Dear Extropes,
This is a long email (partial result of thinking about this topic for
some other work) regarding carbon in comets and dust the solar system.
And I thought to write the long message to the list, so that other
people can see and contribute.
Robert wrote me a while ago asking me about the composition of
solar system dust, in particular, if we see C-compounds in greater
abundance than O-compounds. He pointed to the abundance of water in
comets, which seemed peculiar to him because he would expect from
normal stellar synthesis that the amount of C should be >> amount of
O, so the most abundant compound *should* be CH4, not H2O. And
since, in the sun it seems C ~= O, one could argue that CH4
abundance should be ~= to H2O. Then, when we spoke in email again,
Robert said that he thinks that he understands the discrepancy:
"which seems to be due to the H & O binding to the C, forming CH4 &
CO and it getting blown out of the system due to their low freezing
points. It may get accumulated in the Oort cloud comets, but then
it is likely to blow off on their first past by the sun (on my todo
list is to look at some cometary tail spectra and molecule
abundances)."
So his questions to me are:
"a) Does the carbon show up in the interstellar dust in some way?"
(This ^^ question answered after "b")
"b) Why does oxygen seem to be in excess relative to the carbon?
Then the question is, is it a galactic situation or a local
phonomena?"
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Answering b.
I don't know the answer to b. But here are some pointers.
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Duley, (1984), Quarterly Journal of the Royal Astronomical Society,
Vol 25, 109, shows a table of the depletion of elements in
interstellar space, that might be useful ... I think that it is
generally assumed that the depleted carbon is in the form of CO or
other carbon-bearing molecules such as PAHs.
My old post on carbon in the grand scheme:
http://www.lucifer.com/exi-lists/extropians.4Q99/1109.html
Some of my words about the interstellar medium:
http://www.lucifer.com/exi-lists/extropians.4Q99/3608.html
I told last fall about an important interdisciplinary paper about dust
embedded in the interstellar cloud flowing through the solar system.
"Dust in the local interstellar wind" (1999)
P. C. Frisch, J. Dorschner, J. Geiss, J. M. Greenberg,
E. Gruen, M. Landgraf, P. Hoppe, A. P. Jones, W. Kraetschmer,
T. J. Linde, G. E. Morfill, W. T. Reach, J. Slavin, J. Svestka,
A. Witt, and G. P. Zank. Astrophysical Journal, 525 , 492-516
I put it here for you:
http://galileo.mpi-hd.mpg.de/~graps/frisch.ps.gz
Read this paper once. Then read it again. Then go find a professional
astronomer specializing in the interstellar medium and ask him/her
your questions. I've read this paper 3 times all of the way through,
and I don't understand all of the pieces yet. (Why have a B star as a
reference star for the abundances... what does this mean about
interstellar Kr? Why place so much weight on a MRN distribution for
interstellar dust?)
Frisch et al's paper does and concludes:
* Elemental abundances and the dust-to-gas ratio are calculated towards
two directions ("column densities") in our local interstellar
environment: towards eps CMa and alpha Sco.
* There is not enough mass "missing" from the gas phase to form the dust
grain population observed by the spacecraft.
* Dust grains are overabundant in the cloud feeding interstellar
grains into the solar system.
* The cloud around the solar system is a weakly depleted cloud with
respect to the refractory elements.
* There is evidence for interstellar shock fronts destroying grains in
the local interstellar cloud (LIC).
* The eps CMA LIC component has no carbon available for the dust, a
manifestation of the "carbon crisis" written in Dwek (1997). The lack
of carbon avail for the interstellar dust grains and the presence of
SiC presolar grains in meteorites may indicate an additional
carbon-rich grain source beyond gas condensation from circumstellar
dust grains.
more pointers:
E. Dwek, "Can Composite Fluffy Dust Particles Solve the Interstellar
Carbon Crisis?" Astrophysical Journal 484, p.779
E. Dwek, "The Evolution of the Elemental Abundances in the Gas and
Dust Phases of the Galaxy" (1998) Astrophysical Journal 501, p. 643
some interstellar dust references:
http://galileo.mpi-hd.mpg.de/dune/content/references.htm
Priscilla Frisch's map of our galactic neighborhood:
http://galileo.mpi-hd.mpg.de/dune/content/map.htm
Here are two references that appeared on astro-ph recently.
(I didn't read them myself)
astro-ph/0007178 :
Title: Stellar production rates of carbon and its abundance
in the universe Authors: Heinz Oberhummer (Vienna University
of Technology), Attila Csoto (Eotvos University, Budapest),
Helmut Schlattl (Max-Planck Institute for Astrophysics,
Garching) Comments: 6 pages with 1 figure. Science, 2000
July 7 issue. The postscript file and more information are
available at this http URL, this http URL and this http URL
Journal-ref: Science 289 (2000) 88
The bulk of the carbon in our universe is produced in the
triple-alpha process in helium-burning red giant stars. We
calculated the change of the triple-alpha reaction rate in a
microscopic 12-nucleon model of the C-12 nucleus and looked
for the effects of minimal variations of the strengths of
the underlying interactions. Stellar model calculations were
performed with the alternative reaction rates. Here, we show
that outside a narrow window of 0.5 and 4% of the values of
the strong and Coulomb forces, respectively, the stellar
production of carbon or oxygen is reduced by factors of 30
to 1000. (15kb)
astro-ph/0007314 :
Title: Chemical Abundances in our Galaxy and Other Galaxies
Derived from H II Regions Authors: M. Peimbert, L. Carigi,
A. Peimbert Comments: 10 pages, 2 figures. The evolution of
Galaxies. I- Observational Clues. Conference Proceedings
We discuss the accuracy of the abundance determinations of H
II regions in our Galaxy and other galaxies. We focus on the
main observational constraints derived from abundance
determinations that have implications for models of galactic
chemical evolution: a) the helium to hydrogen abundance
ratio, He/H; b) the oxygen to hydrogen abundance ratio, O/H;
c) the carbon to oxygen abundance ratio, C/O; d) the helium
to oxygen and helium to heavy elements abundance ratios,
Delta Y/ Delta O and Delta Y/ Delta Z; and e) the primordial
helium abundance, Yp. (43kb)
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Answering a.
Here, I point to some references that could be helpful.
Then I attach an essay I wrote ~1 week ago, while thinking
of this topic.
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COMETS
My (admittedly vague) answer to Robert's question is that I don't
think that comets display chemical abundances, including carbon,
that are far from solar abundances. Most references I've skimmed
don't see any problem with carbon abundances. One needs to look at
the gas phase elements versus the solid phase elements, too, of
course.
A detailed reference for chemical abundances in the solar nebula is
Chapter 4: "The Sun and Solar Nebula" in _Physics and Chemistry of
the Solar System_ by John Lewis.
I saw several very helpful chapters to answer some of these
questions in:
_Comets in the Post Halley Era_, Vol 1&2, R.L. Newburn, Jr. M.
Neugebauer, J. Rahe, editors, Kluwer Academic Publishers, 1991.
Hans Rickman's chapter: "The Thermal History and Structure of
Cometary Nuclei" shows a Figure 1, that H2O ice condensed in the
crystalline state at about 150K, and more volatile species (CO or
CO2) were depleted at slightly higher temperatures (confirming what
you said). More figures and discussions in this chapter than what I
have time to look at.., but it probably confirms everything that you
wrote.
Bertram Donn's chapter: "The Accumulation and Structure of Comets"
say that: "inthe case of comets, temperatures had to be sufficiently
low for the normally volatile components of comets (H2, C2H2, CO,
CO2, NH3, HCN) to condense into grains or be adsorbed on or into
grains." Then there is a long discussion of how grains might form.
And in this chapter: E.K. Jessberger and J. Kissel, "Chemical
Properties of Cometary Dust and a Note on Carbon Isotopes" of the
above book, I saw a detailed discussion of elemental abundances of
dust in Halley's comet, that points to some interesting observations
of carbon isotopic ratios.
Some of Jessberger's and Kissel's conclusions:
* Halley's dust is composed of two components: a refractory organic
phase (CHON) and a Mg-rich silicate phase.
* The CHON elements are more abundant than in CI chondrites. The
abundances of C and O approach the solar system (==cosmic)
abundances, N is intermediate between the solar and CI-chondrite
abundances, and H is much closer to the CI-chondrite abunance than
to the solar one.
* Within a factor of two, the abundances of the rock-forming
elements are the same as in the whole solar system (== cosmic).
* The variations of elements Mg, Si, and Fe, as well as the
enrichment of volatile elements (relative to CI chondrites),
provide a link between Halley's dust and interplanetary dust
particles, especially the anhydrous variety.
* On the isotopic compositions: in general no element showed
deviations from the normal solar system composition with the only
exception being carbon, in which the measured 12/13 intensity ratios
range from about 1 to 5000. No simple explanation for that can be
given. The graphite in the Murchison carbonaceous chondrite with
12C/13C was also showed about 5000, which is produced in He-burning
or explosive H-burning processes, so these isotopic anonmalies in
Halley's comet dust probably reveal nucleosynthetic processes.
There's alot more information for you than what I've mentioned about
this topic in this two-volume book series, that would be useful for
you, so I suggest taking a look.
Another more detailed comet dust chemistry reference for you (I'm
weak in chemistry, sorry) is the chapter: "Organic Chemistry in
Comets from Remote and In-situ Observations" by J. Kissel, F.R.
Krueger, and K. Roessler in _Comets and the Origin and Evolution of
Life_ by Springer Verlag, 1997. I don't have time to read this but I
can tell you that you will find 40 pages of detailed organic
chemistry discussions for the comet dust.
>(on my todo
>list is to look at some cometary tail spectra and molecule
>abundances).
Take a look at:
Table 1: "Observed Species in Comet Spectra" of S. Wyckoff's
chapter: "Overview of Comet Observations" in _Comets_ by Laurel
Wilkening, ed., University of Az Press, 1982 would help you answer
this. The table shows about 30 atoms or molecules observed in comet
spectra.
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INTERPLANETARY DUST
No ``loose" dust particles remain from the formation of the solar
system. Poynting-Robertson drag influences the lifetime of
interplanetary dust particles. Micrometer-sized dust particles
spiral into the Sun in only a few tens of thousands of years.
Larger, centimeter-sized particles live a little bit longer -- tens
of millions of years. To find ``primal dust", one must look into
asteroids, comets and meteoroids.
In-situ compositional observations of interplanetary dust is still
in its infancy. Comet Halley was probed by PUMA. Now, we have
Cassini and Stardust dust instruments, both performing compositional
analysis, and the measurements are difficult to interpret, so can't
say much yet about that.
Typical interplanetary dust particles (IDPs) are fine-grained
mixtures of thousands to millions of mineral grains and amorphous
components. Most of the IDPs can have wide ranging elemental
compositions at a given phase in the particle [2].
The great majority of interplanetary dust particles fall into the
chondritic class. The abundant elements Mg, Al, Si, S, Ca, Cr, Mn,
Fe, and Ni are present within a factor of a few of CI chondritic
abundance. Where large deviations from cosmic abundances occur, it
is usually in particles dominated by large mineral grains such as
sulfides, Mg silicates, FeNi metal, phosphides or carbonates. The
presence of large pyroxene, olivine, or other phases commonly cause
deviations from chondritic abundances also [1].
[1] Brownlee, D.E., "The Elemental Composition of Interplanetary
Dust," _Physics, Chemistry, and Dynamics of Interplanetary Dust_,"
ASP Conference Series, Vol 104, 1996, B.A.S. Gustafson, and Martha
Hanner, eds.
[2] Jessberger, E.K., Stephan, T., Rost, D., Arndt, P., Maetz, M.,
Stadermann, F.J., Brownlee, D.E., Bradley, J.P., Kurat, G.,
"Properties of Interplanetary Dust: Information from Collected
Samples", from the book: _Interplanetary Dust_, Gruen, E.,
Gustafson, B.A., ed., in preparation, 2000.
----------------My essay begins----------------------------------------
==================================================
THE DUSTY TRAIL FROM THE SOLAR NEBULA TO THE EARTH
==================================================
by Amara L. Graps
July 16, 2000
Copyright Amara Graps 2000.
All rights reserved.
What are some of the possible paths for the materials of
interplanetary dust particles to arrive at our Earth's doorstep, and
what does the compositional analysis of dust particles tell us about
dust particles' origin?
During our solar system's formation history, the most abundant
element was (and still is) H2. The metallic elements: magnesium,
silicon, and iron, which are the principal ingredients of rocky
planets, condensed into solids at the highest temperatures. The
range of elements of the solar nebula between H2 and (Mg, Si, Fe) is
not known well (Wood, J., 1999). Some molecules such as CO, N2, NH3,
and free oxygen, existed in a gas phase. Some molecules, for
example, graphite (C) and SiC condensed into solid grains. Some
molecules also formed complex organic compounds and some molecules
formed frozen ice mantles, of which either could coat the
"refractory" (Mg, Si, Fe) grain cores.
The formation of these molecules was determined, in large part, by
the temperature of the nebula. Since the temperature of the solar
nebula decreased with heliocentric distance, scientists can infer a
dust grain's origin(s) with knowledge of the grain's materials. Some
materials could only have been formed at high temperatures, while
other grain materials could only have been formed at much lower
temperatures. The materials in a single interplanetary dust particle
often show that the grain elements formed in different locations and
at different times in the solar nebula.
Most of the matter present in the original solar nebula has since
disappeared; drawn into the Sun, expelled into interstellar space,
or reprocessed, for example, as part of the planets, asteroids or
comets.
The major sources of interplanetary dust particles (IDPs) are
asteroids and comets (Leinert and Gruen, 1990). Comet dust studies
are especially interesting because comets preserve some of the
primordial material out of which our solar system formed, and comets
are currently the most abundant source of unchanged nebular
material. Brownlee stated in 1994 that cometary dust might
eventually provide information on matter from: (1) the ancient solar
system, (2) the outer solar system and (3) from pre-solar epochs.
However, before we can make such a conclusion, we must solve a
long-standing question: which interplanetary dust particles are
cometary and which are asteroidal? (Jessberger, 2000)
At the moment, we don't have *direct* laboratory access to cometary
or asteroidal dust material. However, *indirectly*, we do have
laboratory access. Some of those dust particles are swept up by the
Earth, and collected by research aircraft that fly high in the
stratosphere. Other interplanetary dust particles are collected on
the large Earth ice-masses (Antartica and Greenland and Arctic) and
in deep-sea sediments. These stratospheric and polar ice melt
interplanetary dust particles are then examined in the laboratory.
The arrows in the following diagram show one possible path from a
collected interplanetary dust particle back to the early stages of
the solar nebula.
=============== petrologic
TYPES OF / types:
INTERPLANETARY / contains the
DUST PARTICLES / / CI, CM most volatile
=============== /*Carbonaceous -->\ CV, CO elements, and
/ \ CR, CK the largest
*Chondritic ----> 60% \ *Ordinary \ amount of
\ *Enstatite organic compounds
*Iron-Sulfer-Nickel 30%
*Mafic Silicates 10%
The IDPs' elemental composition is one of three major types:
chondritic, 60%, iron-sulfur-nickel, 30%, and mafic silicates, which
are iron-magnesium-rich silicates, (i.e. olivine and pyroxene), 10%
(Jessberger, 1992, Gruen, E. 1999).
Astrophysicists and planetary scientists are especially intrigued by
the chondritic IDPs because they provide a link to the astrophysical
conditions out of which our solar system formed.
We can follow the trail to the right in the diagram above to the
IDPs that contain the most volatile and primitive elements. The
trail takes us first from interplanetary dust particles to
*chondritic* interplanetary dust particles. Planetary scientists
classify chondritic IDPs in terms of their diminishing degree of
oxidation so that they fall into three major groups: the
carbonaneous, the ordinary, and the enstatite chondrites. As the
name implies, the carbonaceous condrites are rich in carbon, and
many have anomalies in the isotopic abundances of H, C, N, and O
(Jessberger, 2000). From the carbonaceous chondrites, we follow the
trail to the most primitive materials. They are almost completely
oxidized and contain the most low condensation temperature elements
("volatile" elements) and the largest amount of organic compounds
(Excell, 1998). Therefore, dust particles with these elements are
thought to be formed in the early life of our solar system. Why? The
volatile elements have never seen temperatures above about 500 K,
therefore, one can conclude that the IDP grain "matrix" consists of
some very primitive solar system material. Such a scenario is true
in the case of comet dust (Gruen, 1999).
We can learn more about these particles' origin, by examining their
surfaces. If we examine, in the laboratory, dust particles' density
of solar flare tracks, their amorphous rims, and the spallogenic
isotopes from cosmic rays (Flynn, 1996), then we have good clues for
how long a particle has been travelling in space. Nuclear damage
tracks are caused by the ion flux from solar flares. Solar wind ions
impacting on the particle's surface produce amorphous radiation
damaged rims on the particle's surface. And spallogenic nuclei are
produced by galactic and solar cosmic rays. A dust particle that
originates in the Kuiper Belt at 40AU would have many more times the
density of tracks, thicker amorphous rims and higher integrated
doses than a dust particle originating in the main-asteroid belt.
One aspect of IDPs that I find especially interesting is the study
of "chondrules". Chondrules are millimeter-sized spherical "drops"
suspended in the younger matrix of dust grain material of chondritic
meteorites and chondritic interplanetary dust particles. The term
"chondrule" is from the Greek word "chondros", which means "grain of
seed." The spherical shape is evidence of a gravity-free formation
environment. (Excell, S. 1998). Chondrules are leftover chaff from
the time period of the collapse of the solar nebula. Chondrules were
once molten, which required temperatures in excess of 1500 to 1900 K
The chondrules must have cooled very rapidly, in about an hour
(Wood, 1999). They could not have been formed in the ambient
temperature of the innermost solar nebula, instead, they were formed
as the result of a quick, high-energy event, for example, a
lightning discharge, an accretion shock, or a magnetic nebular
flare.
Upon further examination of the dust particle "matrix", one
sometimes finds "Calcium-Aluminum-Inclusions" or CAIs, between the
chondrules. CAIs were formed at much higher temperatures than the
temperatures at which chondrules formed, and CAIs may have survived
many multiple high-temperature events (while most chondrules are the
product of a single, transient melting event (Excell, 1998)).
Even more interesting for understanding the origin of CAIs: isotopic
studies show anomalies of excess amounts of oxygen and magnesium.
The large variability in isotopic composition, some factors of 1000
times the solar hydrogen isotope ratios (Gruen, 1999), indicate that
some grains are "not homogenized" with other solar system material,
but have preserved much of their presolar environment. The isotopic
anomalies of CAIs give us a good clue about some of the events of
our solar system's formation because the isotopic anomalies infer
that the solar nebula collapsed shortly after a _nearby supernova
event_. In addition, radiactive dating methods show that the
formation of CAIs preceded the formation of chondrules by about 2
million years (Excell, 1998).
>From a supernova to "us" via dust ...
Dust, dust: around, beneath us,
You swirl and shape and flow;
Hither, thither, yon, beyond,
Your travels I would know.
Dust, dust: my eyes reveal
You cloud and sometimes glow;
As you wander, rest assured
Your matter will unfold.
Tanya Jones
June 2000
REFERENCES
Brownlee D. E. 1994 The origin and role of dust in the Early Solar
System. In Analysis of Interplanetary Dust (M. E. Zolensky, T. L.
Wilson, F. J. M. Rietmeijer, and G. J. Flynn eds.) Amer. Inst.
Physics, New York: 5--8.
Excell, Steven, 1998, Web page: "The Nature and Origin of Chondrules
-- A Survey of the Classic and Current Models of Chondrule
Formation", http://www.arachnaut.org/meteor/chondrules.html
Flynn, G. "Sources of 10 micron Interplanetary Dust: The
Contribution from the Kuiper Belt", from _Physics, Chemistry, and
Dynamics of Interplanetary Dust_, ASP Conference Series, Gustafson,
B.A.S. and Hanner, M.S. ed., Vol 104, 1996, pg. 171-175.
Gruen, E. "Interplanetary Dust and the Zodiacal Cloud",
_Encyclopedia of the Solar System_, Academic Press, 1999 who uses
this reference for Table III: Jessberger, E.K. et al., (1992), Earth
Planet. Sci. Lett. 112, 91-99.
Jessberger, E.K., Stephan, T., Rost, D., Arndt, P., Maetz, M.,
Stadermann, F.J., Brownlee, D.E., Bradley, J.P., Kurat, G.,
"Properties of Interplanetary Dust: Information from Collected
Samples", from the book: _Interplanetary Dust_, Grün, E.,
Gustafson, B.A., ed., in preparation, 2000.
Leinert C. and Gruen E, 1990 "Interplanetary Dust," in _Physics and
Chemistry in Space_ (R. Schwenn and E. Marsch eds.) Space and Solar
Physics, Springer, Berlin: 204--275.
Wood, J.A. "Origin of the Solar System," in _The New Solar System_
Beatty, Petersen, and Chaikin, eds., Sky Publishing, 1999, pg.
13-22.
--------------My essay ends------------------------------------------
Hope that this helps,
Amara
--*************************************************************** Amara Graps | Max-Planck-Institut fuer Kernphysik Interplanetary Dust Group | Saupfercheckweg 1 +49-6221-516-543 | 69117 Heidelberg, GERMANY Amara.Graps@mpi-hd.mpg.de * http://galileo.mpi-hd.mpg.de/~graps *************************************************************** "Never fight an inanimate object." - P. J. O'Rourke
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