Our search for E.T. in the great, dark forest of the universe has us searching for signs of water. We know how important the stuff is for life to thrive. But closer to home there's a more pressing question: Where did Earth's water come from?
A team of international astronomers have taken a few small steps to answering that question by studying the water found within a near-Earth comet, 46P/Wirtanen.
"We have identified a vast reservoir of Earth-like water in the outer reaches of the solar system," said Darek Lis, lead author of the study, in a statement.
The study, published in Astronomy and Astrophysics Letters on May 20, used NASA's Stratospheric Observatory for Infrared Astronomy (SOFIA) to investigate the so-called Christmas comet, as it . The instrument operates out of a specially-designed Boeing 747 up above the clouds, enabling it to see cosmic phenomena with a pair of clearer eyes than some of our ground-based methods.
A long-standing theory posits the Earth's water was delivered by icy comets originating from the outer reaches of the solar system. Comets zip through space full of dust, ice and other chemicals and occasionally, collide with planets. The formation of the Earth likely occurred when small rocky bodies collided with each other -- but these were very water-poor. Thus, the theory suggests that comets may have delivered some of the water that makes up 70% of the planet when they collided with the early Earth.
However, evidence that comets contained the same kind of water found on Earth, good ol' H2O, has been limited. To determine the comet was carrying "Earth-like" water, SOFIA eyed 46P/Wirtanen and examined the ratio between two different types of water.
The water we know and love, H2O, is made of two hydrogen atoms and one oxygen atom. Those hydrogen atoms contain one proton and zero neutrons. That form of water makes up most of the water found on Earth. A less common form of water, "heavy water", is made up of the same atomic structure, but the hydrogen atom contains one proton and one neutron. This form of water is known as deuterium. Studying the ratio of deuterium to hydrogen (D/H ratio) can teach us a little more about its origin.
The D/H ratio in comets usually varies between 1 to 3 times more than the ratio of Earth's oceans, but the astronomers found the ratio in 46P/Wirtanen was basically the same as we see down here on Earth.
Two previous comets have exhibited similar ratios: 103P/Hartley 2 and 45P/H-M-P. Notably, all three are classified as "hyperactive comets", which release water from ice on their surface and in their atmosphere as they heat up on approach to the sun. The team discovered the D/H ratios in all three were related to the water present in their atmosphere. As a result, they believe that all comets might contain Earth-like water locked within their rocky bodies.
Such a finding again opens up the debate regarding the origins of Earth's water. Other comets, originating in the Oort cloud at the edges of the solar system, don't exhibit similar D/H ratios.
Asteroids, which differ to comets in that they are more metallic and rocky, have also been suggested as a source. Of note is the number of asteroids found with Earth-like water in the asteroid belt between Mars and Jupiter.