The Mystery of Solar System Formation Hidden in a Comet
New discovery found in comet dust
How was the solar system formed? This topic is intriguing to not only scientists but also laypersons. Previous researchers have clarified the formation process of the solar system little by little from the results of astronomically observing planets and asteroids, the analysis of meteorites that fall to Earth, and computational simulations.
Although the formation process of the solar system has not yet been completely clarified, researchers have developed a model of solar system formation that they believe is most likely to be true. However, they may have to greatly modify this model because of the following research result. That is, chondrules* were found in the dust of a comet originating from the outer part of the solar system; this cannot be explained using the conventional model. A chondrule is a rock component abundant in meteorites (image on the cover).
image on the cover
“It's where it isn't supposed to be!”
When the solar system was first formed, a huge amount of gas and dust was distributed in the form of a disk centered on the Sun (Fig. 1). At this stage, the inner side of the disk nearer to the Sun is considered to have been in a high-temperature state. Therefore, the dust in the inner side melted and its properties changed upon being heated to 1500°C or higher. When it was rapidly cooled, chondrules were generated.
The dust gradually accumulated to form large masses and a large number of planetesimals with a diameter of about 2-3 km. Such planetesimals developed into planets and asteroids after repeated collision and merging.
According to this model, celestial objects close to the Sun must contain chondrules, as has been demonstrated by, for example, analyzing meteorites obtained from the asteroid belt. On the other hand, no chondrules should have existed in the outer side of the disk because it was distant from the Sun and had an ultralow temperature. In this research, however, chondrules were found in Comet 81P/Wild (Fig. 2), which had an orbit further from the Sun than Uranus.
Fig. 1 Model of solar system formation.
When the solar system was first formed, gas and dust were distributed in the form of a disk (protoplanetary disk) and are considered to have gradually coalesced to form asteroids and planets. Many groups of dust that remained after their formation are distributed beyond the orbit of Uranus; this area is called the Kuiper belt (cited from the website of the Department of Earth and Planetary Sciences, to which Dr. Nakamura belongs).
Fig. 2 Orbits of Comet 81P/Wild and main planets in the solar system.
The orbit of Comet 81P/Wild used to be between the orbit of Jupiter and the Kuiper belt; however, its orbit changed when it approached Jupiter in September 1974, thus causing it to come close to Earth.
The world's-highest-level teamwork
Dr. Tomoki Nakamura, an associate professor at Kyushu University, who has contributed greatly to the discovery of chondrules in comets, is interested in the interactions between materials inside and outside the solar system and has carried out research into cosmic dust. He therefore boasts that his research team has world-leading experimental facilities and a top-class network of engineers for analyzing the dust.
He carried out intensive analysis in the Stardust Project,* in which the dust scattered from Comet 81P/Wild was collected and brought back to the Earth, and achieved significant results ahead of scientists in other countries.
Dr. Nakamura started his analysis by performing a nondestructive test. He carried out an X-ray diffraction experiment using the SPring-8 beamline BL37XU and the synchrotron radiation facilities of the High Energy Accelerator Research Organization. From this experiment, the types of crystal (mineral) that constitute the dust and their abundance ratios were determined. Following his research, Professor Akira Tsuchiyama at Graduate School of Science of Osaka University and his colleagues visualized the internal structure of the dust by X-ray computed topography (CT) using the SPring-8 beamline BL47XU.
In tests that require direct contact with samples, there is no second chance. Therefore, it was necessary to collect as precise information on the internal structure as possible by nondestructive testing. The diameter of each dust particle was only 5-30 μm. The high-brilliance synchrotron radiation at SPring-8 was indispensable for precisely analyzing such ultrafine samples.
After the nondestructive test came a destructive test. First, each dust particle was fixed with epoxy resin to prevent it from dispersing during the measurement. The fixed dust particle was cut into two. One half was sliced to a thickness of 0.1 μm and subjected to transmission electron microscopy to observe its internal structure in detail. This was carried out by Dr. Takaaki Noguchi, an associate professor at Ibaraki University.
The other half was subjected to scanning electron microscopy and isotope measurement using a secondary ion mass spectrometer* to determine its elemental composition and structure. Dr. Nakamura performed the scanning electron microscopy using apparatuses at The University of Tokyo and Osaka University. He also conducted the isotope measurement using the secondary ion mass spectrometer of University of Wisconsin, with the cooperation of Dr. Takayuki Ushikubo and Dr. Noriko Kita; many people had a hand in these series of experiments.
Decisive pattern
Figure 3 shows a cross-sectional image of a comet dust particle, in which an irregular pattern of pale circular areas can be seen. This is conclusive evidence that chondrules are contained in the comet dust. The circular areas are olivine, and the sections surrounding them are pyroxene. When the dust was heated to about 1500°C by the heat radiated from the Sun, olivine, which has a high melting point, maintained its spherical form without melting, whereas pyroxene melted because of its low melting point. This resulted in the pattern of spherical areas after the dust was cooled.
The group led by Dr. Nakamura examined nearly 70 samples by X-ray CT. Among them, 15 samples suitable as analytical targets were sliced and subjected to various measurements. Chondrules were found in 6 of the 15 samples. Because the SPring-8 high-brilliance synchrotron radiation enabled highly precise analysis, this significant result was obtained in a very short time.
In the dust particles, several types of chondrules were observed. The chondrule type was determined by measuring the oxygen isotope ratio, which told us where in the solar system the chondrule was formed. As a result of this measurement, 5 of the 6 samples turned out to be akin to the carbonaceous chondrules that exist in asteroids (Fig. 4). This type of chonrdule is abundant from the center to the outer edge of the asteroid belt (Fig. 5)
Fig. 3 Electron microscopic image of comet dust particle Torajiro.
An irregular circular pattern comprising olivine and pyroxene is confirmed. Aerogel, a material used to trap the dust, is melted because of the energy at the trap and becomes attached to the dust.
Fig. 4
The origin and transition of the solar system can be examined by measuring the oxygen isotope ratio in comet dust particles. All the measured values, indicated by the red circles and green triangles (right figure), are close to those of carbonaceous chondrules. The oxygen isotope ratio is given as the ratio of the abundance of 17O and 18O to that of 16O, which is the most abundant of all oxygen isotopes. CCAM represents the oxygen isotope ratio in the carbonaceous chondrite anhydrous mineral. The black cavities in the dust (left figure) are the marks of irradiated ion beams.
Fig. 5
Carbonaceous chondrites that contain chondrules originating from C-, P-, and D-type asteroids, which are abundantly distributed from the center to the outer edge of the asteroid belt, 3 to 5 times further from the Sun than Earth. Comet P/81 Wild is considered to have been formed in the Kuiper belt (30-50 astronomical units). The chondrules found in the comet are very similar to those in asteroids formed in environments much closer to the Sun. The astronomical unit indicates the ratio of the distance from the Sun to that between the Earth and the Sun.
Necessity of new model of solar system formation
The dust of Comet 81P/Wild was exposed to a high temperature of 1500°C in the past. This is considered to have happened as close to the Sun as the asteroid belt. However, dust was also found in comets originating from beyond the orbit of Uranus. How can this be explained?
According to Dr. Nakamura, it is possible that the migration of chondrules occurred in the primitive solar system formed by the disk of dust. This hypothesis can be partly explained using the current model of solar system formation, but the explanation is not complete. The establishment of a new model of solar system formation* is necessary. Dr. Nakamura said that the analysis of at least 20 samples is required to obtain a statistically significant conclusion and that he would like to improve the precision of analysis by analyzing many more samples in the future. He also said that he is planning to carry out chronological measurements of chondrules in comets to determine precisely when their migration occurred.
In 2010, the asteroid explorer Hayabusa will return to Earth. Dr. Nakamura is involved in the preparations for its return. He will play an active role in the analysis of samples that may have been collected from the asteroid Itokawa.
Column: Travels with Torajiro
Dr. Nakamura said with a wry smile that he sometimes doesn't know where he is when he wakes up in the morning. He rushes about the world, including his home ground Fukuoka, Hyogo, Osaka, Ibaraki, Tokyo, and even the US, to carry out his research on dust analysis, and thus he spends 120 days a year visiting other institutions to carry out his research activities and attend conferences. While he is at Kyushu University, he is so busy giving lectures and supervising students that he cannot devote himself to his own studies.
He has been fond of racing cars since his student days, and is addicted to this hobby because of the excitement of feeling gravity on his body when turning a sharp corner. However, he said sadly, that these days he is too busy to spend time on his hobby. Instead, he flies in airplanes about 60 times a year.
He gets rid of stress by watching the movie series “Otoko wa Tsurai yo. (It's Tough Being a Man.)” He is such a fan of this movie series that he named the comet dust Torajiro after the main character, who was a travelling salesman. He carries DVDs of this movie series with him, and listens to its songs during his travel.
Dr. Nakamura visits the filming location of “Otoko wa Tsurai yo.” on holidays. The shop used as the backdrop of the movie “Otoko wa Tsurai yo. - Torajiro's Holiday” is now closed (Mameda-cho, Hita City, Oita Prefecture).
Interview and original text by Tomoaki Yoshito (Sci-Tech Communications Incorporated)
Glossary
Chondrules
Chondrules are formed when a mixture of a rock containing magnesium (Mg), silicon (Si), and oxygen (O) as its main components and a small amount of iron is rapidly cooled.
Stardust Project
In 1999, the Stardust spacecraft was launched by NASA and returned with dust samples to Earth 7 years later in January 2007.
Secondary ion mass spectrometer
By irradiating a primary ion beam (Cs) onto a sample, the mass of ions (secondary ions) removed by the collision energy is measured. Secondary ions are transported to the measurement stage by electric fields. The percentage mass of ions in the primary ion beam that are reflected by the sample to form a secondary ion beam is called the secondary ion yield. The secondary ion mass spectrometer of University of Wisconsin exhibits the world's highest yield of 70-80%.
New model of solar system formation
Previously, a model was proposed in which a jet, i.e., a strong flow of plasma, flowed from the center to the outside of the solar system during the formation of the solar system because of magnetic fields. However, this model cannot explain why the chondrules in comets are similar to those in a certain asteroid. Therefore, a new model is needed.
This article was written following an interview with Associate Professor Tomoki Nakamura at the Department of Earth and Planetary Sciences, Graduate School of Sciences, Kyushu University. Please also refer to the Research Achievements and Topics section under the title “Investigation of Mystery of the Solar System from a Small Sample at SPring-8,” SPring-8 News No. 33.(*Japanese only)