[cLeEOGPw]

One of the so called "Great Filters" is our Moon. It is providing Earth's metal core with constant moving force and refreshing the dynamo and keeping our atmosphere and water not blown away by solar winds. Since the size of Earths satellite is far from ordinary, this instantly reduces chance of advanced life appearing elsewhere. And if the theory of the source of Moon is right, it formed after collision with object of a size of Earth, the filter is restricted by star systems with the planets within a certain range that had a collision with other planet in a way that left long lasting big object rotating around that planet and generating magnetic field preventing water and atmosphere from escaping. So just by applying only this "Moon filter", reduces the chances of advanced life in our own Milky Way to probably just few cases in whole galaxy. And add the probability of intelligent beings appearing on random planet that has life, it starts to seem pretty realistic that we are only ones not only in Milky Way, but also surrounding galaxies.

The conclusion is we better start working on colonization techniques, because chances are that the future of life in our galaxy is in our hands.

[Udo]

Earth's magnetic field is not powered by the moon. As far as we currently know, the dynamo effect is caused by the action of molten material against a rotating solid inner core. It's true that our magnetosphere keeps the atmosphere in, but having a magnetic field is far from uncommon or unexpected for a planet of Earth's size and composition. In fact, Mars - a planet lacking powerful moons - once had a magnetic field just like ours. The problem here is the size of the planet was not sufficient to keep enough heat inside for as long as ours has.

You might be conflating this with the theory that the Moon stabilizes the Earth's rotational axis and prevents it from wobbling too strongly, which would cause unpredictable summers and winters, kind of like the ones described in A Song of Ice and Fire.

Yes, the Earth-Moon system probably has some uncommon features, but let's not make the mistake to postulate that a world has to be precisely like Earth in order to support intelligent life. It's not at all obvious that our ecosystem represents the only possible solution for an environment that can produce intelligence. All we know is it is one solution.

> The conclusion is we better start working on colonization techniques, because chances are that the future of life in our galaxy is in our hands.

I agree!

[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.

[givan]

We have bacteria that live inside volcanoes or on the bottom of the ocean in extreme conditions, life adapts, without the moon life would only look differently there is no proof that without the moon life on earth would not be possible, looking at life in the universe from a human environment point of view is flawed.

[minikites]

But we only see complex life in a narrower range of much milder conditions. I bet there's lots of space microbes but I agree with the parent to this comment that complex, intelligent life is probably pretty rare.

[givan]

Milder conditions relative to our environment? this is subjective, where do you see complex life and how do you know that complex life requires "milder" conditions or that is rare if we only have ourselves as an example that happen to live in such an environment?

What stops bacteria that live in harsh environments to evolve even if probably slower? harsh meaning hard/impossible for ourselves or for life as we know on earth.

[minikites]

So where is everyone?

http://waitbutwhy.com/2014/05/fermi-paradox.html

> That would mean there were 10 quadrillion, or 10 million billion intelligent civilizations in the observable universe.

>Our sun is relatively young in the lifespan of the universe. There are far older stars with far older Earth-like planets, which should in theory mean civilizations far more advanced than our own.

I think intelligent life is pretty special/rare, given the evidence.

[givan]

What evidence? we don't have enough data to draw conclusions on this matter, we barely sent a very limited robot on mars and some probes in the solar system, the universe is huge.

We can't initiate contact because of primitive technology, we barely made it to the moon with huge costs.

Why aren't advanced civilizations that can reach us contact us? we don't know, there are many theories

http://en.wikipedia.org/wiki/Fermi_paradox#Explaining_the_pa... http://en.wikipedia.org/wiki/Zoo_hypothesis

[minikites]

Exactly, believing in God has about the same scientific support. I wonder how many people don't believe in God but do believe there is advanced, intelligent life that choose not to contact us and how they reconcile that set of beliefs.

[dredmorbius]

NB: The collision genesis theory ("GIH" or Giant Impact Hypothesis) of the Moon's origin involves a body about the size of Mars, referred to as "Theia".

http://en.wikipedia.org/wiki/Origin_of_the_Moon http://www.nasa.gov/topics/solarsystem/features/moon_formati...

Otherwise, yes.