> Wouldn't it take weeks for the surface to cool down since its in a vacuum?
The fact that Mercury has no atmosphere doesn't decrease the rate of heat radiation, it increases it (compared to a planet with an atmosphere). Your question appears to relate to the efficiency of vacuum storage bottles, but that's a different case with a different logic.
For a vacuum thermos, the presence of the vacuum represents a less efficient heat conduction medium than air (or foam), which uses conduction and convection to transfer energy. The difference in temperature between the contents and the environment also plays a part.
But for a planet, no atmosphere means radiation operates at maximum efficiency, and radiation is a very efficient energy transfer method, especially when there's a large temperature difference between the object and space.
Even on earth (with an atmosphere), radiation directly to space turns out to be more efficient than atmospheric conduction and convection, such that objects on the surface can fall well below air temperature if the sky is clear of clouds.
It's not uncommon for earth's surface to drop below air temperature overnight, which can produce such effects as "radiation fog" near the ground, caused by condensation of water vapor in the air cooled by the surface.
It's common to believe that on earth, air conduction/convection is the majority heat transfer method and radiation is less effective, but in fact it's the other way around.
> So I'm still not clear why a vacuum in a thermos is superior then?
It's not just the vacuum, but a combination of a vacuum plus a reflective coating to deal with the effectiveness of radiation as a heat transmission method. In this way a vacuum thermos addresses all three heat conduction methods -- conduction, convection and radiation. More here:
The fact that Mercury has no atmosphere doesn't decrease the rate of heat radiation, it increases it (compared to a planet with an atmosphere). Your question appears to relate to the efficiency of vacuum storage bottles, but that's a different case with a different logic.
For a vacuum thermos, the presence of the vacuum represents a less efficient heat conduction medium than air (or foam), which uses conduction and convection to transfer energy. The difference in temperature between the contents and the environment also plays a part.
But for a planet, no atmosphere means radiation operates at maximum efficiency, and radiation is a very efficient energy transfer method, especially when there's a large temperature difference between the object and space.
Even on earth (with an atmosphere), radiation directly to space turns out to be more efficient than atmospheric conduction and convection, such that objects on the surface can fall well below air temperature if the sky is clear of clouds.
It's not uncommon for earth's surface to drop below air temperature overnight, which can produce such effects as "radiation fog" near the ground, caused by condensation of water vapor in the air cooled by the surface.
It's common to believe that on earth, air conduction/convection is the majority heat transfer method and radiation is less effective, but in fact it's the other way around.