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| <b>Mercury</b> is the closest ] to the ]. It has no ]s and only trace amounts of ] and ] in its ] (temporarily captured from the ]). Its surface appears very similar to ]'s ], but the planet has a much larger iron core and is therefore much denser. Its most distinctive known surface feature is Caloris Basin, a colossal impact crater approximately 1350 km in diameter. Caloris is ] for ''heat'' and the basin is named this because it is near the subsolar point (the point closest to the sun) when Mercury is at perihelion (the closest point in its ] to the sun). | |||
| Mercury has been known of since at least the time of the ] (3rd millennium BC). It was given two names by the ancient ], ] when visible in the morning sky and ] when visible in the evening, but Greek astronomers knew that the two names referred to the same body. ] even believed that Mercury and ] orbited the Sun, not the Earth. | |||
| The orbit of Mercury is highly eccentric, ranging from 46 million kilometers to 70 million kilometers in diameter. The slow ] of this orbit around the sun could not be completely explained by ] ], and for some time it was thought that another planet might be present in an orbit even closer to the sun (sometimes referred to as ]) to account for this perturbation. ]'s ] provided the explanation for this small discrepancy instead, however. | |||
| Mercury is one of the least-studied of the ]'s planets. It was visited by only one spacecraft, ], which flew past it on three occasions in ] and ]. Only 45% of the planet's surface was mapped, and it is located too near the sun for existing telescopes to conduct further mapping from Earth. Until radar observations in ] proved otherwise it was thought that Mercury was ] with the Sun, rotating once for each orbit and keeping the same face directed towards the sun at all times. Instead, Mercury is in a 3:2 resonance, rotating three times for every two revolutions around the sun. The original reason astronomers thought it was tidally locked was because whenever Mercury was best placed for observation, it was always at the same point in its 3:2 resonance, so showing the same face, which would be also the case if it was totally locked. | |||
| Recent radar observations have suggested that water ice may be present in small quantities at Mercury's north pole. Such water, if it exists, is likely located at the permanently shaded bottoms of craters where it is deposited by ] impacts. A new mission to Mercury has been approved by ], named ], which will launch in ] to orbit Mercury starting in ]. | |||
| http://meta.wikipedia.com/upload/mercuryglobe1-thumb.jpg | |||
| ''This mosaic of Mercury was taken by the Mariner 10 spacecraft during its approach on 29 March 1974. The mosaic consists of 18 images taken at 42 s intervals during a 13 minute period when the spacecraft was 200,000 km (about 6 hours prior to closest approach) from the planet. '' | |||
| Other facts: | |||
| * Mass: 3.3×10<sup>23</sup> kg (Approximately 5.5% of the Earth's mass) | |||
| * Diameter: 4878 km | |||
| * Surface Area: ] | |||
| * Length of year: 87.97 Earth days | |||
| * Rotation period: 58.65 Earth days | |||
| * Mean surface temperature: 452 K (]), ranging between 90 to 700 K. | |||
| * Average distance from Sun: ] ] | |||
| **Perihelion: 45 million km | |||
| **Aphelion: 69.7 million km | |||
| ]:<br> | |||
| ] - '''Mercury''' - ] - ] - ] - ]s - ] - ] - ] - ] - ] - ]s | |||
| :''See also :'' ] | |||
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