Why are there no seasons on mercury and venus?

Answer 1

The reason that seasons occur is the orientation of the planet to the Sun, caused by a tilt in its rotation compared to its orbital plane. The Earth's axial tilt is about 23.5° which provides a noticeable variation in temperatures globally depending on the Earth's position in its orbit.

Mercury and Venus are quite different, but neither has much inclination to its rotation, about 2° to 3°, so even if they were Earth-like planets, they would have very little seasonal variation.

However, there are larger factors:

• Mercury's rotation is very slow compared to its year, and Venus has almost no rotation at all.
• Mercury has no atmosphere and experiences vast swings in surface temperature (88 Earth days of blistering heat followed by 88 Earth days of incredible cold).
• Venus is just the opposite: its dense atmosphere does not allow sunlight to reach the surface, but is a year-round pressure cooker, giving the whole planet a temperature around 860°F.

Under these circumstances, a minor change in the angle of sunlight would have negligible effects on the surface conditions.