Why is Mars cold?

  • The surface temperatures of Mars are about -87C to 5C, which is much colder than that of Earth's.
    If Mars has 95% carbon dioxide, which is a Greenhouse gas, why is the surface of Mars so cold? Shouldn't it trap heat and render it hot?


    You should be careful drawing conclusions from percentages. Mars's atmosphere is a lot sparser than Earth's.

    Venus only has 1.1% more CO2, why's it so hot? (spoiler: it ain't because its closer) "Average surface pressure‎: ‎93 bar" ... that's 93 *times* that of Earth. - Density, not percentage.

    @Mazura: Well, it's _partly_ because it's closer; if Venus and Earth had exactly the same atmosphere, Venus would still be warmer (although not by nearly as much) due to its closer proximity to the sun (and it's been postulated that this might actually be the reason Venus got tipped into a runaway greenhouse effect, while Earth hasn't [yet]).

    Sounds like a setup for a Chuck Norris joke. "Chuck Norris has already been to Mars, that’s why there are no signs of life." "Chuck Norris roundhouse kicked Mars and knocked the atmospere off."

    @CrossRoads: Pretty much. The Sun is playing Chuck Norris, doing repeated roundhouse kicks, with Mars undefended due to low gravity and no magnetic field.

    So, we're calling orbital revolutions a roundhouse kick now? Fine, why not.

    I can't help thinking about this story bots clip

  • PM 2Ring

    PM 2Ring Correct answer

    2 years ago

    Firstly, Mars has a mean distance from the Sun of 1.524 AU, so by the inverse square law the energy it gets from the Sun is about 40% of what the Earth gets.



    But the main reason that Mars is so cold is that its atmosphere is very thin compared to Earth's (as well as very dry, see below). From Wikipedia Atmosphere of Mars:




    The atmosphere of Mars is much thinner than Earth's. The surface pressure is only about 610 pascals (0.088 psi) which is less than 1% of the Earth's value.




    In comparison, the mean surface pressure on Earth is 101,300 pascals. So the atmosphere of Mars is barely more than a vacuum compared to Earth's.



    So even though the Martian atmosphere is over 95% carbon dioxide, there simply isn't enough of it to trap much heat.



    Although carbon dioxide is a greenhouse gas, the predominant greenhouse gas on Earth is actually water vapour. However, water is usually cycled in and out of the atmosphere very quickly in response to temperature and pressure changes. Carbon dioxide is a problem because it stays in the atmosphere for a long time, and its presence shifts the equilibrium temperature upwards from that of the plain water cycle.


    But Earth's atmosphere only has 0.04% CO$_2$, so that's an order of magnitude less than Mars, in absolute values.

    @pela True, and that's why I added that info about water. But I suppose I shouldn't make that a "BTW" section.

    Ah okay, I only saw that now.

    Good thing I'm not an observer :D

    I made a small edit, I missed the water part the first time through as well, since you led with absolute pressure, then went on to CO2.

    @uhoh Good idea.

    @pela: So what would the temperature of Mars be if it had no atmosphere, or if the atmosphere was composed entirely of non-greenhouse gasses like nitrogen or argon?

    @jamesqf Probably almost exactly the same. The radiative forcing from carbon dioxide is _tiny_, and quickly drops off (e.g. the difference between 100 ppm and 200 ppm is much larger than between 1000 ppm and 1100 ppm). Water vapour and methane are much more important, but on a planet like Mars, water vapour would be "quickly" lost. On Earth, more carbon dioxide also lets the air to hold more water vapour, which also lets more carbon dioxide into the atmosphere, which is a bit of a positive feedback loop. But geologically speaking, carbon dioxide on its own essentially doesn't matter.

    @jamesqf It would be roughly 5 K colder, which is the difference between Mars' effective temperature and its average surface temperature. For Earth, that difference is 33 K (see my answer below for references).

    Thank you very much for the answer!

    While we are talking percentages, Venus has as much or maybe more nitrogen than Earth. It is just a low percentage compared to the CO2 abundance.

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Content dated before 7/24/2021 11:53 AM