Voyager 2 photo of the blue planet Neptune
Voyager 2 close-up of Neptune showing cloud details and the great dark spot
Voyager 2 view of Neptune and its great dark spot

Voyager 2 image of the
blue planet Neptune

(NASA/JPL)

Voyager 2 close-up of Neptune showing cloud details and the great dark spot
(NASA/JPL)

Voyager 2 view of Neptune and
its great dark spot

(NASA/JPL)

God of the Seas

Leaving Uranus and its moons behind us, we next encounter Uranus's sister planet, Neptune, and its system of moons. Neptune is nearly the same size as Uranus, and its composition is extremely similar. It is also the outermost of the two ice giants. Neptune was named after the Roman god of the seas. It was discovered in 1846 by Johann Gottfried Galle of the Berlin Observatory. Neptune has 13 known moons, most of which were not discovered until after Voyager 2's encounter with the planet in 1989. Most of these moons are quite tiny and are probably captured asteroids. The largest of these moons however, has a diameter of 1,680 miles (2,700 kilometers).

Voyager 2 closeup of Neptune showing cloud formations
Voyager 2 close-up of upper level clouds in Neptune's atmosphere
Voyager 2 close-up image of Neptune's great dark spot

Voyager 2 close-up of Neptune showing cloud formations
(NASA/JPL)

Voyager 2 close-up image of Neptune's great dark spot
(NASA/JPL)

Voyager 2 close-up of upper level clouds in Neptune's atmosphere
(NASA/JPL)

The Big Blue Spot

Since Neptune was known to be so similar to Uranus, astronomers were expecting to find another bland and featureless world when Voyager 2 arrived there in August of 1989. Much to the surprise of everyone on the Voyager team, Neptune would turn out to be anything but bland. The most startling feature to be discovered was a giant blue spot in the planet's southern hemisphere. It was very similar in appearance to the famous red spot on Jupiter, and about half the size. In 1994, observations of Neptune by the Hubble space telescope have revealed that the giant blue spot has disappeared. Recently, a new blue spot was observed in the planet's northern hemisphere. Astronomers are not sure why the spot on Neptune was relatively short-lived, while the great red spot on Jupiter has bee present for hundreds of years. This indicates that the Neptune's atmosphere is highly active and prone to sudden and severe changes. Several smaller spots were also noted, as was a small, fast-moving, cloud that astronomers named "scooter".

This Voyager 2 close-up of Neptune shows upper level clouds in the atmosphere

This Voyager 2 close-up of Neptune shows upper level clouds in the atmosphere. This was a surprise to astronomers who were expecting a more uniform atmosphere similar to that of Uranus. (NASA/JPL)

Features of Neptune

Nearly everything we know about Neptune was learned from the Voyager 2 encounter. Like Uranus, Neptune's blue color is the result of red light being absorbed by methane gas in the planet's upper atmosphere. Neptune's composition is believed to be very similar to that of Uranus. It probably has a small, rocky core about the size of the Earth. This core is thought to be surrounded by an extremely thick layer gases composed mainly of hydrogen and helium with a little methane. Besides the mysterious dark and light spots were other fascinating discoveries. Long, white clouds were seen floating in the planet's upper atmosphere. Neptune has the fastest winds in the Solar System, reaching speeds of 1,200 miles (2,000 kilometers) per hour. Voyager 2 also revealed Neptune's system of delicate rings. There are 4 narrow rings, which are extremely faint. Astronomers think they may be composed of dust particles formed from meteorites smashing into the planet's moons. Like Uranus, Neptune's magnetic field is tilted 47 degrees from its rotational axis. Astronomers believe this is caused by internal forces deep within the planet.

 

 

Statistics for Neptune

Discovered by

Year of Discovery

Diameter

Known Natural Satellites

Mean Distance from the Sun

Rotational Period

Orbital Period

Orbital Eccentricity

Orbital Inclination

Inclination of Axis

Mean Cloud Temperature

Main Atmospheric Component

Atmospheric Pressure

Apparent Magnitude

......

......

......

......

......

......

......

......

......

......

......

......

......

......

Johann Gottfried Galle

1846

30,744 miles (49,492 km)

13

2,798,116,000 miles (4,504,300,000 km)

15.8 hours

165 years

0.009

1.774 degrees

29.31 degrees

-315° F (-193° C)

Hydrogen

2 bars

7.84

 

The Moons of Neptune

Name

Pronunciation

Order

Diameter
(km) 

Orbital Period
(days)

Distance
(km)

Year
Discovered

Naiad

Thalassa

Despina

Galatea

Larissa

S/2004 N 1

Proteus

Triton

Nereid

Halimede

Sao

Laomedeia

Psamathe

Neso

NAH-yad

tha-LAS-uh

des-PEEN-uh

gal-uh-TEE-uh

la-RISS-uh

N/A

PROTE-yoos

TRY-tun

NEER-ee-id

HAL-i-MEE-dee

SAY-oh

LAY-o-mə-DEE-uh

SAM-uh-thee

NEE-soh

1

2

3

4

5

6

7

8

9

10

11

12

13

14

66

82

150

176

194

18

420

2,707

340

62

44

42

38

60

0.29

0.31

0.34

0.43

0.56

0.94

1.12

−5.88

360.13

1,879.08

2,912.72

3,171.33

9,074.30

9,740.73

48,227

50,074

52,526

61,953

73,548

105,283

117,646

354,759

5,513,818

16,611,000

22,228,000

23,567,000

48,096,000

49,285,000

1989

1989

1989

1989

1981

2013

1989

1846

1949

2002

2002

2002

2003

2002

 

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