Earth is a famously wet planet, but where all that water came from in the first place remains a mystery. The most commonly-accepted theory is that comets and asteroids delivered it via impacts during the early days of Earth, and now a NASA study has found new evidence to support that idea. Observations of a comet that whizzed by close to Earth a few months ago show that it contains "ocean-like" water – and this may apply to other previously-dismissed comets too.

Some theories suggest that water has been here more or less since the beginning, when early Earth was just a huge ball of churning, warm, wet mud. Other evidence points to it being delivered when a Mars-sized proto-planet crashed into the young Earth and spawned the Moon.

But the general consensus holds that the life-giving liquid arrived on the backs of asteroids and comets. These collisions occurred far more often in the tumultuous early days of the solar system, and while these space rocks may look pretty dry, water has been detected on them somewhat regularly.

That might sound like an open-and-shut case, but it's more complicated than that. Most comets studied so far have been found to harbor the wrong type of water. The stuff we're used to here on Earth is, as you know, made up of two hydrogen atoms and an oxygen atom. But there's also "heavy" water, which contains a hydrogen atom with an extra neutron.

Water found in ice grains on Comet Wirtanen may suggest that Earth's oceans came from comet collisions NASA/SOFIA/L. Cook/L. Proudfit

Comparing the ratio of heavy-to-regular water between two samples is a solid indication of a shared heritage, kind of like a DNA test. And in almost all cases, water on comets has been found to have very different ratios to Earthly water. In fact, only one out of 11 comets studied is bearing "ocean-like" water.

Until now. Comet Wirtanen, which made its closest approach to Earth in December 2018, is only the second-ever comet to be found carrying ocean-like water. The discovery was made by the Stratospheric Observatory for Infrared Astronomy (SOFIA), which was able to get a clear look at the space rock from its high-altitude perch onboard a modified Boeing jetliner.

But the biggest surprise wasn't just that Wirtanen was home to the right type of water – it might be an indication that every comet is, contrary to previous findings. When the researchers compared the data on Wirtanen to that of other comets, they found that the ratio didn't depend on where the comets came from, as previously believed.

Instead, it seems to be a matter of how much water was being released from ice grains in the vapor cloud around the comet, not the ice on the surface. That means previous comet studies could have been measuring it all wrong, and perhaps they do have more Earth-like water ratios after all.

"This is the first time we could relate the heavy-to-regular water ratio of all comets to a single factor," says Dominique Bockelée-Morvan, second author of the study. "We may need to rethink how we study comets because water released from the ice grains appears to be a better indicator of the overall water ratio than the water released from surface ice."

The research was published in the journal Astronomy & Astrophysics Letters.

Source: NASA