Abundant Ammonia in Ancient Asteroid
The work is being published in the Proceedings of the National Academy of Sciences (PNAS). The paper is titled, “Abundant ammonia in primitive asteroids and the case for a possible exobiology,” and is co-authored by Pizzarello, geologist Lynda Williams, chemists Gregory Holland and Jeffery Yarger, all from ASU and Jennifer Lehman of UC Santa Cruz.
The finding of a high concentration of nitrogen-bearing molecules in an asteroidal environment shown by the new study is very provocative. Besides the noble gases, nitrogen is the fourth most abundant element in the Sun and the universe overall. On the Earth, it is an indispensable ingredient of the biosphere, being essential to DNA, RNA and proteins. In other words, it is necessary for life's information transfer and catalytic processes.
On the early Earth, on the other hand, the prebiotic inventory of reduced nitrogen necessary for the formation of N-containing biomolecules has been difficult to predict. The hypotheses of a reducing atmosphere had initially allowed one to envision considerable ammonia abundance as well as evolutionary pathways for the production of amino acids. However, the current geochemical evidence of a neutral early Earth atmosphere, combined with the known photochemical destruction of ammonia, has left prebiotic scenarios struggling to account for a constant provision of ammonia.
An abundant exogenous delivery of ammonia, therefore, might have been significant in aiding early Earth's molecular evolution, as we should expect it to have participated in numerous abiotic as well as prebiotic reactions.
It also is interesting to note that the new PNAS work was made possible by the finding in Antarctica of these exceptionally pristine, ammonia-containing, asteroidal meteorites. Antarctic ices are good “curators” of meteorites. After a meteorite falls – and meteorites have been falling throughout the history of Earth – it is quickly covered by snow and buried in the ice. Because these ices are in constant motion, when they come to a mountain, they will flow over the hill and bring meteorites to the surface.