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The Forefront of Space Science

The True Face of Comets Illuminated by AKARI
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AKARI is a time machine

Then, what we can discover from such observation? First of all, letís consider where and how cometary nuclei were created. There are many types of comets: some reappear in cycles of several years to tens of years; some come from the outer rim of the solar system and disappear beyond the system without returning. Tracing their origin, it is thought that the home of most comets is a relatively limited region a little outside the rim of the early solar nebula, specifically the region from Jupiter to beyond Neptune. A number of planetesimals from several km to several tens of km in size were born from gas and dust in the disk of the primitive solar nebula 4.5 billion years ago. These planetesimals merged and grew to protoplanets, and grew further to become todayís planets. This is a typical theory of the solar system formation. Planetesimals born in the cold area outside of Jupiter contained much ice. It is thought that many planetesimals collided with each other or were absorbed into protoplanets or planets. Meanwhile, there must have been many planetesimals that did not collide but whose orbits were changed by planetary gravity. Finally, they were blown out to the outer rim of the solar system. They became the origins of comets.

According to this history, comets are considered as fossils of the solar system, and contain various data of the time the system was formed 4.5 billion years ago. The composition of the gas released from these fossils is an important clue to discover the true face of the solar system in the age of the early solar nebula. Cometary nuclei were created in the solar system 4.5 billion years ago and, further, in a deep disk made of gas and dust. We have no means of knowing such place directly. All the information, including the ratio of ice and dust, of water and carbon monoxide, and of water and carbon dioxide, is a clue to know the deepest area of the primitive solar nebula of eons ago.

Fig. 2 is an example of the spectrum of comet Lulin observed by AKARI. Strong radiation is clearly visible around 2.6 to 2.7μm of water and around 4.2 to 4.3μm of carbon dioxide. Meanwhile, it is noticeable that carbon monoxide radiation, which is expected to appear around 4.7μm, is weak. By applying a model assuming the material distribution of cometís coma, we forecast a ratio of molecular abundances in comet Lulinís coma. The result is that, assuming the number of water molecules is 100%, the relative number of carbon dioxide is about 4 to 5% and that of carbon monoxide is less than 2%. These values of carbon dioxide and carbon monoxide are rather low compared to the observation results of comets in the past. Carbon dioxide (dry ice) transforms to gas at a lower temperature than water. Similarly, carbon monoxide changes to gas in lower temperature than carbon dioxide. Considering this fact, we infer that there is a high possibility that Lulinís cometary nucleus was created in a relatively hot place within the primitive solar nebula, in other words, closer to the Sun.

This type of observation and research focusing on carbon dioxide in comets is still in its developmental phase. At this stage, we can identify the ratio of molecule abundances of several comets, but cannot forecast where their cometary nuclei were formed in the past solar system. Nevertheless, AKARI is actively observing comets (e.g., Fig. 3). We expect that observation cases will increase and data accumulate to the degree that we can discuss them statistically. The data will bring important clues to elucidate what evolution occurred in the deeper regions of the dust disk in the primitive solar nebula 4.5 billion years ago, and what materials composed the cometary nuclei (planetesimals) or protoplanets. The combination of comets and AKARI is a kind of time machine to explore the history of the solar system.

Figure 3
Figure 3. Three-colored false image of comet Christensen (C/2006 W3) observed by AKARI/IRC

Takafumi OOTSUBO
(From January 2010) Associate Professor, Astronomical Institute, Graduate School of Science, Tohoku University

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