Halley’s core evaporating

Press, 2 December 1985, Page 16

Halley’s core evaporating

Ultraviolet and radio observations of Halley’s Comet have provided evidence that its core is composed largely of ordinary water ice.

According to the “New Scientist” magazine, astronomers have detected a substance formed by the breakdown of water when exposed to the Sun’s ultraviolet radiation. When Halley’s Comet was still beyond the orbit of Mars, passing below the asteroid belt as it came in towards the Sun, the Sun’s heat already was sufficient to make its frozen material evaporate and form a large gaseous head, or coma.

The latest observations with the British Isaac Newton Telescope on La Palma show evidence for carbon elements in the coma, which stretches out to a total extent of 10 arcminutes (one-third the apparent size of the Moon).

Astronomers expect that these molecules should be only a minor constituent of the “dirty snowball” that lies in the comet’s centre. The major constituent should be water ice, which leaves few telltale signs at optical wavelengths. Evidence of water in the comet shows up at other wavelengths. Radio astronomers at Nancay, in Fragce, were the first to detect

hydroxylions, a breakdown product of water. Late in August, they picked up the characteristic radio wavelengths emitted by hydroxyl, at 1665 and 1667 megahertz. During the ensuing week, the signals increased in strength as the comet came closer to the Sun. The signal strength then indicated that Halley’s Comet was losing 25,000 tonnes of water ice a day. Astronomers using the International Ultraviolet Explorer (1.U.E.) satellite detected hydroxyl by its ultraviolet emission on September 12, and saw additional hydroxyl emission at other wavelengths nine days later. On September 12, the production rate was twice as great as the radio astronomers had found two weeks earlier. By September 21, the rate of evaporation of water ice had doubled again to 100,000 tonnes a day. On this latter date, the I.U.E. team also detected the first signs of emission from hydrogen atoms, the other product of the breakdown of water into hydroxyl. Observations of other comets suggest that Halley will grow a hydrogen “halo” as large as the Sun when it is at its closest point to the Sun, next February. .— London Press Service.