[dredmorbius]

"Mars - a planet lacking powerful moons - once had a magnetic field just like ours."

I think the salient point here is "once had".

The Earths' core remains molten.

Some of that is latent heat of formation, some nuclear decay, some tidal stresses. I don't know the contributions of each. Contribution from Earth-Moon tidal stresses isn't insignificant though.

We _do_ know that tidal stresses can be hugely significant -- Io and Europa are thought to be heated (to differing degrees) by the stresses of orbiting Jupiter. Also one of Saturn's moons, Enceladus, which shows signs of volcanism with ice plumes.

The thought occurs to me that moons of a large gas planet could also harbor formation of life, though they'd have to be rather closer to their parent star than the Solar System's giants are.

[Udo]

Heat from radioactive decay and primordial heat are each thought to be about half Earth's internal energy budget - tidal forces from the Moon, even though they're strong enough the change the elevation of the surface by up to half a meter, do not factor in at a significant ratio as far as I know. Of course I'm only talking about the Earth-Moon system and the claim that without the Moon we would not have any atmosphere or magnetosphere.

> moons of a large gas planet could also harbor formation of life

Indeed, and the fact that there are at least two ways to convey energy to an ecosystem should increase the prevalence of life in general. Moons are an excellent spot for life, in some ways maybe even better than planets. They are more numerous, they can be cozy even outside of the Goldilocks zone, to some degree they can be protected by the magnetic field of their gas giant, they are never tidally locked to the star, and they might have a better chance for volatile chemicals on the surface than your average rocky planet.

Since the thread was originally about the Fermi paradox though, one has to wonder about the likelihood for bearing technological civilizations. How fruitful are ocean worlds for bringing forth tool users? Intelligence may in fact be common, but how conducive is this environment to science and engineering? We don't know.

> though they'd have to be rather closer to their parent star than the Solar System's giants are

If they're experiencing enough tidal stress, such as Europa is, they don't have to be close to the star at all.

[dredmorbius]

I was considering a land-based life-form.

I also suspect that while life could develop within, say, an ice-covered sea on an outside-the-Goldilocks-zone gas giant's moon, that the total available energy flux would likely not be sufficient to support a large or diverse population. Even absent the problems of developing a technological society in an aquatic (or analogous liquid) environment, total biomass, competitive pressures, diversity, and similar stresses which gave rise to humans and intelligence seem unlikely.

Instead I was considering a world with surface conditions similar to Earth, but itself the moon of a larger world.

I've also been reflecting on filters, and one thought that's been occurring to me is that many worlds may simply be subject to sufficiently frequent catastrophes that intelligent life never developed. In the case of humans on Earth, there was a planetwide catastrophe -- the Chicxulub impactor, 66 million years ago. Humans themselves split from their common chimp ancestors 2 million years ago. And complex life emerged about 500 million years ago, after first emergence at least 3.5 billion years ago.

There have been numerous perturbations of terrestrial conditions over that period, but it does suggest that over various intervals, certain degrees of stability are required. Asteroid impacts, nearby supernovae, climate disruption, widespread volcanism, orbital disruptions, variations in solar intensity, and other factors could all reset a nascent emerging life, complex life, intelligence or civilization. There are a lot of opportunities to get it wrong.

At the same time, various stresses also seem to have contributed to humans being, well, human.