H2O & HDO

H2O & HDO

The mystery of why Earth has so much water, allowing our "blue marble" to support an astounding array of life, is clearer with new research into comets. Comets are like snowballs of rock, dust, ice, and other frozen chemicals that vaporize as they get closer to the Sun, producing the tails seen in images. A new study reveals that the water in many comets may share a common origin with Earth's oceans, reinforcing the idea that comets played a key role in bringing water to our planet billions of years ago.

Dirty Snowballs

Planets form from debris orbiting in a disk shape around a star; small pieces of debris can stick together and grow larger over time. Leftover debris remains in regions of our own solar system like the Kuiper Belt, beyond Neptune, or the Oort Cloud, far past Pluto. Comets come from these areas, but we can only see them when their orbits bring them closer to the Sun. The heat from the Sun causes some of the dirty snow to vaporize, creating the fuzzy halo or "coma" of water vapor, dust and ice grains seen in comet images.

Scientists predict that the water in Earth's oceans came from water-carrying bodies in the early solar system that collided with our planet, similar to today's ice-rich asteroids or comets. But scientists do not know where in the formative disk these objects originated.

Water Types

Water is also known by its chemical name H2O because it's made of two hydrogen atoms and one oxygen atom. But using special instruments, scientists can detect two types: regular water, H2O, and heavy water, HDO, which has an extra neutrally-charged particle called a neutron inside one of the hydrogen atoms. Scientists compare the amount of heavy to regular water in comets. If comets have the same ratio of these water types as Earth's oceans, it indicates that the water in both may share a common origin.