The Planet Neptune

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. 

 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.

At first, the "Great Dark Spot" spot appeared to be a very large, cyclonic moving storm. by 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. 

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.  

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.

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