# Why is Mars so cold when it has so much atmospheric $CO_2$?

Mars has 1/10 of Earth's atmosphere but that atmosphere is 95% CO2. Why is Mars so cold when it has so much atmospheric CO2?

• What is the solar constant on Mars? Compared to Earth? – my2cts Oct 30 '18 at 21:02
• That was a false statement: the pressure on Mars is just 6 mbar. About 0.6% of Earth's mean sea level pressure. – Pieter Oct 30 '18 at 22:45
• @mycts: it's about $590\,\mathrm{Wm^{-2}}$: for Earth it's about $1360\,\mathrm{Wm^{-2}}$. – tfb Oct 30 '18 at 22:52
• And, water vapor is also a greenhouse gas, and a good one. There is practically no water vapor in the Martian atmosphere. – David White Oct 31 '18 at 1:47
• So it is a change in water vapour that matters? On Earth we are being cocooned in a layer of increased water vapour resulting from infra red? How does one test for water vapour changes? – CO2 Oct 31 '18 at 11:35

## 3 Answers

There is indeed a greenhouse effect on Mars. I have not been able to find any direct citations but I believe it is several degrees. To understand in detail why it is relatively small (despite there being more CO2 in the atmosphere of Mars in absolute terms than there is on Earth) really requires running a numerical model (these are the references I was looking for but did not find), but it's fairly easy to see some reasons why the effect is relatively small.

Firstly there is almost no water vapour in the atmosphere of Mars, and water vapour is critical to greenhouse effects. In particular on water vapour serves to amplify greenhouse effects on Earth: if temperatures rise then the amount of water vapour in the atmosphere rises (as there's a huge oversupply of water in the oceans!) and this water vapour then works to further raise temperautes. This NASA page describes the effect pretty well.

The second effect is $$\sigma T^4$$: black-body radiation goes as the fourth power of temperature. If you assume a surface temperature of Mars of $$218\,\mathrm{K}$$ and of Earth of $$287\,\mathrm{K}$$ this means that there's about 3 times more thermal radiation from the surface of Earth than from Mars. That means that greenhouse effects, which depend on absorbing & reradiating this upward thermal radiation from the surface, are far less significant.

Slightly peripherally but importantly, the absolute value of greenhouse effects is of somewhat secondary importance. If Earth's average surface temperature was 2 degrees higher than it is, or 2 degrees lower, then it would be perfectly habitable. It probably would be somewhat less habitable if it was 10 degrees higher or lower, but 2 is fine. The problem people worry about is not the absolute effect, it's the rate of change with time. As an example consider recent ice-ages: these involve temperature swings of a little over 10 degrees. Deglaciations happen much faster than glaciations, and the most recent one took about 10000 years. So that's about a degree every 1000 years. (Glaciations take about 100000 years, so a degree of every 10000.) The current rate of warming is something like ten times faster than that.

• So 1) Mars emits less radiation because of the temperature 2) It diminishes the greenhouse effect. But if we ask why its $T$ is less than Earth's $T$, would it be because of the surface mean calorific capacity? – user153036 Oct 31 '18 at 0:17
• @santimirandarp: there's a lot less incoming radiation: the solar constant at the radius of Mars's orbit is about a third of that at Earth's. – tfb Oct 31 '18 at 0:41

The atmosphere is really tenuous. Don't underestimate just how tenuous it is - I think someone above said 0.6% of Earth's atmosphere. So even though the proportion of carbon-dioxide is very much higher than on Earth, there is still not very much of it!

It's so thin, the atmosphere of Mars, that I'm actually quite amazed when I see pictures of duststorms there.

The other answers clarify that the density of the atmosphere and the amount of heat from the sun are very much less than on earth. I would just like to make clear the role of CO2 in the greenhouse gases spectrum.

The main, by an order of magnitude, green house gas is water vapour:

Atmospheric absorption and scattering at different wavelengths of electromagnetic waves. The largest absorption band of carbon dioxide is not far from the maximum in the thermal emission from ground, and it partly closes the window of transparency of water; hence its major effect

The "hence its major effect" needs clarification. The models giving a leading role to CO2 even if it is in percentage something like 5% of the effect of water vapor in the energy balance of radiations, use it as a trigger for increasing the H2O content in the atmosphere, a feedback loop. If there were no water vapour in the atmosphere there would be very little green house effect. The argument goes "increase of CO2 and other gases like methane, increase water vapour in a multiplicative way, called climate sensitivity.

As Mars has very little water vapor the greenhouse engine as modeled for earth cannot work. It is just another gas trapping a bit of the heat in the range shown in the diagram above.

• And Venus? Very hot but no water. very close to sun. Does Venus have a cold side? Like lunar ice caps? – CO2 Oct 31 '18 at 9:31
• One would have to do the specific problem, energy input, radiated etc, it is not my specialty. Certainly no greenhouse feedback engine like the one hypothesized for the earth. Greenhouse effect in a generic form, because the atmpsphere traps heat will certainly exist. – anna v Oct 31 '18 at 10:05
• This is an interesting observation gizmodo.com/5948506/… – anna v Oct 31 '18 at 10:08
• And lunar poles - 220c? Because they never get sun. – CO2 Oct 31 '18 at 10:16