Terrestrial Planet · Innermost
Mercury
The smallest planet and closest to the Sun, Mercury is a heavily cratered rocky world with virtually no atmosphere. Despite its proximity to the Sun, it is not the hottest planet - Venus is - because it has no atmosphere to trap heat.
Overview
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Atmosphere composition
Oxygen
42%
Sodium
29%
Hydrogen
22%
Helium
6%
Potassium
0.5%
Notable features
- Caloris Basin - impact crater 1,550 km wide
- Water ice in permanently shadowed polar craters
- Iron core making up ~85% of the planet's radius
- Cliff-like scarps formed as the planet cooled and shrank
Space missions
| Mission | Year | Notes |
|---|---|---|
| Mariner 10 | 1974-1975 | First spacecraft to visit Mercury; 3 flyby passes |
| MESSENGER | 2004-2015 | First to orbit Mercury; mapped the entire surface |
| BepiColombo | 2018-2025 | ESA/JAXA mission, arrival planned 2025 |
NASA images
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Images courtesy of NASA Image & Video Library.