Water is made of hydrogen and oxygen, but the debate is all about the hydrogen. There are 2 stable isotopes of hydrogen, light and heavy, or 1H and 2D for deuterium. The deuterium to hydrogen ratio in water (and other compounds but water is the most common) has been debated and measured in many locations. The Sun does not make deuterium, but it is the first thing that it burns up (if H is charcoal, D is lighter fluid). The only supply of deuterium–like hydrogen itself–is the Big Bang. So any hydrogen that has been processed through stellar furnaces will lose deuterium.
The earth’s oceans have higher D/H ratios than the sun, so the question has been around for a long time, where did Earth get its oceans? The standard model has been that comets supplied the water for the Earth’s oceans during the “Late Heavy Bombardment” some 3.85 billion years ago. Comets are thought to come from a vast cloud–the Oort Cloud–orbiting the Sun about 1 lightyear away. The gravitational attraction is so weak, that just about anything can change their orbit, and people have suggested passing stars or even gamma-ray bursts are enough to scatter comets, some of which end up making close encounters with the inner solar system and us.
These Oort Cloud comets are thought to predate the Sun, and are leftovers from the “proto-solar nebula” that made the Sun and the planets. Where did this nebula come from? Well our Sun is just 4.5 billion years old, the nebula maybe 5 billion years old, but our Milky Way galaxy is 12 billion years old. So there have been many supernova and Wolf-Rayet stars that shed dust into the galaxy from which our planet’s silicon and carbon and oxygen come from. But the one thing they cannot make, is deuterium. So the deuterium has to be primordial.
Here’s a paper from January 2014 arguing this sort of point: http://arxiv.org/abs/1401.6035 “Chemo-dynamical deuterium fractionation in the early solar nebula: The origin of water on Earth and in asteroids and comets” The word “fractionation” is the keyword–they want to find a way to make the D/H ratio go up by distillation (lighter H heads out toward Jupiter, heavier D stays near the Earth), so that we don’t need extra primordial ice to make the oceans. Their conclusion is that they can explain away the anomaly using locally made materials. This is the standard model, even if it needs a bit of tweaking. After all, if all the silicon and oxygen and carbon was made locally in stars, then most of the deuterium should have been processed locally too.
Then along comes Lauren Ilsedore Cleeves. She is part of team on the Hershel IR telescope that observes a distant proto-stellar nebula with lots of really cold water in it, publishes a 2011 paper, http://arxiv.org/abs/1110.4600 “Detection of the Water Reservoir in a Forming Planetary System”. Her team interprets it as ice grains which sputter water vapor from cosmic rays. And there’s lots of it. It appears that proto-stellar disks are full of primordial ice, rather than recycled supernovae or Wolf-Rayet water. Hence, this paper in Science, where she does a really *big* simulation and says the D/H water in the Earth’s oceans can’t be fractionated in the proto-planetary nebula, it must be interstellar.
Okay, we know why its controversial, but why is this important?
Well, for one thing, if it is interstellar, it means that our galaxy is chock full of water, and then every other planetary system that we’ve spotted could easily have water in it. This is the justification for “planet-finding” missions. (Of course, none of the extra-solar planets have the right temperature, composition, and size to look like a twin of Earth, but that’s a different problem.)
But unfortunately, Cleeves has proven too much. If the water is interstellar, then it wasn’t made in stars–which of course would ruin the D/H ratio too. But if it wasn’t made in stars, then it is primordial, Big Bang created. But if it is primordial, then where did the oxygen in the water come from, since the BB doesn’t make oxygen? Cleeves has solved one problem–the elevated D/H ratio–by creating another–the origin of Oxygen in the interstellar water.
Now as it happens, I’m working on a theory and simulation that says that the Big Bang *did* make oxygen, lots of it, so that the missing “dark matter” of the galaxies is nothing more nor less than ice, as found in comets. And since we have already shown that comets carry fossil bacteria and bio-engineered magnetites, then we don’t need to wait for the planet-finder mission to find us a Cinderella-zone Earthlike planet–we have billions of habitable comets moving in and out of Cinderella zones, flourishing and freezing like the mosquito population of Canada.
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