In late 2011, a team of NASA and European scientists recorded the "fingerprints" of mystery molecules in two distant galaxies, Andromeda and the Triangulum. Astronomers can count on one hand the number of galaxies examined so far for such fingerprints, which are thought to belong to large organic molecules (molecules that have at least 20 atoms or more), said the team's leader, Martin Cordiner of NASA's Goddard Center for Astrobiology. This is quite small compared to, say, a protein, but huge compared to a molecule of carbon monoxide, a very common molecule in space.
Figuring out exactly which molecules are leaving these clues, known as "diffuse interstellar bands" (DIBs), is a puzzle that initially seemed straightforward but has gone unsolved for nearly a hundred years. The answer is expected to help explain how stars, planets and life form, so settling the matter is as important to astronomers who specialize in chemistry and biology as determining the nature of dark matter is to the specialists in physics.
The significance of the first DIBs, recorded in 1922 in Mary Lea Heger's Ph.D. thesis, was not immediately recognized. But once astronomers began systematic studies, starting with a 1934 paper by P. W. Merrill, they had every reason to believe the problem could be solved within a decade or two.
More than 400 DIBs have been documented since then. But not one has been identified with enough certainty for astronomers to consider its case closed.
"With this many diffuse bands, you'd think we astronomers would have enough clues to solve this problem," muses Joseph Nuth, a senior scientist with the Goddard Center for Astrobiology who was not involved in this work. "Instead, it's getting more mysterious as more data is gathered." Detailed analyses of the bumps and wiggles of the DIBs, suggest that the molecules which give rise to DIBs—called "carriers"—are probably large.
Recently, more interest has been focused on at least one small molecule, a chain made from three carbon atoms and two hydrogen atoms (C3H2). This was tentatively identified with a pattern of DIBs.
On the list of DIB-related suspects, all molecules have one thing in common: they are organic, which means they are built largely from carbon. Carbon is great for building large numbers of molecules because it is available almost everywhere. In space, only hydrogen, helium and oxygen are more plentiful. Here on Earth, we find carbon in the planet's crust, the oceans, the atmosphere and all forms of life.