Quote:
Originally Posted by renormalised
Deuterium to Hydrogen ratio. Depending on the amount of the ratio, the higher the water content of the body. Venus has a ratio of 10000:1. The only way to get a ratio that high is to have a very large content of water. The deuterium is preferentially concentrated in the atmosphere as the hydrogen is carried off after being separated from water molecules via UV radiation.
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Done some reading on this aspect since yesterday.
Here is a good, snappy paper explaining the theory (for those interested, and for the record).
Below is my interpretation of how this all works:
The above statements are valid if one starts with the
key assumption that Venus once had a D/H ratio identical to that of Earth.
If this ratio was identical, then it is logical to conclude that Venus may have lost 99.9% of its hydrogen. Since terrestrial planets don't have free hydrogen molecules or atoms in their atmospheres, it is also reasonable to conclude that they probably never did.
On Earth, rocks rarely contain hydrogen unless the hydrogen is derived from water. In the atmosphere of Solar System planets so far sampled, hydrogen is contained mostly within methane (CH4), ammonia (NH3) and water (H2O). On Venus, neither CH4 or NH3 are abundant, so it is concluded that water is the likely reservoir for hydrogen. Since water vapour forms only about 0.002% of Venus' atmosphere, it is concluded that most of the hydrogen has escaped due to photodissociation (UV zapping) of water which,
it is assumed, contains both deuterium and 'normal' hydrogen water
in the same ratio as Earth, Asteroids, Comets and proto-planetary disks.
The deuterium atoms are left behind resulting in the disproportionate ratio, measured by the Venus Pioneer probes dropped into Venus' atmosphere. 'Disproportionate' when compared Earth, Asteroids, Comets and proto-planetary disks, that is.
Seeing as all this depends on the D/H ratio being fixed, how fixed is the ratio throughout the so far explored universe ?
As it turns out, they are finding that the D/H ratio may not be a unique criterion to discriminate between the different origins of water on Earth because new theories are explaining that it is possible for different ratios to have occurred in different temperature regions of proto-planetary disks.
On top of this, the remote measurement of deuterium via spectroscopic measurements is controversial, due to inaccurate assumptions in the models used in its determination.
So, my conclusion is that whilst there is rationale surrounding the theory of 'big water' in the past on Venus, this is all based on the assumption that the ratio of D/H on Venus started out the same as for Earth, which may or may not turn out to be as solid an assumption as first thought.
Cheers
