This question stems from a search for an answer to this other one regarding which molecules are most abundant in the universe (specifically which multielemental compound).

I found an article on the subject of interstellar water chemistry, which states in the introduction that

Molecules such as water form in localized regions of cold and dense gas and dust between the stars (see Figure 1). Typical densities in such interstellar clouds are $10^4$ hydrogen molecules per $cm^3$ and temperatures can be as low as 8 K (pressure∼10−14mbar). The clouds also contain small 0.001–0.1 μm sized solid dust particles or ‘grains’, consisting of amorphous silicates and carbonaceous material. Grains are important because they absorb and scatter the ultraviolet(UV) radiation produced by stars and thereby protect molecules from dissociating photons. Reactions on the surfaces of the dust particles promote the formation of molecules, especially of H2 and other hydrogen-rich species like H2O. [emphasis mine]

But in the paragraph immediately following this statement it says that

Interstellar space is a gigantic ultrahigh vacuum laboratory with densities low enough that molecules can form only through kinetic two-body processes in the gas phase.

The process described in the first paragraph does not seem to comply with the statement in the second one. In particular, involvement of the surface of a sand particle would appear to define (at least) a "three-body process" and while exposure to the gas phase is important to bring reagents together, it would still not seem sufficient to call this a "gas-phase" reaction. Am I wrong? How to reconcile this apparent contradiction?


Interstellar water chemistry: from laboratory to observations. Ewine F. van Dishoeck,Eric Herbst, and David A. Neufeld. Chem. Rev. (2013), 113 (12), 9043-9085, https://doi.org/10.1021/cr4003177.

  • $\begingroup$ The article must have made a mistake. $\endgroup$
    – Paul Kolk
    39 mins ago


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