Spontaneously arising life could have happened after many surface natural (nonlife) molecules on Venus, or subsurface molecular forms on planets or moons farther out gradually got united in various ways with others. It would be warmer ever deeper below the surface which could be at subfreezing temperatures, such as out at Mars or farther out. Warmer means maybe sometimes being more usefully chemically reactive with neighboring molecules.
Lets compare some such subsurface molecules with e.g. castor oil. A castor oil molecule has 11 atoms of carbon, 10 of hydrogen, 10 of oxygen, thus 31 molecular component atoms, totalling 302.2amu (atomic mass units). It's density is .969g/cc (grams per cubic centimeter). This means .5836e24 molecules/cc (e24 means times 10 with exponent 24). (1 cc is about the volume of half a finger tip.) So if one millionth of 1 cc of subsurface matter were molecules at least as significant as castor oil, then 1cc could contain .5836e18 such significantly not too minor molecules, a quite huge number! (Or this means .8357e8 castor oil molecules side by side might total only 1cm length, or 8.357e9 such molecules side by side might total 1 meter length.)
At temperature 25deg C = 77deg F, castor oil has sound speed 1477m/s. That sonic speed spans about 15 sports fields each second! And thus one such castor oil molecule, at average sonic speed, could pass by 1477*8.357e9 = 12.3e12 such molecules in 1 second, while able to give very strong molecular strikes, fast and hard enough to maybe sometimes alter or combine with some other molecules they strike. So its reasonable to expect that many subsurface molecules may possibly over many decades have gradually gotten modified &/or combined bigger.
Let's suppose that some molecules in a few places had eventually combined in ways such that the results were sometimes able to catalyze some other molecules nearby into similarly combining. The ability to catalyze the joining of molecules into forms which might in the future also catalyze more joinings of nearby molecules into like molecules, (which might in the future repeat this forming process), essentially then becomes an effective start of extremely simple "reproductive life".
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