Planet Design - moon(s)

[dayriff]

Would a planet realistically be able to support human life without a moon? Without lunar tides, could life exist on the planet without technological interference (terrraforming, oxygen scrubbers/purifiers, etc)?

Depends, but the answer is "quite possibly not". The tides aren't the issue, it's the fact that our moon was formed when something about the size of Mars slammed into Earth not long after it formed (giant impacts were not uncommon back then - the planets we see today are merely the survivors of that era, the ones in the most stable orbits and that didn't get hit by other worlds that were forming). When that impact happened, most of Earth's primordial atmosphere was also blasted into space and lost - so if we didn't have the moon, we'd have a much thicker primordial atmosphere, which means it'd take much longer for that to evolve into our secondary atmosphere and possibly the greenhouse effect from the thicker atmosphere would have roasted the planet by now.

Some scientists therefore think that a giant impact is required in order to have a habitable, earth-like surface environment.

As I understand it, that last clause is still very much in the realm of, "but we could be wrong". By which I mean that even if trying to run a hard science Traveller Universe, nobody is going to be able to call "bull" just because your habitable planets don't all have giant moons.

(Though actually, making giant moons a common touch-point for all your habitable worlds is probably more cool than not.)

 

[Rikki Tikki Traveller]

Actually, you could use that excuse if you wanted to run a Foundation type game.

Lots of worlds in the habitable zone, but no real life and certainly no intelligent life other than humans. Every world would have to be terraformed at some point and all animal encounters would be variations of Earth life.

If it is good enough for Isaac Asimov it should be good enough for any group of players.

 

[saundby]

There's also the current thought that a sizable satellite is necessary for maintaining surface oceans, and thus a water cycle, on a world in the habitable zone. Basically the satellite is needed to both prevent rotational locking to the star, and to drive tectonics on the world.

Tectonics are required to maintain the H2O/CO2 balance (carbonate subduction and subsequent release via volcanism.) Certainly, without tectonics the "Gibbs Free Energy", i.e. energy available in a system to life as a result of disequilibriums is less.

Here's a source for you:
Evolution of a Habitable Planet

 

[BP]

There's also the current thought that a sizable satellite is necessary for maintaining surface oceans, and thus a water cycle, on a world in the habitable zone. Basically the satellite is needed to both prevent rotational locking to the star, and to drive tectonics on the world.

Tectonics are required to maintain the H2O/CO2 balance (carbonate subduction and subsequent release via volcanism.) Certainly, without tectonics the "Gibbs Free Energy", i.e. energy available in a system to life as a result of disequilibriums is less.

Here's a source for you:
Evolution of a Habitable Planet

Sorry - your direct link did not work for me - (Forbidden Access...). I've read James Kastings material in the past - so I'd be very interested in a good link...

As for the interpretation that a large moon is required for a habitable planet (I suspect your source doesn't actually state this) - just off the top of my head - while a large moon may provide certain benefits conducive to the evolution and conditions of life as we know it - as a requirement none of that sounds particularly convincing given :

a) active volcanic activity observed on Jupiter's moons (Io is quite active); Olympus Mons on Mars is quite large :wink: Subduction is one way to recycle the CO2 in the carbonate-silicate cycle - but to say its the only viable one given our limited knowledge is a bit presumptuous.

b) none of the inner planets are tidally locked IIRC - why would a moon be required to ensure this?

c) Moons go and moons come (especially if prominent Luna creation theory is correct).

d) Moon has responsibility in creating tides - but this is not a known requirement for an ocean or water cycle. Most evidence points and theories point to other 'oceans' in our own solar system...

Tectonics and water cycles seem to be a thermal process (convection in the mantle/convection in the atmosphere, etc.). A large moon such as ours can affect the intensity, but a requirement?

It is nice to fit theory to one set of data - but this often results in the presumption that there is only one way to solve particular requirements. General experience tells all of us that there is more than one way to solve a problem (though Ego often prevents the acceptance of this fact). Looking at nature in particular - you will find numerous and pretty amazing examples of this.

 

[Rikki Tikki Traveller]

Not knocking your statements, but item B) is wrong.

Mercury is locked in a 3:2 resonance, which is tidally locked.

Venus is ALMOST tidally locked. Since it's day is just a bit longer than it's year and it rotates backwards, it is almost there. Probably another couple million years and it will be in a 1:1 resonance and tidally locked.

 

[phavoc]

Unless you are trying to develop native species, then it doesn't really matter. Humans that are inhabiting the planet wil have imported plants and animals to the planet. So the planet itself could show little to no nateive life, but as long as the environment is able to support it, then it should be considered to be supportable.

 

[BP]

Not knocking your statements, but item B) is wrong.

Mercury is locked in a 3:2 resonance, which is tidally locked.

Venus is ALMOST tidally locked. Since it's day is just a bit longer than it's year and it rotates backwards, it is almost there. Probably another couple million years and it will be in a 1:1 resonance and tidally locked.

Knock away - mine are just the opinions of a layman trying to understand.

Thanks for the correction (hence my IIRC - sometimes my memory is tidally locked :wink - however Mercury is not tidally locked in the classic - one side always facing definition - since it is 3:2 spin-orbit resonance. (Though that resonance is due to eccentric orbit and basically tidal forces - but tidal forces effect all orbits). Because we couldn't make out any details on the surface (and the nature of the eccentric 3:2 orbit) it was originally classified as tidally locked till radar pulses were used ('60s I think - Mariner probes? were in the 70's) - again IIRC.

(Of course my definition of tidally locked may not match the document that was linked - but I couldn't check it.)

Venus at closest approach is sometimes referred to as 'tidally locked' with Earth, but I'm not sure whether tidal forces are actually at play with that... and one theory I recall does put Venus tidally locked prior to its retrograde rotation.

My understanding is that - given enough time - ALL bodies will become tidally locked. Most close orbiting bodies in our solar system (moons) are tidally locked to their primaries - but then we are looking billions of years after their probably formations.

In reference to habitable planets - I believe the common notion was that tidally locked planets would lack an atmo due to temperature extreme on the 'day' versus 'night' sides - however, computer models have shown that wind circulation could prevent this from being an absolute problem (upper atmo super-rotating and lower at more moderate speeds).

(Bear in mind my limited knowledge comes from over 2 decades ago and occasional reads regarding more modern probes and theories and is amateur at best... not to mention memory challenged! I could google - but then I'd probably get stuck for hours!)

 

[saundby]

Sorry the link didn't work for you, but as far as I can tell it's a working link to a PDF with no logins, subs, or anything req'd. The reference is the best I could pull on a short basis, I had a more specific paper on some current work that I've misplaced. I don't know whether there's an electronic edition and I failed to note the authors. My ESP was faulty that day, didn't realize this topic would be coming up here in the near future.

Anyway, the gist of it was that the chances of life on a particular exoplanet otherwise considered a good possibility were largely written off as a result of the determination tha the planet has no moon of sufficient size. I was supposed to look over the paper to get the info on the quality of their measurements WRT some design work I have going. Fortunately it looks like it's superfluous for that but I'd still like to read it if I can fish it up again.

On tidal locking, that includes the resonance locks, and the timeframes were for planets in the HZ considering the number of GY it takes for complex life to rise (enough for a Cambrian explosion, IIRC.)

Granted situations like hot subsurface life has not really been considered in most of the models, and all models at present come with bucketloads of assumptions. But we're still at the point of fitting curves to one point of data so it's the best we've got.

Also not considered is life based on solvents other than water, etc.

Anyway, I've been "reading a vibe" among some in the field (I'm not in it myself, just an interested bystander with some intersection of professional efforts) that large moons are, if not required, then very, very important for complex life and possibly even simple life. Rotation and tectonics yielding water/carbon cycles as well as release of other volatiles have been the reasons I've overheard.

If I find anything else I'll try to post it.

 

[BP]

Sorry the link didn't work for you...

Anyway, the gist of it was that the chances of life on a particular exoplanet otherwise considered a good possibility were largely written off as a result of the determination tha the planet has no moon of sufficient size.

On tidal locking, that includes the resonance locks, and the timeframes were for planets in the HZ considering the number of GY it takes for complex life to rise (enough for a Cambrian explosion, IIRC.)

Granted situations like hot subsurface life has not really been considered in most of the models, and all models at present come with bucketloads of assumptions. But we're still at the point of fitting curves to one point of data so it's the best we've got...

Thanks saundby - the link is working for me now! (no longer getting a 403 Forbidden message).

And thanks for elaborating - such is the actual position I presumed...

As you stated so well - this is based on a bucketload of assumptions and a single highly in flux 'data point' (Earth). Part of the problem with many of the current theories is they keep requiring addendum - i.e. more 'explanations' to account for observed data that is exceptional to the theories. As an analyst I question placing restricting habital exo-planet limitations based on a number of layered assumptions requiring a large moon - because we have one and we can 'work' it into our history.

In light of theories of our Moon as an artifact of a collision; Venus rotation/orbit due to impact and collision with its own moon; possible large impact on Mars, etc - there is a good chance that collisions with large bodies would cause the same effects (for habitability/evolution) and possibly be more common than retaining a large moon.

One commonly sees references to evolution requiring billions of years. Yet, our fossil record indicates that evolution on earth largely came in sporadic leaps related to major events (ice ages, cap carbonate, impacts). Evolution comes about in large part in response to or as a result of conditions in the environment (chemicals, radiation, temperature, competition). The jump from single cell organisms to human may well have occurred in only the last 500~600 Ma (Cambrian explosion on up). The billions of years of single celled organisms evolution may have largely been due to no environmental demand or accident necessitating multi-cellular organisms (or an incomplete fossil record). And from single celled to amphibian (oxygen breathing) was only 200~300 million years. And from the first amphibians to mammals with a neocortex was only another 100 Ma. The most rapid evolution appears to have occurred in the last 25 Ma due to warmer climate (though this may also speak to quality of our geological/fossil records).

Correlating large moons with habitable planets might yield up a better chance of locating earth like planets - but I think current theories are a long way from indicating that it is the only probably way habital environs can exist or even the most likely.

 

[saundby]

Still not the paper I was seeking, but directly related to the topic:

TIDAL LIMITS TO PLANETARY HABITABILITY

 

[Sevya]

I was building a couple planets yesterday and got to thinking. Would a planet realistically be able to support human life without a moon? Without lunar tides, could life exist on the planet without technological interference (terrraforming, oxygen scrubbers/purifiers, etc)?

Since at present, we only have one planet as a model, I think it's pretty hard to say definitively what is or is not possible on this issue. We can look at what scientists THINK happened here. There are too many variables to allow a certain answer. The last thought on the issue I've heard is "No", but the reasons given involve processes that the moon is not solely responsible for. I personally don't believe in "Impossible", so I'd say yes it probably could, but it would happen differently than it did here.

Sevya

 

[Sevya]

Eventually, the Earth will become Tidally locked to the Moon, like the moon is now. At that point, the moon will no longer be moving away from the Earth. Then, due to friction, the two will slowly begin to move towards each other, eventually crashing together, they will still be tidally locked the entire time.

This idea seems suspect to me. The moon's angular velocity is far slower that the rotation of the earth. As the moon gets farther away, that angular velocity will drop. In order for the earth to become tidally locked to the moon, the moon would have to get a lot closer, to about the distance of geostationary satellites.

Just my thought on it...

Sevya

 

[hdan]

Actually, from what I understand, since the Earth rotates much more quickly than the moon orbits, and the moon interacts with the tides, the Earth is actually causing the moon to accelerate, and thus the moon's orbit is expanding. This increase in distance has actually been measured with the lunar laser reflectors left by the Apollo missions.

 

[EDG]

Since at present, we only have one planet as a model, I think it's pretty hard to say definitively what is or is not possible on this issue. We can look at what scientists THINK happened here. There are too many variables to allow a certain answer.

Actually, we know a hell of a lot about what is possible regarding this issue, and indeed about a lot of astronomical and physical phenomena. Even given some unknowns, we can make some pretty damn good educated guesses based on what we know. And nobody's really giving a "certain answer" - of course it depends on the variables involved, and every planet and system is unique and has a unique history. But that doesn't stop us giving general answers that are likely to be valid in these situations (given certain assumptions).

 

[EDG]

This idea seems suspect to me. The moon's angular velocity is far slower that the rotation of the earth. As the moon gets farther away, that angular velocity will drop. In order for the earth to become tidally locked to the moon, the moon would have to get a lot closer, to about the distance of geostationary satellites.

No, it won't.

What's happening is that while the moon is being pushed away from Earth by tides, it is also slowing down the earth's rotation. Eventually (billions of years from now) the Earth's rotation will slow down so much that it will be the same length as the time it takes for the moon (in an even further orbit than it is now) to orbit the Earth.

 

[Sevya]

Actually, from what I understand, since the Earth rotates much more quickly than the moon orbits, and the moon interacts with the tides, the Earth is actually causing the moon to accelerate, and thus the moon's orbit is expanding. This increase in distance has actually been measured with the lunar laser reflectors left by the Apollo missions.

Yes, exactly. As the moon accelerates, it moves to a higher orbit, which takes longer to complete, even at the higher speed. The actual speed (km/hour) is higher, but angular velocity (degrees/hour) is lower. Orbital mechanics can be counter-intuitive that way.

Sevya

 

[Sevya]

No, it won't.

What's happening is that while the moon is being pushed away from Earth by tides, it is also slowing down the earth's rotation. Eventually (billions of years from now) the Earth's rotation will slow down so much that it will be the same length as the time it takes for the moon (in an even further orbit than it is now) to orbit the Earth.

Ok, that makes sense. The energy to accelerate the moon comes from the earth's rotation slowing down.

Sevya

 

[EDG]

Yes, exactly. As the moon accelerates, it moves to a higher orbit, which takes longer to complete, even at the higher speed. The actual speed (km/hour) is higher, but angular velocity (degrees/hour) is lower. Orbital mechanics can be counter-intuitive that way.

Ok, that makes sense. The energy to accelerate the moon comes from the earth's rotation slowing down.

Again, not correct.

The moon is not "accelerating" at all (well, it is if you're talking about its velocity changing over the course of its normal eccentric orbit, but that's not what we're talking about here). While its orbit is expanding, the angular and linear velocity of its motion around the earth are both decreasing. What's happening is that (ignoring solar tides) the angular momentum in the earth-moon system is being conserved - if the spin angular momentum of the earth's rotation is decreasing as the earth's rotation slows, the orbital angular momentum of the moon orbiting the earth has to increase by the same amount - and the way to do that (via arcane maths) is by increasing the distance of the moon from earth.

The complication is that we're not just talking about two bodies here, but three - the Sun is the third body. Solar tides are (very slowly) reducing the total momentum in the earth-moon system too.