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.”

Notes for Editors

For more information about the topic please check our web site
(available not before 11th October, 9 pm CET)

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The moon and the earth seen from outer space: research done at the ETH
Zurich shows that the difference between the two celestial bodies cannot
be a result of the raw material they were formed out of. The material was
identical. (Figure: NASA)


Prof. Alex Halliday

Tel. +41 (0)1-632 75 25

Fax +41 (0)1-632 11 79


Dr. Uwe Wiechert

Tel. +41 (0)1-632 05 98

Fax +41 (0)1-632 11 79