[meteorite-list] Following the Dust Trail

[Ron Baalke]

http://www.universetoday.com/am/publish/following_dust_trail.html

Following the Dust Trail
Tammy Plotner 
Universe Today
June 2, 2005

Summary - (Jun 2, 2005) On March 13, 1986, the ESA probe, Giotto, had a
close encounter - a close encounter with a visitor from the Oort cloud
spewing 18 metric tons of gas every second and pouring 30 metric tons of
dust from its nucleus. It's name? Comet Halley... And following its
trail was one of the world's foremost experts on cometary dust
properties - Dr. Jochen Kissel. "Historically comets have always been
unusual bodies, as they seemed to appear out of the nothing and also
disappear like that. " But the real mystery is the dust.

Full Story -
As Professor Emeritus of the Max Planck Institute, Dr. Kissel has a
life-long devotion to the study of comets. "In the early 20th century
the comet tails lead to the postulation and later to the detection of
the 'solar wind', a stream of ionized atoms constantly blown away from
the sun. As astronomical observations became more powerful, more and
more constituents could be identified, both solid state particles and
gaseous molecules, neutral and ionized." As our techniques of studying
these outer solar system visitors became more refined, so have our
theories of what they might be comprised of - and what they look like.
Says Kissel, "Many models have been proposed to describe the dynamic
appearance of a comet, from which Fred Whipple's was apparently the most
promising. It postulated a nucleus made up from water-ice and dust.
Under the influence of the sun, the water-ice would sublime and
accelerate dust particles along its way."

Still, they were a mystery - a mystery that science was eager to solve.
"Not until Halley was it known that many comets are part of our solar
system and orbit the sun just like the planets do, just on other type
orbits and with additional effects due to the emission of materials."
comments Kissel. But only by getting up close and personal with a comet
were we able to discover far more. With Halley's return to our inner
solar system, the plans were made to catch a comet and its name was Giotto.

Giotto's mission was obtain color photographs of the nucleus, determine
the elemental and isotopic composition of volatile components in the
cometary coma, study the parent molecules, and help us to understand the
physical and chemical processes that occur in the cometary atmosphere
and ionosphere. Giotto would be the first to investigate the macroscopic
systems of plasma flows resulting from the cometary-solar wind
interaction. High on its list of priorities was measuring the gas
production rate and determining the elemental and isotopic composition
of the dust particles. Critical to the scientific investigation was the
dust flux - its size and mass distribution and the crucial dust-to-gas
ratio. As the on-board cameras imaged the nucleus from 596 km away -
determining its shape and size - it was also monitoring structures in
the dust coma and studying the gas with both neutral and ion mass
spectrometers. As science suspected, the Giotto mission found the gas to
be predominantly water, but it contained carbon monoxide, carbon
dioxide, various hydrocarbons, as well as a trace of iron and sodium.

As a team research leader for the Giotto mission, Dr. Kissel recalls,
"When the first close up missions to comet 1P/Halley came along, a
nucleus was clearly identified in 1986. It was also the first time that
dust particles, the comet released gases were analyzed in situ, i.e.
without man made interference nor transportation back to ground." It was
an exciting time in cometary research, through Giotto's instrumentation,
researchers like Kissel could now study data like never before. "These
first analyses showed that particles are all an intimate mixture of high
mass organic material and very small dust particles. The biggest
surprise was certainly the very dark nucleus (reflecting only 5% of the
light shining onto it) and the amount and complexity of the organic
material."

But was a comet truly something more or just a dirty snowball? "Up until
today there is - to my knowledge - no measurement showing the existence
of solid water ice exposed on a cometary surface." says Kissel,
"However, we found that water (H2O) as a gas could be released by
chemical reactions going on when the comet is increasingly heated by the
sun. The reason could be 'latent heat', i.e. energy stored in the very
cold cometary material, which acquired the energy by intense cosmic
radiation while the dust was traveling through interstellar space
through bond breaking. Very close to the model for which the late J.
Mayo Greenberg has argued for years."

We now know Comet Halley consisted of the most primitive material known
to us in the solar system. With the exception of nitrogen, the light
elements shown were quite similar in abundance as that of our own Sun.
Several thousand dust particles were determined to be hydrogen, carbon,
nitrogen, oxygen - as well as mineral forming elements such as sodium,
magnesium, silicon, calcium and iron. Because the lighter elements were
discovered far away from the nucleus, we knew they were not cometary ice
particles. From our studies of the chemistry of interstellar gas
surrounding stars, we've learned how carbon chain molecules react to
elements such as nitrogen, oxygen, and in a very small part, hydrogen.
In the extreme cold of space, they can polymerize - changing the
molecular arrangement of these compounds to form new. They would have
the same percentage composition of the original, but a greater molecular
weight and different properties. But what are those properties?

Thanks to some very accurate information from the probe's close
encounter with Comet Halley, Ranjan Gupta of the Inter-University Centre
of Astronomy and Astrophysics (IUCAA) and his colleagues have made some
very interesting findings with cometary dust composition and scattering
properties. Since the beginning missions to comets were "fly-bys", all
the material captured was analyzed in-situ. This type of analysis showed
that cometary materials are generally a mixture of silicates and carbon
in amorphous and crystalline structure formed in the matrix. Once the
water evaporates, the sizes of these grains range from sub-micron to
micron and are highly porous in nature - containing non-spherical and
irregular shapes.

According to Gupta, most of the early models of light scattering from
such grains were "based on solid spheres with conventional Mie theory
and only in the recent years - when the space missions provided strong
evidences against this - have new models have been evolved where
non-spherical and porous grains have been used to reproduce the observed
phenomenon". In this case, linear polarization is produced by the comet
from the incident solar light. Confined to a plane - the direction from
which the light is scattered - it varies by position as the comet
approaches or recedes from the the Sun. As Gupta explains, "An important
feature of this polarization curve versus the scattering angle (referred
to the sun-earth-comet geometry) is that there is some degree of
negative polarization."

Known as 'back scattering', this negativity occurs when monitoring a
single wavelength - monochromatic light. The Mie algorithm models all of
the accepted scattering processes caused by a spherical shape, taking
into account external reflection, multiple internal reflections,
transmission and surface waves. This intensity of scattered light works
as a function of the angle, where 0° implies forward-scattering, away
from the lights original direction, while 180° implies back scattering -
back awards the source of the light.
According to Gupta, "Back scattering is seen in most of the comets
generally in the visible bands and for some comets in the near-infra red
(NIR) bands." At the present time, models attempting to reproduce this
aspect of negative polarization at high scattering angles have very
limited success.

Their study has used a modified DDA (discrete dipole approximation) -
where each dust grain is assumed to be an array of dipoles. A great
range of molecules can contain bonds that are between the extremes of
ionic and covalent. This difference between the electronegativities of
the atoms in the molecules is sufficient enough that the electrons
aren't shared equally - but are small enough that the electrons aren't
attracted only to one of the atoms to form positive and negative ions.
This type of bond in molecules is known as polar. because it has
positive and negative ends - or poles - and the molecules have a dipole
moment.

These dipoles interact with each other to produce the light scattering
effects like extinction - spheres larger than the wavelength of light
will block monochromatic and white light - and polarization - the
scattering of the wave of the incoming light. By using a model of
composite grains with a matrix of graphite and silicate spheroids, a
very specific grain size range may be required to explain the observed
properties in cometary dust. "However, our model is also unable to
reproduce the negative branch of polarization which is observed in some
comets. Not all comets show this phenomenon in the NIR band of 2.2 microns."

These composite grain models developed by Gupta et al; will need to be
refined further to explain the negative polarization branch, as well as
the amount of polarization in various wavelengths. In this case, it is a
color effect with higher polarization in red than green light. More
extensive laboratory simulations of composite grains are upcoming and
"The study of their light scattering properties will help in refining
such models."

Mankind's successful beginnings at following this cometary dust trail
started with Halley. Vega 1, Vega 2 and Giotto provided the models
needed to better research equipment. In May 2000, Drs. Franz R. Krueger
and Jochen Kissel of Max Planck Institute published their findings as
"First Direct Chemical Analysis of Interstellar Dust". Says Dr. Kissel,
"Three of our dust impact mass spectrometers (PIA on board GIOTTO, and
PUMA-1 and -2 onboard VEGA-1 and -2) encountered Comet Halley. With
those we were able to determine the elementary composition of the
cometary dust. Molecular information, however, was only marginal." Deep
Space 1's close encounter with Comet Borrelly returned the best images
and other science data received so far. On the Borelly Team, Dr. Kissel
replies, "The more recent mission to Borrelly (and STARDUST) showed
fascinating details of the comet surface such as steep 200m high slopes
and spires some 20m wide and 200m high."

Despite the mission's many problems, Deep Space 1 proved to be a total
success. According to Dr. Mark Rayman's December 18, 2001 Mission Log,
"The wealth of science and engineering data returned by this mission
will be analyzed and used for years to come. The testing of high risk,
advanced technologies means that many important future missions that
otherwise would have been unaffordable or even impossible now are within
our grasp. And as all macroscopic readers know, the rich scientific
harvest from comet Borrelly is providing scientists fascinating new
insights into these important members of the solar system family."

Now Stardust has taken our investigations just one step further.
Collecting these primitive particles from Comet Wild 2, the dust grains
will be stored safely in aerogel for study upon the probe's return.
NASA's Donald Brownlee says, "Comet dust will also be studied in real
time by a time-of-flight mass spectrometer derived from the PIA
instrument carried to comet Halley on the Giotto mission. This
instrument will provide data on the organic particle materials that may
not survive aerogel capture, and it will provide an invaluable data set
that can be used to evaluate the diversity among comets by comparison
with Halley dust data recorded with the same technique."

These very particles might contain an answer, explaining how
interstellar dust and comets may have seeded life on Earth by providing
the physical and chemical elements crucial to its development. According
to Browlee, "Stardust captured thousands of comet particles that will be
returned to Earth for analysis, in intimate detail, by researchers
around the world." These dust samples will allow us to look back some
4.5 billion years ago - teaching us about fundamental nature of
interstellar grains and other solid materials - the very building blocks
of our own solar system. Both atoms found on Earth and in our own bodies
contain the same materials as released by comets.

And it just keeps getting better. Now en route to Comet Comet 67
P/Churyumov- Gerasimenko, ESA's Rosetta will delve deeper into the
mystery of comets as it attempts a successful landing on the surface.
According to ESA, equipment such as "Grain Impact Analyser and Dust
Accumulator (GIADA) will measure the number, mass, momentum, and
velocity distribution of dust grains coming from the comet nucleus and
from other directions (reflected by solar radiation pressure) - while
Micro-Imaging Dust Analysis System (MIDAS) will study the dust
environment around the comet. It will provide information on particle
population, size, volume, and shape."

A single cometary particle could be a composite of millions of
individual interstellar dust grains, allowing us new insight on galactic
and nebular processes increasing our understanding of both comets and
stars. Just as we have produced amino acids in laboratory conditions
that simulate what may occur in a comet, most of our information has
been indirectly obtained. By understanding polarization, wavelength
absorption, scattering properties and the shape of a silicate feature,
we gain valuable knowledge into the physical properties of what we have
yet to explore. Rosetta's goal will be to carry a lander to the a
comet's nucleus and deploy it on the surface. The lander science will
focus on in-situ study of the composition and structure of the nucleus -
an unparalleled study of cometary material - providing researchers like
Dr. Jochen Kissel valuable information.

On July 4, 2005, the Deep Impact mission will arrive at Comet Temple 1.
Buried beneath its surface may be even more answers. In an effort to
form a new crater on the comet's surface, a 370 kg mass will be released
to impact Tempel 1's sunlit side. The result will be the fresh ejection
of ice and dust particles and will further our understanding about
comets by observing the changes in activity. The fly-by craft will
monitor structure and composition of the crater's interior - relaying
data back to Earth's cometary dust expert, Kissel. "Deep Impact will be
the first to simulate a natural event, the impact of a solid body onto a
comet nucleus. The advantage is that the impact time is well known and a
spacecraft properly equipped is around, when the impact occurs. This
will definitely provide information of what is below the surfaces from
which we have pictures by the previous missions. Many theories have been
formulated to describe the thermal behavior of the comet nucleus,
requiring crusts thick or thin and or other features. I'm sure all these
models will have to be complimented by new ones after the Deep Impact."

After a lifetime of cometary research, Dr. Kissel is still following the
dust trail, "It's the fascination of comet research that after each new
measurement there are new facts, which show us, how wrong we were. And
that is still on a rather global level." As our methods improve, so does
our understanding of these visitors from the Oort Cloud. Says Kissel,
"The situation is not simple and as many simple models describe the
global cometary activities pretty well, while details have still to be
worked, and models including the chemistry aspects are not yet
available." For a man who has been there since the very beginning,
working with Deep Impact continues a distinguished career. "It's
exciting to be part of it" says Dr. Kissel, "and I am eager to see what
happens after the Deep Impact and grateful to be a part of it."

For the very first time, studies will go well beneath the surface of a
comet, revealing its pristine materials - untouched since its formation.
What lay beneath the surface? Let's hope spectroscopy shows carbon,
hydrogen, nitrogen and oxygen. These are known to produce organic
molecules, starting with the basic hydrocarbons, such as methane. Will
these processes have increased in complexity to create polymers? Will we
find the basis for carbohydrates, saccharides, lipids, glycerides,
proteins and enzymes? Following dust trail might very well lead to the
foundation of the most spectacular of all organic matter -
deoxyribonucleic acid - DNA.