Final answer:
Mercury's high daytime temperatures are mainly due to its close proximity to the Sun and its slow rotation resulting in long days. The lack of a significant atmosphere leads to extreme temperature fluctuations between day and night.
Step-by-step explanation:
The extreme heat experienced during the daytime on Mercury can be attributed primarily to its proximity to the Sun. As the closest planet to the Sun, Mercury is subjected to intense solar radiation, resulting in high surface temperatures that can reach up to 430°C (700 K) at noon.
Moreover, its slow rotation results in long days, causing one side of the planet to face the Sun for extended periods.
However, Mercury lacks a significant atmosphere to retain this heat, leading to drastic temperature drops during its long nights, with temperatures plummeting to -170°C (100 K) just before dawn.
The combination of its close distance to the Sun and long days due to its rotation enhances the duration and intensity of the sunlight it receives, explaining why daytime temperatures on Mercury are so high.