One of the primary missions of the Rosetta spacecraft, which was launched in 2004 and is now orbiting comet 67P/Churyumov-Gerasimenko, is to examine the composition of the comet up close. One of the many questions scientists hoped the probe would answer is one that is seemingly very simple – where did Earth’s most important asset – liquid surface water – come from.

For years scientists theorized that water was delivered to earth by a series of comet impacts during the late heavy bombardment, a period in early Earth history where impacts were relatively commonplace. The theory made sense – comets are the only extraterrestrial bodies known to contain large amounts of water.

That’s why, when scientists got the results back from the spectroscopes aboard the orbiter and its lander, they were in for a surprise: while the comet did contain large amounts of water, the isotopic signature of that water differed markedly from that of Earth’s water (it contained over three times as much deuterium as water on Earth).

While some scientist now suspect water was delivered by asteroids (which, they claim, may have once contained more water than they do now), a new theory has emerged from research carried out at Ohio State University and presented at the American Geophysical Union conference this December in San Francisco.

Computer models and high pressure, high temperature laboratory studies suggest that the earth itself may have generated the water now found in the oceans. The results that Ringwoodite, a mineral that is common in the mantle, is capable of harboring hydrogen ions at the depths and pressures common in the mantle. A series of recently described geochemical pathways triggered by plate tectonic mantle circulation can cause the hydrogen to bond with oxygen, which is ubiquitous in the planet’s crust. The study suggested that a volume of water equal to that of the Pacific Ocean may currently be locked within the earth’s crust.

These findings, if accurate, suggest two things.

First, they suggest that the hydrogen making up the water in today’s oceans was present at the time of our planet’s formation.

Second, they suggest that plate tectonics is even more critical to life on earth than previously thought. In addition to providing a magnetic field and controlling greenhouse gas concentrations, plate tectonic processes may be responsible for the presence of water on Earth’s surface.

A better understanding of the origins of water will help us understand our own planet, but, equally importantly, it will allow us to hone our search for life beyond our Solar System.