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.
Red/green stereo anaglyph of Comet Wild 2
"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
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
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:
JPL Media Contact for Stardust:
Last Updated: December 14, 2006