I looked this up on Wikipedia and read about the “Girdler sulfide process”. I didn’t fully understand what I was reading, but the articles made it very clear that these processes are indeed energy-intensive. 
And the Girdler sulfide process requires two different components to have a temperature difference of 100 degrees Celsius. :’(
Another isotope difference is that helium-3 has a lower boiling point than the more common helium-4. In this case, though, it’s due to quantum mechanical effects, not the mass difference.
So is it true that if you take D2O ice cubes and stick them in liquid H2O the D2O cubes sink? (I’ve heard that’s actually true.)
There aren’t any organic processes that change the nucleus of an atom. All organic processes rely on the electromagnetic forces and don’t utilize either the strong or weak nuclear forces. So it would really be a stretch to have anything that could produce heavy water.
I think your best bet for a high concentration of heavy water is going to involve starting with gravity-based separation (like Chronos suggested) and then coming up with some way to strip out the normal water. So… maybe a giant planet starts off liquid and separates things, then eventually freezes solid and is smashed apart. An ocean-sized piece of heavy water ice (or heavy methane or heavy ammonia would work too) from the lower layers of those frozen oceans might hit a moon or other planet and there you go.
Of course, you have to remember that hydrogen isn’t just in the water. Pretty much every carbon- and nitrogen- bearing compound has hydrogen in it. So even there, you’d have deuterium exchanging with protium and reaching some kind of equilibrium. A batch of heavy water wouldn’t stay that way easily.
Heavy water is actually not blue in the same way regular water is; the extra mass of the deuterons causes the absorption frequencies to shift. If the ocean were made of heavy water, it wouldn’t be blue. See here for some details.
In one sense, isotopes always have the same “chemical properties”, because chemical reactions occur using only the valence electrons; having a different number of neutrons will not change whether an atom will form various compounds. However, all physical properties will be different, including the rate of chemical reactions responsible for life. Since these processes were evolved to happen at a certain rate, significant complications can occur when their speeds are changed.
If something could explode a neutron star, would the result be a nebula where all the elements are in their most neutron-rich stable isotope? So you’d have an entire solar system where all the hydrogen was deuterium, all the carbon was carbon-13, all the oxygen was oxygen-18, etc?
Most likely, you’d just end up with a huge mess, mostly of iron.
Here’s a half-baked idea. NoriMori said abiotic processes, but would s/he be willing to accept ones involving simple life forms? Don’t enzymes often slightly favor one isotope over another? If so, one could maybe imagine an iterative process in which the local algae equivalent had selectively bound all the normal hydrogen in, say, coal, and some bacteria equivalent came along and selectively released the deuterium. Rinse, repeat for a few aeons. In the natural course of things, evolution would fix the selectivity before deuterium predominated, but maybe NoriMori can think of some explanation for why that wouldn’t happen.
Yes, that is perfectly true.
Ahaha, interesting! I didn’t understand all that, but enzymes are something I could definitely use! I hadn’t thought of that…
And by the way, I’m a girl. 
Well, what is it about hydrogen that makes it not want to keep a neutron? Is there nothing that could reverse that?
Ok, how about this: early in the planet’s history the temperature is stable and such that H2O is liquid and D2O is solid. There is the circulation of some sort of liquid metal in the planet’s core that generates a healthy magnetic field. Everything is nice and stable.
Then one day the circulation stops. The magnetic field is gone. Solar winds strip the planet of it’s atmosphere. The liquid H2O quickly evaporates and is also stripped away. There is no water left except D2O.
Then one day the circulation begins again. A nice healthy magnetic field protects the planet. D2O evaporates and a new atmosphere forms. Then life evolves on this planet in the presence of only D2O, not H2O.
Farfetched, I know, because the quantity of deuterium would be very small, but it’s something…
I don’t think it would work, because the D2O wouldn’t spontaneously freeze out of a solution of H2O. Also even in solid form D2O has a vapor pressure, and anything that could evaporate the water would also heavily evaporate the D2O. There’d be a tiny amount of enrichment, but no more than a single step of a cascade process makes. The last gram of water on the planet might be 1/300 deuterated instead of 1/3000.
There’s nothing that makes it not want to keep a neutron. If a hydrogen nucleus happens to glom onto a neutron somehow, it’ll hold onto it just fine. It’s just that the conditions under which it would glom onto a neutron in the first place are scarce in the current Universe, and at the time when it could happen, the abundances were such that it didn’t happen very often.
Ohhhhhhh…
Well, darn! Ay-yi-yi…
I know that some nuclides emit neutrons during radioactive decay, and that neutrons can also be emitted during spontaneous fission. Is there any way that, on some planet where spontaneous neutron-emitting fission/radioactive decay was happening all the time, the excess neutrons could “glom onto” hydrogen atoms/water molecules?
“In health physics neutron radiation is considered a fourth radiation hazard alongside the other types of radiation. Another, sometimes more severe hazard of neutron radiation, is neutron activation, the ability of neutron radiation to induce radioactivity in most substances it encounters, including the body tissues of the workers themselves. This occurs through the capture of neutrons by atomic nuclei, which are transformed to another nuclide, frequently a radionuclide. This process accounts for much of the radioactive material released by the detonation of a nuclear weapon. It is also a problem in nuclear fission and nuclear fusion installations, as it gradually renders the equipment radioactive; eventually the hardware must be replaced and disposed of as low-level radioactive waste.”
~ Wikipedia, “Neutron radiation”
It seems that a lot of materials absorb neutrons pretty easily. Why is it so difficult for hydrogen?
If water were bombarded with neutrons like in the situation I described in my previous post, could it eventually become heavy water?
It’s timed, you have only a 5 minute period to edit anything. This keeps dishonest people from editing their own previous statements in debates and claiming that they never said what they are accused of; something I’ve seen on boards that allow unlimited editing.
And it’s my understanding that replacing regular water with heavy water does have effects on Earth life. IIRC bacteria tend to grow huge for example. Well, huge for bacteria.
cite?
That might work, but I’m not a nuclear physicist: I’d have to do a fair bit of digging to find cross-sections and such. I would expect, though, that hydrogen would absorb neutrons less readily than heavier elements, because of the smaller number of bodies in the interaction. One neutron hitting one proton is likely to just bounce off, while when one neutron hits a bunch of protons and neutrons, the neutron’s initial energy can get spread out among a whole bunch of particles, such that none of them has enough energy to be knocked out.