Mercury: the quick little world
Mercury has a large metallic core and a rocky outer shell. Its very thin exosphere does little to move heat around or retain it overnight, producing a large day–night temperature contrast.
Closest does not mean hottest: Venus has a higher average surface temperature because of its dense atmosphere. Mercury’s rotation and orbit combine to make its sunrise-to-sunrise day longer than its year.
Wonder about this
Why does being nearest the Sun not make Mercury hottest?
A science clue
Venus’s dense atmosphere traps much more heat; Mercury has almost no atmosphere to retain it.