A trail of discovery
RECENTLY, two groups of researchers published their findings on comet Hyakutake. They have detected certain compounds in the comet which could provide us with valuable information about the composition of the nebula from which our Sun and the planets emerged. Comets are also believed to have been a source of many of the organic compounds seen on the terrestrial planets (Nature, Vol 383, No 6599).
In March 1996, the comet Hyakutake (officially termed as comet c/Hyakutake 1996 132) was at its perigee or the closest distance to the earth. It came as close as about 10 per cent of the distance between the Sun and the earth. This close range gave astronomers a unique opportunity to observe the comet with various instruments.
William Irvine from the Paris Observatory, France, and his collaborators have detected hydrogen isocyanide (HNC) molecule in comet Hyakutake. Using the Caltech Submillimeter Observatory (cso), California, us, the group detected and identified a spectral line at a frequency of 362.630092 giga- hertz, which is characteristic of the compound.
The group has also measured the abundance of another organic molecule, hydrogen cyanide (HCN) in the comet. The surprising result is that the abundance of HNc relative to HCN is very similar to that seen in interstellar molecular clouds. Furthermore, the ratio is very different from that expected in the material in the outermost reaches of our solar system, from which the comets are thought to have been formed.
A comet has a solid nucleus composed of ice mixed with micrometer-sized dust grains. As the nucleus approaches the Sun, the temperature rises and the ice evaporates. The resulting gas escapes with the dust particles trailing in a gigantic tail, millions of km long.
The studies have lead the researchers to conjecture that some of the interstellar gases could have been incorporated into the comet's nucleus as ice on the interstellar grains. This hypothesis, if verified by other data, will provide an important clue to our understanding of the conditions prevalent in the solar nebula at the time when the planets and the comets formed.
Close on the heels of this comes another report of the discovery of x-ray and extreme ultraviolet emission from Hyakutake. c m Lisse and his collaborators have studied the comet at its perigee using the Roentgen x-ray satellite and the Rossi x-ray Timing Explorer. Lisse and his team's observations are significant as x-ray and ultraviolet emission from comets had not been observed before.
The researchers believe that the interaction between the comet and the solar wind (the constant stream of energetic particle being emitted by the Sun) or the solar magnetic field could be a possible explanation of the emission. They have also found x-ray emission from three other comets as well as Hyakutake in its post perihelion (the closest distance to the Sun) phase. The perigee of the comet Hale-Bopp will provide another opportunity for scientists early next year to confirm these findings.