Saturn’s moon, Titan, must have formed in an entirely different way to Earth and yet they share one thing in common: thick, nitrogen-rich atmospheres that are seething with organic compounds.
Today, Josep Trigo-Rodriguez at the Institute of Space Sciences (CSIC-IEEC) in Barcelona and Javier Martín-Torres at the Centro de Astrobiología in Madrid, both in Spain, point out that this might be an important clue. Their thinking is that common features point to a common past, meaning that the atmospheres of Earth and Titan must have formed in similar ways.
At first glance, that seems unlikely. The conventional thinking is that the Earth formed in the inner part of the Solar System from the accretion of rocky planetesimals. Titan, on the other hand, formed in a melee of comet-like iceballs that orbited Saturn in the early Solar System.
Nevertheless, Earth’s early atmosphere has always puzzled planetary geologists. One theory is that it formed from the outgassing of rocks as they bound together into the early Earth. But these rocks are thought to have been relatively poor in light elements such as hydrogen, carbon and nitrogen because these would have been preferentially blown away from the inner disc of the early Solar System.
So it’s a surprise to see similar distribution of these elements in the atmosphere of Titan which formed from iceballs much further away.
Trigo-Rodriguez and Martín-Torres conclude that the similarity means that Earth’s atmosphere must also have formed from comets, probably during the late heavy bombardment, a period some 4 billion years ago when the inner Solar System is known to have been showered with ice and rocks from further away.
That seems sensible. Indeed it matches the conclusions from another line of thinking: isotopic analysis of the ratio of N14 and N15 in the Earth’s atmosphere.
And that means it looks increasingly likely that life on Earth, made of star stuff, has evolved breathing comet gas.
Ref: arxiv.org/abs/1102.4198: Clues On The Importance Of Comets In The Origin And Evolution Of The Atmospheres Of Earth And Titan