Rosetta scientists say Earth's water did not come from comets

Rosetta scientists have said that Earth's water probably didn't come from comets, by analysing water vapour emanating from the comet being tailed by the spacecraft.

According to the researchers, comets Comet 67P/Churyumov-Gerasimenko like couldn't have been the source of the water in Earth's oceans, and that suggests our H2O was more probably delivered by asteroids, during a time when they were much icier than they are now.

The key finding has to do with the balance of two types of hydrogen bound up in those H2O molecules. The most common type of hydrogen has one proton in the atom's nucleus, but on Earth, about three molecules out of 20,000 instead have deuterium, a different hydrogen isotope with a proton and a neutron in the nucleus. Measuring the deuterium/hydrogen ratio, or D/H ratio, is one of the primary tasks for ROSINA.

As the car-sized Rosetta spacecraft homed in on Churyumov-Gerasimenko, ROSINA got an increasingly precise fix on the D/H ratio in water molecules wafting off the comet. The data showed that there were about three times as many deuterium atoms as there would be in an earthly sample.

The results announced on Wednesday indicate that there can be quite a bit of variation in the signature of water from Jupiter family comets, but based on the ROSINA analysis, it would be hard to explain how cometary water could have been the main contributor to Earth's oceans.

The findings leave the asteroids of the early solar system as the best candidates for furnishing Earth's water and based on the cratering rates observed on the moon, scientists have figured out that our planet was hit by a wave of cosmic impacts during a period known as the Late Heavy Bombardment — about 800 million years after the solar system was formed.

The study was published online in the journal Science.