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How big a difference does the Earth's liquid outer core and mantle make for the evolution of complex life?

If all of Earth's parameters would be the same, but it had a solid interior just like Mars, would we still have an atmosphere, liquid water, and a moon? How much weaker would our magnetic field be? Or would we have none?

I seem to remember that Mars has a very thin (or no) atmosphere because it has no liquid core and thus no magnetic field, but could it be that our greater planetary mass would compensate for that? Venus has an atmosphere after all, even though its core seems to be only partially liquid.

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  • $\begingroup$ Gravity is a big factor in atmosphere retention as well. $\endgroup$ – Tim B Sep 30 '14 at 12:48
  • $\begingroup$ You mean the mantle is liquid- I understand that geo-scientists are quite sure the core is solid. $\endgroup$ – user2617804 Sep 30 '14 at 13:49
  • $\begingroup$ @user2617804: You are absolutely right, I wasn't really aware that the inner core is solid, whereas everything between outer core and crust is liquid. The question is still the same, though. $\endgroup$ – Florian Hollandt Sep 30 '14 at 14:13
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    $\begingroup$ Well, according to mainstream accepted geology and planetary evolution theory, I'd say there'd be little to no life naturally evolved on a completely solid planet. No magnetic field means no protection from the solar wind, which can mean little to no atmosphere. No volcanic activity means less volanic layer materials, including metals etc. many of which are necessary for life - it might also prevent or prolong the time between extinction events, thus disallowing the survival of more complex life forms (because the simpler ones stay dominant). All speculative of course. $\endgroup$ – mechalynx Sep 30 '14 at 17:41
  • $\begingroup$ @ivy_lynx: I think you should make that into an answer. $\endgroup$ – celtschk Sep 30 '14 at 21:23
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I agree with much of the other answer but did want to contest one point, as the effect of the solar wind on atmospheres is not fully understood. It is entirely possible that lighter gasses such as hydrogen are affected more by the solar wind than heavier ones. What is known for sure is that Venus has very little magnetic field and still has an atmosphere much much denser than Earth's.

http://en.wikipedia.org/wiki/Atmospheric_escape

So life would evolve much like it did here on earth, the atmosphere may be thinner. Water would be rarer (due to the lack of hydrogen), and radiation levels would be much higher. Expect most creatures to be resistant to radiation damage or have breeding strategies that avoid it. During solar storms animals may well retreat into burrows and there could well be a lot of nocturnal animals as well.

With no magnetic field compasses would not work. Electronics would be harder to develop and less reliable until proper shielding is developed.

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Keep in mind that I'm not an expert and I'm going with what is generally considered acceptable by the mainstream, both for what the planets are like and the causality we've attached to these characteristics. I don't necessarily agree with all these explanations and correlations, but it corresponds to common understanding.

Based on what we know about planetary formation and geology, the mantle and liquid core provide us with a magnetic field. Without one, we'd have no protection from the solar wind:

Solar wind attacking Earth's atmosphere

That means the Sun would evaporate the atmosphere and hence you get less protection from solar and cosmic radiation. You also get less weather, such as lightning, rain etc. - without these, you don't get much chance for evolution of complex life, though of course extremophile bacteria can still survive. So you might have "life" in the strict sense, such as bacteria landing through space, but you wouldn't call the planet "alive" in any sense.

Liquid water is somewhat related. Considering that the primitive Earth is assumed to not have water the way we have it today, but a much more toxic mix that slowly got changed into a combination of mostly water, due to environmental change which came about from terraforming bacteria (this is how I remember it roughly, not sure on the details), I'd say we're back at square one - you won't have vast populations of microorganisms doing their business on a planet unshielded from solar radiation (especially when the temperature range would be much more than 5-10 degrees Celsius between night and day - it'd be closer to 100 or more, making any adaptation difficult). You could however have ices, accumulating from space debris over a long period of time. The extra problem here is, where would it collect?

The reason we have vast oceans is due to mantle activity - tectonics, volcanos etc. Without a liquid mantle, you don't have those as far as we can understand. So you'd just be a smooth rocky planet like Mars. No basins.

The last but not least problem I can think of is the evolutionary cycle. Major extinction events have been linked to the emergence of new and more complex life. In our case it was probably related to environmental changes, correlated with environmental weather and atmosphere cycles, as well as solar cycles. This requires enough stability to preserve life after a disaster, but not enough to prevent the cycle from moving on, to allow newer life to take over. In other words, even if you got to dinosaurs, they could just stay like that for much longer (assuming extinction events are not caused by biological populations and instabilities inherent to evolutionary stages).

A planet without an atmosphere, no mantle movement and eruptions and little shielding would be pretty much like Mars - rocky, barren, relatively smooth and practically lifeless.

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    $\begingroup$ Liquid water can't exist in low pressure environments, it either turns to ice or sublimes directly to vapor (and then escapes). This means that if you lose the air you also lose the water, unless the water is covered in an ice sheet (i.e. Europa) $\endgroup$ – Tim B Sep 30 '14 at 22:52
  • $\begingroup$ The atmosphere does a better job at protecting the Earth's surface from solar radiation than the Earth's magnetic field. $\endgroup$ – user2617804 Oct 1 '14 at 5:16
  • $\begingroup$ Thank you for the elaborate answer. While intuitively making sense, I think this is not the entire picture. I read about Venus on Wikipedia, and- liquid interior or not- it kept its atmosphere despite having a weak magnetic field. Sure, none of it is water... But I would guess that this is due to Venus' composition and not to the particular vulnerability of atmospheric water. $\endgroup$ – Florian Hollandt Oct 1 '14 at 6:16
  • $\begingroup$ @GaspardMonge Many, many obvious objections to this perspective are quite valid - I don't agree with half of this myself and you're both right. However, I'm basing this on what I understand to be the mainstream view. There's too many details to examine - I could go on for ages, but that's not what you need out of an answer :P A bit of googling will give very detailed explanations, but don't expect clean-cut answers. $\endgroup$ – mechalynx Oct 1 '14 at 10:01

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