NEPTUNE

NASA image of Neptune from Voyager 2     Neptune is the fourth and last of the "gas giant" planets in proximity to the Sun.  Neptune is the eight planet in our solar system, at an average distance of 2.8 billion miles from the Sun.  Like the other gas giants, its rotation is fast, at about 19 hours.  Neptune takes 165 years to make one orbit around the Sun and has 13 known satellites. 

Atmosphere and Weather:   Much of what we know about Neptune's atmosphere is from the Voyager II flyby in 1989.  Like Uranus, Neptune has a dynamic atmosphere with a shroud of cold clouds.  The average cloud temperature ranges from about -240 degrees to -330 degrees F.  Voyager II found the clouds to be ever changing and rapidly circling the planet.  Scientists were amazed at how much changeable weather Voyager II observed on Neptune.  A couple of interesting features found from Voyager II's flyby were very bright, white cirrus clouds circling the globe rapidly. One cirrus cloud, nicknamed "scooter" was observed to move across the planet every 16 hours!  These very high altitude clouds are made of methane ice crystals.  The other interesting feature was dubbed the "Great Dark Spot", moving westward at 700 mph.

"Scooter" just underneath the Great Dark Spot At first, the "Great Dark Spot" spot appeared to be a very large, cyclonic moving storm, similar to Jupiter's  "Great Red Spot" (although the Great Red Spot spins anti-cyclonically).  But upon closer inspection, it is likely that Neptune's "Great Dark Spot" is a hole in the methane cloud deck, similar to the ozone hole experienced on Earth.  After the Voyager II mission, the Hubble Telescope found that the "Great Dark Spot" had disappeared, but a new oval shaped dark spot had formed in a different location in Neptune's northern hemisphere. The disappearance and subsequent reformation of these dark spots on Neptune is in stark contrast to the stationary "Great Red Spot" storm on Jupiter.  Whether the dark spots on Neptune are cyclonically rotating storms or atmospheric holes, Neptune nevertheless displays an extremely dynamic atmosphere, with changes in temperature and very fast wind speeds.  The highest winds observed in the solar system have been measured on Neptune with speeds near 1,200 mph near the "older" Great Dark Spot.  The equatorial regions of Neptune possess average wind speeds of over 700 mph, which is faster than the speed of sound here on Earth.  Like Jupiter and Saturn, Neptune radiates more hear than it receives.  Therefore it has an immense internal heat source.   It is hypothesized that Neptune's internal heat source adds to vertical convection, extreme winds speeds and the overall dynamics of the blue planet.  The close up view photograph (above) shows Neptune's "Great Dark Spot" (shown slightly left of center) with "scooter" the bright cloud patch, just to the south. 

Neptune's thick atmosphere is mostly hydrogen, with smaller amounts of helium and methane.  It is the absorption of red light by methane which gives Neptune its very blue coloration.  The average temperature on Neptune is a brutally cold -373 degrees F.  Triton, Neptune's largest satellite, has the coldest temperature measured in our solar system at -391 degrees F.  That is only 68 degrees Fahrenheit warmer than absolute zero, a temperature in which all molecular action stops.   

QUICK FACTS
(Data is from NASA Goddard)

Average distance from Sun 2.8 billion miles
Perihelion 2.76 billion miles 
Aphelion 2.82 billion miles
Sidereal Rotation 16.11 Earth hours
Length of Day 16.11 Earth hours
Sidereal Revolution 164.9 Earth years
Diameter at Equator 30,707 miles
Tilt of axis 28.32 degrees
Moons 13 known
Atmosphere Hydrogen (80.0%), Helium (18.5%), Methane (1.5%) 
Discoverer Johann Gottfried Galle
Discovery Date September 23, 1846

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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