The research also indicates that theories of how comets were formed may need to be revised.

Comets are lumps of ice, gas, rock, and dust - frozen relics from the birth of our solar system - that orbit the Sun. Scientists now believe comets could have formed at different times during the evolution of the solar nebula, and may reveal their age by the structure of the dust they carry.

Within a comet's cosmic cloud, astronomers have found two kinds of dust grains; grains with their molecules stuck together every which way, called amorphous, and grains with molecules that have an orderly, crystalline structure. The dust emit light of various colors at different intensities, allowing astronomers to distinguish between the two.

The researchers believe molecular clouds, like the one that collapsed to form the solar nebula, contain only amorphous dust. Crystalline grains formed later, as the dust clouds were heated by the forming Sun.

The research, to be published in the July 20 issue of Nature, indicates that comets with mostly amorphous dust are ancient because they formed early in the solar nebula's evolution, before the Sun had time to heat and distribute very much crystalline dust. Comets with a large proportion of crystalline dust formed later as the nebula evolved and crystalline grains became more common.

"The fun part of laboratory work like this comes when you try to tie it together with observations, and you run into an interesting problem," said lead author Dr. Joseph Nuth, Supervisory Astrophysicist at NASA's Goddard Space Flight Center, where the laboratory research was conducted.

"Observations in 1989 found crystalline olivine dust in comet Halley. Our research placed severe constraints on how fast this dust crystallizes, and we realized that Halley could not have formed the way astronomers think it formed," added Nuth.

Astronomers believe Halley formed exclusively from material present in the region of the giant planets (Jupiter and Neptune), then was ejected to the cold fringes of the outer solar system, well beyond Pluto.

"We know that these dust grains change from amorphous to crystalline as they are heated, and our laboratory research revealed that the rate at which they change is extremely sensitive to temperature," Nuth added.

"At the very low temperatures, where water-ice and the other volatile components of comets are frozen, the time required for amorphous silicate dust grains to change to the crystalline olivine found in comet Halley is many times longer than the age of the universe," he added.

The crystalline olivine dust must have been made much closer to the Sun, where temperatures were higher, and olivine could have been formed in hours, days, or years.

However, at these warm temperatures, the ices that make up the bulk of Halley could not form. Researchers believe the crystalline dust formed near the Sun and was thrown out to the region near the giant planets, where it was incorporated into Halley.

This outflow of material is a new twist on models of comet formation and the solar nebula's evolution.

"Previously, it was believed that most of the matter in the nebula ended up in the Sun and planets. By and large, this is still correct, but we now realize that there must have been a small amount of material flowing away from the forming Sun as well," said Nuth.

Since we get regular visits from comet Halley, it is no longer in "cold storage" in the Oort Cloud. Its orbit has been disturbed and now takes it into the inner solar system.

"Another suggestion to explain the presence of crystalline grains might be that the crystalline dust forms on the surface of Halley as its orbit takes it closer to the Sun.

"This is unlikely, since at Halley's closest approach to the Sun, the temperature is still so low that the crystalline olivine would take billions of years to form. If Halley has been visiting the inner solar system for this long, it would have been vaporized already," said Nuth.

The researchers hope future comet observations will help test their theory. "As the solar nebula evolved, organic molecules present in the gas became progressively more complex. The Sun supplied the energy, and the dust grains acted as a catalyst to promote complex organic molecule formation.

"We expect ancient comets - those with a lot of amorphous dust grains - to have few complex organic molecules. Comets that formed more recently should have a large proportion of complex organic molecules along with their crystalline dust," said Nuth.

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