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I recently discovered that there is another kind of water : heavy water. Heavy water is made of deuterium, a heavier isotope of hydrogen. Heavy water itself is 11% percent heavier than "normal" water.
This got me wondering :
Is a world where all the water is heavy water possible ?
Is a world with no protium, only deuterium (and the unavaoidable contamination of a tiny trace of tritium), hence all the water is "heavy water" possible? Sure it is.
All that's needed to be plausible is a plausible mechanism to concentrate the heavier isotope, probably in multiple stages (like multi-step distillation to reduce impurities further and further).
A potential mechanism might be a naturally occuring mass spectrometer, perhaps in orbit of a neutron star. Gas ionized by x-rays and UV, orbiting through the star's hyper-strong magnetic field, will tend to separate into streams of heavier and less heavy atoms. A few million years of this might plausibly separate the gas enough that some later event that strips it away from the neutron star and makes it available for star and planet formation preferentially grabs the denser deuterium.
Now, whether a star capable of sustaining life on a planet can form from deuterium only is a separate question -- at the very least, it wouldn't be where its mass says it should be on a Hertzsprung-Russel chart.
As noted in comments, this may not be a mechanism that can produce enough mass to form an entire star, but might well do to form a rogue planet that can later be captured. After capture, there's still the possibility of surface contamination by comet bombardment, but that's generally an event from early in stellar system formation, where rogue capture might well occur after primary accretion is well over with.
One of the less-known strange things about Venus is that it has 120 times more deuterium than Earth. Now, Earth is 1 D to 6420 H, so that is still only 1 in 50 hydrogens, but it is the start of something.
We're going to take Venus, and put it around a nice stable red dwarf that can survive for trillions of years. Venus may have been wet just 1 billion years ago, so I imagine we can let it simmer for maybe 10 billion years (part of me is still sore from where I pulled that number) to roast off most of the light H. We can also start by tinkering with the mass to make sure none of our precious deuterium escapes, so hopefully we retain a bit more than Venus has of hydrogen now.
Now what? We have to get rid of the acid, so let's drop, oh, some sulfur-metabolizing bacterium on our Venus that grabs up all the S, causing its atmospheric H2SO4 to become plain H2O + 1 1/2 oxygen molecules. Per ancient terraforming schemes for Venus, we'll say the bacteria grow in the atmosphere but fall to the ground as cinders, taking not just the S but also those 90 atmospheres of CO2, or at least the carbon from them.
When all is said and done, you have a very old, dry planet with some nice lakes of heavy water under a heavy oxygen atmosphere. Almost everything will burn on contact; perhaps you can drop some metallic meteorites that take up a lot of oxygen but don't react that much with the water.
Heavy water is D2O. You need a large quantity of deuterium to get large quantities of heavy water.
Deuterium is a precious gift from the Big Bang
Most of the deuterium present today was produced during the Big Bang nucleosynthesis event. Deuterium is made by separating naturally-occurring heavy water from a large volume of natural water. Deuterium could be produced in a nuclear reactor, but the method is not cost-effective.
So a large quantity of heavy water is possible only in an imaginary world. In real universe, it occurs only in very small quantities. On earth, the natural abundance of deuterium in the ocean is approximately 156.25 ppm.
Biochemical reactions
Deuterium behaves differently from normal hydrogen in biochemical reactions. While it's not deadly to drink a small amount of heavy water, ingesting a large quantity can be lethal.
In your imaginary world, full of heavy water, biochemical reactions will be different.
