Every chemical reaction in nature (i.e., where chemical bonds are formed or broken) is either what is called an endothermic reaction, or an exothermic reaction. The words tell you which way the heat flows during the reaction: Endo - heat is flowing into the reacting substances... Exo - heat is flowing out of the reacting substances. Another way of saying it is that heat is given off during an exothermic reaction and absorbed during an endothermic reaction.
The formation of water is an exothermic reaction.
Breaking chemical bonds takes more energy than forming them. So during the formation of water, we wouldn't expect any external heat to be needed. All the energy you need is already stored internally, as a sort of potential energy. In fact, any energy flowing into the reacting elements during formation would only act to disrupt the formation of new bonds, not assist in making them, because you need to get the excess energy out and get the reacting substances to settle down, or you'll never get them to bond.
Now, there is a little thing called the "energy of activation," which is a little preliminary shot of energy the reactants may initially need to get the reaction going, but it is a minor amount of energy compared to the total energy bonding substances need to fully bond. I imagine our happy H2 and O pair could get the activation energy to kick start things at their wedding from any number of sources in deep space, what with all the different kinds of radiation and subatomic particles zipping around out there at every conceivable velocity.
I saw something the other day that said that the total internal energy of a collection of bonding substances is always greater before bonding occurs than after (You wanted to know where the energy comes from?... It comes from within). And that makes sense in light of the above: the system, or collection of substances, must cool down, settle down, and give off some of that heat (exothermic!) in order for the bonding to take place. You don't want a lot of excess heat coming in and shaking everything up. Otherwise, the bonds will never form.
Another thing you alluded to was how different ices can be stuck onto the surface of a dust particle (particles whose average size is about the size of smoke particles wafting up from a cigarette... Hey, what do we expect when something the size of a star explodes in just about biggest explosion imaginable, blasting its outer layers to smithereens, literally to dust?) yet can still form water. While cemented to the surface of the particle! I do not think scientists have fully answered all those kinds of questions yet, which are still an active area of research.
Here's something that's kind of fun to do: Ask Google for examples of common, everyday endothermic and exothermic reactions. The lists that come up during that search never fail to surprise and sometimes confuse the heck out of me!
I apologize if I've messed up this explanation in any way, or added to your confusion in any way. But if I have, let me know and I'll definitely ask the mods to remove the comment. The last thing I want to do is add to the clouds of gaseous crap out there in interstellar space that seem to have enveloped the earth and somehow gotten onto the internet. Cheers!
I confused molecular “endo” and “exo” with nuclear “fusion” and “fission” in my first Comment.
I actually understand the difference, but, 50 years after my last chemistry class, I often lack the fluency to clearly express it.
Two of the links I read stated that H and O combine into water inside the proto-planetary clouds that give birth to new solar systems.
Until those clouds start to heavily condense into stars and planets, I presume their temperature is close to absolute zero, and their diffuse mass creates almost no pressure.
I have no problem understanding how liquid and gaseous elements react with other liquid and gaseous elements.
But I still struggle to understand how ultra-cold micron-sized specks of solid hydrogen and solid oxygen combine into H2O in the absence of external energy.
Maybe they crash into each other, and kinetic energy starts the bonding process?
Bottom Line - I have no doubt there are excellent explanations on the Web. When I have time to find one, I will send you the link.
Thanks again for your extra effort.