About 70% of the surface of our planet Earth is covered in water. We are nestled in our solar system at just the right distance from the Sun for this liquid water to exist. Any farther and that water would be frozen in ice. Any closer and temperatures would be too hot and we would be at risk for a runaway greenhouse effect similar to what’s happening on the scorching surface of Venus. Our not-too-cold, not-too-hot position in the so-called ”Goldilocks zone” is a pretty good thing because, of course, water is necessary for life.
But how did that water get here? Water is a defining characteristic of our planet and it plays such an important part of our daily lives. Understanding how water arrived on Earth is a key part of understanding how and when life evolved here as well. But we don’t even know how it where it came from. Scientists are still actively researching how our planet got to be so wet in the first place.
The Early Earth
Our current picture of planet formation starts with a protoplanetary disk—that’s a large disk of gas and dust swirling around our newly-formed Sun. As the grains of dust and ice in the disk interact with themselves, those grains begin to form bigger and bigger clumps. Eventually those clumps form what we call planetesimals, the building blocks of rocky and giant planets.
But in the early period of our solar system’s formation, that disk was much hotter at the position where our Earth sits now. So even though there were most likely water molecules present in the mess of debris that made up the disk, it was too hot for water to condense into a liquid, causing it to evaporate instead. What’s more, the early Earth did not yet have an atmosphere making it easier for any liquid water droplets to be blown off into space. This leaves us with a bit of a puzzle. If the Earth could not have formed from the disk with its oceans already intact, how did they get here?
Comets vs Asteroids
If Earth’s water wasn’t formed along with the Earth, then, planetary scientists suspect, it must have been delivered later via extraterrestrial messenger. Both asteroids and comets visit the Earth and are known to harbor ice. (Not sure of the difference between an asteroid and a comet? Check out my earlier episode.) In fact, models of the compositions of asteroids and comets suggest that they even harbor enough ice to have delivered an amount of water equal to Earth's oceans....
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