìè¶ÌÊÓƵs estimate that Earth’s iron core makes up about half the diameter of the planet. In contrast, the Moon is much less dense – only 3.34 grams per cubic centimetre – and must be mostly rock, with little iron. Like Earth, and unlike the Moon, Mercury has a high density, 5.44 grams per cubic centimetre, so it must have plenty of iron.
In fact, Mercury has far more iron than Earth. Part of the reason for Earth’s high density is its size, more than 12 000 kilometres across. The weight of the planet itself compresses the materials within because Earth is so big. But Mercury is small, just 4878 kilometres across, so its weight does not affect its density too much.
It is possible to correct planetary densities by ‘uncompressing’ them – that is, by determining what the densities would be without the effect of the planets’ weight. Earth has an uncompressed density of only 4.4 grams per cubic centimetre. Mercury’s uncompressed density is higher than that of Earth or any other planet, at 5.3 grams per cubic centimetre. The high density is a true reflection of the planet’s composition and means that Mercury must have an enormous iron core, almost as big as the planet itself. The iron core could have a diameter three-quarters that of Mercury as a whole.
How did Mercury get so much iron? All the planets in the Solar System formed from a spinning disc of dust and gas that surrounded the Sun at its birth – which explains why all the planets lie close to a single plane. Particles in the disc collided, forming aggregates that grew in size and decreased in number until there were only nine principal planets left. In 1972, John Lewis, of the Massachusetts Institute of Technology, proposed a theory for the Solar System’s origin that attempted to explain the differences in composition between the planets.
Lewis believed that different substances condensed out of the primordial disc at different, but orderly, distances from the Sun. Substances with high melting points – iron, alloys and silicates – condensed close to the Sun, where the disc was hot, whereas substances with lower melting points – ice, for example – could do so only far from the Sun, where the disc was cold. According to this theory, the inner planets should be iron and rock, and the outer worlds should also contain substances such as ice and ammonia.
But Mercury is impossible to fit into Lewis’s model. If the planet formed as close to the Sun as it is now, it came from a very hot part of the primordial disc. Iron melts at a high temperature and Lewis’s model predicts that Mercury should be rich in iron, but the planet has far more iron than the model says it should.
Although Lewis’s model explains many of the Solar System’s patterns, it must be modified to account for Mercury’s extreme composition. One possibility is that Mercury was scorched by the young Sun. In this theory, Mercury was once a bigger planet, with the same iron core it has now but with a larger mantle of rock on the outside, giving the planet a ratio of iron to rock acceptable for the model. But intense heat from the young Sun could have blasted away much of the rocky surface, leaving a smaller planet with a higher proportion of iron. Today, astronomers see stars that undergo huge increases in brightness when they are young. If the Sun ever erupted like this, Mercury could have been a victim. But in recent years another scenario has been gaining favour among astronomers, led by George Wetherill of the Carnegie Institution in Washington DC. It also starts with a bigger Mercury, with its present iron core and a larger mantle of rock. This time, the rocky outer layer is removed not by the Sun but by a large asteroid. This asteroid smashes into Mercury and ejects much of the planet’s rocky envelope, leaving the surviving Mercury with its huge iron core.
Although this impact made Mercury smaller, Wetherill believes that it was unusually small to begin with. In fact, Mercury’s size may have been instrumental in producing its present composition. If Mercury had been bigger, like Earth, such a collision would not have affected it much, and the planet would have kept a lower proportion of iron. If, on the other hand, Mercury had been smaller, the collision could have destroyed the planet altogether. If Wetherill is right, the formation of the Solar System was not as orderly as Lewis envisaged. Whereas Lewis’s model predicts that similar planets would arise around other stars similar to the Sun, Wetherill’s scenario means that many features of the Solar System may be due to random acts of violence.
Mercury’s iron bears not only on the planet’s origin but also on another Mercurian mystery, its magnetic field, which in theory should not even exist. Planetary magnetic fields are thought to arise from electric currents stirred up in a planet’s liquid core if it rotates fast enough. This model seems to work for those planets that have strong magnetic fields – Earth, Jupiter, Saturn, Uranus and Neptune. All five planets spin fast, and part of their interiors are molten. Mercury should not have a magnetic field, because it spins too slowly. And the planet is so small that its core may be completely solid. Though both Mercury and Earth started out hot and molten, Mercury cooled faster, just as a freshly baked roll cools faster than a large loaf of bread.
But Mariner 10 surprised astronomers with its discovery that Mercury does have a magnetic field, albeit one that is only 1 per cent as strong as Earth’s. They still cannot explain how the magnetic field arises from a slowly spinning world that may have a solid interior. Some astronomers hold out the hope that part of the interior may be molten, especially if some substance, such as sulphur, lowers the melting point of the planet’s iron-rich interior. Sulphur lowers the temperature at which iron melts in the same way as salt keeps ice from freezing on roads. Sulphur is a logical candidate to keep Mercury’s core liquid, because it is soluble in iron. But its presence would raise more problems than it solves, for according to Lewis’s model Mercury should not have any sulphur. The planet is too close to the Sun to have sulphur, because it has a relatively low melting point.
Last year, the puzzle of Mercury’s magnetic field intensified. Jack Burns of the New Mexico State University, Michael Ledlow of the University of New Mexico and their colleagues observed radio waves from Mercury that told them that the planet is certainly hot, but that its heat comes entirely from the Sun. None emerges from the core of Mercury. This implies the planet’s interior is completely solid, with none of the molten material scientists believe necessary to generate a magnetic field. So the puzzle remains.
Another discovery from Mariner 10 was that Mercury has an atmosphere of hydrogen, helium, and oxygen. The atmosphere is quite tenuous, with a pressure no more than a million millionth (10-12) of Earth’s. But its stranger components, sodium and potassium, were discovered in 1985 by Andrew Potter of NASA’s Johnson Space Flight Center in Houston, Texas, and Thomas Morgan, also of NASA. Sodium and potassium are metals – not the sort of thing one expects in an atmosphere. They could come from meteoroids that strike Mercury, vaporise and eject sodium and potassium into the atmosphere, both from themselves and from the surface. In addition, the solar wind may blast sodium and potassium from the surface into the atmosphere. Or the two elements may arise from some unknown process.
Mercury presents so many unanswered questions about its origin, magnetic field, and atmosphere that researchers need new data to resolve them. In particular, they would like to see the other 55 per cent of the surface that Mariner did not photograph. The data that astronomers already have is so intriguing that they are starting to call for another mission to Mercury. But the mission they want will not be like Mariner, which simply flew past the planet. Instead, scientists want a spacecraft to orbit Mercury. An orbiter could photograph the entire surface of Mercury and study the planet, its magnetic field and atmosphere for months or even years.
But there are problems, both practical and political. First, the practical: Mercury is a tough planet to orbit. It is close to Earth, so it is easy to send a spacecraft there, provided it flies past the planet the way Mariner did. But getting a spacecraft into orbit is much harder, for the same reason that we can learn little about Mercury by observation from Earth: the planet is so close to the Sun. A spacecraft travelling directly from Earth to Mercury feels so much gravitational pull from the Sun that by the time it gets to Mercury it is moving very quickly – far too quickly to slow down and orbit the planet. It’s like a car that races down a steep hill and then tries to turn sharp left just before it reaches the bottom. To make matters worse, Mercury is small, with little gravity of its own to slow down and capture a visiting spacecraft.
One very expensive solution would be to carry a huge amount of fuel aboard the spacecraft. When the craft reached Mercury, it could use this fuel to slow down enough to go into orbit. But more fuel means more weight, so much more that a direct Earth-to-Mercury spacecraft would be incredibly expensive. Even Mercury enthusiasts might balk at the price tag for such a mission.
But in the mid-1980s, Chen-Wan Yen of the Jet Propulsion Laboratory in Pasadena, California, discovered that she could buy a cheap ticket to Mercury. She found that a spacecraft bound for Mercury need not carry a lot of fuel to slow itself down if instead it flew past Venus and Mercury several times before making its final approach. Each encounter would slow the craft down; when it reaches Mercury for the final time, it would need to fire only a little fuel to go into orbit.
Yen’s work means that a Mercury orbiter could be cheap. The only penalty is that a flight from Earth to an orbit around Mercury would take between three and five years. Mariner 10 made the trip in just under five months, but even three or five years is short compared with flight times to the outer planets.
Despite Yen’s work, there remains a political problem: Mercury just isn’t a sexy planet. Questions concerning the planet’s origin, magnetic field and atmosphere may fascinate Mercury afficionados, but when most people think of this planet, if they think of it at all, they see the lunar landscape and inhospitable climate. When money is tight, politicians would probably prefer to spend the money on more glamourous and photogenic worlds, such as Venus, Mars, Jupiter, Saturn – or even Earth.
Ken Croswell is a writer specialising in astronomy and physics.
Further reading: Mercury: The Elusive Planet, by Robert G. Strom, published by the Smithsonian Institution Press, London, 1987.
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The slow emergence of the swift planet
Mercury is one of the most difficult planets to see from Earth. In fact, its elusive character has made it one of the more obscure parts of the Solar System. Before the voyage of Mariner 10, astronomers had collected little conclusive information about the planet.
Although Mercury is not far from Earth – only 77 million kilometres away at closest – it is even closer to the Sun. Viewed from Earth, Mercury is never more than 28 degrees away from the Sun in the sky. So without a telescope, an observer can see Mercury only near the horizon, for just a short time at dawn or dusk. In contrast, planets like Jupiter and Saturn are there to be seen all night long.
When Mercury does emerge from the Sun, the planet moves so fast that it can be viewed for only a week or two before it vanishes back into the Sun’s glare; like its namesake, the messenger of the gods, the planet is swift. Mercury revolves around the Sun faster than any other planet, completing an orbit in just 88 days.
There is much else about Mercury that is extreme. Its orbit is both tilted and elongated; only Pluto’s is more elliptical and highly inclined. Mercury’s orbit is tilted at 7 degrees to the plane of the Earth’s path, and although the planet lies 58 million kilometres from the Sun on average, its separation varies from 46 million to 70 million kilometres. Because Mercury’s distance from the Sun varies, so does the intensity of sunlight striking the planet’s surface, ranging from 4.6 to 10.6 times what it is on Earth.
Other basic data about Mercury eluded astronomers for many years. Though the length of the planet’s year was known, the length of its day was not. Astronomers believed that Mercury rotated on its axis only once each orbit, making its day also 88 days long. If so, Mercury and the Sun were like the Moon and Earth: one side of Mercury must continually face the Sun, just as one side of the Moon continually faces Earth.
Only in the 1960s did we learn the truth: Mercury actually rotates every 58.6 days, which is two-thirds of its year. In retrospect, the first hint that the conventional picture was wrong came in 1962, when astronomers reported that the night side of Mercury was warmer than it should be if that side never faced the Sun. But rather than question the planet’s 88-day rotation period, scientists thought Mercury might have an atmosphere thick enough to carry warmth from the day side to the night side.
Then in 1965, Gordon Pettengill and Rolf Dyce, then at Cornell University, pointed the 1000-foot Arecibo radio telescope in Puerto Rico at Mercury. They bounced radar signals off the planet and found that the returned signals did not agree with a period of rotation lasting 88 days. They found that Mercury rotates at a rate close to the presently accepted value of 58.6 days.
Even though Mercury’s day does not equal its year, Mercury nevertheless spins more slowly than any other planet except Venus. Therefore, the days and nights are long, and the temperature range at the surface is huge. During the long day, Mercury reaches a sizzling 450 °C. At night, the planet plunges to -180 °C. Only Venus is hotter; only Uranus, Neptune and Pluto are colder.