The Rosetta Space Probe has not found comet water matching the type available on Earth, meaning the water we drink may have come from another source.
There are still all kinds of mysteries science is trying to solve about our planet Earth, including this one: How the heck did we end up with so much water? The prevailing theory was that comets brought this water to Earth during one of our historic large-scale impacts. But now that the Rosetta space probe had a chance to look at a comet up close, we might be back at the drawing board again.
The problem? The European Space Agency's Rosetta probe brought back results that the comet's water is heavy water, which means it contains an above-average amount of deuterium, an isotope of hydrogen. Scientists had hoped that comets from the Kuiper Belt would be a perfect match for Earth's water, much like one that we observed three years ago. But the heavy water content from this comet suggests the previous observations might be an anomaly, meaning we need to think of other possibilities.
One popular theory is that water was brought here by asteroids instead, since they likely carried it roughly 4 billion years ago. But not all scientists are in agreement with the asteroid idea. Another theory is that Earth carried its original water in its crust or ice at the poles, that melted and spread over the course of several warming and cooling periods.
Another possibility is that Earth just happened to be hit with a comet that, like the one observed in the Kuiper Belt, had the correct water content. But that's something difficult to prove reliably. Regardless, Rosetta is clearly already expanding our knowledge of outer space, an incredibly exciting prospect even if we were originally wrong.