# What is the minimum pressure of a purely CO₂ atmosphere on Europa that can retain enough heat for surface liquid water?

*Ignoring external factors such as Jupiter's magnetosphere blowing this new atmosphere off. I'm not sure where to begin the maths for that

• For surface liquid water you need not only a temperature above 0.01 °C, you also need a surface pressure of more than 0.006 bar. But the surface pressure of Europa is much lower, about 100 nPa or 1 pbar (yes 1 picobar).
– Uwe
Commented Aug 14, 2023 at 20:20
• @Uwe my engineering final project in uni was a Europa lander (DORRA the Europa Explorer, the paper is out in the wild) and every time someone said Europa has an atmosphere I reminded them that there's more atmosphere in a light bulb Commented Aug 14, 2023 at 23:14
• @ErinAnne An Ultra High Vacuum (UHV) with 1 µbar is ten times the surface pressure of Europa.
– Uwe
Commented Aug 15, 2023 at 12:56

## 1 Answer

A very rough starting point is how much this atmosphere would differ from a perfect blackbody.

The melting point of water is roughly 270 kelvin for a wide range of pressures, so with that as a surface temperature, an object would radiate 300 W/m^2

Compare that to the solar energy flux of 50 W/m^2, and keeping in mind that the cross section area of a sphere receiving light is four times smaller than the total surface area, that means the atmosphere must reduce the thermal radiation to 1/24th of a blackbody.

For comparison, for well insulated Venus, the same numbers are 2600 W/m^2 emitted and 2600 W/m^2 received for a blackbody with the same temperature. Factoring in the area difference again, the heavy atmosphere of Venus is only improving upon a blackbody by a factor of 4. (The equality of the two numbers is a coincidence).

So probably we need a stronger greenhouse effect than Venus, but I stress that this is only a very approximate estimate.