A Beautiful Message from 4.5 Billion Years Ago

— The Mystery of the Earth's Formation Gets Decoded from Meteorite Samples

Takashi Mikouchi
Associate Professor,
Department of Earth and Planetary Science,
Graduate School of Science

Meteorites born 4.5 billion years ago still shine brightly. Takashi Mikouchi, an associate professor at the Department of Earth and Planetary Science, is conducting research on these beautiful meteorites, which are in fact the igneous rocks formed by the cooling of the melted magma in a celestial body that was created soon after the formation of the solar system. "What makes meteorites attractive is that information from several billion years ago has been saved in them. By analyzing them, we can learn more about the environment at the time of the formation of the solar system, which opens up the possibility of solving the mystery of the formation of the Earth," says Mikouchi.

To observe meteorite fragments under a polarization microscope, they have to be sliced as thin as possible so that sunlight can pass through them. Remarkably, Mikouchi slices them by hand to a thickness of only 0.03 millimeters. He can then identify the kinds of minerals contained in the slices by checking the differences in the birefringence and polarization properties of the light passing through them.

How do we get these meteorites? The sight of the asteroid probe Hayabusa returning to Earth with a lot of tiny particles from the asteroid Itokawa in its on-board sample container is still fresh in our minds. But we do not have to go into space to find meteorites: they also fall from the sky. Where do they actually come from? How do scientists analyze the stone-chip fragments of meteorites? Mikouchi's scientific exploration is to look for the answers to the mysteries of the meteorites.

Often Found in Antarctica!

Meteorites vary in size from as big as several meters in diameter to as tiny as a grain of rice. In most cases, the surfaces of meteorites are colored black, but some of them are as shiny as bright metal.

We may find it surprising that many meteorites have been found in Antarctica, where it is said that more than half of the meteorites that have been collected so far were first found. Mikouchi explains: "An undeniable reason is that tiny black meteorites can be easily found on a white surface covered with ice and snow. However, there is a bigger reason than that. Antarctica is covered with glaciers that have been moving towards the ocean for tens of thousands of years. If they bump into mountain ranges before they reach the ocean, they get stuck there. Meteorites that fall onto inland glaciers are randomly swept towards the foot of mountain ranges by the movement of the glaciers, which means meteorites have accumulated in these areas over tens of thousands of years."

In 1969, more than 10 years after the first Japanese Antarctic research expedition in that area, members of an expedition team who were conducting a geological investigation at an area called the "Yamato Mountain Range," which is located near the Showa Station, Japan's main research station in Antarctica, found nine meteorites. These meteorites were all of different kinds. After this discovery, a lot of other meteorites were found in Antarctica.

Besides Antarctica, meteorites are often found in the Sahara. On some days, up to ten meteorites are found there, and "meteorite markets" have been opened in Morocco since the latter half of the 1990s. Researchers like Mikouchi often buy meteorites from the dealers in these markets.

Analysis Begins from Mere 0.2-Gram Samples

Mikouchi conducts analyses of meteorites using mineralogical and crystallographical methods. He says that samples that are 0.2–0.3 grams in weight and 5 millimeters square are large enough for him to use for his research. He analyzes the minerals and crystal structures in the samples and tries to find out through what kinds of processes they were formed. For his analyses, he uses such equipment and methods as a polarizing microscope, SEM (Scanning Electron Microscope), TEM (Transmission Electron Microscope), EPMA (Electron Probe Micro Analyzer), and x-ray diffraction.

By analyzing the composition and crystal structure of meteorites based on these methods, Mikouchi can infer the mechanisms of meteorite formation. For example, the meteorite shown in the image was formed by the cooling of hot melted magma, and this can be proved by artificially making a mineral with a similar structure by cooling it at a certain speed from a hot temperature. Using these experimental procedures and methods allows Mikouchi to understand the process of meteorite formation. He can fix the age of a meteorite at 4.5 billion years by estimating the half-life of a slight amount of radioactive isotopes contained in the meteorite.

99.5% of the Meteorites Came from Asteroids

So far, 40,000–50,000 meteorites have been found all over the world. It is believed that more than 99% of them fell from asteroids. The basis for that conclusion is the remarkable similarity between the reflected spectra of asteroids observed through a telescope and the spectra of meteorites found on the Earth observed by the same method. This should be proved by the analysis of the particles from the asteroid Itokawa, which were brought to the Earth by Hayabusa.

Meteorites are much bigger than shooting stars. While shooting stars, which are comet dust, are burned up when they enter the Earth's atmosphere, meteorites fall to Earth without burning up since they are larger fragments of asteroids, which are much bigger than shooting stars. Mikouchi says, "When we calculate the trajectory of the falling meteorites from the video images we sometimes come across, the results lead us to conclude that the asteroids are located between Mars and Jupiter."

Are there any meteorites that came from bodies other than asteroids? It is reported that there are about 100 meteorites that fell from the Moon and 50 from Mars. Celestial objects such as asteroids collide with the Moon and Mars, and the impact of the collisions throws stones and boulders on the Moon and Mars into space, whence they fall onto Earth. Until the 1970s, it was not believed that a strong enough energy could be created to hurl large stones on Mars into space. However, it was found that stones on Mars could break away from the gravity of Mars and reach Earth if they were propelled by an unusually strong collision. In the ways described above, it is possible to investigate how our solar system was born and how the existing planets and moons were formed by examining the meteorites that fell from asteroids, the Moon, and Mars. The field of Earth and planetary sciences opens up a wondrous world of research that tries to solve the mysteries of the Universe and the Earth.