Scientists don't know how it happened, but the oxygen-starved atmosphere of Saturn's moon Titan contains chemicals that on Earth can serve as building blocks for complex organic molecules.

Scientists used an instrument on the Saturn-orbiting Cassini spacecraft and found heavy negatively charged ions in Titan's upper atmosphere, despite the absence of oxygen.

On Earth, oxygen is needed to ionise molecules in the lower ionosphere.

"Their existence poses questions about the processes involved in atmospheric chemistry and aerosol formation," says Dr Andrew Coates, with University College London's Mullard Space Science Laboratory.

The molecules have additional rings of carbon and can become what are called polycyclic aromatic hydrocarbons.

Scientists believe these polycyclic molecules can serve as a starting point for simple microbial life.