MOUNTAIN VIEW, Calif. -- Slight temperature differences in the two comet-forming regions of the solar system cause the water-ice largely comprising comets to form in different ways, NASA researchers says.
"We predict that comets from the Kuiper belt and Oort cloud contain structurally different forms of water ice," Peter Jenniskens at Ames Research Center said. Jenniskens, with David Blake at Ames, published their results in today's issue of the journal Science.
Comets are thought to be pristine chunks of debris left over from the solar system's formation about 5 billion years ago. They are made of more than 40 percent water ice and come from exceedingly cold regions of the solar system.
According to Jenniskens, two populations of comets exist, based on their present location in the solar system and where scientists think they originated.
Some comets formed in the Kuiper belt, which is located in the outer region of the solar system beyond Pluto's orbit. These comets, known as short-period comets, probably formed at temperatures colder than minus 370 degrees Fahrenheit, he said.
Oort cloud comets probably formed in the Neptune-Uranus region and were then expelled to much greater distances from the sun. Oort cloud comets, known as long-period comets, were probably formed at temperatures warmer than minus 320 degrees Fahrenheit, Jenniskens said.
Most Oort cloud comets come from a solar "sphere" around 30,000 astronomical units (AU) away, but scientists think the cloud itself extends almost halfway to the nearest star, Alpha Centauri, which is four light years from the sun. One AU equals the distance from the Earth to the sun. There are about 60,000 AU in a light year.
Water vapor frozen onto the rocky grains which coalesced to form comets is frozen like a glassy film rather than a crystalline solid, Jenniskens said. This glassy ice has the same basic structure as liquid water. In this form, the water molecules are connected to each other by four strong hydrogen bonds in an open cage-like structure.
At the very low temperatures of comet formation in the Kuiper belt -- colder than minus 380 degrees Fahrenheit -- some water molecules are trapped in the cages of this structure during freezing. But comets formed at slightly higher temperatures in the Oort cloud expelled the water from the cages. Kuiper belt comets, therefore, consists of a different form of water ice than Oort cloud comets.
Comet Shoemaker-Levy which recently crashed into Jupiter, was most likely a lower temperature, short-period comet formed in the Kuiper belt, Jenniskens said. Because short-period comets orbit the solar system faster and more often, Jupiter's gravity is more likely to capture them, he said.
Jenniskens' and Blake's prediction are based on laboratory
simulations of cometary ice formation under conditions thought to exist
when the objects formed. They discovered the water-trapping process of
the lower temperature comets by simulating the freezing process in an
Ames' laboratory and observing the process with a transmission electron
microscope. Their research is conducted in the Space Science Division
at Ames.