Over the span of at least 2 million years, the hydrothermal system would have cooled as it aged. And eventually, the water would have reached the ideal thermal window for hosting heat-loving, or thermophilic, organisms — between about 106 F (41 C) and 252 F (122 C).
Such systems were prevalent during the impact bombardment that shaped the Hadean. Estimates of the size and frequency of impactors vary, but one model suggests our planet was resurfaced by about 6,000 impactors, each larger than the roughly 6-mile-wide (10 km) Chicxulub impactor. Those impactors may have produced some 200 impact craters 620 to 3,100 miles (1,000 to 5,000 km) in diameter, each a potential incubator for microbial life. These impact-generated hydrothermal systems may have been far more expansive (and common) than volcanic systems, like those at Yellowstone and along mid-ocean ridges today.
The right ingredients
Having the proper temperature is only part of the recipe for cooking up life — the right ingredients in the Earth’s crust are also necessary.
While today’s atmosphere is mostly nitrogen and oxygen, the Hadean atmosphere may have instead been dominated by hydrogen, carbon dioxide, carbon monoxide, and ammonia, before being filled with steam and rock vapor produced by the largest impact events. As intense ultraviolet rays from the young Sun beat down on that post-impact, debris-filled atmosphere, it could have generated a hydrocarbon haze in the sky, casting a deep yellow-orange smog that eventually settled to the surface, forming hydrocarbon-rich sediment layers on top of multi-mile thick layers of impact ejecta.
Hot, mineral-rich water venting through those rubble piles of hydrocarbon-rich sediments would have been chemical factories for organic reactions, providing the necessary feed stock for microbial ecosystems. If one had an ear for hydrothermal activity following a strike, one might even hear roaring gases venting at the surface of ring-shaped island chains surrounding the centers of impact sites, with plumes of bubbling fumes and dissolved pollutants thrumming above the seafloor, and Earth itself creaking as the crater settled.