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May 31, 1996

Douglas Isbell
Headquarters, Washington, DC
(Phone: 202/358-1753)

Jim Sahli
Goddard Space Flight Center, Greenbelt, MD
(Phone: 301/286-0697)

RELEASE: 96-108

Chemical Measurements of Comet Hyakutake Suggest a New Class of Comets

Astronomers observing the close approach of Comet Hyakutake to the Earth in March discovered large quantities of the gases ethane and methane in the comet. This is the first time these or other molecules classified as "saturated hydrocarbons" have been found in a comet, strongly suggesting that at least two basic types of comets inhabit the Solar System.

This conclusion also has potentially profound implications for scientific theories that describe the primordial conditions that led to the formation of the Sun and the planets.

The discovery by a team of NASA and university researchers using the NASA Infrared Telescope Facility at Mauna Kea, Hawaii, will be published in today's issue of Science magazine.

Ethane has never before been detected in comets or in interstellar matter, the ultimate source material from which the Solar System was formed. Yet, comet investigators found levels of ethane in Comet Hyakutake that are about 1,000 times greater than can be explained if the molecules were formed by normal physical processes within the gases of the primordial solar nebula, the birth cloud of the Solar System.

"The discovery of ethane was a blinding surprise," according to research team leader Dr. Michael J. Mumma of the Laboratory for Extraterrestrial Physics at NASA's Goddard Space Flight Center, Greenbelt, MD. The spectral lines, or identifying signature of ethane gas, "were so bright they seemed to leap off the computer screen when we got the first observation," Mumma said.

The discoveries were made on March 24, 1996, with the three-meter diameter telescope of the NASA Infrared Telescope Facility atop Mauna Kea. The investigators used a state-of- the-art instrument known as a high-resolution infrared spectrometer. The device was cooled to about minus 400 degrees Fahrenheit to achieve the needed sensitivity to infrared light, which has a longer wavelength than red light and cannot be seen with the human eye.

The unexpected ethane discovery came as the observers searched for evidence of molecules of methyl alcohol, a known constituent of other comets. However, "the emissions of methyl alcohol that we first looked at were much weaker than expected, so we decided to search for other signatures of the alcohol," said research team member Dr. Michael A. DiSanti of the Catholic University of America, Washington, DC. "But after reprogramming the spectrometer, instead of detecting methyl alcohol, we discovered ethane."

Further observations and analysis showed that ethane and methane each constitute about one percent of the frozen gases in Comet Hyakutake. (The astronomers measured radiation from gases released from their frozen state as the solid nucleus -- or "dirty iceball" -- of the comet was warmed by the Sun.)

"Comets that are rich in ethane must have experienced very different conditions during their birth than comets that do not contain it," Mumma said. One theory is that ethane- rich comets formed in the warmer region near the primitive Saturn and Jupiter, while those without it formed farther away from the young Sun, near the primitive Uranus and Neptune.

Another possibility is that cometary ices formed even earlier, in different layers of the original interstellar gas and dust cloud that led to the solar nebula. An even more challenging concept is that the vast sphere of comets that are believed to surround the Solar System, called the Oort Cloud, may contain comets that formed from different solar nebula -- that is, stars other than the Sun. Chemical and physical processes may have been at work in any scenario, altering the properties of the material that now makes up the comet's ice.

The discovery of ethane in Comet Hyakutake will spur scientists to go back and review measurements of other comets to see if unusual blips in their data contain hints of ethane. "For example, we're going to go back and look at Comet Halley data again," Mumma said. Similar measurements of Comet Hale-Bopp, which will pass closely by Earth in March and April 1997, are scheduled for June, he added.

As a comparison to comets, there are three major categories of asteroids. Some of the rocky bodies now considered to be asteroids may in fact be dead nuclei of short-period comets.

Both ethane and methane occur naturally on Earth and some other planets, and in certain meteorites, including the Murchison meteorite that fell on Australia in September 1969. While ethane is much less common than methane in the planets, it is almost equally abundant to methane in both Comet Hyakutake and in the Murchison meteorite, the researchers note. "Therefore, it is possible that the gases found in the Murchison meteorite and those found in the comet had a common origin," according to Dr. Marina Fomenkova of the University of California at San Diego. "However, the diversity of organic material in primitive meteorites and in comets shows that they formed under a wide range of conditions," she cautions.

Science team members including Drs. Karen Magee-Sauer, Rowan College of New Jersey, Neil Dello Russo and David X. Xie of the Goddard Space Flight Center, and Charles Kaminski of the NASA Infrared Telescope Facility office, Hilo, Hawaii, are continuing to investigate the questions raised by the cometary ethane discovery.

"This is the type of finding that makes a person excited to be a planetary scientist," Mumma said. "It may open a new window on our understanding of comets and their role in shaping the world in which we live."