October 20, 2021

I, Science

The science magazine of Imperial College

Why does Saturn’s largest moon have a nitrogen atmosphere like Earth?  It’s a question that has stumped astronomers since the Voyager spacecraft went to Titan in 1981.  However, laser gun-toting scientists from Japan think a catastrophic volley of meteorites might hold the answer.

Titan’s air is so thick and its gravity is so low that if we went there and strapped wings to our arms, we’d be able to fly like birds.  But why is the air there?  None of Saturn’s other satellites have atmospheres.  In fact Titan is the only moon in the Solar System that’s surrounded by anything other than tiny traces of gas.

Looking at Earth’s history provides no clues.  Earth’s atmosphere formed billions of years ago when our then-molten planet underwent what’s known as differentiation – a grand, geological tidy-up when heavy elements like iron and nickel sunk to what is now the core, while lighter elements like silicon, carbon, and nitrogen formed the mantle, crust and atmosphere.  Importantly, Titan’s differentiation was incomplete, and astronomers think its nitrogen stayed locked in its crust as ammonia ice.  Something must have set it free.

Researchers from Osaka, Tokyo and Chiba believe impacts during the Late Heavy Bombardment – a period between 4.1 and 3.8 billion years ago when the solar system was pummelled by hundreds of thousands of meteorites – were responsible.

Testing this hypothesis was not easy.  The team, led by Dr Yasuhito Sekine, had to simulate 300,000 trillion-tonne lumps of rock smashing into Titan at 25,000 miles per hour.  To do so, they blasted ammonia and water ice with gold and platinum projectiles fired using pulses from a high energy laser gun.  The team’s results, published this week in Nature Geoscience, show that ammonia ice readily converts to nitrogen gas on impact.  Their calculations suggest that the Late Heavy Bombardment supplied enough impacts to generate sufficient nitrogen to form the moon’s dense atmosphere.

The scientists are now tuning their methods to assess the effects of meteorite impacts on other moons and planets in the solar system.