A team of international astronomers has taken some small steps to answer this question by studying the water of a near-Earth comet, 46P / Wirtanen.
"We have identified a huge 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, which approached Earth in December 2018. The instrument works with a specially designed Boeing 747 above the clouds, which allows it to see cosmic phenomena with a pair of clearer eyes than some of our ground-based methods.
A long-standing theory states that the water of the earth was delivered by icy comets that emanate from the outer reaches of the solar system. Comets race through the room full of dust, ice and other chemicals and occasionally collide with planets. The Earth was probably formed when small rocks collided – but these were very poor in water. The theory is that comets may have released some of the water that makes up 70% of the planet when they collided with the early Earth. However, there is evidence that comets contained the same type of water found on Earth, good old H2O has been limited. To determine that the comet bore "earth-like" water, SOFIA studied 46P / Wirtanen and examined the relationship between two different water types .
The water we know and love, H2O, consists of two hydrogen atoms and one oxygen atom. These hydrogen atoms contain a proton and zero neutrons. This form of water makes up most of the water on earth. A less common form of water, "heavy water," consists of the same atomic structure, but the hydrogen atom contains a proton and a neutron. This form of water is known as deuterium. Examining 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 and 3 times the ratio of oceans on Earth. Astronomers found that the ratio of 46P / hostanes was basically the same as we are here to see the earth.
Two earlier comets showed similar ratios: 103P / Hartley 2 and 45P / H-M-P. Remarkably, all three are classified as "hyperactive comets" that release water from the ice on their surface and in their atmosphere as they warm as they approach the sun. The team found that the D / H ratios in all three cases are related to the water present in the atmosphere. As a result, they believe that all comets could contain earth-like water trapped in their rocky bodies.
Such a finding re-opens the debate on the origin of water on earth. Other comets originating from the Oort cloud at the edges of the solar system have no similar D / H ratios.
The source also suggested asteroids that differ from comets in that they are more metallic and rocky. Noteworthy is the number of asteroids found with earth-like water in the asteroid belt between Mars and Jupiter.