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TIME TRAVEL THROUGH A TRAIL OF COMET DUST

Dolores Beasley
Headquarters, Washington, DC              July 19, 2000
(Phone: 202/358-1753)

Bill Steigerwald 
Goddard Space Flight Center, Greenbelt, MD
(Phone: 301/286-5017)

RELEASE:  00-111

TIME TRAVEL THROUGH A TRAIL OF COMET DUST 

     If you've seen one comet, have you seen them all? Not 
according to new NASA research.

     Scientists believe they may one day be able to travel through 
time by looking more closely at the dust swirling with a comet as 
it hurdles through our galaxy. 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."

     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."

     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. 

     For images and more information, refer to: 

http://www.gsfc.nasa.gov/gsfc/spacesci/solarexp/cometage.htm

                          -end-

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