"...Height 250 km
Average surface pressure 93 bar or 9.2 MPa
Mass 4.8 × 1020 kg
Composition[1][2]
Carbon dioxide 96.5%
Nitrogen 3.5%
Sulfur dioxide 150 ppm
Argon 70 ppm
Water vapour 20 ppm
Carbon monoxide 17 ppm
Helium 12 ppm
Neon 7 ppm
Hydrogen chloride 0.1–0.6 ppm
Hydrogen fluoride 0.001–0.005 ppm..."
https://en.wikipedia.org/wiki/Atmosphere...
https://en.wikipedia.org/wiki/Atmosphere...
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Argent say: lanab's answer is mostly right, except that Mercury *does* rotate. It's not tidally locked, so there is no permanent dark side. It rotates 3 times for every 2 of its years.
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Metalplanttag say: Greater greenhouse effect of the atmosphere. The atmosphere of Venus is thick and mostly carbon dioxide, 96.5% by volume. Most of the remaining 3.5% is nitrogen.
Mercury has a atmosphere that is almost negligible, with some hydrogen, helium, oxygen, sodium, calcium, potassium and water vapor.
The greenhouse effect is the process by which solar radiation from the sun causes the planet's atmosphere to warm and slows down the escape of the heat into space.
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