MERCURY

NASA Mercury image from Mercury Messenger     At an average distance of 36 million miles, Mercury is the closest planet to our Sun.  It revolves around the Sun every 88 days, but its rotation takes nearly 59 days.  Therefore Mercury completes three rotations for every two orbits around the Sun.  Mercury is one of only three planets in our solar system which has a perfect circular sphere (the other two being Venus and Pluto).

Atmosphere and Weather:  Mercury has an extremely thin and non-protective atmosphere.  For all practical purposes, the atmosphere is nearly a vacuum.  The sparse atmosphere is primarily composed of oxygen, sodium and hydrogen.  Mercury's atmospheric atoms are continuously being lost to space.  These atoms are then believed to be replenished by the solar wind and possibly through "outgassing", which is the release of gas from a solid substance and increases as temperatures warm.  

Due to the tenuous atmosphere, Mercury really has no weather to speak of other than wild fluctuations in temperature.  In fact, Mercury has the largest diurnal temperature spread of any planet in our solar system.  This is explained by a few factors.  First, as stated above, Mercury rotates on its axis every 59 days relative to fixed stars such as our Sun.  However a fictitious observer on Mercury would see that its actual length of day from sunrise to sunrise would take about about 176 Earth days to complete due to perspective.  Why is this?  Mercury's orbital-rotational resonance ratio is 3:2.   All other planets in our solar system have a ratio of 1:1.  Therefore Mercury's very long solar day,  close proximity to the Sun and very thin atmosphere, all combine to produce the largest diurnal temperature spread 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.  In addition, since Mercury has virtually no atmosphere to scatter light, the sky would be black, even though the Sun's disk itself would be over twice as large as what we observe from the Earth.

With Mercury's orbital-rotational resonance ratio at 3:2 and highly elliptical orbit, an observer at certain latitudes could watch the Sun rise, move directly overhead and stop, retrograde back, then proceed back on its westward track.  

Fly-bys by the Mariner 10 spacecraft in 1974 and '75 and recent photos from the Mercury Messenger reveal a cratered surface, marked with cliffs and ridges very much like our Moon's.  Lava flows have also been discovered.  In some parts of Mercury, these lava flows have "buried" ancient scarred terrain producing a smoother surface which has not been greatly pock-marked by meteors yet.  Mercury is the most dense planet in our solar system when not accounting for gravitational compression.  If you count the force of gravitational compression, then Earth is the most dense.  It is believed that Mercury's dense iron core is larger than Earth's and likely compromises most of the planet itself.  This would give Mercury relatively thin mantle and crust layers.  

Interestingly, complex radar observations found evidence of ice water on the shady side of craters on Mercury's north pole.  This could be possible, since 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.  One theory of how this ice water could have been placed on Mercury is from comet residue.  

Mercury is the least explored "rocky" planet of our inner solar system (Mercury, Venus, Earth and Mars).  However it appears that Mercury holds a lot of information about how all of the terrestrial planets have evolved since the beginning times of the solar system.  To complete an in-depth investigation of Mercury's secrets, NASA has launched the "Messenger" space probe.  Messenger should establish orbit around Mercury in March 2011 and study it for a year.

QUICK FACTS
(Data is from NASA Goddard)

Average distance from Sun 36,000,000 miles
Perihelion 28,600,000 miles
Aphelion 43,400,000 miles
Sidereal Rotation 58.6 Earth days
Length of Day 175.94 Earth days
Sidereal Revolution 88.0 Earth days
Diameter at Equator 3,025 miles (2nd smallest planet)
Tilt of axis Essentially none - 0.01 degrees
Moons None
Atmosphere Very thin, nearly a vacuum.  42% Oxygen, 29% Sodium, 22% Hydrogen, 6% Helium, 0.5% Potassium 
Discoverer Unknown
Discovery Date Prehistoric

DEFINITIONS:

Average distance from Sun:  Average distance from the center of a planet to the center of the Sun. 
Perihelion:  The point in a planet's orbit closest to the Sun.
Aphelion:  The point in a planet's orbit furthest from the Sun. 
Sidereal Rotation:  The time for a body to complete one rotation on its axis relative to the fixed stars such as our Sun.  Earth's sidereal rotation is 23 hours, 57 minutes.
Length of Day:  The average time for the Sun to move from the Noon position in the sky at a point on the equator back to the same position.  Earth's length of day = 24 hours
Sidereal Revolution:  The time it takes to make one complete revolution around the Sun.
Axis tilt:  Imagining that a body's orbital plane is perfectly horizontal, the axis tilt is the amount of tilt of the body's equator relative to the body's orbital plane.  Earth is tilted an average of 23.45 degrees on its axis.


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