Where did Earth’s water come from?

Oceans cover 71% of our planet.  About 60% of the human body is water (more for kids, less for adults).  Beer is 90-97% water.

But, on a global scale, Earth is actually very dry.  Earth is only about 0.1% water by weight.  If all of Earth’s water were smushed into a ball it wouldn’t even be the size of the US.  And without this water we certainly wouldn’t be a pale blue dot.

Ball containing all of Earth's water compared with the planet (credit: Jack Cook/WHOI/USGS)
Ball containing all of Earth’s water compared with the planet (credit: Jack Cook/WHOI/USGS)

Still, Earth has much more water (by weight and in total) than Venus or Mars.  So, is Earth a water-rich planet or a water-poor one?  And why does Earth have any water at all?  Let’s rewind the clock a little and ask the question: where did Earth’s water come from?

To address this question we’ll have to delve way back into the mists of time (ooh la la!).  Back to when the terrestrial planets were forming from a disk of gas and dust orbiting the young Sun.  Close to the Sun the disk was so hot that the only solid bodies were made of rocks and metals.  Farther away the temperature dropped low enough for other, more “volatile” molecules to condense.  Beyond the “snow line” (aka “ice line” aka “frost line”) water condensed as ice.  At hotter temperatures water could still exist but it was in vapor form (not liquid).

The “snow line” in a planet-forming disk is the distance beyond which it is cold enough for water to condense as ice. Closer to the star water exists as vapor.

Earth is located in the rocky part of this picture.  [And so is the habitable zone, for that matter.]  We think that Earth was delivered water from the outer icier parts of the planet-forming disk.  Contrarians among you might ask, “Well, maybe Earth was in the rock-ice part of the disk”!  BUT,  if Earth did form in such a cold environment then we should have a lot more water than we do.  Hydrated asteroids from past where we think the snow line was are typically about 10% water.  That is 100 times more water by weight than Earth!  If Earth was 10% water the planet would be covered in global oceans of about 270 km in depth!  [Nerd alert: the exact ocean depth depends on how much of the water is stored in the mantle as hydrated rocks vs. sitting on the planet’s surface as water/ice/vapor.  If all the water were on the surface the oceans could be as deep as 1000 km.]

What Solar System bodies delivered Earth’s water? This is a mystery with 2 clues.  First, Earth’s water has a distinct chemical signature.  So, the culprit water delivery boy must be a chemical match.  Second, the orbital dynamics must allow enough of these objects to bash into Earth and do their thing.  Very distant or very low-abundance delivery boys don’t do much good.

The top suspect are asteroids. C-type asteroids have the right chemical signature.  Comets are measured to have the wrong chemical signature (see image below), although a couple of recently-measured comets actually have the same signature as Earth.

The Hydrogen chemical signature (D/H) for different Solar System bodies.  Earth is the blue line, wet asteroids are the green square, and the orange and purple symbols represent measurements of comets.  Credit: Paul Hartogh / Nature.
The Hydrogen chemical signature (D/H) for different Solar System bodies. Earth is the blue line, wet asteroids are the green square, and the orange and purple symbols represent measurements of comets. Credit: Paul Hartogh / Nature.

It is a lot easier to get asteroids to crash onto Earth than comets.  Comets come from farther out and have to pass Jupiter’s gravitational barrier.  During planet formation there is enough mixing that some water-rich asteroids are naturally delivered to the growing Earth.  [Click here to see some movies of water delivery in the “classical” (old) picture of Solar System formation.]

In the fancy new Grand Tack model, comets and water-rich asteroids actually come from the same place!  The outer part of the asteroid belt was deposited there by Jupiter as the gas giants migrated outward.  For every C-type asteroid that ended up on a stable orbit in the asteroid belt, ten were scattered onto orbits that crossed those of the growing terrestrial planets.  So, in this model, Earth was delivered water by the same parent population as the parent population of C-type asteroids.  So, the chemical signatures also match!

THE PUNCHLINE: Earth formed where it was too hot for ice.  Most of Earth’s water probably came from asteroids, although the distinction between asteroids and comets may be smaller than generally thought. 



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