Molecular oxygen, O2, is vital to life on Earth and possibly on other
planets. Although the biogenic processes leading to its accumulation in Earth's
atmosphere are well understood, its abiotic origin is still not fully
established. Here, we report combined experimental and theoretical evidence for
electronic-state-selective production of O2 from SO2, a major chemical
constituent of many planetary atmospheres and one which played an important
part on Earth in the Great Oxidation event. The O2 production involves
dissociative double ionisation of SO2 leading to efficient formation of the
O2+ ion which can be converted to abiotic O2 by electron neutralisation.
We suggest that this formation process may contribute significantly to the
abundance of O2 and related ions in planetary atmospheres, especially in
those where CO2, which can lead to O2 production by different mechanisms,
is not the dominant component