Observations made with the High-Dispersion Spectrograph (HDS) of Subaru Telescope have, for the first time, allowed astronomers to measure the formation temperature of ammonia ice in a comet. The temperature of 28 +/- 2 Kelvin (about -245C or -410F) suggests that this comet, Comet LINEAR (C/1999 S4), was formed between the orbits of Saturn and Uranus. These observations provide us with not only direct evidence of the environment in which the comet was born, but also establish brand new methods for probing the origin of comets.

Two images of Comet LINEAR observed in 2000 with Subaru Telescope. Credit: National Astronomical Observatory of Japan

Comet LINEAR was discovered in 1999 by the Lincoln Near Earth Asteroid Research project (LINEAR), operated by the MIT Lincoln Laboratory. Figure 1 above shows two images of Comet LINEAR obtained by Subaru Telescope in 2000. A team of researchers from the National Astronomical Observatory of Japan, the HDS group, and the Gunma Astronomical Observatory made spectroscopic observations of Comet LINEAR on July 5th, 2000, during the commissioning phase of HDS, when the comet was bright.

The team concentrated on the emission lines produced when NH2 molecules which have been previously excited, lose some energy and emit light at a series of characteristic wavelengths. Previous studies indicate that NH2, which consists of one nitrogen and two hydrogen atoms, is produced when the powerful Solar UV rays free a hydrogen atom from the ammonia (NH3) gas which is constantly boiling off the comet. The emission lines of the NH2 molecules should therefore contain information on their parent ammonia molecules.

Molecules like NH2 and NH3 which contain two or three hydrogen atoms are classified as either "ortho" or "para", depending on whether the quantum mechanical spins of the hydrogen atoms are aligned or not. The ortho-to-para ratio strongly depends on the physical environment, and would have been preserved when the molecules were confined into the icy cometary nuclei. The observed ratio can therefore reveal the temperature at the time the ice was formed.

Molecules in the ortho and para states emit radiation at wavelengths which are very close together, but subtly different due to the differences in alignment between the spins of the hydrogen atoms. The resolving power of HDS is high enough to separate these lines and determine how much light is being emitted by molecules in the ortho and para states. Using code written by Mr. Hideyo Kawakita of the Gunma Astronomical Observatory, the strengths of the emission lines from NH2 could be modeled and compared with the observations to determine the ratio of ortho to para molecules in Comet LINEAR. Furthermore, the team investigated the ortho-to-para ratio of the parent NH3 molecules and estimated that the formation temperature of the ammonia ice to be 28 +/- 2 Kelvin, which suggests that Comet LINEAR was formed between the orbits of Saturn and Uranus in the primordial Solar System nebula.

Until now, the formation temperature had only been determined for water ice in comets, and this is the first time that it has been measured for another molecule. Dr. Jun-ichi Watanabe of the National Astronomical Observatory of Japan, and a member of the team who performed this research, says "The brand new methods using NH2 molecules have great potential for studying the origin of comets. I have a high expectation for future results obtained by these methods, especially for short-period comets which are thought to have a different origin from long-period comets such as Comet LINEAR."

This result has been published in Science, November 2nd, 2001 Issue.

Note: Comet LINEAR (C/1999S4) was an unusual comet because it disintegrated about one month after the HDS observations. A 50-cm telescope at the Mitaka campus of the National Astronomical Observatory of Japan monitored this disintegration.