The massive nitrogen atmosphere of Saturn's moon Titan could be the result of a series of giant impact events four billion years ago, according to new research conducted by scientists based in Japan.

Titan is the only moon in the Solar System with a dense atmosphere, so dense that its surface pressure is 50 percent greater than on the Earth. But this presents something of a conundrum; large atmospheres usually only form after a planet or moon has differentiated into a core, mantle and crust, yet the Cassini spacecraft, which has been exploring the Saturnian system since 2004, found that Titan is not completely differentiated.

Before Cassini arrived in the Saturn neighbourhood it was speculated that Titan's nitrogen was produced by the breakup of atmospheric ammonia by sunlight, or through outgassing of ammonia in volcanic systems, but both of these models require Titan to have formed at high temperature, and thus be differentiated.

The idea that Titan got its atmosphere long after the moon formed is supported by measurements made by the Huygens probe as it descended through Titan's atmosphere in 2005, which found an extremely low abundance of primordial argon (36Ar) – it would be expected in much larger amounts if the atmosphere had formed along with the moon 4.6 billion years ago.

Sekine used laser gun experiments to demonstrate that nitrogen is formed during impacts into ammonia ice representing Titan's crust. Calculations based on the experimental results show that Titan would acquire enough nitrogen in this way to sustain the current atmosphere.

But the ratio of nitrogen isotopes on Titan are different to that of Earth, which too has an atmosphere dominated by nitrogen, suggesting that the atmospheres of the two bodies were derived from different sources. More detailed study of the atmospheres and ices of outer Solar System bodies, including comets, will help unveil exactly where Titan got its atmosphere from.