This tiny planet – the smallest in the solar system – is only slightly larger than our moon (and its surface is the spitting image of our moon’s too), so its gravity is too weak to grip a heat-trapping atmosphere.
Because Mercury has no atmosphere, it can’t hold onto its heat very well. The night side of Mercury gets very cold, which brings down the average temperature of the planet and makes it only the second hottest planet in the Solar System after Venus.
Picture Credit : Google