The deuterium anomoly

[farmerben]

What could explain the different ratios of deuterium and protium on different astronomical bodies?

My leading hypothesis is that deuterium sinks deeper into the core than protium due to gravity and thus we observe more protium on larger worlds.  Or perhaps planetary magnetic fields sweep up more protium than deuterium.  Neither hypothesis matches the data perfectly.  

Also the few papers published online dealing with the issue do not seem to be using these hypotheses.  

 

 

Edited August 23, 2019 by farmerben

[DDE]

Well, the theory for Venus appears pretty established - deuterium is less vulnerable to solar wind erosion.

[farmerben]

The solar wind erosion hypothesis looks pretty solid.  I found some data suggesting that Venus has 10x the D/H ratio of Earth, and Mars has 5x.

The proposed mechanism is that radiation breaks up atmospheric water molecules to create free hydrogen.  The protium will on average acquire twice the velocity of deuterium from collisions and therefore be disproportionately ejected from the planet.  

It is anomalous that comets and asteroids have D/H ratios slightly higher than Earth, but less than Venus and Mars.  

We might assume that small asteroids have almost zero isotope separation, and reflect the deuterium ratio of the early solar system.  But then one would predict Earth to be deuterium enriched compared to asteroids, which is not the case.  

Earth does have a much stronger magnetic field than Venus and Mars.  So it is possible the Earth is capturing massive amounts of protium from the solar wind.  The loss rate must be greater than the capture rate.  But the capture rate for Earth would be large compared to Venus.  

There are some papers online suggesting that the deuterium ratio could tell us something about the formation of the early solar system.  Perhaps there were other proto-planets now lost.  But I have yet to find any published research saying what type of data would support conclusions about missing proto-planets.  

[magnemoe]

The 4 fat guys holds on to any hydrogen, Uranus and Neptun has less gravity and some losses and Jupiter eats stuff. 
Some recent studies indicate Jupiter ate an planet. 

[lajoswinkler]

It's not really about sinking (difference is tiny), but more about the ease of stripping it away by solar wind.