  Molecular oxygen, O$_2$, 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 O$_2$ from SO$_2$, a major chemical
constituent of many planetary atmospheres and one which played an important
part on Earth in the Great Oxidation event. The O$_2$ production involves
dissociative double ionisation of SO$_2$ leading to efficient formation of the
O$_2^+$ ion which can be converted to abiotic O$_2$ by electron neutralisation.
We suggest that this formation process may contribute significantly to the
abundance of O$_2$ and related ions in planetary atmospheres, especially in
those where CO$_2$, which can lead to O$_2$ production by different mechanisms,
is not the dominant component.
