What is interstellar dust made of?

[t@nn]

I've been intrigued by the latest findings that interstellar dust may take the form of helical structures. I've never thought much about this dust and I am uncertain what it comprises of. Does anyone here know what the dust is made of?

[iantresman]

I believe it is made of rocky material, but it may form helical structures because it is surrounded by an interplanetary, interstellar, or intergalactic plasma, which together with light and friction, causes the dust to charge, resulting in a complex (dusty) plasma, that behaves as a plasma in a magnetic field. See for example:

• Some aspects of dust-plasma interactions in the cosmic environment,
  Mendis, D. A.; Rosenberg, Marlene, in ''IEEE Transactions on Plasma Science'' (ISSN 0093-3813), vol. 20, no. 6, p. 929-934, (1992)

[astromark]

Interstellar dust is made of almost every thing that can be found in the universe. Some of it is gaseous and some as solids. Debris from spent nova events and leftover material from star formation. Yes some will be charged particles and even plasma state matter. Millions of cubic tonnes of all sorts of stuff and then just one or two handfuls of spent carbon... Interstellar dust is interstellar dust. It would be wrong to sagest it is all this or that.

[t@nn]

Okay, thank you.

[Amber Robot]

Interstellar dust is made of almost every thing that can be found in the universe. Some of it is gaseous and some as solids. Debris from spent nova events and leftover material from star formation. Yes some will be charged particles and even plasma state matter. Millions of cubic tonnes of all sorts of stuff and then just one or two handfuls of spent carbon... Interstellar dust is interstellar dust. It would be wrong to sagest it is all this or that.

I don't really understand this response. What we normally think of as "dust" is not gaseous. Dust particles may be charged but not actually a plasma. The fact that dust absorbs light at a continuum of wavelengths is what suggests that it is a solid. The particular spectral response of this absorption gives clues as to both the composition and the size distribution, suggesting that there are dust particles ranging from microns in size down to what might better be called large molecules.

The study of interstellar dust is an area of active research. Determining the exact composition of dust is not trivial. We do know that there are a lot of silicates and dust from iron. Also, there are most likely also carbonaceous grains. The large molecules are dominated by what are called PAHs - polycyclic aromatic hydrocarbons. They have very distinct emission features in the infrared. There is also a near-blackbody emission from warm dust down in the even longer wavelengths.

And the dust characteristics differ from region to region and may be different in molecular clouds, the diffuse interstellar medium, and nebular regions.

[iantresman]

I don't really understand this response. What we normally think of as "dust" is not gaseous. Dust particles may be charged but not actually a plasma.

Charged dust may behave exactly as a plasma, responding collectively in response to electromagnetic forces, and also the particles may be the dominant charge carriers; which is why it is called a dusty plasma (or complex plasma, now). It's only when the grains reach a certain size, does gravity become the dominant force. See the following papers, both of which are online:

• Dust in cosmic plasma environments
  Mendis, D. A., Astrophysics and Space Science, vol. 65, no. 1, Sept. 1979, p. 5-12
  
  
• Charged dust in the outer planetary magnetospheres. I - Physical and dynamical processes
  Hill, J. R.; Mendis, D. A, Moon and the Planets, vol. 21, Aug. 1979, p. 3-16.

[01101001]

What, nobody's suggesting space-alien vehicle exhaust?

For all the claims of intergalactic visitors having come here, the interstellar routes must be clogged with exhaust -- and super-fast-food litter!

Sorry. We now return you to your regularly scheduled mainstream-science answers.

[mugaliens]

What, nobody's suggesting space-alien vehicle exhaust?

Ok, I think it's space-alien vehicle exhaust. Non-existant exhaust, mind you, as their technology is so advance they never pollute.

[astromark]

...for Amber Robot, So, just as random clouds of interstellar gas are found to contain some particles of solid mater. The word 'dust' is not to be confused with the contents of my vacuum cleaner. Some of this stuff is raw basic elements while as much can be the spent mater from a nova event... dust can be all of these things. The very stuff we are made of included. Interstellar dust is a nonspecific description of all that is detectable floating about in the void between stars. If you were to focus your attention on the large gaseous regions of the Orion Nebular you will find huge amounts of interstellar dust. Suspended in the clouds of interstellar gas... these things are not uncommon across the universe. Just made harder to detect because they are unlit. defuse. random. (and exhaust gasses.)

[Amber Robot]

Interstellar dust is a nonspecific description of all that is detectable floating about in the void between stars.

That's not true. Though what constitutes "dust" may have a bit of leeway, it's certainly not *everything* between the stars. There is plenty of gas, of varying ionization states, between the stars too.

[neilzero]

If the temperature is 3 degrees k or less, all the compounds and elements except helium are frozen solid, but perhaps not. Temperature has strange meanings when there are only three particles per cubic meter including electrons and hydrogen nuclii, which some have suggested is 90% of what is in intersteller space on the average. Some cubic kilometers likely have lots more particles varing widely in composition. Becase of the high speeds and very long mean free path, compositions of cubic kilometers would change by the minute, perhaps by the second. One particle with a mass of one picogram or heavier particle per cubic kilometer may be typical. That is about the lightest dust particle we experience on Earth. As I suggested in another thread, does a single atom of osmium behave like a gas or vapor? Perhaps. Neil

[neilzero]

Halfway to Earth's moon, a picobar may be typical, but the pressure is much lower between the stars.
On another thread someone found a source (for intersteller space) which gave the pressure as 1.46 times ten to the -18 pascals. A picobar is almost 10 billion times that much pressure if my arithmetic is correct. Neil

[Bearded One]

How is the word "dust" meant here? I think the normal definition isn't going to be much help in the interstellar medium. I think interstellar dust would comprise everything below a certain size. Good luck defining a size. I would place no lower limit on the size though, so an atom (or molecule) of oxygen qualifies as a dust particle, as would free particles. Don't ask about photons.

Now, how close do molecules of oxygen have to get, on average, to be called a gas and not just a bunch of randomly separated oxygen molecules?

[TheOncomingStorm]

According to wikipedia dust is from a few molecules to 0.1 mm in size and can be made from any material.

http://en.wikipedia.org/wiki/Interstellar_dust

[TheOncomingStorm]

Here is another article on the different types of interstellar medium, go figure no mention of plasma.
http://en.wikipedia.org/wiki/Interstellar_gas

[trinitree88]

Seems there little concensus, but here's a good dust guy...Draine. see:ftp://ftp.astro.princeton.edu/draine...RAA_41_241.pdf

[Amber Robot]

I think interstellar dust would comprise everything below a certain size. Good luck defining a size. I would place no lower limit on the size though, so an atom (or molecule) of oxygen qualifies as a dust particle, as would free particles. Don't ask about photons.

In standard astronomical parlance, dust does have a lower limit. A single atom or molecule does not qualify as a dust particle. I'm not sure what you mean by "free particle". The exact placement of that lower limit is a bit fuzzy but as Bruce Draine says in that ARAA article to which trinitree88 links, "the term 'dust grain' is understood here to extend down to molecules containing tens of atoms, as there is no discontinuity in the physics as the particle size decreases from microns to Angstroms."

]Now, how close do molecules of oxygen have to get, on average, to be called a gas and not just a bunch of randomly separated oxygen molecules?

How are randomly separated oxygen molecules* not a gas? I don't think there's a lower limit on the density of a gas.

*there are actually very, very few oxygen molecules in the interstellar medium -- oxygen tends to be in carbon monoxide and dust grains or appear in its atomic form.

[Tim Thompson]

Interstellar dust is a big field in astronomy. Molecular hydrogen will not form in gas phase chemical reactions, it's a 3-body problem that requires a surface, supplied by dust grains. So the microphysical details of the surfaces of grains is key to understanding the abundance of H₂ in the interstellar medium. Indeed, there is a vast array of low temperature chemical reactions on grain surfaces, responsible for many of the 129 molecues detected so far in the interstellar & interplanetary medium. Dust is also responsible for broad band absorption & extinction, so its optical properties are as important as its chemical & physical properties. There is an enormous scientific literature on interstellar dust.

Interstellar dust grains are generally modeled as silicate mantles, with various coatings of organics, oxides, iron, and ices.

A brief list of recent books

• An Introduction to the Physics of Interstellar Dust, Endrik Krugel, Taylor & Francis, 2007 (an updated version of his earlier book from 2002, The Physics of Interstellar Dust).
• The Physics and Chemistry of the Interstellar Medium, A.G.G.M. Tielens, Cambridge University Press, 2005
• Interplanetary Dust, Eberhard Grun, et al., editors, Springer, 2001

A brief list of recent journal papers.

• Composite interstellar grains, Vaidya, Gupta & Snow, Monthly Notices of the Royal Astronomical Society 379(3): 791-800, August 2007 (Silicate spheroid with graphite inclusions, modeled to reproduce observed interstellar extiction & polarization).
  
• Dust Masses, PAH Abundances, and Starlight Intensities in the SINGS Galaxy Sample, B.T. Draine, et al., Astrophysical Journal 663(2): 866-894, July 2007 (Dust models & dust to gas ratios for 65 observed galaxies; grains are found to contains Mg, Si & Fe, as well as PAH).
  
• Infrared Emission from Interstellar Dust. IV. The Silicate-Graphite-PAH Model in the Post-Spitzer Era, Draine & Li, Astrophysical Journal 657(2): 810-837, March 2007 ("IR emission spectra are calculated for dust heated by starlight, for mixtures of amorphous silicate and graphitic grains, including varying amounts of PAH particles. The models are constrained to reproduce the average Milky Way extinction curve.").
  
• Solid Solution Model for Interstellar Dust Grains and Their Organics, Freund & Freund, Astrophysical Journal 639(1); 210-226, March 2006 (The authors show that dust grains formed in the gas rich environment of AGB stars are solid solutions which contain the usual oxides & silicates, but also gas phase components & organics, such as CH₂ & CH₃)
  
• Probing the surfaces of interstellar dust grains: the adsorption of CO at bare grain surfaces, Fraser, et al., Monthly Notices of the Royal Astronomical Society 356(4): 1283-1292, February 2005 (The authors propose that an observed feature of the interstellar spectrum about wavenumber 2175 cm^-1 is due to CO adsorbed onto the bare surface of dust grains; their conclusion is supported by laboratory measurements of CO adsorbed onto a zeolite substrate).
  
• Formation of molecular hydrogen on analogues of interstellar dust grains: experiments and modelling, Vidali, et al., Journal of Physics: Conference Series 6(1): 36-58, January 2005 (Laboratory chemical kinetics of H₂ formation on grrain surfaces).
  
• H₂ Formation on Grain Surfaces, Cazaux & Tielens, Astrophysical Journal 604(1): 222-237, March 2004 ("We have developed a model for molecular hydrogen formation on surfaces.")
  
• Interstellar Dust Grains, B.T. Draine, Annual Review of Astronomy & Astrophysics 41: 241-289, 2003 ("This review surveys the observed properties of interstellar dust grains." This is the same paper reference by trinitree88, but this link to ADS allows you to follow the 211 papers which cite this one)

[trinitree88]

In standard astronomical parlance, dust does have a lower limit. A single atom or molecule does not qualify as a dust particle. I'm not sure what you mean by "free particle". The exact placement of that lower limit is a bit fuzzy but as Bruce Draine says in that ARAA article to which trinitree88 links, "the term 'dust grain' is understood here to extend down to molecules containing tens of atoms, as there is no discontinuity in the physics as the particle size decreases from microns to Angstroms."



How are randomly separated oxygen molecules* not a gas? I don't think there's a lower limit on the density of a gas.

*there are actually very, very few oxygen molecules in the interstellar medium -- oxygen tends to be in carbon monoxide and dust grains or appear in its atomic form.

Amber Robot. While molecular oxygen is a pretty good oxidizing agent, atomic oxygen, produced by actinic ultraviolet irradiation, is a bit more ferocious....and space parts need protection. Gold is good as a thin film(which is why you see those gleaming gold clad satellites in pre-launch photos).
see;http://www.grc.nasa.gov/WWW/RT/2004/RP/RPY-degroh.html