再Astrochemistry, March 16th 2013 P77 宇宙化学
Most people think of the empty space between the stars as being, well, empty. But it is not. It is actually filled with gas. Admittedly, at an average density of 100-1,000 molecules per cubic centimetre (compared with 100 billion billion in air at sea level), it is a pretty thin gas. But space is big, so altogether there is quite a lot of it. Most of it, about 92%, is hydrogen. A further 8% is helium, which is chemically inert. But a tiny fraction - less than one-tenth of a percent - consists of molecules with other elements, such as oxygen, carbon and nitrogen, in them. Though these other elements are mere soupcon of the interstellar soup, they do give it real flavour. So far about 180 types of these molecular ingredients have been detected in space from their chemical bonds. There are two reasons for wanting to study them. One is that these molecules are probably the precursors of life. The other is that the rarefied nature of astrochemistry changes the way processes work. It means the individual steps in chemical reactions can be disentangled from one another in a way that is hard - and sometimes impossible - on Earth. And it allows reactions to happen there that are unknown on Earth. Now the astrochemists have a new tool：the Atacama Large Millimetre/submillimetre Array (ALMA) in northern Chile. It was officially opened on March 13th but has already been making discoveries, including the most intense burst of star birth in the early universe. ALMA consists of 66 dishes and is the world's most powerful radio telescope. At a cost of $1.3 billion it should provide a hundredfold increase in sensitivity and resolution over the best older instruments. Those older telescopes had to focus on nebulae, where the interstellar gas is most concentrated (a familiar one, visible through binoculars, is the gas cloud around the stars that make up Orion's sword). And older telescopes can detect only strong, simple signals of the sort emitted by small molecules like carbon monoxide and hydrogen cyanide, which have two and three atoms respectively. Spotting more complex substances was almost impossible because their rotational energy is scattered by their numerous bonds across a wide range of frequencies. ALMA, by contrast, can detect such things routinely. It has already identified glycolaldehyde and acetone, molecules that have eight and ten atoms respectively. In particular, ALMA's masters, a consortium of research agencies from Canada, Chile, Europe, Japan, Taiwan and the United States, hope to find simple sugars and organic acids - molecules most researchers in the field believe were needed to get life going on Earth. The ability to study chemical reactions stage by stage will be eqtally important. High-school chemistry lessons, with their neat equations transforming, say, 2H2+O2 into 2H2O, miss out a plethora of intermediate steps as (in this case) the formation of hydroxyl, OH. In a lab, these intermediater are often too short-lived to be detectable. But in space an intermediate may hang around a long time before it encounters its partner in the next stage of a reaction. ALMA can see the microwave traces of such intermediates, and thus gain a better understamding of them. There are also completely new reaction to discover. Anthony Remijan, of America's National Radio Astronomy Observatory, who is one of the astronomers putting putting ALMA through its paces, is studying the formation of methyl formate, compound widely used on Earth in applications from insulation to insecticides. Usually it is synthesised either from methanol and formic acid, or methanol and carbon monoxide. But there is, in theory, a third route that uses formic acid and an un stble substance made from methanol and hydrogen. This has not been seen in an Earthy laboratory, but Dr Remijan thinks it is an important pathway in space, and ALMA should soon tell him if he is right. Probably, that particular discovery will have no practical consequences. The known syntheses are effective, and methyl formate is already cheap. But it will prove a principle about using the cosmos as a chemistry laboratory, and the hope is that similar findings about other molecules that are harder to make may allow chemical engineers at home to reformulate their processes. If that happens, the test tube in the sky really will have proved its worth.
