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?
