May 25, 2020

04.11.09:Did Ancient Comets give Earth its Seas?

Comets continue to be at the center of controversy concerning the early Earth and life. If you’ve been following our recent reports as of late, you know that opinions run the gamut, from ancient cometary impacts being relatively rare, to comets being crucial to life as we known it. Now, researchers at the Niels Bohr Institute in Denmark have scored one for the comet camp. Recent studies of ancient rocks in Greenland suggest that the primordial Earth may have undergone a massive cometary bombardment early in its history, about 3.85 billion years ago. Were talking waaaaay back in the Archean period, before life had even taken hold. The conclusion is based on our friendly elemental smoking gun, Iridium. Rare on Earth, what little iridium is found in the Earth’s crust is almost certainly of extraterrestrial origin. Asteroid impacts generally distribute about 18,000 parts per trillion, while comets, due to a higher impact velocity and icy rock composition, produce amounts much lower, about 130 parts per trillion. The team found a ratio of 150 ppt, strongly suggesting that comets were the primary constituents of the Late Heavy Bombardment. This is a tantalizing clue in two enduring mysteries concerning the early Earth; how did we our get our large oceans, and how did life start? Looking out into the solar system, we are the only planet with a large surface covering of liquid water. Could it have been deposited by comets? That’s a lot of dirty snowballs… there is some thought that life itself, or at least amino acids, known as the chemical building blocks of life, might have been deposited in the same fashion by a method known as panspermia. Not all scientists remain convinced, however, and for now, spinning cometary hypotheses remains a sure way to generate scientific controversy. Are we all “comet-stuff?”