Near infrared spectroscopic observation of Comet LINEAR (C/2002T7), which will approach the Earth in May 2004 and is expected to be very bright, was carried out with the Subaru Telescope in September 2003. After analyzing the spectral data, we discovered water icy grains in the coma of Comet LINEAR. Comet Hale-Bopp was the first example of the detection of water icy grains in a cometary coma, and this time is the second one. Cometary nuclei are thought to be remnants of planetary sources (called planetecimals) existed in the early solar system. Since it is considered that the cometary nuclei were formed from icy dust (nonvolatile dust covered with ice mantle) in the interstellar medium or the solar nebula, research into such water icy grains in the cometary nuclei is very important to reveal the physical conditions of the early solar system.

Fig. 1: Close-up view of Comet LINEAR with CISCO (Range: 25000km×25000km) Low Res. (166 KB) Middle Res. (311KB)

It is known from past studies that cometary ices consist of water (H₂O) ice (more than 80 %), and the remainder is carbon monoxide (CO) and carbon dioxide (CO₂) (about 20 % or less). The CO and CO₂ ices are higher volatile materials than water ice, and they are easy to volatilize even in the low-temperature region far from the Sun. It is therefore possible that water icy grains can directly be observed from comets beyond the area of 3 Astronomical Units (AU: 1 AU = approximately 150,000,000 km) from the Sun where water icy grains do not volatilize, because the CO and CO₂ ices may be the power to release the water icy grains and dust from the cometary nucleus.

We observed Comet LINEAR approaching the Earth with the Cooled Infrared Spectrograph and Camera for OHS (CISCO) attached to the Nasmyth focus of the Subaru Telescope on September 14, 2003 and discovered water icy grains in its cometary coma. The comet was at about 3.5 AU from the Sun, and the brightness was only 1/100 of that of Comet Hale-Bopp at the same distance. Comet LINEAR is supposed to become brighter when the comet is getting closer to the Sun; however, it is difficult for the water icy grains to exist in the cometary coma since the water icy grains are heated by the Sun and easily vaporized.

The successful detection of the water icy grains in Comet LINEAR was achieved by the high collecting power of the Subaru's 8.2m-primary mirror and the high spatial resolution to resolve the small region where the water icy grains existed. The Subaru Telescope will conduct observational studies of water icy grains in many other comets.

Fig. 2: This figure shows the observed spectrum of comet LINEAR taken with the Subaru' CISCO (red points with error-bars), and modeled spectra (purple and blue solid lines). The purple line corresponds to the case of pure water icy grains (1 micrometer in diameter: 1 micrometer = 0.001 millimeters). The red line corresponds to the case of water icy grains (5 μm in diameter) with dark materials. The model results are based on the optical constants of crystalline water ice, and the absence of 1.65 μm absorption feature in the observed spectrum indicates that the cometary water ice was in amorphous state at the observation. This means that the water ices formed under low-temperature condition (colder than -150 deg-C).