Temperature difference between Venus and Mercury

[Agent Smith]

TL;DR

Venus is hotter than Mercury

Why Venus is hotter than Mercury
In the video linked to above, the author explains why Venus, though it's farther away from the sun, is hotter than Mercury. It squares with what I read thousands of years ago. I hope the explanation is correct.

If the video is accurate then I have a question. Is there a similar anomalous temperature-pair in the outer planets? For example, wholly hypothetical, is it (possible) that Uranus is colder than Neptune?

Muchas gracias.

 

[Vanadium 50]

The mean temperature of Neptune is 5C colder than Uranus. However, temperatures vary with time and position, so it is possible that the coldest spot on Uranus is colder than the warmest spot on Neptune.

 

[DaveC426913]

Venus has a thick gas atmo. Mercury has no atmo.

I did not see the video; is it safe to say that - after distance - presence of an atmo is the primary driver for temps?

Both Uranus and Neptune are gas giants, similar in size and composition. They will have temps completely dominated by distance/insolation.

I'm not sure how there's any useful comparison.

 

[Vanadium 50]

To add to my message, one might ask "why are Uranus and Neptune so close in temperature?", it's not all that close: say 50K and 55K. It only looks close if you are in celsius. Just from radiative considerations, you'd expect ~9K difference; we see ~5K.

 

[Agent Smith]

@DaveC426913 . That's correct; as per the video greenhouse effect on Venus (atmosphere is nearly all $CO_2$.

To add to my message, one might ask "why are Uranus and Neptune so close in temperature?", it's not all that close: say 50K and 55K. It only looks close if you are in celsius. Just from radiative considerations, you'd expect ~9K difference; we see ~5K.

Cogito ... this is a good question. Apparently, the temperature difference is too small to be explained by their relative distances from the sun.

 

[Ken G]

On the original video, it's always hard to say how deeply into an explanation does one need to go. This video contents itself with comparing atmospheres to clothing, and calls that "the greenhouse effect." So OK, the greenhouse effect can indeed be thought of as a slowing of the escape of heat, which is also what clothes do. So one must decide if that analogy is enough, or if a little more detail is required. The video makers decided not to include the importance of the fact that the heat for the Earth comes from the Sun originally, unlike the heat escaping from our body that we generate metabolically. Again, one always has to decide what's important.

But if you really want to know what the greenhouse effect actually is, it is more than just having an atmosphere. It is having an atmosphere that contains "greenhouse gases" (i.e., not nitrogen or oxygen), which are gases that are transparent to the visible light that brings the heat in, but opaque to the infrared light that lets the heat out-- something clothing does not do. One reason this distinction is important is it explains why carbon dioxide is important to the greenhouse effect on Earth and to global warming, even though it is a trace component of the nitrogen+oxygen atmosphere.

Diving even deeper, the temperature of a planet also depends on something called the "scattering albedo", which has to do with how much visible light is reflected away from the planet before it is ever absorbed as heat. A high scattering albedo will reduce the temperature of a planet, even before you consider any greenhouse effects. I suspect differences in scattering albedo is what explains the temperature difference of Neptune and Uranus, though it is probably explained somewhere. (I did a quick search, and the first hit https://www.hull.ac.uk/work-with-us...hat-is-the-coldest-planet-in-the-solar-system points to the possibility that whatever kicked Uranus over on its side might have also caused the interior to cool, and a larger part of the surface temperature of the outer planets is due to escaping internal heat.) I actually think it would be fair to say the relative temperatures of Uranus and Neptune are not completely understood at present. (We don't even know why their color looks so different.)

 

[Agent Smith]

@Ken G , asante sana. But if you took a thermometer to outer space and measured the ambient temperature, it would record a decrease in temperature as it moves away from the sun and an increase as it moves towards the sun, si? The only source of heat being solar radiation and the absence of other modulating factors would imply that to be the case. I believe.

So, minus all known temperature-determining factors, is the earth supposed to be hotter/colder? My money's on colder (there's significant amounts of $CO_2$ in the earth's atmosphere).

 

[Vanadium 50]

Is the temperature of the Earth's core the same as the moon's? After all, they are the same distance from the sun.

If not, why not?

Now apply this reasoning to other planets.

 

[Ken G]

So, minus all known temperature-determining factors, is the earth supposed to be hotter/colder? My money's on colder (there's significant amounts of $CO_2$ in the earth's atmosphere)

Yes, the greenhouse effect warms the Earth considerably, otherwise we'd have frozen oceans. Water vapor dominates it, but CO2 is the thing that's rising. So the current global warming we are seeing is a case of too much of a good thing.

 

[DaveC426913]

Yes, the greenhouse effect warms the Earth considerably, .

Earth's core is very active. Moon's core is dead.

 

[Vanadium 50]

That's true but not relevant, and it confuses effect with cause.

 

[DaveC426913]

That's true but not relevant, and it confuses effect with cause.

Which? Ken's GE comment, or my active core comment?

 

[Vanadium 50]

Yours.

 

[Ken G]

Earth's core is very active. Moon's core is dead.

Heating of the surface due to the Earth's interior is a weak effect. For example, the total energy balance depicted at http://www.climate.be/textbook/chapter2_node8.xml doesn't ever bother to include it at all! (But some of the reasons given for the Uranus/Neptune temperature ratio suggest that for the outer planets, where solar insolation is obviously way lower, the interior heat flux is more important. It is often said that Saturn is still contracting, for example, so that's an additional factor to consider for gaseous planets.)

 

[Agent Smith]

Is the temperature of the Earth's core the same as the moon's? After all, they are the same distance from the sun.

If not, why not?

Now apply this reasoning to other planets.

Yes, the greenhouse effect warms the Earth considerably, otherwise we'd have frozen oceans. Water vapor dominates it, but CO2 is the thing that's rising. So the current global warming we are seeing is a case of too much of a good thing.

Replying to both.

With minimal to 0 knowledge of planetary science, it's difficult if not impossible for me to answer this question at even an elementary level. It's just odd that the sun doesn't completely decide which planet has life and which doesn't. The Sun has always been worshipped as the life-giver; turns out it's $CO_2$ that keeps the earth warm at this distance.

But the sun on the skin during summer feels warmer than 30 degree Celsius. I'm missing a lot of variables aren't I?

 

[Ken G]

Yup. Even weather forecasters have a "feels like" variable they try to report, there is lot more going on that just average temperature, but the factors that control average temperature are starting to become better understood. There's always a ways to go, and a way to go, yet.