Hi all. Long-time lurker/guest, long-time amateur astronomer, first-time BA poster.

Like the subject says, what would Venus's surface temperature run if it had an Earth-like atmosphere? I've googled this and searched the BA archives before posting the question, so please forgive me if this has an easily accessible answer.

Hi davidhw, welcome to the forum! I am moving this to the Q&A section, as you are more likely to get a responce.

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Sorry, Duane. Didn't mean to cause extra work.

It should not be too difficult to answer, remember our Earth also has a greenhouse effect sometimes for better ( keeps the ice age away ) sometimes for worse ( can cause tropical stroms, heatwaves, flooding )
If the planet had no atmosphere you'd be looking at solar flux density for a body and simple albedo or surface reflection and you'd find Merucry was very dark and absorbs a high percentage of radiation, but Mercury is cooler than Venus which has thick Venusian air surrounding the planet.

So for Earth you would equate the power given off by our Earth with the Solar power absorbed, you'll find our planet should have a sub zero-figure but thanks to some of the greenshouse effects the temperature of our Earth's surface is higher.
but apart from the Sun you must also consider internal heat sources such as volcanic heat from the planets interior

A very complex question. Venus receives about twice as much solar energy as the Earth, so would be twice as hot if neither planet had an atmosphere; but the effect of the atmosphere is very great. If Venus had an atmosphere like Earth's and extensive seas, the water vapour content would be much higher.

It is possible that Venus would become shrouded in white reflective water clouds, which could reflect away much of the Sun's energy. In fact if these water clouds reflected as much energy as the current sulphuric acid clouds reflect today, the surface of Venus would be cooler than the surface of Earth (around 270 K as opposed to 288 K). But that would involve a continuous, unbroken cloud cover from pole to pole.

The excess water vapour in the atmosphere would add a lot of heat due to increased greenhouse effect, so Venus would be very sultry indeed; but I don't know quite how to factor that in.

This is much too simplistic a figure, however- the very slow rotation of Venus would make the weather of Venus entirely different to Earth's, with days and nights which last four months each. The daytime temperature could become very, very hot, and the night time temperature very cold. To compensate for this, the atmosphere would probably superrotate like it does today, so the planet would distribute heat more evenly, but the continuous linear hurricane around the equator would not make conditions very comfortable.

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Perhaps it would help to simplify my question and just ask what Earth's temperature would be if it were Venus's distance from the sun. Would it roughly be:

1/(Venus distance from sun/Earth distance from sun)^2

or

1/(108.2 million kms/149.6 million kms)^2

=

1.911 x whatever the Earth's average temperature is (say, 56 F)

or

107 F?

So the question is. What would the environment of Venus be like if it had a Earth like atmosphere?
Given that its closer to the sun and has less gravity because it has less mass. Its rotational speed. Distance from the sun might sagest the uncomfortably hot end of the scale of survivability. More than likely its going to be to hot for us without considerable shielding. Under ground we might be able to survive. Earths Van-Allan belts protect us from much of the suns deadliest radiation. Venus does not seem to have the level of active magnetic shielding. This is a problem for us. Venus has a very hostile environment. Mars might be a better proposition for us.

Wow. Now there's something I never thought about -- Venus having had the chance at being cooler than the Earth.

Makes the notion of a "habitable zone" much more elastic than I realized before, i.e., in our searches for Earth-like planets around other stars, simple distance from the star will not tell the entire story.

Temperature varies with the fourth root of solar energy flux, so it varies with the inverse square root of the planet's distance from the sun. So for Venus' orbital distance, that's a factor of 1.17 in absolute temperature. (You can't do the sums on anything but the absolute temperature scale.)
Earth has a black-body temperature (neglecting greenhouse effect, allowing for its albedo) of 250K, so that puts Venus at ~290K. Naively add ~30K for an Earth-level greenhouse: ~320K.

Grant Hutchison

You can't do this kind of math on Fahrenheit temperatures. You need to convert to Kelvin first.

for the science. I left math and physics behind in high school -- looking forward to remembering what I can around here with the help of others. :-)

Astromark the lack of magnetic field doesn't make Venus more hositile than Mars, the Red-planet almost has no magnetic field aswell
it is other inhospitable factors that make us call Venus a hellish planet.

Davidhw, if you take the rough estimate of grant hutchison it gives you an idea of how hot Venus would be
Record hot temperatures have been recorded in America during heatwaves - NYC once had temps of 108F, 42 Celsius or 315K

So only some of the very, very hottest places on Earth would be close to grant hutchison's Venus-Earth-Atmo estimate of 320K

Very hot, yes, but isn't 320 K about 116 F? If water were present, then life could become very abundant even at such a high temperature.

Venus black body temperature can be derived from the Stefan Boltzman law to be written as:

Te=(S*(1-A)/4*sigma)^1/4

with S solar flux, A albedo and sigma the Stefan Boltman constant

For Earth we substititute A = 0,3 and S = 1367 w/m2 This gives us a black body temperature of 255 K. With an average temp of 288K (15C) we infer that there is 33C greenhouse effect. Which is an extreme simplification disdaining all kind of heat tranfer processes but anyway.

For Venus with S = 2660 w/m2 and A = 0.72 we would get 239K. That's chilly indeed but what if Venus looked like Earth with identical albedo of 0.3, then we would get an blackbody temp of 301K (28C) to which Greenhouse effect (33C?) still would be added., But then we would have a lot of clouds increasing the albedo again to say 0.5. Then the BB temp would be 276K (3C) with the greenhouse adding of some 33C would likely bring us in the goldilocks zone.

My two cents: 30-40C

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Wow. This is really fascinating. All else being equal, Venus would not be nearly as hot as I thought with an Earth-like atmosphere.

I suppose my next question would be: why is Mercury so hot? As I understand it, Mercury's sun-facing side runs around 700K. But it's only twice as close to the sun as Venus. Or to put it another way, if Venus's blackbody temp is similar to Earth's and Mercury has no atmosphere, why does Mercury jump in temp so markedly? Does it have something to do with the "fourthing" Grant mentioned?

The fourth-power law actually reduces the responsiveness of planetary temperature to distance from the sun: it's an inverse square-root response, instead of the inverse-square we're used to for other parameters. Reduce the distance to a quarter and the energy flux will increase 16-fold, but the absolute temperature will merely double.

For Mercury, a couple of things are going on:
It has a low albedo (0.06), and so absorbs almost all the solar energy that arrives at its surface, in contrast to Earth and Venus.
The calculations that Andre and I did are mean temperatures for the whole planet. They assume a spherical object that is absorbing energy evenly all over its sunwards side, and radiating energy evenly over its whole surface. Earth approximates this because it rotates reasonably fast, Venus approximates it because its massive atmosphere transports heat efficiently from dayside to nightside. Mercury does neither, and so has a big difference in temperature between dayside and nightside.

Using the same formulae as previously, Mercury's mean temperature comes out to be around 440K. If we average a dayside max of 700K and a nightside min of 100K, that seems to be reasonably near the mark.

Grant Hutchison

Ah, that's the variable I wasn't factoring in: albedo.

So theoretically, if an Earth-like planet were in orbit around a G2-class star like our sun at Mercury's distance, perhaps with an even higher albedo than Earth, temperatures could remain such that liquid water could exist on the surface, thereby allowing for the development of life as we know it?

I guess I'm asking these questions in order to get a better grasp on the range of habitable zones in a solar system like ours. I seem to recall seeing a figure of 0.9 - 1.6 AU; however, it seems it should extend closer, given what I've learned here about Venus.

No, I don't think so.

Let's move earth up to the orbit of Venus. First, water vapor is a very efficient greenhouse gas, and the amount of water vapor would quickly increase, so the temperature would go up. Also, there is a great deal of carbon stored away that would be released, so the CO2 levels would increase quickly, eventually reaching levels similar to that of Venus. Before long, the oceans would be boiling away.

In the longer run, the large amounts of water vapor reaching high altitude would be split by UV, the hydrogen would escape, and there would be less and less water.

The point is that an earthlike atmosphere and environment, given the various constituents of the planet just wouldn't be stable.

Now, you might imagine the situation if you could remove most of the carbon and water. If there was just a little water, most of it very saline, which would increase the boiling point, you might have a somewhat more stable situation, but over geological periods it is still unlikely to be stable.

At Mercury's orbit it would be much worse.

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Ah, that makes sense. Thank you. This explains the habitable zones erring on the side of farther, not nearer, distance.

This is true. Although a hot Earth would have more water vapour in its atmosphere, and therefore more clouds and a higher albedo; it would also suffer from the greenhouse effect of the additional water vapour. Perhaps Andre's estimate of 40C would need to be revised upward to about 50C, more or less the same as Grant's estimate.

Then we think about the loss of water by the effects of UV as described by Van Rijn, and see that the situation would not be stable.
There is a school of thought that Venus had water and a slightly more Earth-like atmosphere for several billion years; it all depends on how rapid the hydrogen loss was in the early history of the planet. According to Bruce Fegley in this PDF the rate of hydrogen loss nowadays is far too slow to account for the loss of all Venus's oceans; billions of years ago the UV was apparently more intense, and hydrogen loss was much faster (even though the Sun was somewhat less luminous at that time). If Venus had an ocean today it would probably keep it for quite a long time.

A slightly cooler K-class star would (I believe) emit less UV even when the star is younger, so a planet in the same temperature range as Venus should retain its water longer. But the habitable zones of K class stars are already quite close to the star, and quite narrow; so we are not gaining much there.

How about increasing the gravity of our planet?
A large terrestrial in a Venus-like orbit with a gravity somewhat greater than the Earth's would be able to hang on to its water for quite a bit longer. the surface temperature would still be similar to Death Valley, but the planet would be wet for a longer period, and there could be the possibility of enough of a biosphere to maintain Earthlike oxygen levels.

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Yes, all this stuff you're hearing from Van Rijn and eburacum is why I described my greenhouse jigger factor of ~30K as "naive". You were asking us just to transport Earth to Venus' orbit, so I left the greenhouse contribution of Earth's atmosphere unaltered.
In fact, neither the albedo nor the greenhouse effect can stay unaltered in a water world if you heat it up: water contributes strongly to the albedo as both ice and cloud, and to the greenhouse as vapour. It also serves to dissolve carbon dioxide and sequester it as carbonate rocks: if you lose water as the others have described, you'll begin to see CO2 building up in the atmosphere, increasing the greenhouse.

Grant Hutchison

Funny, I heard almost the same question from a cowerker at my workplace some days ago.

I would like to add something here along the line of Van Rijn's last post: It is plausible to assume that Earth and Venus actually started out with very similar atmospheres - and look what has become oft them! What seems safe to say is that neither the Earth's nor Venus's current atmospheres are what they started out with.

So the question, what happens to Earth if you move it to Venus's orbit is not only purely theoretical but also a bit misleading. If Earth were in Venus's orbit, it couldn't ever have ascquired the atmosphere it has now.

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And here's another effect to chew on. The Sun should be quite a bit brighter now than it was, say, 3 billion years ago. I don't recall a more exact figure, but this would move in the sense of saying that Venus 3 billion years ago might have been in a more similar situation as Earth is now. That ignores a lot of other issues-- would such a young Venus have time to build up the nitrogen Earth has, perhaps without plate tectonics to boot? Would the internal heat of the cooling planet still make it too hot for liquid water? Most likely, Venus 3 billion years ago was not like Earth is now, but it might not have been like Venus is now either!