[meteorite-list] Stardust Findings Suggest Comets More Complex Than Thought

[Ron Baalke]

http://stardust.jpl.nasa.gov/news/news110.html

Stardust Findings Suggest Comets More Complex Than Thought
December 14, 2006

Comets may be more than just simple conglomerations of 
ice, dust and gases. Some may be important windows on 
the early solar system. Others may have contributed 
materials necessary to the development of life on our 
own planet.  Scientists have found a wide range of 
compositions and structures for the comet Wild 2 
particles that were captured and returned to Earth by 
NASA's Stardust spacecraft. Their findings indicate the 
formation of at least some comets may have included 
materials ejected from the inner solar system to the far 
and cold outer edge of the solar nebula.

Minerals formed near the sun or other stars were in the 
samples returned to Earth by the Stardust spacecraft in 
January 2006. The findings suggest materials from the 
inner regions of the solar system could have traveled 
to the outer reaches where comets formed. This may alter 
the way scientists view the formation and composition of 
comets. 

"We have found very high-temperature minerals, which 
supports a particular model where strong bipolar gas 
jets coming out of the early sun propelled material 
formed near to the sun outward to the outer reaches of 
the solar system," said Michael Zolensky, NASA cosmic 
mineralogist and Stardust co-investigator at the 
Johnson Space Center (JSC), Houston. "It seems that 
comets are not composed entirely of volatile rich 
materials but rather are a mixture of materials formed 
at all temperature ranges, at places very near the early 
sun and at places very remote from it."

Zolensky is lead author of a paper on the mineralogy of 
the captured comet particles, one of seven reports about 
the mission's initial findings appearing in the Dec. 15 
issue of Science Express, the online edition of the 
journal Science.

Scientists have long thought of comets as cold, billowing 
clouds of ice, dust and gases formed on the edges of the 
solar system. But comets may not be so simple or similar. 
They may prove to be diverse bodies with complex 
histories. Comet Wild 2 certainly is made up of 
components with a more complex history than thought.

Scientists found a wide variety in particle composition 
and size in the Wild 2 samples. Most of the Wild 2 
samples appear to be weakly constructed mixtures of very 
small grains with a few larger grains. Also, a wide range 
of high- and low-temperature minerals, from olivine to 
low- and high-calcium pyroxene compositions, is present 
in the Wild 2 samples. 

Such a diversity of high- and low-temperature minerals 
requires a wide range of formation conditions, probably 
reflecting different formation locations. Many particles 
did not form in the cold environment and locations where 
cometary ices condensed. 

Instead, they needed high temperatures to form, as well 
as complex and as yet little understood dynamical 
processes to end up where comets actually formed. Also, 
particles from different environments must have undergone 
some process of accretion to end up as aggregates composed 
of different minerals.

One of the major discoveries from the analysis of the comet 
samples was finding particles rich in organic matter. 
"Comets are believed to have brought water and organic 
matter to the early Earth, and it is important to 
understand the nature of these materials because they 
are necessary ingredients for the origin of life," said 
Lindsay Keller, NASA scientist at JSC and Stardust 
co-investigator. "One of the first analyses we obtained 
on the samples showed abundant hydrocarbons in many of the 
particles."  

Subsequent analyses revealed that some of the organic 
matter formed in the cold cloud of dust and gas that was 
the precursor to the solar system. 

Keller led a team of U.S. and international scientists 
using spectroscopic techniques to study the Stardust samples. 
Spectroscopy uses different wavelengths of light and other 
radiation to discover the chemical makeup of materials. 
The team's results also appear in this week's issue of 
Science.

In addition to finding these varied compositions in the 
Wild 2 samples, Stardust investigators found a wide 
diversity of particle densities. The captured Wild 2 
samples are predominantly fine-grained, loosely bound 
aggregates, most also containing much larger individual 
crystals of olivine, pyroxene and iron/nickel sulfides. 
All analyses suggest that small and large Wild 2 
particles are composed of a similar, if not identical, 
suite of minerals.

"Many researchers, but not all, have thought that 
cometary solids are similar to interstellar dust, which 
is generally fine grained," said Friedrich Horz, NASA 
planetary scientist at JSC and lead author of a paper on 
impact features on Stardust and comet Wild 2 dust. "These 
models entail that such particles existed during the 
formation of comets, as 'leftovers' from the initial gas 
and solid mixture that were not processed further during 
comet formation in the cold environment from the sun."

Scientists found a much wider diversity of particle 
densities, including dense minerals, than advocated 
earlier by some researchers. 

"We see and demonstrate for the first time that there is a 
continuum between fine particles and more dense objects, 
the latter including pure minerals of, for example, 
olivine and iron sulfide," said Horz. "The range in 
density and cohesion that we saw and infer thus 
disagrees specifically with the popular model that 
comets are composed of only fine grains. A minority, 
however, did allow for particles of variable density. 
We have confirmed the latter viewpoint."

Stardust's pre-mission predictions included a wide range 
of sizes. Thus the findings substantiate the view of 
highly variable particle sizes. The specific size 
distribution found in Wild 2 samples differs from that 
of comet Halley dust (Stardust has fewer fine-grained 
particles) and from that of comet Grigg-Skjellerup 
(Stardust is more fine-grained). 

NASA's Jet Propulsion Laboratory, Pasadena, Calif., 
manages the Stardust mission for NASA's Science Mission 
Directorate, Washington. Dr. Peter Tsou of JPL is the 
deputy principal investigator and is a co-author on 
seven reports about the mission's initial findings 
appearing in the Dec. 15 issue of Science Express, 
the on-line edition of the journal Science.

For more information about Stardust studies and other 
mission information, visit:  

http://stardust.jpl.nasa.gov/

JPL Stardust Media Contact:  
DC Agle
818-393-9011
agle at jpl.nasa.gov