# What if Mars were the same size as Earth with an atmospheric pressure of 1 atm?

With the abundance of carbon dioxide, and yet being 1.52 times farther from the sun, would it make the Martian temperature cold or hot?

I am assuming that the composition of the atmosphere remains the same, but at a much greater density.

• Just to clarify, you're talking about a planet in Mars orbit the same size and mass as Earth with 1 atm of pressure at sea level, but the same atmospheric composition as the real-world Mars? Commented Jul 7 at 22:31
• It would be Earth-like. Note, the mass of the planet is not important directly, the greenhouse effect depends on the height and density of the CO2 layer. Thus, it could work also with the current Mars, although probably you would have a hard time to extract the required amount of CO2 from its soil. It is there, but you would likely need to process maybe some ten m height upper layer of the whole planet. Commented Jul 7 at 23:41
• Note, CO2 is a poison for us even in much smaller concentration, even if we have enough oxygen beside it. Our exhaled breath has 4% CO2 and it is already too much for us in the long-term. It binds too strongly to our hemoglobin and barriers the O2 transfer of our blood, above some percent of long-term CO2, we die. Quite a painful death, because suffocation receptors in our brain are triggered by the excess CO2 and not by the lack of O2. Commented Jul 7 at 23:43
• Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking.
– Community Bot
Commented Jul 8 at 3:02
• If Mars had always been the size size as Earth, it would probably have had a stronger magnetic field, kept its atmosphere and oceans, had moving tectonic plates, and so on. Simple geometry suggests the equator in Mars would be the same temperature as 66 degrees latitude, but the atmosphere would have clouds which would reflect some heat back into space. Commented Jul 8 at 8:46

### Yes, an 1atm CO2 atmosphere would likely cause an Earth-like temperature there.

My educated guess is based about the known pressure and temperature gradient of the Venus, combined with rough estimation of basic thermodynamics.

You need to count with 2 important factors:

1. Incoming solar radiation decreases with the square of the distance. Thus, Mars with 1.5x farther than Earth, gets about 2.25x lesser sweet sunshine.

2. Outgoing thermal radiation of a planet depends on the fourth power of its absolute temperature. Absolute temperature is the celsius degree + 273, its unit is Kelvin. If you are an American, here you need to calculate more. For example, absolute temperature of the Earth is about 288K.

Because the temperature of the planets is roughly constant, their incoming and outgoing radiation must be the same.

These two result that if you have a planet x times farther than Earth, its temperature will be $$\sqrt{x}$$ lesser (in K units).

(Third important factor is the albedo of the planet, i.e. the amount of sunshine it reflects back without being heated by it. Fourth important factor is its greenhouse effect generated by its atmosphere.)

Note, also the Venusian atmosphere is not so hot - at least where its pressure is 1 atm. Problem is "only" that this is 60km above its soil.

Here is the temperature and pressure of the Venusian atmosphere in function of the height:

Temperature:

Pressure:

Here we can see that the Venusian atmosphere is 1 atm at a height of 50km. At this height, its temperature is 400K, also about 130°C.

However, Mars is about 2x farther than Venus, resulting in an $$\frac{1}{\sqrt{2}}$$ multiplier to everything. For this reason, my educated guess of your Mars is about 284K which is probably far closer to the average temperature of the Earth, as the inaccuracy of my this estimation.

• +1 for a good rationalization based on real data.
– JBH
Commented Jul 8 at 6:56
• Well thought through. This reference consensus.app/home/blog/… suggests it would take 1-5atm of CO2 to keep Mars above 0 degrees C. Commented Jul 8 at 21:47

The universal gas law determines the state of the atmosphere: PV = nRT, where P is the atmospheric pressure, V the gas volume, n the number of moles of gas, R the gas constant, and T the temperature. So for a given volume of a gas, ANY gas, you get a given temperature at the surface.

• Which is why the temperature never varies here on Earth Commented Jul 10 at 13:11

Martian atmosphere at 1 atm, and with the same composition as it has, would quickly cause most of the CO2 to freeze. CO2 freezes solid below minus 78 Celsius, and Mars is around minus 60C on average, and minus 125C at the poles. Thick, pressurized CO2 would slowly migrate to the poles and freeze there solid, halting its own possibility of global warming. This would level eventually, because the pressure would drop, causing a plateau between CO2 freezing at the poles, and heating the planet up at the equator.

The Earth-sized Mars would then achieve a kind of equilibrium, in which the polar regions are more or less permanently around minus 78-ish or lower, the equatorial regions are likely hovering in the temp ranges we know from Earth's Siberia, Greenland and North Canada, while the tropical regions are about as hospitable as Antarctica.

That is unless life emerges there, and starts eating CO2 and farting methane, then the game changes, but not much.

There is no option, short of purposeful terraforming, for Mars to be truly Earth-like. Best case scenario is a Mars with a "Siberian Strip" between its tropics, and everything north and South of them being inhospitable frozen hell.

If you want a warmer Mars, you need it to either have a pretty robust core activity to generate heat, or drop a methane-rich asteroid on it.