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rfmcdonaldEmily Lakdawalla's recent post at the Planetary Society Blog raises an interesting question. How small can a life-sustaining world be?
Aluminum-26, the radioactive isotope most likely to have heated young asteroids, is likely to produce enough heat to melt ices and energize interesting chemical reactions. Unfortunately, the asteroid starts out as being too hot, then cools too rapidly to let water and potential life migrate inwards.
But, large asteroids like Ceres are a different story.
When the solar system was very young and still very hot, could medium-sized asteroids have been habitable abodes for life? It's not a crazy question, because there's abundant mineralogical evidence for what we consider the building blocks of life in asteroids, specifically carbonaceous asteroids. They contain lots of different organic molecules including amino acids; there's evidence for the presence of liquid water within asteroids in the past, and many carbonaceous asteroids still retain quite a lot of water bound within their minerals; and the residual heat from their formation and from the decay of radioactive isotopes give you the "food," water, and energy that are necessary for life. But despite searches there's been no convincing evidence that life ever arose on an asteroid. Given what we think are the right ingredients, why didn't life arise?
Aluminum-26, the radioactive isotope most likely to have heated young asteroids, is likely to produce enough heat to melt ices and energize interesting chemical reactions. Unfortunately, the asteroid starts out as being too hot, then cools too rapidly to let water and potential life migrate inwards.
But, large asteroids like Ceres are a different story.
Big asteroids are a different story. Big, ice-rich bodies like Ceres, or the body whose destruction gave rise to the Themis family of asteroids, would have differentiated, separating into layers of liquid water above a rocky interior, solving the pore space problem. Their long-lasting internal heat would likely have kept those oceans liquid for millions or tens of millions of years; Ceres may even have retained an internal ocean right down to the present day. So the moral of the story is: despite the existence of habitable zones within medium-sized asteroids for many millions of years, it just doesn't seem like life had a chance to get started there. But we ought to check for ancient life on Ceres and Themis!
