Important, but elusive

Doing astrochemistry in a lab is difficult, but can be important.

We’ve mentioned helium hydride as an exception to one of the more reliable rules of chemistry, that noble gases (of which helium is the noblest) do not form compounds.  It is also, as one would expect, very difficult to find; it was an important discovery when it was actually detected out in space earlier this year.  It requires a peculiar combination of density, radiation field and chemical composition for this molecule to form, conditions that are rare even if we look throughout the universe.

But at a certain time in the past helium hydride was extremely important.  It was the very first molecule formed, before stars or galaxies appeared, and its characteristics are vital to our understanding of the cosmic “Dark Ages” (between the Big Bang and the first stars).  So the fact that it’s hard to find now is rather inconvenient, since we’d really like to study its behavior.

It can, however, be made in a (special kind of) laboratory, one that combines low density (a vacuum by any other name) and low temperature.  A recent experiment on laboratory helium hydride found that a certain reaction involving it takes place much more slowly than had been calculated before, with implications for much of astrochemistry.

This experiment did not exactly recreate Dark Age conditions.  Instead, it prepared molecules in a certain way and measured how they interacted, coming up with numbers different from those extrapolated from other experiments.

The episode shows, in part, how difficult it is to duplicate astronomical conditions in the laboratory.  In fact this is one of the salient points of the science (either convenient or inconvenient, depending on one’s point of view): astronomers deal routinely with conditions that cannot be matched here at home, testing our theories with extremes and inspiring new ones with strange phenomena.  And if important regimes are inaccessible both to observations and experiment, our numbers may be off.  Our astronomer remembers much uncertainty in the arcane field of solar neutrinos some years ago, because the speed of a certain nuclear reaction deep in the Sun was not very well known theoretically and could not be measured on Earth.

So the ingenuity of laboratory chemists will continue to be called on to help out astronomers.  In return, the astronomers can provide, well, challenges.