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Some Problems With Low-Gravity Planets

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Re: Some Problems With Low-Gravity Planets
Post by Tenshinai   » Sun Oct 29, 2017 2:33 pm

Tenshinai
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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.


Why would it be small? Size is not the sole determinator of gravity.

Size and density, not just one or the other.


Mars has 0.38g gravity and has definitely been volcanically active in history(and may still be).
Io has 0.18g gravity and HAS volcanic activity NOW.
The moons Enceladus and Triton also have lower gravity yet are still active volcanically. Triton has merely 0.08g.

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.


Not an automatic correlation.

Venus is has 0.9g gravity yet has 93 times more atmospheric mass than Earth.
Despite that its magnetic field gives negligable protection against stellar winds, being drastically weaker than that of Earth.

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.


Again, you assume automatic correlations that isn't real.
You can find contradictions to all your statements so far just in actually observed facts in just our own solar system.

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.


:shock:

Wait, whaaat?

Nope nope nope nope nope. You're about as wrong as is possible.
Just go look into the few examples i used to disprove you above.

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.


It has already been reasonably well established that the gravity of Mars, which is 0.38g, should be enough for human needs without too extreme special requirements.
Earth's moon has been noted that WITH such requirements, like exercise regimens not too far from that on space stations and the like, even it should be realistic for human longterm settlement as far as gravity goes.

Cosmic radiation is either solved(or mostly) by getting a proper atmosphere, or being indoors and using protective equipment outside.

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.


Gross exaggeration. That would be 2500 sqm/person. That is more than a magnitude greater than what you can get away with even if you have a closed system, as long as the system as a whole is large enough or has artifical systems to help regulation.

When research into the concept of selfsufficient space stations or spacecraft were looked into, they were able to cram enough "farmland" into less than 50 sqm to support a grown human.
It wasn't entirely preferable, but it could be reduced even more.
Give a thousand sqm and you can have it used as a 4-5 stage full circle life support with plenty to spare.

Well, those are some reasons I wouldn’t want to live on a 0.73 G planet.


You're basically quoting the absolute worst possible cases for every possibility, so yeah, of course you wouldn't.

Your predictions have absolutely nothing to do with average or "most likely" however.
In fact, i think you're actually closer to the "least likely".

Merely having a 0.7g or 0.8g gravity is absolutely in no way a hindrance to human colonization.

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.


OR, you could have something with even LESS gravity, like say, Titan.
Which just happens to also have a higher atmospheric pressure than Earth and about 20% greater atmospheric mass.
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Re: Some Problems With Low-Gravity Planets
Post by Lord Skimper   » Wed Nov 01, 2017 7:35 am

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Imaginos1892 wrote: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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It rains liquid methane (Natural gas), not liquid Nitrogen. Titan has 11-12 times the water of Earth. Its ground is water Ice, it's lava is saline water heated by the pressure of 70-100 miles of ice floating on top of it.

A 0.14G planet at 1AU would be just fine if it has a Magnetosphere. And if it doesn't it would be like Mars. Mars has 7/1000 of Earth's Atmosphere. Titan at 1 AU qualifies as a planet, without a magnetosphere it would lose a lot of atmosphere unless it had it's own moon which generated a magnetosphere then it would be fine. Put 16 Psyche orbiting Titan at 1 AU and your 0.14G world is just fine. You wouldn't even see it from the surface of Titan. Titans atmosphere is very thick. 400 miles thick. Yellowy orange. With wings a person can fly on Titan. And if you fall you max out at about 20 mph.

Now Titan at 1 AU would have less Ice, more water, might even be a water world. Hydrogen and Oxygen, Nitrogen, Methane etc... Winged finned humanoids could swim and fly.
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