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Observations of Mercury from ground-based observers and the Mariner 10 spacecraft have not shown evidence of a significant atmosphere (compared to other planets). Mariner 10 observef a tiny amount of helium 1000 km above the surface, but this is probably caused by the solar wind and the breakdown of Mercury's crust. Sodium, potassium and oxygen were also detected in this weak atmosphere, but these elements can be lost when they react with the Sun and Mercury's magnetic field.
Temperatures at the surface range between -300 F and 800 F, which is hot enough for lead to melt. This large range in surface temperature is possible because Mercury is close to the Sun (a year is only 88 Earth days long) and does not have sufficient atmosphere present to moderate the range in surface temperature.
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Mercury is the only inner planet other than the Earth that has a significant magnetic field (220 nT). This field, along with the planet's high density and small size relative to the Earth, indicates that it probably has a molten iron core. The magnetic field has two poles, and is tilted in the same direction as Mercury's axis. Mariner 10 observed a shock wave called a "bow shock" in front of the planet, where the planet's magnetic field meets the solar wind.
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Mercury is the closest planet to our Sun at 36 million miles. It revolves around the Sun every 88 days, but its rotation takes nearly 59 days. Therefore, the planet's day is two-thirds the length of its year.
Mercury has an extremely thin and non-protective atmosphere. Fly-bys by the Mariner 10 spacecraft in 1974 and '75 revealed a cratered surface very much like our Moon's. It appears that hydrogen and helium (likely from the solar wind) make up what thin atmosphere Mercury has. Even though Mercury rotates on its axis every 59 days, its orbital period is longer than its sidereal rotation. Therefore a solar day on Mercury (sunrise to sunset), takes about 176 days. Due to this fact, its closeness to the Sun and a very thin atmosphere, Mercury has the largest diurnal temperature spread of any planet in our solar system. It is understood that the sunny side may reach temperatures of 750 to 800 degrees F., while the nighttime temperature plummets to nearly -330 degrees F. The average temperature on Mercury is a balmy 354 degrees F.
Interestingly, complex radar observations found evidence of ice water on the shady side of craters on Mercury's north pole. This could be possible, because Mercury does not tilt on its axis and therefore rotates perpendicular to the Sun. Thus, the Sun will always be at an extremely low angle at the poles and certainly not high enough to melt or sublimate ice. One theory of how this ice water could have been placed on Mercury is from comet residue.
Mercury has a highly eccentric orbit and a long orbital period. Therefore at certain latitudes, an observer on the surface could watch the Sun rise, move directly overhead and stop, then retrograde back a bit. After retrograding, the Sun would then proceed back on its westward track. In addition, since Mercury has virtually no atmosphere to scatter light, the sky would be black, even though the Sun disk itself would be over twice as large as what we observe from the Earth.
It has no moons.
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| Name |
Start Time |
Ending Time |
Year |
Region |
Topic |
| Discovery |
265 BC |
265 BC |
- |
Mercury |
Timocharis made the first recorded observation of Mercury. |
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Planet
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Diameter: 4,878 km (3,030 miles)
Mass: 0.33x10^24 kilograms (0.06 x Earth's)
Density: 5,427 kg/m^3
Minimum Distance from Sun: 45.9 million km
Maximum Distance from Sun: 69.7 million km
Minimum Distance from Earth: 45 million km
Name in Roman/Greek Mythology: Mercury/Hermes
Rotation Period around Axis: 58.65 days
Revolution Period: 0.24 years
Surface Gravity: 3.7 m/s^2 (0.38 x Earth's)
Temperature Range: -184 C to 427 C
Average Surface Temperature: 440K
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