RELASE NO: 98-199

SHORT LIVED MOLECULE FIRST SEEN IN ATMOSPHERE OF NEPTUNE

Methyl, a short-lived product of the breakup of methane molecules, has been detected in the turbulent outer atmosphere of Neptune. This marks the first time a highly reactive and therefore short lived molecule of hydrogen and carbon, called a hydrocarbon radical, has been seen in the atmospheres of the outer planets.

The discovery, made by the European Space Agency's Infrared Space Observatory (ISO), may help explain the puzzling abundance of another hydrocarbon molecule in atmospheres of these vast but remote worlds. Methyl may also serve as a tracer to measure the turbulence in their extensive atmospheres.

"The mystery is that there is too much ethane in the outer planets for our model to explain. Sunlight is needed to make ethane; it breaks down methane to methyl, which combines to give ethane. The concentration of atmospheric ethane from Jupiter to Neptune is about the same," said Dr. Paul Romani of NASA's Goddard Space Flight Center in Greenbelt, Md. The problem is that Neptune is much farther from the Sun than Jupiter, so the sunlight reaching Neptune is about 33 times fainter. Detecting methyl on Neptune may help us understand how Neptune's atmosphere makes so much ethane when it receives so little light."

A methyl molecule consists of a single carbon atom and three hydrogen atoms, while a methane molecule is made up of a carbon atom and four hydrogen atoms, and ethane has two carbon atoms and six hydrogen atoms.

Romani's model of the formation of methyl on Neptune helped convince the ISO team, led by Dr. Bruno Bezard of the Paris Observatory, Meudon, France, to look for the scarce gas. The model told the team where methyl could be found in Neptune's atmosphere and what amount of infrared light the molecule would emit.

"We took Dr. Romani's prediction and determined that there should be enough methyl in the right part of the atmosphere for ISO to be able to see methyl emissions from Neptune, which is about three billion miles from Earth. We also want to look at the other gas giant planets for comparison studies. If we can see how different atmospheres produce methyl, we can refine our model of its formation. We have detected it on Saturn recently as well," said Dr. Bezard.

The observations revealed that methyl has an average concentration of one part in 30 million in Neptune's atmosphere, and two to three times higher concentration in Saturn's atmosphere. Infrared light, invisible to the human eye, is perceived as heat. The infrared light from methyl is blocked by Earth's atmosphere and required a sensitive observatory in space to detect.

Methyl may also be used as a tracer to monitor the turbulent weather in the upper atmospheres of the outer planets.

"Telescopes using ordinary, visible light can view clouds, but the region where methyl is detected is above them. Thus, methyl observations reveal turbulence ordinary telescopes can't see," said Dr. Romani.

Massive storms, some almost the size of the Earth, churn in Neptune's atmosphere. These storms may inject methane into the upper atmosphere, where sunlight breaks it down and produces methyl.

"The amount of methane in Neptune's upper atmosphere is 10 to 100 times over the saturation level. We think it gets there due to the action of these enormous storms. A similar situation occurs on Earth, when huge thunderstorms over the equator inject water into the stratosphere. Observing methyl may therefore tell us about the weather on Neptune," said Dr. Romani.