Astronomers have long known that much of the universe is filled with diffuse hydrogen. In fact, they can see ionised hydrogen gas by the electromagnetic waves it gives off.
But back in the 1960s, some astronomers suggested that the interstellar medium might also be filled with solid hydrogen ice too. Various others later pointed out that this was unlikely because the ice ought to sublimate, even in the extreme cold of interstellar space.
More recently, astronomers have taken a second look at this idea and the pendulum of scientific opinion has begun to swing back in favour of hydrogen ice. That’s because chemists have discovered that hydrogen ice is more stable if it contains impurities. The extra ions in the lattice help to stabilise H2 ice.
So if hydrogen ice forms in space in the presence of other impurities, interstellar space could be full of it.
That raises an interesting question. Hydrogen ice is more or less transparent at optical frequencies. So how can we detect it in space?
Today, Ching Yeh Lin at the Australian National University in Canberra and a few buddies make an interesting suggestion. They say that when photons bombard hydrogen ice, they ought to ionise it creating ionised clusters of hydrogen and in particular H6+. This ion cluster does not form in hydrogen gas so its presence is a good marker for hydrogen ice.
The problem is that nobody knows what H6+ looks like–this work hasn’t yet been done in the lab. So Ching Yeh Lin and co have calculated from first principles its vibrational transitions. Their conclusion is that H6+ (and its deuterated cousin (HD)3+) ought to produce various emissions in the infrared part of the spectrum.
They then go on to compare their predictions with the emissions that astronomers can see coming from interstellar space.
TechReview
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