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Some Problems With Low-Gravity Planets

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Some Problems With Low-Gravity Planets
Post by Imaginos1892   » Sun Oct 15, 2017 11:04 pm

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I’m trying to convince a FanFiction author that having humans live on a planet with a surface gravity of 0.73 G is detrimental to his story. He seems to think low gravity is cool. I did manage to talk him up from 0.64 G.

I have thought up a number of reasons why an ‘Earth-like’ 0.73 G planet orbiting in the ‘life zone’ of a G-type star would not be suitable for humans. Some of them are difficulties with the planet itself, others are problems humans would have living at 0.73 G. Have a look, tell me if I’m right, or full of shit, or if I’ve missed anything.

First, a planet that small would almost certainly have a cold core. No volcanic activity, no plate tectonics, and no significant magnetic field. After a few hundred million years all mountains are worn down to hills, and there are no major rock masses remaining on the surface. The only forces altering the surface to more than twenty meters deep are meteoric impacts. Limited to no outgassing from deeper than 1,000 meters except as a result of those impacts. At least you don’t have to worry about earthquakes.

With weak gravity and no magnetic field, stellar wind blows away most of the atmosphere. Surface atmospheric pressure would only be 200 to 300 millibars. Water boils at 60 to 65 degrees C, and evaporates easily at all temperatures. Intense ultraviolet light penetrates deep into the atmosphere and dissociates water molecules. Hydrogen rises and blows away, oxygen reacts with surface minerals. Atmosphere consists almost entirely of nitrogen, carbon dioxide and sulfur dioxide. There is no water on the surface, and very little under it.

The thin atmosphere holds very little heat and night-time temperatures drop below -100 C. At least 70% of cosmic radiation reaches the surface. The planet would be considered an airless, waterless frozen Hell, not suitable for terraforming because it just would not hang on to an atmosphere if you gave it one.

Taking all of that into consideration, it looks like The Earth must be at the small end of the ‘Earth-like planet’ range. I’d put that at 9 to 12 meters-per-second-squared surface gravity, making the median around 1.07 G. I would expect most of the ‘Earth-like’ planets out there to be slightly to significantly bigger than The Earth.

All right, say some group is stupid or desperate enough to colonize the damn place anyway. What happens to them?

The human body goes a little…off at reduced gravity. Our bones get weak, our muscles atrophy, we get fat, and our immune systems become less effective. Fluids build up in places they shouldn’t. Include the increased cosmic radiation and you’ve got a population with a lot of health problems.

On a planet with an unbreathable atmosphere, you live in transparent domes to contain your air, right? You’re going to need at least a square kilometer of dome for every 400 people — don’t forget the farmland. A few big domes are much more efficient than a lot of little domes. With the low outside pressure, those domes are pushed upward with a force of 7.2 to 8.3 million tons per square kilometer. Better set your anchors really deep, because bedrock is a long, long way down.

Well, those are some reasons I wouldn’t want to live on a 0.73 G planet. And no, being able to jump really high for the first few months wouldn’t make up for it. How about you?
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Re: Some Problems With Low-Gravity Planets
Post by dscott8   » Mon Oct 16, 2017 9:17 am

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My ice cream would drift right off the cone.
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Re: Some Problems With Low-Gravity Planets
Post by cthia   » Mon Oct 16, 2017 1:56 pm

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The Mars Trilogy by Kim Stanley Robinson does a superb job on the subject. One of my favorite reads. And authors.

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Re: Some Problems With Low-Gravity Planets
Post by Dilandu   » Mon Oct 16, 2017 2:39 pm

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First, a planet that small would almost certainly have a cold core. No volcanic activity, no plate tectonics, and no significant magnetic field.


Hm, I disagree. Tidal effects from the star and other heavy bodies could keep core warm for relatively long time, even if overall mass is low. Mercury, for example.

With weak gravity and no magnetic field, stellar wind blows away most of the atmosphere


What about the relatively light moon inside the magnetic field of large gas giant or even brown dwarf, orbiting the primary star?
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Re: Some Problems With Low-Gravity Planets
Post by The E   » Mon Oct 16, 2017 3:13 pm

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Dilandu wrote:
First, a planet that small would almost certainly have a cold core. No volcanic activity, no plate tectonics, and no significant magnetic field.


Hm, I disagree. Tidal effects from the star and other heavy bodies could keep core warm for relatively long time, even if overall mass is low. Mercury, for example.


True, but that also has consequences for habitability, and probably not good ones at that.
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Re: Some Problems With Low-Gravity Planets
Post by aairfccha   » Tue Oct 17, 2017 3:47 pm

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You are implicitly assuming a small planet the age of Earth in an approximation of steady-state conditions. A younger small planet could be (a lot) better suited to human life than Mars today. However, another question would be whether those life-sustaining conditions would remain im place long enough for life to actually develop.
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Re: Some Problems With Low-Gravity Planets
Post by pappilon   » Thu Oct 19, 2017 3:08 am

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aairfccha wrote:You are implicitly assuming a small planet the age of Earth in an approximation of steady-state conditions. A younger small planet could be (a lot) better suited to human life than Mars today. However, another question would be whether those life-sustaining conditions would remain im place long enough for life to actually develop.


And who knows about Titan (Saturn) and Europa (Jupiter)
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Re: Some Problems With Low-Gravity Planets
Post by Imaginos1892   » Thu Oct 19, 2017 10:41 am

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I'm considering the issue mainly in terms of a planet's suitability for human colonization. That's what the story is about.
pappilon wrote:And who knows about Titan (Saturn) and Europa (Jupiter)

Any life there would be constrained by the low level of available energy. Not saying it's impossible, but they would have to have very slow metabolisms.
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Re: Some Problems With Low-Gravity Planets
Post by Lord Skimper   » Fri Oct 27, 2017 11:07 pm

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Gravity doesn't affect the atmosphere. Thin atmospheres come from solar wind.

Titan which has 0.14G has an Atmosphere 145% that of Earth. Titan is cold, which is good otherwise it would blow up. It is cold because it is far away from the Sun. Saturn itself is cold.
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Re: Some Problems With Low-Gravity Planets
Post by Imaginos1892   » Sun Oct 29, 2017 11:44 am

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There are a number of factors that influence atmospheric density. It is stupid to claim that gravity is not one of them.

Titan 9.5 AU from the sun. It has 1/7th of Earth’s gravity, but the intensity of sunlight, and the solar wind, are both 1/90th of what we get here at 1 AU. Its average surface temperature is 180 degrees below zero. On cold nights in Saturn’s shadow, it rains liquid nitrogen.

Titan’s atmosphere is 98% nitrogen, and N2 is an extremely stable molecule, with very high binding energy. The weak sunlight at 9.5 AU doesn’t split a significant quantity of them apart. It is shielded from the solar wind by Saturn’s magnetosphere, so those that get split don’t get blown away.

If a body having 1/7th of Earth's gravity were orbiting the sun at 1 AU, its atmosphere would be extremely thin, like, oh, say, THE MOON. Gravity doesn't affect the atmosphere…yeah, riiiiiight.
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