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Moon and Earth Formed out of Identical Material

The "Giant Impact" theory has been known and accepted by scientists for more than a decade because it provides an explanation for the low density of the moon and the angular momentum of the earth-moon system.

Zurich - Oct 14, 2001
According to the "Giant Impact" theory the moon was formed by a collision between a proto-earth and a smaller planet. In the October 12 issue of Science, ETH researchers present results showing that the composition of the oxygen isotopes of the moon and the earth are identical.

This is a strong indication that the proto-earth and the planet with which it collided were formed from an identical mixture of components and that they orbited the sun at similar distances.

The research group from the ETH Zurich have presented results of their analysis of 31 samples of various types of lunar rocks, which were brought back from the Apollo missions 11, 12, 15, 16 and 17. Isotopes are atoms of the same element but with different mass because of differing numbers of neutrons.

The oxygen isotopes O16, O17 and O18 in the samples were measured using laser fluorination. This technique was developed over the past decade and is ten times more precise than previous methods.

The oxygen isotope composition of lunar rocks had been measured long before but, encouraged by the availability of the new method, the ETH researchers decided to carry out new measurements.

They were astounded by their results. Uwe Wiechert, senior assistant at the ETH Zurich and first-named author of the Science paper, explains: "We wanted to investigate how homogeneous the moon is and whether the samples contained parts of the proto-earth and the planet with which it collided. We already knew that the earth and the moon have a very similar isotopic composition. We never expected, however, that they would turn out to be identical."

The composition of the oxygen isotopes can be used to determine the origin of the rocks within the solar system because the oxygen isotope compositions are not uniformly distributed. If two rocks have identical oxygen isotopic composition then the probability is great that they were formed from the same "parent planet" or formed from the same average mixture of solar system debris.

The characteristic composition of meteorites from Mars, for example, is different from that of the earth and the moon, or from any other bodies in the solar system. If two big bodies, such as the earth and the moon, have an identical oxygen isotope composition, they formed from an identical mixture of components and were formed at very similar distances to the sun.

Gigantic impact of a planet the size of Mars
The "Giant Impact" theory has been known and accepted by scientists for more than a decade because it provides an explanation for the low density of the moon and the angular momentum of the earth-moon system.

The theory states that, about 50 million years after the birth of the solar system, a planet of the size of Mars -- called Theia after the mother of the Greek moon goddess, Selene -- collided with the proto-earth.

At this time the earth was in the latter stages of its formation and had about 90% of the mass it has today. Due to the enormous mass involved the collision must have been very energetic. Most of the mass of the earth probably melted and major portions may have evaporated.

The debris from the collision formed a ring around the earth, out of which the moon was formed. Subsequently, the moon moved further away from the earth and slowed down the rotation of the earth. These processes are still on-going.

On the one hand, the results now published in Science fit the "Giant Impact" theory very well. On the other, the virtually identical composition of oxygen isotopes are a strong indication that proto-earth and Theia were formed from the same mixture of components and that they probably orbited the sun at very similar distances -- like twin planets.

Another explanation could be that the moon and the earth (as it is today) both received the same proportion of material from proto-earth and Theia.

Computer simulations of the "Giant Impact", however, refute this hypothesis. They show that the moon must have been formed for the main part out of Theia's silicon-rich outer portions.

In line with what is known today about the formation of planets in the solar system, the collision of two bodies with similar orbits is not an unusual occurrence. Planets are formed in three stages. In the last stage, they can only grow by colliding with other planets.

Next question: Where does the earth's water come from?
If the moon and the earth are composed of the same material, then the question arises as to why these two celestial bodies are so different today.

Earth, for example, is covered for the main part with water, while practically no water is found on the moon -- estimates suggest that the entire water content of the moon is about a third of the volume of Lake Zurich.

A research team at the ETH is looking into the question of the origin of water on earth. There are a number of theories. For example, one hypothesis is that the water came from a body from the asteroid belt. Others have proposed that comets delivered water to the Earth.

Uwe Wiechert says: "There are a number of theories on the origin of water. It would be exciting to examine the water recently found on the moon and compare it with earth's water. At present, we presume that the water found on the moon was formed by solar winds, but perhaps here too, we will find the unexpected."

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