Was the water in your tea delivered by an asteroid?

Water, water everywhere but no one's sure how it all got here. When Earth formed 4.6 billion years ago it was incredibly hot from gravitational contraction and continuous meteorite and asteroid bombardment. Any of the planet's original water should have boiled away into space.

To replace what went missing, it's widely thought that water was delivered by a fusillade of asteroids and comets after Earth's had cooled down enough for water to pool on its surface. The question has always been which was the main contributor: asteroids, composed of mostly rock, or comets, made mostly of ice. At first blush, comets seem the logical choice. Melt ice and you get water. But it's not that simple. New data from Rosetta's comet may point otherwise.

Water's made of two hydrogen atoms bonded to one oxygen. Together they make H2O. The key to determining where the water originated is in its 'flavor', in this case the proportion of deuterium – a form of hydrogen with an additional neutron – to normal hydrogen in water. Because that extra neutron makes a deuterium-based water molecule heavier, it's often referred to as "heavy water".

Heavy water comprise just 0.2% of all naturally occurring water on Earth. Ice cubes made of it sink in a glass of ordinary water.

The proportion of heavy vs. normal water is an important indicator of the formation and early evolution of the solar system, with simulations showing that it should change with distance from the Sun. This makes it a key diagnostic to determining where an object originated.

Of the 11 comets for which measurements of deuterium to hydrogen (D/H ratio) have been made, only Comet 103P/Hartley 2 was found to match the composition of Earth's water, in observations made by ESA's Herschel Space Observatory  in 2011. The real surprise are the asteroids. Despite generally being poor in water except for the carbon-rich carbonaceous variety, the rocky asteroids may very well have delivered much of our planet's water after its hot-headed birth.

"Our finding also rules out the idea that Jupiter-family comets contain solely Earth ocean-like water, and adds weight to models that place more emphasis on asteroids as the main delivery mechanism for Earth's oceans," said Kathrin Altwegg, principal investigator for ROSINA (Rosetta's comet atmosphere analyzer) and lead author of the paper reporting the results in the journal  Science this week.

So despite the fact that asteroids have a much lower overall water content, impacts by a large number of them could still have resulted in Earth's oceans.

Wonderful, isn't it, that the answers to some of the simplest but most profound questions are sometimes found right over our heads.

And yet. And yet I can't help think that at least a portion of our water came right out of the ground. There's no question that volcanoes were active on Mars 1-2 billiion  years ago; Earth must have been belching out gases and vapor at least that early or perhaps even earlier. Since the most common gas vented from volcanoes is water vapor followed by carbon dioxide it would seem we should also consider water supplied from deep magma sources that fed early volcanoes.

Not only did volcanic activity build Earth's early atmosphere but it could have also provided a healthy dose of earthy H2O. Just a thought.