Bernard Foing is the principal scientist for the European Space Agency's SMART-1, a spacecraft currently in orbit around the moon. He is also the director of the International Lunar Exploration Working Group. In this essay, he describes how the moon formed billions of years ago, when a Mars-sized body collided with the young Earth. While the moon has undergone a different evolutionary history than the Earth, its close connection to our planet continues to this day.

You could say that the Earth gave birth to the moon. The moon formed 4.55 billion years ago when a Mars-sized impactor hit the Earth. A few hours after impact, floating lava and rocks orbited around the Earth, and then this ring of debris eventually re-condensed under the effect of gravitation to form the proto-moon.

A current numerical simulation indicates that 85 percent of the mass of the present day moon came from the impactor, and the remaining 15 percent came from the Earth. But just as some of the Earth became the moon, some of the impactor partly contaminated the Earth with iron that is now part of our planet's core, as well as with other material, some of which turned into gas after the impact.

The Earth itself formed very soon after the formation of the sun. There were a number of embryo planets, or planetesimals, which condensed to form the proto-Earth. After 40 or 50 million years, the event that gave birth to the moon took place on the young Earth.

When the impactor hit the Earth, both were molten due to the thermal heat released by the accretion of the embryo planets. Also, at the beginning of the solar system, there were still a lot of radioactive elements that released energy embedded into the rocks. So we believe that, at least in the first 100 million years of Earth history, it could have been covered by a magma ocean. We believe this was also the case for the moon.

When the moon had a magma ocean, the lighter minerals floated up to form the original crust, and the heavier elements sank into a core. There are indications that the moon's core is 300 to 400 kilometers thick, from a radius of 1,737 kilometers. That's a very small volume, because it's only a fifth of the size of the moon. It's one-hundredth of the volume.

When this core was molten, it was subject to currents that could have generated a magnetic field in the first 100 million years of lunar history. After this core cooled down, it was no longer able to generate those currents.

We found a similar thing for Mars. On Mars there are indications that, in its first 500 million years, there was a molten core that could generate a current and a magnetic field. This would have shielded the planet from the influence of the solar wind, and therefore helped to protect the early atmosphere from being eroded. So during this time, Mars could have had a dense atmosphere, warmer conditions, and maybe even wetter conditions.

Mars had volcanic activity during this early period, and there are also signs of early volcanism on the moon. Some of these signs of lunar volcanism are surface wrinkles and tectonic deformations that occurred as magma under the crust cooled and compressed.

Some volcanic activity took place on the moon because of giant impacts. When the planet embryos that were still wandering around bombarded the moon, magma between the core and the crust extruded in some areas. This gave rise to volcanoes and lava deposits that filled the giant impact basins to form the Maria. Most of the cliffs and mountains on the moon that we see today are also the result of these giant impacts.

In fact, most of what we see on the surface is from this period of heavy bombardment. The surface of the moon is therefore like a record book of the small bodies that were wandering around at this time. These bodies left scars on the moon, but we don't know exactly when they occurred. We also don't know the diversity of those bodies. So the next phase of exploration would be to look at these different impact basins to determine their ages, and also to determine their compositions by measuring, for instance, the composition of the impact melts.

The same population of bodies that hit the moon could have impacted the Earth, except that the Earth, being bigger, attracts even more bodies at higher speeds. So from the lunar record, we could reconstruct the type of bodies that have impacted the Earth. By determining the compositions, we could see how much these bodies contributed to the reservoir of material on Earth.

One question we have is, what was the original composition of the impactor that resulted in the moon? Did it come from a different place in the solar system, or did it come from a parent orbit? There are theories indicating there was material at the same distance from the sun as the Earth, but ahead of us on a so-called Lagrange point, and as a result of this material wandering on its orbit, it impacted the Earth.

But a new theory says that rocky planets are not formed only from material at a given distance. Instead, they are a mixture from bodies wandering around on a large range of distances. For instance, there were rocky, iron, and even water-rich bodies that accreted to form the rocky planets. So the final composition would be more a result of chance, a result of different random collisions, than something extremely systematic.

The isotopic signatures of elements can provide some clues. The ratio of, for instance, oxygen 18 and oxygen 16, tells you about the reservoir of material from which the moon formed. One of the surprising discoveries from the analysis of the lunar samples from Apollo and from the Soviet lunar missions is that the isotopic ratios for oxygen are very similar for the moon and the Earth. This may indicate that they were formed from very similar reservoirs.

The volatiles that are present can provide more clues about the formation of the impactor, but you have many things that can reset the volatile content. Even if there was water in this impactor, a large part would have evaporated during the moon-forming impact. Also, in the first tens of millions of years of the Earth and moon history, other sources of water could have been added to the outer layers. There were a number of icy bodies that were present beyond the snow line in the solar system, where ice can be stable, and some of these icy bodies were perturbed in their orbits and impacted the Earth and the moon. There are also some water-rich bodies -- not really comets, but asteroids with water-rich minerals -- which could have enriched the Earth and moon in water.

Because the moon is small, it could not retain an atmosphere or very much water that may have been brought by comets and asteroids. In fact, because of its weak gravity, when the moon receives an impact, it loses more mass than it receives. So the moon is slightly smaller today than it was in the past, while the Earth is big enough to retain more mass from impacts.

But whenever impacts occurred on the Earth or the moon, there was an exchange of matter between the two bodies. There was a cataclysm in the solar system 3.8 billion years ago, when a number of bodies, possibly coming from the asteroid belt, were suddenly ejected in larger numbers into the inner solar system. This created all the impact basins that we now see on the surface of the moon.

Even more impacts must have occurred on the Earth, but our planet's active tectonics has since erased these ancient impact basins. Based on the speed and energy of these impactors, a significant amount of the Earth probably was transferred to the moon at this time -- we estimate about 200 kilograms of Earth per square kilometer. So there could be a thin layer of Earth material on the moon which has subsequently been buried by the lunar soil.

Interestingly, 3.8 billion years ago is also the time of the oldest fossils of life on Earth. So there's a possibility that if we retrieve some of the early Earth samples on the moon, we could determine by their mineral composition and organic content whether there were any pre-biotic forms. This would give us information on whether there is a missing link between pre-biotic chemistry and life.

So the lunar record could give us access to these early Earth samples, but it will be very difficult search. The challenge now is to look for these layers of Earth that are buried under several tens of meters of lunar dust. To excavate this in a smart way, maybe we could make use of impact craters that have already done the work for us.

There is only a tiny amount of material to be found, and we'll need a way to distinguish Earth material from moon material. But because Earth may have been quite wet 3.8 billion years ago, we may find some hydration signatures in this material. We could look at isotopic signatures as well. We'll need a series of missions to identify places of high potential for these layers, measuring from orbit and deploying some rovers to make measurements. Eventually humans can make a more detailed search and bring some of those samples back to Earth.

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