Thicker air and lower gravity

January 8, 2007 3:35 AM

Imagine a world in which the atmospheric density is somewhat greater than ours, but gravity is somewhat lower. How might our experience be different on that world?

For example, I imagine that the greater air resistance would mean a lower terminal velocity for falling objects. Is this correct or am I confused?

Assuming a mix of gasses in the same proportion to our own atmosphere, would the higher pressure of oxygen at sea level cause physiological problems, or would the higher pressure of nitrogen force you to decompress gradually if you, say, went up in a balloon, or risk an attack of the bends? What other physiological effects might occur?

How would gunfire, explosives and other incendiary reactions be affected?

How would climate and weather differ? Would clouds form more readily? Would they form at higher or lower altitudes?

What sort of technologies might be favored?

What other interesting effects might this environment have?

A world with lower-than-Earth gravity probably can't retain a denser-than-Earth atmosphere, I realize. Be gentle in the face of my naivety.

posted by Ritchie to science & nature (16 comments total)

There's probably not enough information here to answer the question. It all depends on questions like "how dense", "what mix of gases" or "what value of g".

For example within your parameters we could assume an atmosphere that supports fire, or one that does not. Obviously those are going to have very different technological outcomes.

Also, when you say "our experience", do you mean that of homo sapiens, or that of a man-like creature that evolved natively?
posted by Leon at 3:53 AM on January 8, 2007

A world with lower-than-Earth gravity probably can't retain a denser-than-Earth atmosphere, I realize. Be gentle in the face of my naivety.

Ever heard of Venus? Slightly below Earth gravity at the surface, atmospheric pressure about 90 times higher than Earth's.
posted by edd at 3:58 AM on January 8, 2007

I imagine that the greater air resistance would mean a lower terminal velocity for falling objects.

Depends on how much you simultaneously change gravity by. Terminal velocity arises from the equilibrium of downards gravitational force (mg) and upwards viscous force (a function of viscosity, η). In most circumstances, as you say, increasing atmospheric density would increase η. So the upward force goes up, but so does the downward. Which wins depends on how much you change each thing by.

How would climate and weather differ? Would clouds form more readily? Would they form at higher or lower altitudes?

Normal clouds in the troposphere, i.e. near the Earth's surface, form when the air pressure is low enough to allow condensation of water. This is why, when cloud coverage is total, you get a "deck" at one height. In reduced gravity this deck would normally be higher, but in an atmosphere of increased density is might be lower. Again, it depends on which effect is greatest.

What sort of technologies might be favored?

High altitude travel, since in the reduced gravity it would be relatively inexpensive to get above the densest regions of the atmosphere (this is the main cost of space travel), and once there you'd be able to travel very quickly in the thin atmosphere.

However, as you suggest, there is a problem with the whole idea of reducing g while increasing the mean atmospheric density. You need a bigger planet to pull the atmosphere closer to the surface, making it denser, but that bigger planet has stronger gravity. The only practical way of having your cake and eating it that I can think of is to change to the chemical composition of the atmosphere so that it (or components of it) are pulled closer to the surface of the planet by a smaller force, cf. Venus.
posted by caek at 4:01 AM on January 8, 2007

I missed the bit about "same mix of gasses" in your question. Sorry about that.

In that case, things are going to burn a lot more easily, and a lot faster. Raise the pressure enough, and I suspect you'd see near-constant fires due to lightning strikes, and I'd expect the local flora and fauna to have evolved to survive that. (Hmm. Would that kind of mechanism eventually lock away atmospheric oxygen in the form of charcoal?)
posted by Leon at 4:04 AM on January 8, 2007

(oh and someone's about to point out to me that pressure isn't density, but pressure was easier to find, and it's an indication the density is going to be higher)
posted by edd at 4:05 AM on January 8, 2007

Sorry Leon, I left the parameters purposely vague. I was thinking of an environment that will support human life - generations of them, actually - without special life-support equipment. Not knowing human tolerances, I can't very well lock myself into specifics. I'm more interested in likely interactions and phenomena.

But your question raises another question. What adaptations might homo sapiens exhibit after some time alone in this world?
posted by Ritchie at 4:05 AM on January 8, 2007

Well, one obvious adaptation would be smaller lungs; a denser atmosphere wouldn't need as much lungpower.

That said, I *believe* atmospheric pressure is a function of the weight of the upper atmosphere pressing on the lower. If you lower the gravity, you will lower the pressure, because the air doesn't weigh as much. Gradually, the upper layers would escape. Even a small reduction in gravity, over geologic time, could change the air density quite a bit.

It sounds like you're trying to write a story. If so, unless someone comes up with a brilliant idea, I don't think you can have both things and still call it science fiction. You can make it vaguely sf-like, but unless I'm missing something here, a high density atmosphere of the same composition as Earth's, but in a low-gravity environment, would have to be done with handwaving/magic.
posted by Malor at 4:50 AM on January 8, 2007

Malor: I suppose you could put the entire planet in a glass jar, and pump up the atmospheric pressure until it met requirements...
posted by Leon at 5:11 AM on January 8, 2007

There is research [ 1 | 2 ] that suggests widely higher atmospheric oxygen and carbon dioxide percentages supported plant and animal species with wildly larger physiologies.
posted by Blazecock Pileon at 5:14 AM on January 8, 2007

That research refers to the fractional amount of oxygen in the entire atmosphere, not the amount of oxygen per unit volume at the surface. It's not obvious that the effects would be the same.
posted by caek at 5:36 AM on January 8, 2007

Not sure if this helps, but:

In reality, the higher up in the atmosphere a girl lives, the later the onset of her period. So, girls raised at high altitude typically get their periods closer to 18 than 12. Note that this is a *within* lifetime adaptation: if a girl is born at high altitude and brought down before her period begins, the onset will be sooner and vice versa.

Not sure if this would have a huge effect in your scenario, unless people are living higher up in order to avoid something about the lower areas (or perhaps if there were a conflict between those who lived high up and those who live low).
posted by carmen at 6:15 AM on January 8, 2007

It's hard to point to how things would be with lower gravity, since gravity is a function of mass. A lowering of gravity over time would require an according loss of mass over time. There's nothing in what we understand about recent geology to suggest a more-than-negligible increase in earth mass, which could be somehow tied to the fossil record in such a a way as to answer the second half of your question.
posted by Blazecock Pileon at 6:36 AM on January 8, 2007

One obvious thing is flight would be easier. You'd see bigger birds and flying mammals, man would develop powered flight earlier because cruder less powerful engines would suffice, LTA flight would be easier and areodynamics of all kinds would be more important.
posted by Mitheral at 6:37 AM on January 8, 2007

Do some research on scuba diving.
Assuming the same gas mix, an increased atmospheric pressure could cause a regular human to suffer nitrogen narcosis.
If there is a different gas mix w/ a higher O2 % a person would need to worry about oxygen toxicity.
Also long term exposure to high pressures can cause aseptic bone necrosis, see saturation diving.
posted by mister e at 8:25 AM on January 8, 2007

There's gonna be a lot more flying things: more traction due to the density of air and less bone mass required to keep structures stable (lower gravity). On earth, it requires stuff like hollow bones to get yourself in the air, in this postulated world it may well be possible for unmodified humans to fly using some sort of strap-on flappy thing like people were imagining we could a couple hundred years ago.

Smaller lungs.

Wider variety of animal forms: denser/rounder will work without collapsing under gravity, i.e. whale-sized things surviving on land without requiring water for support. Light & spindly things might also become more common due to the ease of flight or even the possibility of "treading water" in air - imagine an air-going seahorse or much larger (metres across) butterflies.

Flying things requiring speed (e.g. predators) are gonna be much more streamlined, with stubby wings and loads of muscle. Wing shapes for high speed flight would be possibly quite different, needing to provide more thrust than lift.

Likewise flora would vary too: much taller structures would be able to support themselves in reduced G. Same applies to buildings: awesome architecture becomes possible with some imagination at low-g.

Bullets, etc would decelerate more rapidly; this would likely favour heavier rounds or non-kinetic weapons. Shock-waves from explosions would be much more damaging at range. Fires would likely burn more rapidly. Massive swords would be much easier to wield. Oxidation engines (internal combustion, jet, whatever) would be smaller for a given power.

I reckon you need to read Raft, by Stephen Baxter. It describes a situation very much like what you're talking about. In that book, it's set in an air nebula in a different universe (don't ask) that has a higher G constant - not g, but G so that a pair of masses is are more attracted to each other than in this universe. That leads to crazy stuff like independently-flying trees, people sticking to each other slightly and people on a metal plate (the raft) that provides enough gravity for you to walk on. As you get near the edges of the raft, the direction of gravity is no longer normal to the surface.

So there are (assuming you're writing SF) ways around the high-density-atmosphere/low-g issue; just depends on how crazy you wanna get. Some of the best hard SF (I think) results from changing one tiny fundamental thing and then exploring the consequences by illustrating a world. Changing G is more fundamental than your atmosphere/low-g thing but in the same vein - nothing to say you can't have a high-pressure bubble environment on a tiny & otherwise inhospitable planet.
posted by polyglot at 3:46 PM on January 8, 2007

You want lower G and higher pressure?

How about huge canyons in the surface of an Earth-like (but drier) planet? Deep canyons. Miles and miles deep. You'd have lower G, since there'd be less of the planet between you and the center of mass. But, if you had a bountiful atmosphere at "ground level", you'd have massively increased pressures at the bottom of the canyons.

You need a special kind of planet to support this, as it needs to be geothermally dead or your critters are going to have to constantly contend with lava flows. I'd say that it'd be something with a massively old biosphere, full of deep-sea creatures that evolved lungs as their environment dried up.

Fun.
posted by Netzapper at 4:14 PM on January 8, 2007