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The Moon in pictures

It's just a lump of rock, and it isn't very far away. So one might think we know everything there is to know about the Moon. Not so. We may have studied it, orbited it and landed on it, but the Moon's origin, shape, geology and very identity still pose questions.

In the beginning�
The leading theory of the Moon's creation involves a big bang¹. Some 4.5 billion years ago, the Solar System was filled with objects occasionally smashing into each other - 'planetisimals', from which the planets eventually built up. During this chaotic period, an object about the size of Mars (one-tenth Earth's mass) probably slammed a glancing blow into an early Earth, giving our planet its spin. The iron cores of both objects merged into a single planet, while a great swoosh of lighter, outer bits were flung into orbit (the simulation pictured shows the first 24 hours of this process). Some fell back to Earth; about half would eventually coalesce into today's Moon, at an eightieth of the Earth's mass. The early Moon would have been very close to the Earth - perhaps just four Earth radii away - as it coalesced from a ring of debris. This scenario accounts for why the Moon has no substantial metallic core, as the planets of our Solar System do, and why its rocks are similar, but not identical, to those of the Earth's mantle. But it does not command universal assent. Others theorize that the Moon was simply a bit of passing rock that fell into the Earth's gravity, or a blob spun out from the early Earth when it was forming.

Reference
1. Canup, R. M. & Asphaug, E. Nature 412, 708-712 (2001). | link

How the Moon got squished
Like the Earth, the Moon isn't a perfect sphere. Instead it bulges like a rugby ball into two points, thanks to the tidal pull of our planet. And it is also flattened a bit by its own spin. The Moon's current orbit would be expected to squash the 3,500-km-across Moon by metres; but in fact it is bent kilometres out of shape. Ian Garrick-Bethell and his colleagues at the Massachusetts Institute of Technology have worked out one explanation for this¹. The Moon, they say, could have gone through a period some 100-200 million years after it formed when it was in an oval-shaped orbit that took a week, rather than a month, to do a lap of the Earth. The longest axis of this orbit was about 50 Earth radii - putting the Moon, at all times, much closer to the Earth than it is today. It would also have spun more quickly, turning 1.5 times for every orbit, with the pointy ends facing towards the Earth when at its closest approach (see diagram: start at number 1 and follow it counter-clockwise). The shape that the early, malleable Moon adopted would then have been frozen into the rock, in a 'fossil' that we see today, in a staid circular orbit some 120 Earth radii across. This theory isn't inconsistent with the idea that the Moon formed in a giant collision, or with the Moon's current orbit. But we can't yet explain how or why its orbit would have varied so much over time.

Reference
1. Garrick-Bethell, I., Wisdom, J. & Zuber, M. T. Science 4, 652-655 (2006). link

Near� and far
The insistent gravity that changed the Moon's shape also had effects on its innards. As a result, the far side of the Moon is a very different place from the familiar scarred face that stares down on the Earth each night. It is much more uniform, without the dark 'seas' that make up the face of the Man in the Moon (or the rabbit in the Moon, if you are Asian). This is because of massive differences between the near and far side that were locked in as the Moon cooled and hardened between 4 and 3.5 billion years ago. Its centre of mass is shifted about 2 kilometres off-centre towards the Earth, and the outer crust is also lopsided: it is 60 kilometres thick on the nearside, relatively dense and packed with a greater proportion of radioactive elements; 100 kilometres thick and less dense on the far side. This probably has something to do with gravitational pulls exerted on the young Moon. The lopsided Moon went though a period of 'heavy bombardment' some 3.8-3.9 billion years ago. Along with the rest of the Solar System, the Moon was smacked by a passing barrage of unknown origins. The thinner crust on its nearside (made thinner by impacts) and the extra heat provided by radioactive elements encouraged hot magmas from within the still-warm Moon to bubble up to the surface on the nearside. The oozed volcanic melt then cooled into the 'seas' that look down on us today.

All shook up
Today, the Moon isn't exactly a hot place, geologically speaking. The youngest known volcanic rim is about a billion years old. But what it does have is quakes. Meteor impacts, monthly deep quakes caused by competing gravitational pulls from the Earth and Sun, and thermal quakes caused by the thermal expansion and contraction of rock during the day/night cycle, can rattle the Moon with shakes of up to magnitude 3 on the Richter scale. But the Moon also gets shaken by real whoppers, of up to magnitude 6, by forces of unknown origin. A seismic network set up by the Apollo missions (see picture) recorded 28 of these shallow moonquakes between 1972 and 1977 - seven of them greater than magnitude 5. Researchers speculate that these may be caused by a slow relaxation of rock deformed by ancient impacts, in the same way that continents once depressed by glaciers on Earth are still rebounding today. Or it could be strange quark matter whacking into the unprotected Moon. Clive Neal from the University of Notre Dame, Indiana, is developing a proposal to put a new seismic network on the Moon to study these in more depth. Before we know what causes them, he notes, we won't know how to quake-proof any future Moon bases.

Caught on camera
Most of the Moon's large craters were made a very long time ago, and today's impacts are rather small. Nevertheless, amateur astronomer Leon Stuart managed to catch a pretty big impact on camera in 1953. The photo that Stuart took was long controversial: was it really an impact, or just a meteor burning up in the Earth's atmosphere in line with the Moon? In 2003, Bonnie Buratti, a scientist at NASA's Jet Propulsion Laboratory in Pasadena, and Lane Johnson of Pomona College, Claremont, California, tracked down a crater they think was made by 'Stuart's Event'¹. From the photo, they calculated that the object creating the flash was probably 20 metres in diameter, making a crater some 1-2 kilometres across. They then trawled through the data from the Pentagon's Clementine spacecraft, which photographed the Moon in 1994. Their search revealed a perfect candidate - a fresh-looking 1.5-kilometre-wide crater in just the right spot. The team estimates that the impact had an energy of about 0.5 megatons - 35 times the energy released by the Hiroshima atomic bomb. Such events are thought to occur perhaps once every 50 years. Smaller events than this are spotted more regularly by NASA telescopes. On 2 May 2006, NASA scientists captured one particularly clear video recording of a 25-centimetre meteoroid hitting the Moon's Sea of Clouds (Mare Nubium).

Reference
1. Buratti, B. J. & Johnson, L. L. Icarus 161, 192-197 (2003).

Elementary analysis
The lunar crust is made up of much the same stuff as Earth's outer layers: mostly oxygen and silicon, along with doses of elements such as aluminium. But some of the mineral compositions are distinctly different from their Earthly cousins - volcanic magmas on the Moon have been found to contain oddly high levels of titanium or iron, for example. This image, made from multiple infrared images snapped in 1992 by the Galileo spacecraft, highlights some major composition differences in elements that might be usefully mined (for future locals or for export). Areas appearing red generally correspond to the lunar highlands, whereas blue to orange shades indicate the ancient basaltic lava flow of a low-lying lunar 'sea'. Bluer areas contain more titanium than do the orange regions, which hold more iron. The Moon does have some material that the Earth has little of, particularly helium-3, which it picks up from the solar wind bombarding its unprotected surface.

Is that cheese?
Most of the Moon's surface is made of grey, volcanic basalts, covered with a layer of dust created by a constant bombardment by micrometeorites. But astronauts have also stumbled on patches of colour, including this 'orange soil' - particles of glass made in the fiery fountain of an ancient volcanic eruption. This picture shows a particularly fine-grained set of samples, with particles of glass and volcanic melt just 20-45 micrometres across (about the same size as Earthly silt). Chemical analysis of the orange soil material has shown the sample to be rich in titanium, iron oxide and zinc.

A planet or a moon?
Our Moon is bigger than Pluto, and about a quarter of the diameter of the Earth itself, giving some astronomers the notion that it might better be described as a planet than a satellite. That might seem ridiculous, but astronomers have been confused about what a planet is, exactly, and have spent much time recently debating the issue. A definition for planets in our Solar System - objects in orbit around the Sun that are big enough to be made round by gravity and that have cleared their neighbourhood of debris - was finally arrived at in August 2006, but proved highly controversial from day one. The definition as it stands means that the Moon isn't a planet. But, bizarrely, it could also be used to argue that the Earth isn't a planet (the Moon being evidence that it hasn't cleared its orbit of debris). The debate is obviously due to continue.