Why Mercury Isn't The Solar System's Hottest Planet (Synopsis)

“There is no question that climate change is happening; the only arguable point is what part humans are playing in it.” –David Attenborough

If the Earth didn't have any global warming at all, our planet's mean temperature would be 255 K, or about -1º Fahrenheit: the mean temperature of the Antarctic continent. As it stands instead, our planet is much warmer than that, owing to the warming, insulating effects of the atmosphere, which is largely transparent to (incoming) visible light, but traps a fair amount of the (outgoing) infrared radiation.

Natural color image of Venus from Mariner 10 data. Image credit: © 2005 Mattias Malmer, from NASA/JPL data.

This effect is even more spectacular on our inner, sister planet, Venus. While Venus might be more reflective and twice as far from the Sun as Mercury, its greenhouse gases and insulating cloud layers are so effective that it's actually the hottest planet in the Solar System, outstripping Mercury day or night.

Transits of Venus (top) and Mercury (bottom) across the edge of the Sun. Note how Venus’ atmosphere diffracts sunlight around it, while Mercury’s lack of atmosphere shows no such effects. Images credit: NASA / SDO / HMI / Stanford Univ., Jesper Schou (top); NASA’s TRACE Satellite (bottom).

Go get the whole story on the physics of why and how this is so over on Forbes!