It seems from the recent Science paper by Altwegg et al (2014) that the commonly accepted source of Earth's water being the comets might not be (completely) true. Their study suggest that the water in the comet they studied is different from Earth's (higher Deuterium content). The alternative source of water that Altwegg suggests is from asteroids. My perception is that there is not that much water in asteroids or at least not sufficient to create Earth's oceans. How much water is in asteroids now? How much water was there in asteroids reaching Earth when the oceans were formed?


2 Answers 2


This is far too soon to draw sweeping conclusions. The Philae probe has only measured deuterium for one comet. But comets are diverse, of diverse origins. We need to take a lot more samples, from many different comets, before we can conclude firmly that water has, or has not, come from comets.

The only conclusion we could draw so far from this study is that earthly water has not come exclusively from comets similar to 67P/C-G.

What you're seeing now is the usual distortion that occurs in the media after some hugely popular experiment is completed - its conclusions are blown out of proportion. But that's not how science works.

Let's write this one down, and wait for further science to be put forth. We don't know enough yet about these objects.

  • $\begingroup$ I agree with your view and I was surprised to hear the comments by the lead author at the press conference. My question remains, whether asteroids can be considered as a feasible source or not. More science is needed. $\endgroup$
    – arkaia
    Dec 11, 2014 at 19:31

“More science is needed” is sometimes (not always) moving the goalposts on the part of the speaker. Worse, it’s sometimes deliberate subterfuge, sometimes inadvertent “Rash Dictum”-ing, sometimes inadvertent Dunning-Krugering on the part of the speaker.




Meteorites per se contain, in many cases, a fraction of one percent water. Given that Earth’s oceans per se represent 0.0X percent of Earth, asteroids can EASILY explain Earth’s oceans. A better question is the infall of the asteroids that immediately formed Earth (see two or more works of L. Piani) or the lesser, later flux of impacting asteroids. That “late veneer” includes some carbonaceous chondrites, with >1 to several percent water:

Van Schmus, W. R., Wood, J. A. (1967). A chemical-petrologic classification for the chondritic meteorites. Geochim. Cosmochim. Acta 31, 747-765

Assuming all carbonaceous chondrites, Earth water requires only 1 to a few percent mass contribution by asteroids. More likely, the infall was a mixture of multiple classes, requiring several percent of Earth’s mass. This is not recent publication.

What IS recent are bounds on the ice currently surviving in asteroids, including NEOs (not NEAs… “object” was chosen to reflect asteroid-comet continuum bodies).

Snodgrass, Agarwal et al. 2017 The Main Belt Comets and Ice in the Solar System The Astronomy and Astrophysics Review, 25, 5

Schorghofer, Hsieh 2018 Ice Loss From the Interior of Small Airless Bodies According to an Idealized Model Journal of Geophysical Research: Planets 123, 9 2322-2335

…and follow-on works in the past ~2 years.


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