The simple organic compounds that exist in interstellar space and the solar system are thought to originate from late-type giant stars. Anthracene is one such compound, and contains 14 carbon atoms and 10 hydrogen atoms arranged into three rings. Micrometre-sized dust grains in the solar system often contain frozen anthracene and water.

To simulate conditions in interplanetary space, Gabla and colleagues cooled a thin layer of anthracene to 150 kelvin and placed it inside a vacuum. The sample was bombarded with 3.5 keV protons, a similar energy to those in the solar wind. A stream of water vapour directed at the sample mimicked the effect of the evaporating water.

After the anthracene had received around 1017 protons per square centimetre - equivalent to around 100 years of solar wind, under current conditions - Gabla's team found that a yellowish-brown crust had emerged on the previously white surface. Mass spectrometry revealed that the crust consisted of a wide variety of quinone molecules.

"To our knowledge, this is the first time it has been shown that biological molecules can be formed from the interaction of low-energy protons and non-biological molecules", Gabla told PhysicsWeb.

Low-energy protons such as those found in the solar wind can only penetrate a few hundred nanometres into the surface of a comet. This means they would have a negligible effect on the overall composition of the comet. But interplanetary dust particles - usually debris from collisions between comets - are typically just a few micrometres across, and these react readily with protons because of their large surface-to-volume ratio. Gabla and colleagues point out that over three thousand tonnes of dust enter the atmosphere every year, and that such small grains of organic matter could enter the Earth's atmosphere without burning up.

However, astrophysicist Chandra Wickramasinghe of Cardiff University in Wales remains cautious. "This shows that biochemical monomers can form under solar-system conditions. Although this process must occur, its contribution to the origin of life remains conjectural." Wickramasinghe and colleagues recently suggested that fully fledged microbes may have initiated life on Earth.