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Earth's atmosphere is 0.038% carbon dioxide. Mars's atmosphere is 95.3% carbon dioxide. Venus's atmosphere is 96.5% carbon dioxide.

If Earth's climate is controlled by CO2, then why is Mars so cold in comparison to Venus? Mars is very cold (average about –60°C) and Venus is very hot (+460°C).

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    $\begingroup$ climate isn't controlled by CO2... it is just sensitive to it... just as it is sensitive to many other factors. $\endgroup$ – farrenthorpe Mar 8 '17 at 18:47
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Firstly, Mars is farther away from the sun than Venus or Earth, so it gets less heat from the sun. Secondly, Venus & Earth are volcanically active, whereas Mars is volcanically inert. Thirdly, the atmosphere on Mars is much thinner than those on Venus and Earth.

The density of the atmosphere on Venus is approximately $65\ \mathrm{kg/m}^3$, whereas, the density of Earth's atmosphere is $1.217\ \mathrm{kg/m}^3$ and the density of Mars's atmosphere is $0.02\ \mathrm{kg/m}^3$. Earth's atmosphere is 60 times denser than that of Mars; Venus's atmosphere is over 3000 times denser.

With such a thin atmosphere on Mars, there is little atmospheric mass to retain heat, despite the atmosphere being composed of 95.3 percent carbon dioxide.

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  • $\begingroup$ So to rephrase a take home point: percentage is often a poor metric, because 98% of a sliver is still a sliver, and it's not enough to have a big impact. $\endgroup$ – JeopardyTempest Mar 8 '17 at 8:01
  • $\begingroup$ Coincidentally, just in case you happen to know... any idea which of your three factors dominate the others Fred? If Mars had 1 of the 3, which would warm it the most? Its always nice to get a scale idea when possible on lists of synoptic factors. $\endgroup$ – JeopardyTempest Mar 8 '17 at 8:05
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    $\begingroup$ iven if water is not mentioned in the question it do play a big role in combination whith co2 on the earths atmosphere .water on mars is too cold to stay liquid and end up in the atmosphere and venus is too hot for liquid water to form. $\endgroup$ – trond hansen Mar 8 '17 at 15:20
  • $\begingroup$ I think you should mention that Venus is not confirmed to be active it is believed but it has not been confirmed. $\endgroup$ – Lucian09474 Mar 9 '17 at 10:25
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    $\begingroup$ Just to elaborate the first point, the mean distances are 0.723 AU, 1 AU and 1.524 AU for Venus, Earth, and Mars, respectively. Radiation is proportional to the square of the distance, so the radiation received by Venus is 1.524^2 / 0.723^2 ~= 4.44 times as much as the radiation received by Mars. $\endgroup$ – Victor Engel Jul 26 '18 at 16:03
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In addition to what has been said by @Fred, I'd like to add that CO2 doesn't actually generate heat, it is just very good at trapping heat. Mars doesn't get as much heat as Earth, and it also doesn't have enough CO2 to effectively trap heat that has been received, as a result, it's much colder than Earth.

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    $\begingroup$ And Venus gets lots more heat from the sun, and has lots more CO2 to trap that heat. Which is why the surface is hot enough to melt lead, instead of the steamy tropical jungle imagined by 1930s SF writers - which it probably would be if it had an Earthlike atmosphere (and water, of course). $\endgroup$ – jamesqf Mar 8 '17 at 19:29
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    $\begingroup$ @jamesqf I was going to post an answer to that effect, but when I did the math, it turns out that isn't exactly true. Venus' albedo is very high (0.7) due to its clouds, while Earth (0.3) and Mars (0.16) are much lower. Therefore, in the climate model sense, it receives less heat than Earth and only a bit more than mars. The effective temperature of Venus is 232K, while Earth is 254K and Mars is 207K. $\endgroup$ – kingledion Mar 9 '17 at 17:35
  • $\begingroup$ @kingledion: But effective temperature and surface temperature are two quite diffenent things, no? Perhaps I'll have a better answer after I work my way through "Principles of Planetary Climate", but this thread is likely to be a bit dated by then :-) $\endgroup$ – jamesqf Mar 9 '17 at 23:45
  • $\begingroup$ Effective temperature is not very useful. It bears no relation to the thing that is really of interest and that is the temperature of the very tiny thin region centered about the surface where the biological entities may live, fly or swim. Surface temperature is "everything" pretty well, surface is where you land the probes, explore, build your colony etc. $\endgroup$ – user7733 Apr 14 '17 at 19:34
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Hasn't anything to do with CO2. It's the atmospheric pressure. Venus receives very little sunlight but still very hot due to extreme pressure. Jupiter is extremely hot due the intense atmospheric pressure despite being much further from the sun. CO2 has very little effect trapping heat beyond 50ppm. The earth was in an ice age with 10 to 20 times the current CO2 levels

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    $\begingroup$ This is quite trivially incorrect - you would infer from that argument that if the sun was removed from the system, the planets would not freeze. For instance, Titan has an atmospheric surface pressure 1.5 times that of Earth, but is cold enough for liquid methane lakes to form. Jupiter is hot because it is still cooling down from it's formation. $\endgroup$ – Andrew Jon Dodds Jan 10 '18 at 14:30
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The important difference between Mars and Venus is obviously not CO2 or water, or distance from the Sun for that matter, but the atmospheric pressure. You can easily determine the surface temperature if you know a few parameters like the molecular composition/weight and the pressure using the Boltzmann constant. Surface temperature is on no planet determined by only greenhouse gasses.

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