Saturn's moon Titan contains chemicals that on Earth can serve as building blocks for complex organic molecules.

The atmosphere of Saturn's moon Titan, seen here, contains hydrocarbons that could be the starting point for microbial life .

Saturn moon home to life's building blocks

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.

Titan's atmosphere is mostly nitrogen and methane, a combination that is leaving scientists wondering how the negative ions, which have 10,000 times the mass of hydrogen, formed.

Cassini collected the data during 16 different encounters of Titan.

"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.

The research, reported in the journal Geophysical Research Letters, builds on an earlier discovery of smaller complex molecules called tholins above Titan's surface.

Tholins, which have up to about 8000 times the mass of hydrogen, are believed include some chemical precursors to life.

"Understanding how they form could provide valuable insight into the origin of life in the solar system," says planetary scientist Dr Hunter Waite, with the Southwest Research Institute in San Antonio, Texas.


Murky brown mist
Scientists suspect organic solids from the murky brown mist that surrounds Titan have been raining down on its surface for billions of years.

The late Professor Carl Sagan coined the term tholins from the Greek word tholos, or mud. Scientists believe Titan's natural tholins could be part of the chemistry needed for life.

Titan is the second largest moon in the solar system and the only one with a dense atmosphere.

It is believed to resemble primordial Earth and scientists are studying it to try to understand how life evolved here.

Cassini, a joint mission of NASA and the European Space Agency, has been circling Saturn and its eclectic collection of moons and rings since 2004.

Its next pass by Titan is scheduled for later this week, when the probe's infrared camera will attempt to image what may be a large lake of hydrocarbons on the moon's southern hemisphere.