Complex carbon-based molecules, including many that are important for life on Earth, could have formed in the early solar system. That's according to a paper by Fred Ciesla and Scott Sandford in this week's issue of the journal Science, based on their computational simulations.
The origin of these prebiotic molecules has been a long-standing puzzle. In order for simple stable molecules such as carbon dioxide and methanol to rearrange themselves into more complex configurations, two conditions are thought to be needed.
In order to break the simple configurations apart, ultraviolet light is needed, and in order for the fragments to then be rearranged into different configurations, a heat source is needed. Until recently it was thought that all of the organic material on Earth had originated in situ, helped by lightningstorms in the Earth's early history.
However, recent evidence is that organic molecules are quite widespread in the Universe. They have been found on interplanetary dust grains and in meteorites arriving on Earth; furthermore, their spectral signatures can be seen in many star-forming nebulae far beyond our own solar system. This suggests that they can form in a much more widespread environment than simply the atmospheres ofplanets.
Until now, the early solar system had been ruled out as such an environment. The protoplanetary disk of gas and dust which formed into the planets was so dense that only its innermost parts would have been exposed to the Sun's ultraviolet radiation.
But writing in Science this week, Ciesla & Sandford report on computational simulations of the migration of material within such disks, taking into account effects such as the evolution of the disk and the turbulence within it.
They conclude that there would have been very significant mixing of material as the solar system was forming, and that much of the material would at some point, by random chance, have found itself elevated slightly out of the top or the bottom of the disk. Here, it would have had a clear line-of-sight to the Sun.
Even if this only happened for a thousand years for a typical dust grain, out of a total lifetime of around a million years for the disk, that would have been enough time for substantial chemical changes to occur.
By comparing their simulations with laboratory experiments to determine the amount of ultraviolet irradiation needed to produce chemical changes, Ciesla & Sandford conclude that this would have been ample time to account for all of prebiotic molecules found in the solar system today.
The question remains as to how these molecules went on to form still larger and more complex molecules, such as amino acids and proteins, and how these eventually led to life on Earth, but forming the prebiotic building blocks is an important step along the way.
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