Why isn’t Mercury the hottest planet in the Solar System?

Why isn't Mercury the hottest planet in the Solar System?

In the grand scheme of the Solar System, by far the greatest source of energy is the Sun. And, generally, we assume that the hottest planet in the Solar System is Mercury, because it is the one that is closest to the Sun.

However, this is not so, and that award must be taken by another world.

Mercury is very hot, completing a full orbit of the Sun in just 88 Earth days, reaching temperatures that exceed 400 ° C in its hottest region .

However, as Mercury rotates very slowly on itself, the dark areas where the sun does not shine remain that way for a long time, which translates into temperatures below -100 ° C there.

Venus the fiery

Comparisonmevetima Comparison of the inner planets, from left to right Mercury, Venus, Earth and Mars.

But what about Venus? It is approximately twice as far as the Sun as Mercury. It took 225 Earth days to orbit the sun king. And it still spins on itself more slowly than Mercury.

And yet, when the temperature of Venus is measured, there is a surprise: Venus has the same temperature at all times, both day and night, at an average 462 ° C, which makes it a planet warmer than Mercury

This fact puzzled astronomers, because Venus was not large enough to generate its own heat, yet it was hotter at midnight than the temperature recorded at noon on Mercury . What was the problem?

The important difference here between Mercury and Venus can be divided into two:

  • Mercury is much less reflective than Venus.
  • Mercury has no atmosphere, while Venus has a very thick atmosphere.


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Calculating how much an object reflects or absorbs radiation is known as its albedo, which comes from the Latin word albus , which means white. An object with an albedo of 0 is a perfect absorber, while an object with an albedo of 1 is a perfect reflector. All physical objects have an albedo between 0 and 1 .

For example, despite its nocturnal whiteness, the Moon’s average albedo is only 0.12, which means that only 12% of the light that reaches it is reflected, while the other 88% is absorbed.

Mercury turns out to be similar to the Moon (albedo 0.119), while Venus’s albedo is by far the highest of all planetary bodies in the Solar System (0.90). So not only does Mercury receive four times more energy per unit area, but it absorbs almost nine times more sunlight than Venus receives .

If we look then at the atmosphere of both planets, as we have said, Mercury has hardly any, and Venus has a very dense one. In other words, all the heat that Mercury receives and that is reflected is radiated into space. But in the case of Venus, the heat bounces off the atmosphere. Therefore, due to the intense greenhouse effect to which Venus is subjected, it thus becomes the hottest planet in the Solar System .

Via | Medium