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The Forefront of Space Science

The True Face of Comets Illuminated by AKARI
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AKARI has observed comet Lulin several times at various timings. This article reports the observation results of March 30 and 31, 2009, after the comet passed through the perihelion (the nearest point to the Sun). The InfraRed Camera (IRC) onboard AKARI is able to conduct both imaging and spectroscopic observation. On March 30, we conducted near-infrared spectroscopic observation in the 2 to 5μm wavelength. On the next day, March 31, we performed three-color imaging observation in 2, 3, and 4μm. Fig. 1 is a false-color picture by three colors. The field of view (FOV) in images by AKARI is about 10arcmin x 10arcmin. The coma of comet Lulin extending to the full FOV can be clearly seen.

Figure 1
Figure 1. Three-colored false image of comet Lulin (C/2007 N3) observed by InfraRed Camera (IRC) onboard AKARI
Image was taken in 2, 3, and 4μm and falsely colored by blue, green, and red respectively to be overlapped. A coma extending more than 100,000 km is visible.

It was mentioned above that comet ice mostly consists of water, carbon dioxide, and carbon monoxide. With near-infrared or radio wave, we can observe molecules of water and carbon monoxide from the ground. We have been actively researching to estimate their abundances. However, it is impossible to observe carbon dioxide, the source of dry ice, from the ground. Carbon dioxide radiates in around 4.26μm and 15μm induced by its molecular vibration. To observe such radiation, we need to use a rocket or satellite to avoid the earthís atmosphere. The near-infrared wavelength region (i.e. 2 to 5μm) in AKARIís observation covers the 4.26μm radiation by molecular vibration of carbon dioxide. AKARI can also cover both the 2.66μm radiation of water molecules and 4.67μm of carbon-monoxide molecules. Thus, AKARI is the ideal satellite for observation of molecules contained in the cometary nucleus (Fig. 2).

Figure 2
Figure 2. Near-infrared spectrum of comet Lulin observed by AKARI/IRC
This shows that intensity in around 2.66μm and 4.26?m is strong, suggesting that much water (vapor) and carbon dioxide are emitted.

There have been only several observational cases of the carbon-dioxide molecules of comets across the world in the past. The cases are: two comets by nearby and in-situ observations by explorers (i.e. comet Halley by the Vega explorer of the former Soviet Union and comet Tempel 1 at the time of its collision with the impactor of NASAís Deep Impact mission); and comet Hale-Bopp and comet Hartley 2 by Europeís infrared astronomical satellite ISO. In addition, there was only one case with comet Hale-Bopp by ISO where the three main molecules of water, carbon dioxide and carbon monoxide were ďsimultaneouslyĀEdetected ďwith the same instrument.ĀEIf we could observe many comets from various heliocentric distances, we could determine more accurately the ratio of these molecules. AKARIís observation is revolutionary. It allows us in a short time to increase observational data of carbon dioxide contained in cometary nuclei more than fivefold.

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