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I've already posted something similar recently, but I don't think I was really asking the right question. I'm designing a hypothetical lower-gravity planet with 0.47M, 0.79r and 0.76g, with a similar density to Earth. I've already determined that this mass, radius, and density will allow my planet to sustain a long-lived internal dynamo and strong magnetic field, as well as have an escape velocity that will permit the stability of water vapor in the atmosphere. I want to give the planet an Earth-like atmosphere (in terms of composition), but the fact that the gravity is lower might mean that the atmosphere would be less dense and expand farther out. I'm worried this might make the planet too cold, and I don't want a snowball planet scenario.

My question is - if I want the planet to be at least as warm as Earth was during the last ice age, what factors should I tweak to make the planet realistically warm enough? (I'm thinking maybe increasing the total mass of the atmosphere, ocean size/depth, rotation rate, etc). I have some general ideas but no confidence that they make sense.

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More atmosphere

https://www.coursehero.com/study-guides/astronomy/the-massive-atmosphere-of-venus/

...as percentages, the proportions of the major gases are very similar for Venus and Mars, but in total quantity, their atmospheres are dramatically different. With its surface pressure of 90 bars, the venusian atmosphere is more than 10,000 times more massive than its martian counterpart.

You can have as much atmospheric gas on your 0.76g planet as you want. Venus has 0.9g but it has a metric boatload of gas in its atmosphere. You can use that to warm your planet up too - more total gas in the atmosphere, more greenhouse effect, higher air pressure at sea level and all that.

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Under your planetary scenario, the escape velocity would be 8.626 km/s, which is 77 percent of the escape velocity of Earth (11.184 km/s), but higher than the escape velocity of Mars (5.025 km/s). This means there's a chance of having a reasonable atmosphere.

One way to have a warmer planet would be to have a higher concentration of carbon dioxide in the atmosphere, but not too high. You want a nice greenhouse effect. One way to achieve this would be to make the planet volcanically active, with volcanoes emitting enough carbon dioxide to produce a warmer planet.

Something like a tropical jungle world might be possible, were there is enough jungle to prevent an escalating greenhouse situation where too much volcanic carbon dioxide accumulates in the atmosphere, but volcanoes keep emitting enough carbon dioxide so an equilibrium is achieved.

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The simplest answer is the planet has evolved a biosphere that produces an output of sufficient methane to warm the atmosphere to the degree you require.

Methane is a greenhouse gas that retains heat over twenty times more of that of carbon dioxide. It's not the only greenhouse gas that can raise a planet's global temperature. Earth would have a global temperature of around minus 30 degrees centigrade if it wasn't for our atmosphere's carbon dioxide content.

The main source of methane is from biological systems. That's why a biosphere is the probable source.

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It sounds like you need some atmospheric control satellites. Or if the sun is hotter/closer that would help too. Lots of volcanic activity or internal friction like on the moons of Jupiter might also help.

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    $\begingroup$ Hey, you did a nice job for a first answer, I had an even worse one. But you might need a little more info. $\endgroup$ May 24 at 23:44
  • $\begingroup$ It's not the greatest answer, just a few ideas. I would have made it a comment but apparently I'm not allowed to do that yet. $\endgroup$ May 24 at 23:49

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