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Solid Hydrogen Ice May Explain Interstellar Glow, Say Chemists

Astronomers have never been able to explain the weak glow they can see in interstellar space. Now new evidence suggests that solid hydrogen ice may be responsible

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.

It turns out that interstellar space glows faintly, producing a complex mixture of frequencies. These emissions are called the diffuse interstellar bands or DIBs and their origin has long puzzled astronomers.

Now Ching Yeh Lin and co say the predicted emissions of H6+ closely match those that astronomers can see. “We conclude that serious consideration should be given to the possibility that solid H2 is abundant in the interstellar medium,” they say.

That’s exciting progress. Hydrogen must be an important component of the interstellar medium but astronomers know that gaseous hydrogen cannot produce the observed emissions. That’s led astronomers to imagine that all kinds of other more complex molecules might be present, even large organic molecules, such as amino acids and polyaromatic hydrocarbons, the building blocks of life.

There is good evidence that these molecules are present in some dense clouds but it seems a stretch to argue that they are spread throughout interstellar space.

So the possibility that solid hydrogen ice might be responsible instead is compelling.

What we need, of course, is more data. Somebody needs to make H6+ from solid hydrogen and accurately measure the emissions it would produce in the conditions of deep space.

Not an easy experiment, to be sure, but one that is surely possible with today’s technologies.

Ref: arxiv.org/abs/1105.1861: Interstellar Solid Hydrogen

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