Reading the Dust from Asteroid Itokawa First Analysis of the HAYABUSA Sample First Analysis of Tiny Particles from Itokawa Itokawa, the asteroid visited by HAYABUSA, is completely rocky, but its central region is covered in regolith - sand, pebbles and small rocks. This area, called the Muses Sea, is where HAYABUSA picked up the sample it brought back to Earth. Although the initial analysis of these dust particles is still in progress, they have already provided a lot of useful information.

Meteorites are fragments of asteroids

We know that 80 percent of meteorites that fall to the Earth are made up of a rocky material called ordinary chondrites (a group of stony meteorite). We also know that Itokawa is an S-type asteroid, based on our analysis of the sunlight it reflect - a property known as solar spectral reflectance. Since ordinary chondrites and S-type asteroids have similar solar spectral reflectance, scientists had hypothesized that these meteorites originate from such asteroids. But since the solar spectral reflectance between the two is not a 100-percent match, they were unable to prove it.

Our initial analysis of dust particles from Itokawa used the following four methods to find out whether the particles have the same composition as ordinary-chondrites.

First, Professor Akira Tsuchiyama and his group at Osaka University used an X-ray micro-CT scanner to map the dust particles' shape and inner structure in three dimensions. They also identified the minerals the particles are made of, and mapped their three-dimensional spatial distribution.

Next, Professor Tomoki Nakamura and his group at Tohoku University used electron microscopes in conjunction with an X-ray diffraction technique to investigate the specific chemical composition of the minerals that make up the dust particles. They learned that the particles are made up of minerals such as olivine, pyroxene, plagioclase, troilite (iron sulfide), taenite (an alloy of iron and nickel) and chromite. This combination of minerals is not found on Earth; it is unique to extraterrestrial ordinary chondrites.

Professor Hisayoshi Yurimoto and his group at Hokkaido University examined the abundance ratio of oxygen atoms of different weight (isotopes). Oxygen, the main element in the composition of planets, has three isotopes of different weights, and each planet has different abundance ratios of each isotope.

Finally, Professor Mitsuru Ebihara and his group at Tokyo Metropolitan University used neutron activation analysis to study the concentration of different elements in the particles. This is a process in which the sample is bombarded with neutrons. By measuring the minute amounts of radiation released when the neutrons strike the sample, scientists can determine the precise amount of each element in the particle.

The studies found that Itokawa has LL-type mineralogy, a type found in the ordinary-chondrite group, because of its oxidation environment, which is one containing an abundance of oxygen trapped inside crystals. This is proof that the ordinary chondrites that frequently strike the Earth come from S-class asteroids.

X-ray CT imaging of two different wavelengths, or different energies. Minerals are identified by looking at the subtle differences in the brightness of the light they reflect. (Courtesy of Osaka University and JAXA)
X-ray CT imaging of two different wavelengths, or different energies. Minerals are identified by looking at the subtle differences in the brightness of the light they reflect. (Courtesy of Osaka University and JAXA)
Oxygen isotope ratios of Itokawa dust and terrestrial material show the differences between the two. (Courtesy of Hokkaido University and JAXA)
Oxygen isotope ratios of Itokawa dust and terrestrial material show the differences between the two. (Courtesy of Hokkaido University and JAXA)

Itokawa experienced frequent collisions with small astronomical objects

Using electron microscopes, researchers discovered that the Itokawa particles have broken crystals and white grains, indicating that strong impacts partially melted the rock and produced bubbles. This means that the parent object from which Itokawa originated was subjected to powerful collisions with other astronomical objects.

Also, an X-ray micro-CT analysis confirmed the presence of particles with sharp edges, as well as others with rounded edges. Researchers believe that the sharp edges were caused by angular particles when the original asteroid smashed into another object, and that the rounded edges were caused by oscillations during subsequent collisions with other asteroids. When an asteroid collides with another astronomical object, the material on the asteroid's surface is smashed into smaller pebbles, sand and dust - a material known as regolith. It may be that when Itokawa collides with another asteroid, the impact shakes the whole asteroid in a way that pushes the regolith into relatively flat regions such as Muses Sea. That's where HAYABUSA collected its sample.

Electron microscope image. White grains (indicated by arrows) are where bubbles once formed. (Courtesy of Tohoku University and JAXA)
Electron microscope image. White grains (indicated by arrows) are where bubbles once formed. (Courtesy of Tohoku University and JAXA)
Particles with sharp edges (left) and rounded edges (right). (Courtesy of Osaka University and JAXA)
Particles with sharp edges (left) and rounded edges (right). (Courtesy of Osaka University and JAXA)

Parent body was 10 times larger than Itokawa

Analysis of the chemical composition of the dust particles has found evidence that the inside of the asteroid was once hot - about 800°C. To reach that kind of inner temperature, an asteroid would have to be around 20 kilometers in diameter, so scientists estimate that Itokawa's parent body was once at least that large. They think that heat was generated by the decay of radioactive elements inside the asteroid. Today, Itokawa only about 500 meters across, just 1/40th the diameter of the parent body.

Itokawa dust that was once heated to 800°C. (Courtesy of Tohoku University and JAXA)
Itokawa dust that was once heated to 800°C. (Courtesy of Tohoku University and JAXA)

Image from an electron microscope. (Courtesy of Tohoku University and JAXA)
Image from an electron microscope. (Courtesy of Tohoku University and JAXA)

Burned asteroid surface

Space is full of high-energy particles (i.e. radiation) from the Sun and from objects outside our solar system. Particles from the Sun are called solar wind, while those from outside the solar system are called galactic cosmic rays, and astronomical objects are constantly bombarded by both types. So an asteroid's rocky surface is altered not just by other objects, but by solar wind and galactic cosmic rays, which turn the asteroid's surface dark, as if it were scorched. This phenomenon is known as "space weathering."

Scientists have believed for some time that the reason spectral-reflection values of meteorites and asteroids do not completely match is that asteroids are subject to space weathering. Now we have discovered direct evidence that this hypothesis is correct.

To investigate space weathering on Itokawa, Professor Takaaki Noguchi and his group at Ibaraki University stabilized the particles by placing them in a resin, and sliced them into ultra-thin cross-sections. They then studied the cross-sections, which are approximately 0.1 µm (0.001 mm) thick, with an electron microscope. The group found bright white spots in an area ranging from the particle's surface down to approximately 50 nm (0.05 µm) inside. They identified these as nanoparticles containing iron, which were created by space weathering. Scientists think that when charged (ionized) particles from solar wind strike the surface at high speed in vacuum, the impact creates nanoparticles mainly composed of metallic iron. These iron-bearing nanoparticles are what causes the varying hues of Itokawa's surface. And a chondrite that has been subjected to space weathering has the same spectrum as an S-type asteroid.

Close-up study of Itokawa particles by slicing them into cross-sections. (Courtesy of Ibaraki University and JAXA)
Close-up study of Itokawa particles by slicing them into cross-sections. (Courtesy of Ibaraki University and JAXA)
White spots in this photo are nanoparticles that contain abundant iron and were created through space weathering. (Courtesy of Ibaraki University and JAXA)
White spots in this photo are nanoparticles that contain abundant iron and were created through space weathering. (Courtesy of Ibaraki University and JAXA)

Itokawa may disappear due to weathering

Three Itokawa dust particles (#0015, #0053, #0065) containing neon gas that originated on the Sun. (Courtesy of the University of Tokyo and JAXA)
Three Itokawa dust particles (#0015, #0053, #0065) containing neon gas that originated on the Sun. (Courtesy of the University of Tokyo and JAXA)
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Analysis by Professor Keisuke Nagao and his team at the University of Tokyo detected in the Itokawa dust particles roughly the same ratio of noble gases, such as neon, and their isotopic components as that found in solar wind. This confirms that Itokawa has been subjected to space weathering. Noble gases from solar wind can only penetrate the very top of the dust particles' surface. The abundance of the noble gases tells us that Itokawa's surface has been struck by solar wind for hundreds or thousands of years, and is thus heavily weathered.

On the other hand, the study did not detect any evidence of Itokawa being struck by galactic cosmic rays from outside the solar system. Galactic cosmic rays can penetrate the surface of an astronomical object as deep as about one meter, but this effect does not show up if the surface has been exposed to the space environment for less than one million years. This means that the dust from Itokawa that scientists analyzed appeared on the surface less than one million years ago. In addition, we have learned that these particles were not constantly on the surface. Instead, they moved between the surface and underneath.

Scientists speculate that while moving in this manner, the dust on Itokawa's surface eventually, under the influence of space weathering, was hurled into space at a rate of a few dozen centimeters every million years. In other words, Itokawa is shrinking. Its diameter is now about 500 meters, but if the surface is shaved away at the current rate, the asteroid will gradually become smaller, and in about one billion years may disappear entirely.

Disparate pieces gathered together to form Itokawa

Timeline of Itokawa's history. (Courtesy of Tohoku University and JAXA)
Timeline of Itokawa's history. (Courtesy of Tohoku University and JAXA)

Early analysis has revealed the history of Itokawa's formation. The figure on the right is a timeline of the asteroid's evolution.

1. After the birth of the solar system about 4.6 billion years ago, dust and gas - the initial material in the solar system - coalesced and formed Itokawa's parent body.

2. The parent body was at least 20 km in diameter, and its internal temperature peaked at around 800°C. After that, the parent body gradually cooled down.

3. Other small astronomical objects collided with Itokawa's parent body.

4. The parent body was completely broken apart by the impact of collisions.

5. The gravity of some of the disparate pieces pulled them back together to create Itokawa. At that time, the asteroid's surface had not yet been affected by space weathering, so it had a lighter hue than now.

6. Space weathering has darkened the color of the surface and reduced Itokawa's diameter to around 500 meters.

Elemental analysis also found that the Itokawa particles contain traces of iridium, which is a product of the solar system's formation 4.6 billion years ago. The trace elements will help scientists study the evolution of the solar system as they continue their analysis of the particles.

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