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Habitable Venus
#1
There's now speculation that Venus was habitable-ish to within 700 million years ago:
https://www.sciencealert.com/venus-may-h...-years-ago

Given the power of Alien Space Bats, what conditions would be required on Venus to keep it habitable to the current period? Thin atmosphere, 24-hour rotation, large land area for high albedo? Anything else?

Or is Venus just too close to the sun to stay human-habitable for 4.5 billion years?
Mike Miller, Materials Engineer
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"Everbody's always in favor of saving Hitler's brain, but when you put it in the body of a great white shark, oh, suddenly you've gone too far." -- Professor Farnsworth, Futurama
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#2
I think that in the EWoCS system early Venus (and Early Mars) are both considered to be Gaian worlds. I think both worlds would be significantly different from each other, and from the Early Earth; but they would probably both have some quantity of liquid water. There is no guarantee that Venus ever held lifeforms, but it would be very interesting if it did. There might even be some relict lifeforms there now, but I am skeptical about that.
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#3
Here's an atmosphere diagram from Forbes.com that shows how Earth's atmosphere has changed over time; Venus's atmosphere may have been similar, especially for the first couple of billion years. But too much greenhouse gas would accelerate the change to a Cytherian-type atmosphere, so either the water disappeared long before 700 mya, or the greenhouse gas content of Venus became very low very early on.
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#4
(05-26-2020, 07:08 PM)stevebowers Wrote: Here's an atmosphere diagram from Forbes.com that shows how Earth's atmosphere has changed over time; Venus's atmosphere may have been similar, especially for the first couple of billion years. But too much greenhouse gas would accelerate the change to a Cytherian-type atmosphere, so either the water disappeared long before 700 mya, or the greenhouse gas content of Venus became very low very early on.

So to get a Venus that's human habitable c2000AD, it'd need to be something like a Campian Gaian world (low in surface water, high in albedo), perhaps specifically like Cenote where most carbon is locked up as carbonates?
Mike Miller, Materials Engineer
----------------------

"Everbody's always in favor of saving Hitler's brain, but when you put it in the body of a great white shark, oh, suddenly you've gone too far." -- Professor Farnsworth, Futurama
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#5
IIRC, the current thought is that a major cause of the Venus hothouse is that it rotates extremely slowly. This leads to just about no magnetic field, and in turn that leads to the lighter components of the atmosphere being stripped away by the solar wind. This apparently happened because solar UV split water in the upper atmosphere and the hydrogen was blown away. Which in turn leads to very low water levels. Early life lived in the sea, a bit difficult if there isn't much sea.

So would a Venus rotating at something close to the rate Earth does remain habitable to date? Has any simulation work been done?

Incidentally, no water leads to no plate tectonics.
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#6
(05-27-2020, 03:47 PM)iancampbell Wrote: IIRC, the current thought is that a major cause of the Venus hothouse is that it rotates extremely slowly. This leads to just about no magnetic field, and in turn that leads to the lighter components of the atmosphere being stripped away by the solar wind. This apparently happened because solar UV split water in the upper atmosphere and the hydrogen was blown away. Which in turn leads to very low water levels. Early life lived in the sea, a bit difficult if there isn't much sea.

So would a Venus rotating at something close to the rate Earth does remain habitable to date? Has any simulation work been done?

Incidentally, no water leads to no plate tectonics.

It's debatable. A friend of mine had seen like five competing theories on why Venus lacks a magnetosphere:
1) The Venus core is completely solid
2) The Venus core is completely liquid
3) A collision changed Earth's core's composition, allowing it to persist longer than Venus'
4) Venus spins too slow (yours)

They couldn't remember the fifth, so I'll add what I've seen to complete it:
5) Venus' lack of plate tectonics lead to low temperature gradient, resulting in no convection in the Venus mantle

I believe some studies show that magnetic fields are not a major factor anyway. What prevents water from escaping to the upper atmosphere in the first place and be split by radiation is a cold trap.

On slow rotation, some say a Venus rotating close to Earth's rate would not be habitable due to the lack of cloud formation on the dayside to reflect sunlight, which is twice as strong. A slow-rotating Venus would still have the cloud formation which is helpful.
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#7
(05-27-2020, 06:58 PM)The Astronomer Wrote: I believe some studies show that magnetic fields are not a major factor anyway. What prevents water from escaping to the upper atmosphere in the first place and be split by radiation is a cold trap.

A low inclination planet creates relatively large arctic regions because the poles never get tilted toward the sun for half the year. They get perpetual twilight and night time. Large ice caps would be a self-reinforcing cold trap: high albedo lowers temperatures and accumulates more ice.

Quote:On slow rotation, some say a Venus rotating close to Earth's rate would not be habitable due to the lack of cloud formation on the dayside to reflect sunlight, which is twice as strong. A slow-rotating Venus would still have the cloud formation which is helpful.

I have to wonder about that. At Earth-like levels of illumination and 24-hour rotation, there's plenty of time to develop large clouds. It's a daily cycle in Florida during the summer: clouds boil up from water throughout the day, then rain out in the evening. With twice the sunlight, you should be able to get more water into the air in less time. However, I'm not a meteorologist.

So, some factors toward a 0AT habitable Venus:
1. Low inclination for proportionally large ice caps (vs. any other planet at 0.7AU from a G2V star)
2. 50% or less water coverage, which is enough (?) for plate tectonics but would increase albedo, land generally having a higher albedo than water
3. Thinner atmosphere, perhaps 0.5 to 0.7-bar at sea level

Here's a fun page: https://www.astro.indiana.edu/ala/PlanetTemp/index.html

Using values of a bond albedo of 40 (vs 29 for Earth), average distance of 0.723AU, and 0.5 greenhouse effect (vs 1 for Earth), the result is rather warm: 38C average versus Earth's average of 15. Do those input values look sensible?

What would a 38C planet be like?
Mike Miller, Materials Engineer
----------------------

"Everbody's always in favor of saving Hitler's brain, but when you put it in the body of a great white shark, oh, suddenly you've gone too far." -- Professor Farnsworth, Futurama
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#8
(05-27-2020, 10:16 PM)Cray Wrote:
Quote:On slow rotation, some say a Venus rotating close to Earth's rate would not be habitable due to the lack of cloud formation on the dayside to reflect sunlight, which is twice as strong. A slow-rotating Venus would still have the cloud formation which is helpful.

I have to wonder about that. At Earth-like levels of illumination and 24-hour rotation, there's plenty of time to develop large clouds. It's a daily cycle in Florida during the summer: clouds boil up from water throughout the day, then rain out in the evening. With twice the sunlight, you should be able to get more water into the air in less time. However, I'm not a meteorologist.

Oh, yeah I worded it wrong. The idea is, the winds on fast-rotating worlds would blow clouds away, leaving less cloud coverage than worlds with slow rotation, where milder winds allow more clouds to pile up. Although I do imagine that at some point the slow rotation is going to heat the subsurface point sufficiently to cause winds similar to very slow-rotating tidally-locked worlds. I have no clue on this one.
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#9
(05-27-2020, 10:16 PM)Cray Wrote: Using values of a bond albedo of 40 (vs 29 for Earth), average distance of 0.723AU, and 0.5 greenhouse effect (vs 1 for Earth), the result is rather warm: 38C average versus Earth's average of 15. Do those input values look sensible?

What would a 38C planet be like?

I'm fairly sure the Earth has been at least ten degrees warmer in the past, especially during the Mesozoic era. With no ice at the poles and an inhospitable desert at the centre of Pangaea, our planet would have been very different then.
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