Story research - need to know what is affected by a lower gravity environment

[sinister42]

The story I'm writing takes place on a planet with lower gravity than Earth - enough to be noticeable to colonists on a generational ship that spins to simulate Earth gravity.

What effects, besides bouncing a little and feeling lighter and the obvious ones like that, would the colonists notice? If they lit a bonfire on Landing Night, would the fire look different if the gravity were, say, 80% or 70% that of Earth? Would water flow differently?

 

[PTeppic]

If the people are from Earth, they'd probably have extra "strength", because they're used to the stronger gravity: they'll be able to jump further, throw further, etc. If they take plants and animals, they might grow taller on the new planet. Would humans? But they would perhaps have greater "bone density" (and plant equivalent) than any native species.

 

[chrispenycate]

There'd be very little noticeable difference from the absolute gravity. Oh, all their reflexes would be wrong for sports, as much from the lack of Coriolis effect as absolute magnitude of the forces, but playing volleyball would be an interesting experience. They'd be able to lift heavier weights, but this is hardly going to revolutionise life. Water would flow slightly slower, produce strange-shaped waves, but only subliminally disturbing; after all, their reference is largely films shot on Earth, streams and oceans on your self-propelled space habitat are tame, planned features.

In fact, more of the shock differences are going to be due to being on a planetary surface, rather than in a synthetic environment, and overhead, rather than being a ceiling with a convenient light source is weather, and, during the dark bits, you can see for ever. And planetary gravity is constant; it doesn't drop off as you go uphill. With lots of falling over and fractures, not because people are jumping higher, but they're not used to irregular surfaces planets specialise in. But all of these would be the case on any planet (less on one without a breathable atmosphere, where you could set up domes and a synthetic environment simulating the trip).

More directly evident in mood and general physical ability would be atmospheric consistency, totally independent of gravity. A bit less oxygen would compensate for all the increased agility, a bit more and you're exhilarated, bouncing, almost drunk.

 

[Aleph]

Or WOULD they be able to lift heavier weights?

Humans are all muscle to account for Earth gravity, we need to fight it every day. As everything is lighter, we'd quickly become very much weaker. Instead of lifting a 20 kilo/50 pound block of, I dunno, Camembert cheese with a bit of effort, we would only have to possess the muscle strength to lift the 5 kilo block. They're both the same size on our lighter-gravity planet, you see.

Muscle wastage would then start happening instantly. Unless our visitors keep exercising hard-core every day, they'll quickly adjust to become flabby 90 pound weaklings.

Their bones, no longer having to support muscular frames under gravity would also start to lose density. As others have posted, fractures will be more common. A kick in the skins may break your leg, as you are no longer built to withstand these forces.

 

[Mirannan]

I think a fire would look and act differently, specifically less intense, although I'm by no means sure.

My reasoning: The driving force behind the convection currents that keep a fire from going out immediately is the difference between the weight of the air above the fire and the weight of the air around it. For a fire in (say) 70% gravity, the mass density difference would be the same as on Earth - but the weight difference would be that 70%, assuming the fire didn't change.

The extreme of this is trying to get a fire to burn in microgravity. If there is no forced air circulation, the fire will go out almost immediately once the air immediately around the fire becomes exhausted of oxygen.

Lastly, I think it's likely that the spin gravity of the ship would be gradually adjusted to match the gravity of the destination (if, of course, it's known) to get people's reflexes and walking motions, that sort of thing, used to the conditions at the destination.

 

[psychotick]

Hi,

Yes we'd adapt quickly enough, depending on how low G it was. Soon it would seem to us as if it was normal.

So there would be natural muscle atrophy. Bone density would slowly decrease though this would take years. Probaby it would be the next generation that would really notice this problem. Fractures are likely to be more of an issue as the years grow for two reasons. First muscle stength would remain relatively strong compared to bone strength. And second, even though you tend to think of light gravity as a softer way to fall, it isn't necessarily. Gravity may not pull you down as hard, but your body's mass remains the same and inertia when you hit something remains the same. Falls may be less dangerous but running into something could become more so.

Of interest I think one thing humans would keep would be our relatively quick reflexes. The higher the gravity the fast reflexes have to be because of the danger of falls etc.

To add to this with gravity comes air pressure. I would guess that all things being equal this lighter gravity world would have thinner atmosphere. (Note that it doesn't have to, Venus has the same gravity as Earth but I think 400 times the air pressure.) Thinner atmospher means more difficulty finding enough air to breathe, so over time I would expect people to have to adapt as they do to live in higher altitudes on Earth with changes in lungs and blood.

Cheers Greg.

 

[Aleph]

Well, for the purposes of fiction it's probably okay there's not really that much of a correlation between gravity and atmosphere - some planets have a similar gravity to ours (like the poster above mentioned, Venus was cited in the NASA document I googled with having high atmosphere and lower gravity). You can play around with the figures there in your imagined world.

I always remember the images of people who had been in low-gravity environments for a while returning to Earth and having to be carried out of their re-entry vehicles because they couldn't walk!

 

[sozme]

Well the truth is there would be a great deal of physiological changes just going down 20% G.

The list is pretty long, but the obvious ones would be bone density (decrease) and skeletal muscle tone (it would decrease, especially in the lower extremities).

Without exercise and proper nutrition in this environment, in the long-term, there would be some relatively interesting and not so minor medical problems.

Someone said something about the oxygen composition changing? If its higher than that at atmospheric pressure (roughly a mole fraction of .21 or 21%), it would be bad. If its very slightly lower, it wouldn't be a problem in healthy people (i.e. those that don't have sickle cell trait or metabolic problems). The human body has some very good control mechanisms for adjusting to minor decreases in atmospheric pressure and pO2. Contrary to popular belief, too much oxygen is very bad. A cool factoid - humans on fictional colonies where there is a higher partial pressure of oxygen would age more rapidly than those on earth.

- CB, M.D. student, M.S. in physiology

 

[Bowler1]

Gravity is very important factor in keeping an atmosphere in place (a magnetic field helps us as well) and without strong gravity a planet will lose its atmosphere. We don’t have enough knowledge to know where the common tipping point would be for planets to lose their atmosphere (Chrispy, could you provide a good guess?) but clearly Mars (38% earth) lost all its air, Mercury (38% earth) too, while Venus (90% earth) has an atmosphere. Sadly that’s about all the examples we have. So somewhere between 38% earth gravity and 90% planets seem to lose all their atmosphere.

Me, I think anything less than 70% earth gravity will be airless as the planet will be too small for a magnetic field will lose all its air or whatever to solar wind. I can’t prove this of course, just guess work and not even educated guess work.

But Chrispy once built a small world for me (the size of Brazil) with a fully working atmosphere. It was perfect, except the air never got above my knees. I still have it and visit once in a while, not very often, as I get tired of crawling around everywhere!

 

[chrispenycate]

It also depends on temperature (cold places keep gas molecules better than hot ones) and the molecular weight of the gasses making up the atmosphere. Thus Mars has lost almost all its water vapour (H2O, low molecular weight) but kept a fair amount of the heavier CO2, while Titan, quite a bit smaller than Mars has a largely methane (CH4, nearly as light as water vapour) atmosphere, but has a surface pressure (as I remember it) higher than Earth's. And timing; if you generated your volatiles by crashing a couple of gigatonnes of comet into it (comets being built largely out of things that would be gasses in a more hospitable region in the solar system) you could get several million years of use out of it before slow (but inevitable) attenuation cleared enough of the lighter gasses to render it unusable.

It's all to do with escape velocity, and molecules at a particular temperature having the same kinetic energy, so lighter ones have to be moving faster (you don't want me to give you the formulae, do you? And lower gravity means your atmospheric pressure drops off slower, so more of the atmosphere is further from the planet (unless you're talking about gas giants where the atmosphere is the planet, but who'd want to live there? Better the Niven gas torus). Which makes it easier to blow off.

Sorry about the inconvenience Bowler, forging neutronium is a bit of a speciality. Trouble is, you wanted a breathable, nitrogen/oxygen type atmosphere. Had I been allowed to use heavier gasses (like neon. I designed a moon {for somebody here, actually} with a neon/argon atmosphere, and when the charged particles from the solar wind struck it it lit up in great whorls and flashes of turbulence, but I haven't a story of the first intrepin explorers to set foot there {and that was to get a moon subtending a larger solid angle of surface without increased tides; I do get excessive, but decorative}. Enough digression from the digression). You merely have to make your characters Chrispiform – half an inch tall, and six feet wide, see convention photos – to avoid any problems of them sticking out out of the breathing space – mountain climbing is out, I'm afraid, as is skiing (except cross-country).

 

[Bowler1]

Titan is an unusual example of a planet sized moon with an atmosphere because of its very unusual circumstances. I think the tidal forces of Jupiter keeps the core hot and the volcanos erupting and the atmosphere going. It’s gravity is very low because it’s not a very dense moon and I strongly suspect that in the absence of Jupiter it would be airless. I’m still holding to my tipping point of around 70% earth gravity to keep an atmosphere on a planet, clearly in the absence of interfering gas giants that are making a mockery of my newly invented rule. As Chrispy has stated, losing an atmosphere takes time, as in Mars may have been very nice in the past so if your timing was right we could have danced on the surface of Mars. A nice way of saying, when it comes to SciFi all bets are off, even my brand new 70% rule.

However, Titan as an example does offer other possibilities for generating and renewing an atmosphere on a body with low gravity. I can’t imagine a body that relies on volcanos’ like on Titan to be very useful to us, as in breathing and running around the surface, but could easily develop other types of life. Even Titan is a candidate for some life, but I think the odds may be against it.

But, (rule number two today, I’m on a roll) I’m going to say that having a moon in orbit around the planet would be good for plate tectonics and general recycling of and renewing of elements in an atmosphere. Additional (possible) side benefits of tides, a stable axis and a pretty night time sky.

Half an inch tall and six feet wide really does bring a new meaning to the word “fat”.

 

[ralphkern]

As well as physiology remember as well structures would be adapted. A nice bit of world building might be some mention of tables and chairs being more spindley as they don't have to take the same amount of weight. People would develop a more loping walk, no where as extreme as moon astronauts but noticeable.

Also centrifugal 'artificial gravity' is different from regular gravity. After generations on a spinning ship they would be used to dropping objects and seeing them curve down. They would be used to how their inner ears were reacting under a spin and might find it awkward switching to mass gravity etc.

 

[psychotick]

Hi,

So as a minor aside, how does Venus with slightly less than Earth's gravity and a horrendous temperature manage to keep such a massively dense atmosphere?

Cheers, Greg.

 

[Bowler1]

Atmosphere of Venus - Wikipedia, the free encyclopedia -By the power of Google!

The dense atmosphere is because Venus has lost the lighter elements over time, while still having enough of an ionoshere (I only learned all this this morning, so I'm just pretending to be bright) to keep the heavier elements in. I may be wrong in this, but I'm sort of thinking its like the static you get when you rub a ballon hard and it makes your hair stand on end. The solar wind is rubbing Venus and creating a lot of static, or enough to keep the heavy elements in and maintain an atmosphere.

Excuse me while I chant a little. By the power of Chrispy, please come and confirm or dispel and comments.

Right, fingers crossed the great Chrispy appears, you just never know.

 

[chrispenycate]

You rang, outan?

For a molecule to leave its gravity well (which might be a planet, a moon, a comet or even a star) it must achieve escape velocity (a misnomer; it is speed, a scalar quantity, that counts, not the vector direction with velocity).

The average momentum(I can't remember which curve defines the probabilities of so much faster and so much slower in the speed distribution) of a molecule at a particular temperature is constant, so the speed it is travelling is the inverse of its mass. Is that clear? I hate to use the freeway analogy, with trucks, automobiles and bikes, as this involves several thousand collisions per second, which is worse even than LA.

Venus' atmosphere contains lots of heavyweight molecules, like formaldehyde, sulphuric acid and various long-chain hydrocarbons which would be liquids or even solids on any reasonable planet (if you succeeded in cooling the place down you probably wouldn't need to bituminise the roads and parking lots; the entire surface would already be black). Any lightweight molecules, like hydrogen and helium, have long since gone exploring the cosmos, and any captured solar wind had better find something to compound with pretty fast, or it'll be on its way in a matter of decades. Medium weight molecules, like oxygen or water, coming from breakdown of the larger lumps by actinic sunlight have their emigration papers, too. Perelandra is no place to plan a beach holidy.

Personally, without a worthwhile magnetic field (which Venus lacks) I don't see the ionised layer of the atmosphere stopping much egress. Oh, the sunlight itself will dissociate many atoms, and the solar wind itself is largely ionised, but unlike Earth, where the path of such charged objects is curved by magnetism, half of the ions will be given extra speed by electrostatic repulsion, balancing those who are attracted. Net gain, not enough to worry about.

So an atmosphere on a low gravity world, even quite a warm one, would be quite possible; breathing it might be a bit problematic. Particularly water vapour would diappear about twice as fast as oxygen. Inconvenient, in the long run.

 

[Bowler1]

I've had to reread the Great One's post a few times, but from what I can gather using the static on a ballon (after you've rubbed it really, really hard and used it to make your hair stand on end, unless you use gel or hairspary, of course) as an anology of the upper atmosphere of Venus to partially deflect the solar wind is a good one. Lighter items have been blown away over time leaving the heavier elements behinds hense the hot house affect.

I also think he called me a monkey and a hairy one at that - how does he know these things, he's right again!

So in golfing terms and my ballon thing, I've gone and hit a hole in one on a par 15 or something. Don't worry everyone, normal service will resume directly - have I mentioned RAY GUNS yet?

 

[Ursa major]

Instead of lifting a 20 kilo/50 pound block of, I dunno, Camembert cheese with a bit of effort, we would only have to possess the muscle strength to lift the 5 kilo block. They're both the same size on our lighter-gravity planet, you see.

To be (very) pedantic** - and using the scientific, rather than the everyday, meaning of the unit - the kilogram is the unit of mass, not weight, so 20 kilograms of something cannot be the same size as 5 kilograms of the same thing. The newton (N) is the unit of weight***.

So, on Earth, cheese of mass 20kg will weigh 196.133N; on the low-gravity planet (one-quarter of Earth's), 20kg of cheese will weigh 49.03325N.


To be more serious, inter-planetary trade of bulk goods should always be in terms of mass, so it's a good thing that when we buy things by "weight", we do it mostly in kilos. (Luckily for at least some of those over the Pond, the pound is also a unit of mass. )



** - I blame chrispy....

*** - Strictly speaking (or typing, to be even more strict ), the newton is the unit of force, but it's being used here in the context of gravity acting on mass.

 

[sinister42]

Wasn't the discussion of mass/weight, newtons/kilos on QI recently?

 

[Mirannan]

Ursa major - Interestingly enough, the question of what we are measuring when we "weigh" something depends on the equipment used. Most scales today use strain gauges/force cells to measure the weight (the downforce) of an object - but a traditional set of scales, using what are normally called weights but are actually standard masses, measure mass directly.

For precision work, a newly obtained scientific scale needs to be calibrated with standard masses because gravity does vary over the Earth by 1% or so.

On Earth, for most practical and non-scientific purposes, this distinction doesn't matter. On a spinning habitat, it might.

 

[psychotick]

Hi,

Thanks Chris. C.S. would be so dissappointed. No picnics on Perelandra, no breathing on Malacandra and the silent planet ain't so silent either. What's a philologist supposed to do?

Cheers, Greg.