Water is believed to be a necessity for life, without water there would be no life on Earth. So where did our water come from and is it unique to the planet we call home?
Well in short, no, water is not unique to Earth. Through analysis of rock samples we have of other inner solar system objects, the amount of water per body is roughly relative to its size. Earth has the greatest amount of water followed by Mars, while the Moon and asteroids are relatively dry.
While we do not have any known rock samples of Venus, analysis of the Venusian atmosphere indicates that water was present during formation.
There are many competing hypothesis on the origins of our water however the two most prominent views are that;
1. Earth accreted as a dry body during planetary formation and water was delivered in the last stages of accretion via cometary impacts.
2. Planetesimals accreted water-bearing grains during the early stages of planetary formation
While these are both strong hypothesis evidence from hydrogen isotopes indicate that the bulk of our water could not come from comets.
Comets
It is believed comets contain significant quantities of water and that the heavy bombardment of Earth by cometary bodies in the late stages of planet formation led to water being delivered to our surface.
However, hydrogen isotopes indicate that there is a high chance this is not the source of the bulk of water on Earth.
By comparing the ratio of two stable hydrogen isotopes 1H, hydrogen and 2H, deuterium (known as the deuterium/hydrogen ratio) in Earth’s oceans and samples from comet 1P/Halley and comet Hyakutake we can see that the comets contain roughly twice as much deuterium relative to 1H than Earth’s oceans.
Body D/H isotopic ratio
Bulk Earth 1.5×10^-4
Comet 1P/Halley 3.16×10^-4
Comet Hyakutake 2.82×10^-4
If the ratio of the above comets is representative of all comets this would indicate that terrestrial water would have come from alternative sources to comets. However this is not yet testable.
Water-bearing grains
This model for the origin of volatile materials on Earth claims that, during the first stages of accretion water-bearing grains were part of the material that formed planetesimals and then planetary embryos.
During the later stages of differentiation the water-bearing grains released their volatiles by outgassing.
Outgassing rates for early Earth would have been much higher than what they are today because the internal convection rate was much higher and by 4 Ga ago most of the Earth’s atmosphere and hydrosphere would have formed, after the formation of the core and the giant impact which formed the Moon.
Evidence of hydrated minerals comes from meteorites and while this appears to be the most plausible model based on current evidence it is not without uncertainties. One such uncertainty is whether water-bearing minerals could have formed at 1 AU or if they would have needed to form much further out.
References
Rothery, D., Gilmour, I., Sephton, M. and Conway, A. (2011). An introduction to astrobiology. Cambridge: Cambridge University Press.
exobuzz 