ABECASSIS Elodie

BRUN Caroline

DECQ Clémence

(Teacher : Michel FAYE)

Lycée Louis-le-Grand

123, rue Saint-Jacques 75005 Paris FRANCE

NEPTUNE

            Neptune is the fourth largest planet in the solar system, and the eighth major planet in order of increasing distance from the sun. Discovered in 1846 by Johann Gotfried Galle, whose observation was based on mathematical predictions, Neptune is the furthest giant gaseous planet. It has got eight moons, and its atmosphere is the most agitated of the nine planets’.

I. Historic and explorations

            In Roman mythology, Neptune (Greek : Poseidon) was the god of the sea. The name of Neptune comes from its blue colour.

            1) Discovery

            After the discovery of Uranus, it was noticed that its orbit was not as it should be in accordance with Newton’s laws. It was therefore predicted that another more distant planet must be perturbing Uranus’ orbit. Neptune was first observed by Johann Gotfried Galle and Louis d’Arrest on 1846 September 23 very near to the locations independently predicted by John Adams and Urbain Joseph Le Verrier from calculations based on observed positions of Jupiter, Saturn and Uranus. An international dispute arose between the English and French (though not, apparently between Adams and Le Verrier personally) over priority and the right to name the new planet ; they are now jointly credited with Neptune’s discovery. Subsequent observations have shown that the orbits calculated by Adams and Le Verrier diverge from Neptune’s actual orbit fairly quickly. Had the search for the planet taken place a few years earlier or later it would not have been found anywhere near the predicted location.

            More than two centuries earlier, Galileo observed Neptune when it happened to be very near Jupiter, but he thought it was just a star. On two successive nights he actually noticed that it moved slightly with respect to another nearby star. But on the subsequent nights it was out of his field of view. Had he seen it on the previous few nights Neptune’s motion would have been obvious to him. But, alas, cloudy skies prevented observations on those few critical days.

            2) Explorations

            Neptune has been visited by only one spacecraft, Voyager 2, on August 25 1989. Much of we know about Neptune comes from this single encounter. But fortunately, recent ground-based and HST observations have added a great deal, too.

  Neptune
Voyager 2 took this view from Neptune on August 20 1989. In the
center of the photograph we can see a spot which remind us of Jupiter’s Big Red Spot, and which seems to be, just like that of Jupiter, a huge everlasting storm.

  Observations of Neptune. HST (Hubble Spatial Telescope)
On those almost real-coloured pictures, we can see a brilliant cloudy structure on the South pole, right at the bottom of the picture. There are brilliant stripes of clouds, at South latitudes 30° and 60°, and at North latitude 30°. The second picture shows the opposite side of the planet, nine hours after the first picture was taken.

The storm system, known as the « Great dark spot », wasn’t observed, as the second little dark spot, DS2, which had been previously observed by Voyager 2. The lack of these spots was one of the most surprising events of this program. These dramatic changes of the huge storm systems are not yet understood, but point to the dynamic nature of Neptune’s atmosphere.

  The HST observes the high altitude clouds
These three pictures were taken on October 10, October 18 and November 2 1994, illustrating the dynamic changes of Neptune’s atmosphere. The pink spots are clouds of methane crystals.

 
  The HST discovers a new dark spot
In June 1994, the HST showed that the Great Dark Spot, first observed by Voyager 2 had disappeared. This new picture taken on November 2, shows a new spot, which might be an area of light gas, through which we can observe clouds further down in the atmosphere.


    Cirrus-shaped clouds
This picture shows cirrus-shaped clouds in the North atmosphere of Neptune. These stripes are 30 to 100 miles wide, and thousands kilometers long.
(Crédit: Calvin J. Hamilton)

  Real-coloured picture   (Courtesy of NASA/JPL)


  Great Dark Spot
White feather-shaped clouds delimit the dark and light blue areas inside the Great dark Spot. The shape of these areas’ limits suggests a direct rotation. (Courtesy of NASA/JPL)


  Changes of the Great Dark Spot
The bright cirrus-shaped clouds change rapidly on Neptune, in a few score hours. The surprisingly quick changes observed on these three pictures, taken at 18 hours intervals, show that in this area, the atmospheric conditions of Neptune may be as dynamic and changing as on the Earth.  (Courtesy of NASA/JPL)


  Last glance
On this picture taken by Voyager 2 on August 30 1989, we can see a beautiful view of crescents of Neptune and its moon, Triton.
(Crédit: Calvin J. Hamilton)


  Little Dark Spot
This picture shows the Little Dark Spot, south from the Great Dark Spot. It seems to be a storm in Neptune’s atmosphere, similar to Jupiter’s Big Red Spot. (Credits : Calvin J. Hamilton)


  Neptune’s rings
These two views of Neptune’s rings were taken by Voyager 2 on August 26 1989, at a distance of 174,000 miles. We can see the two main rings, clearly visible in their whole span, and the little inside ring, 25,000 miles distant from Neptune’s center. A lot of stars can be seen in the background.
(Courtesy of NASA/JPL)

  Twisted rings
This part of one of Neptune’s rings seems to be twisted. Scientists think that this characteristic is due to the rotation of the rings around Neptune during their formation. (Courtesy of NASA/JPL)

II. Characteristics

            1) Main characteristics

Neptune is the last gaseous planet in the solar system. The mean distance of Neptune from the sun is 4,5 billion km ( 2,796 billion miles), and its mean linear diameter is approximately 49,000 km or about 3,8 times that of the earth. The average stellar magnitude of the planet is 7,8, and it is therefore never visible to the naked eye, but it can be observed in a small telescope as a small, round, greenish-blue disk without definite surface markings. The temperature of the surface of Neptune is about –218°C, much like Uranus, which is more than one billion miles closer to the Sun. Scientists assume, therefore, that Neptune must have some internal heat source. The atmosphere consists mostly of hydrogen and helium, but the presence of up to three percent methane gives the planet its striking blue colour.

DATA

Mass (kg)

1,024.10 26

Mass (Earth = 1)

17,135

Equatorial radius (km)

24 746

Equatorial radius (Earth = 1)

3,8799

Mean density (g / cm 3 )

1,64

Mean distance from the sun (km)

4,504.10 9

Mean distance from the sun (Earth = 1)

30,0611

Rotational period (hours)

16,11

Orbital period (years)

164,79

Mean orbital velocity (km / s)

5,45

Orbit eccentricity

0,0097

Axial inclination (degrees)

28,31

Orbital inclination (degrees)

1,774

Surface equatorial gravity (m / s 2 )

11,0

Visual geometrical albedo

0,41

Magnitude (Vo)

7,84

Mean cloud temperature (°C)

-193 to –153

Atmospheric pressure (bars)

1-3

Composition of the atmosphere

Hydrogen

Helium

Methane

85 %

13 %

2 %

            2) Dynamic

            Neptune is a dynamic planet with several big dark spots, which remind us of Jupiter’s storms. The biggest, called the Big Dark Spot, is the same size as the earth. Voyager 2 discovered a small irregular-shaped spot, scooting to the east. This scooter , as it has been nicknamed, might be a trail of clouds rising up above a deeper cloudy layer. Long bright clouds, similar to the cirrus on Earth, have been observed in the high atmosphere of Neptune.

            The most violent winds ever observed on a planet have been measured on Neptune. The winds blow essentially to the west, in the opposite direction of the planet’s rotation. Close to the Big Dark Spot, the winds blow up to 2,000 km/h.

            3) Rings and moons

            The Voyager 2 planetary probe discovered eight moons in 1989. The following table summarize the data concerning Neptune’s moons. Further information will be given about Neptune’s moons in the following paragraph.

Moon

#

Radius (km)

Mass (kg)

Distance (km)

Discoverer

Date

Naiad

III

29

?

48,000

Voyager 2

1989

Thalassa

IV

40

?

50,000

Voyager 2

1989

Despina

V

74

?

52,000

Voyager 2

1989

Galatea

VI

79

?

62,000

Voyager 2

1989

Larissa

VII

104×89

?

73,600

Voyager 2

1989

Proteus

VIII

200

?

117,600

Voyager 2

1989

Triton

I

1,350

2,14.10 22

354,800

W. Lassell

1848

Nereid

II

170

?

5,513,400

G. Kuiper

1949

 

            Neptune is also circled by five thin rings. The following table presents the main characteristics of these rings.

Name

Distance

Width

Thickness

Mass

1989N3R

41,900 km

15 km

?

?

1989N2R

53,200 km

15 km

?

?

1989N4R

53,200 km

5,800 km

?

?

1989N1R

62,930 km

< 50 km

?

?

III. Moons

            1) Despina

Despina, only 27,700 kilometers (17,200 miles) from Neptune's clouds, orbits every 8 hours. Its diameter is about 150 kilometers (90 miles). It is irregularly shaped and shows no sign of any geological modification. Despina circles the planet in the same direction as Neptune rotates, and remains close to Neptune's equatorial plane.

Despina Statistics

 Discovered by

Voyager 2 

 Date of discovery

1989 

 Mass (kg)

 Equatorial radius (km)

74 

 Mean distance from Neptune (km)

52,500 

 Orbital period (days)

0.334655 

 Mean orbital velocity (km/sec)

11.41 

            2) Galatea

Galatea [gal-eh-TEE-eh] lies 37,200 kilometers (23,100 miles) from Neptune. It has a diameter of 180 kilometers (110 miles) and completes an orbit in 10 hours, 18 minutes. It is irregularly shaped and shows no sign of any geological modification. Galatea circles the planet in the same direction as Neptune rotates, and remains close to Neptune's equatorial plane. The above image is smeared so that Galatea appears highly elongated.

Galatea Statistics

 Discovered by

Voyager 2 

 Date of discovery

1989 

 Mass (kg)

 Equatorial radius (km)

79 

 Mean distance from Neptune (km)

62,000 

 Orbital period (days)

0.428745 

 Mean orbital velocity (km/sec)

10.52 

            3) Larissa

Larissa [LA-ree-suh] is only about 48,800 kilometers (30,300 miles) from Neptune's clouds, and circles the planet in 13 hours, 18 minutes. Its diameter is about 190 kilometers (120 miles). It is irregularly shaped and shows no sign of any geological modification. Larissa circles the planet in the same direction as Neptune rotates, and remains close to Neptune's equatorial plane.

Larissa Statistics

 Discovered by

Stephen Synnott 

 Date of discovery

1989 

 Mass (kg)

 Radius (km)

104x89 

 Mean distance from Neptune (km)

73,600 

 Orbital period (days)

0.554654 

 Mean orbital velocity (km/sec)

9.65 

This image of Larissa was acquired by the Voyager 2 spacecraft on August 24, 1989. (Credit : Calvin J. Hamilton)

  Shaded Relief Map of Larissa This image is a shaded relief map of Larissa, a small inner satellite of Neptune. As with all maps, it is the cartographer's interpretation and not all features are necessarily certain given the limited data available. This interpretation stretches the data as far as is feasible. The leading side faces forwards in the orbit of Larissa. The trailing side faces backwards along the orbit. Longitude 0 is at the righthand end of the leading side, and faces Neptune. As with all conformal (true shape) projections, the scale in these maps varies, increasing from the centre to the outer edge. (Courtesy Phil Stooke)

            4) Naiad

Naiad, the last satellite discovered, is about 54 kilometers (33 miles) in diameter and orbits Neptune every 7 hours and 6 minutes about 23,200 kilometers (14,400 miles) above the clouds. It is irregularly shaped and shows no sign of any geological modification. Naiad circles the planet in the same direction as Neptune rotates, and remains close to Neptune's equatorial plane. The above image has been smeared so that Naiad appears highly elongated.

Naiad Statistics

 Discovered by

Voyager 2 

 Date of discovery

1989 

 Mass (kg)

 Equatorial radius (km)

29 

 Mean distance from Neptune (km)

48,000 

 Orbital period (days)

0.294396 

 Mean orbital velocity (km/sec)

11.86 

 

            5) Thalassa

Thalassa appears to be about 80 kilometers (50 miles) in diameter. It orbits Neptune in 7 hours, 30 minutes about 25,200 kilometers (15,700 miles) above the cloud tops. It is irregularly shaped and shows no sign of any geological modification. Thalassa circles the planet in the same direction as Neptune rotates. The above image has been slightly smeared so that Thalassa appears highly elongated.

Thalassa Statistics

 Discovered by

Voyager 2 

 Date of discovery

1989 

 Mass (kg)

 Equatorial radius (km)

40 

 Mean distance from Neptune (km)

50,000 

 Orbital period (days)

0.311485 

 Mean orbital velocity (km/sec)

11.68 

            6) Triton

Triton [TRY-tun] is the largest moon of Neptune, with a diameter of 2,700 kilometers (1,680 miles). It was discovered by William Lassell , a British astronomer, on October 10, 1846 scarcely a month after Neptune was discovered. Triton is colder than any other measured object in the Solar System with a surface temperature of -235° C (-391° F). It has an extremely thin atmosphere. Nitrogen ice particles might form thin clouds a few kilometers above the surface. The atmospheric pressure at Triton's surface is about 15 microbars, 0.000015 times the sea-level surface pressure on Earth.

Triton is the only large satellite in the solar system to circle a planet in a retrograde direction -- in a direction opposite to the rotation of the planet. It also has a density of about 2.066 grams per cubic centimeter (the density of water is 1.0 gram per cubic centimeter). This means Triton contains more rock in its interior than the icy satellites of Saturn and Uranus do. The relatively high density and the retrograde orbit has led some scientists to suggest that Triton may have been captured by Neptune as it traveled through space several billion years ago. If that is the case, tidal heating could have melted Triton in its originally eccentric orbit, and the satellite might even have been liquid for as long as one billion years after its capture by Neptune.

Triton is scarred by enormous cracks. Voyager 2 images showed active geyser-like eruptions spewing nitrogen gas and dark dust particles several kilometers into the atmosphere.

Triton Statistics

 Discovered by

William Lassell 

 Date of discovery

1846 

 Mass (kg)

2.14.10 22

  Mass (Earth = 1)

3.5810.10 -3

 Equatorial radius (km)

1,350 

 Equatorial radius (Earth = 1)

2.1167.10 -1

 Mean density (gm/cm 3 )

2.07 

 Mean distance from Neptune (km)

354,800 

 Rotational period (days)

-5.87685 

 Orbital period (days)

-5.87685 

 Mean orbital velocity (km/sec)

-4.39 

 Orbital eccentricity

0.0000 

 Orbital inclination (degrees)

157.35 

 Escape velocity (km/sec)

1.45 

 Visual geometric albedo

0.7 

 Magnitude (Vo)

13.47 

 Mean surface temperature

-235°C 

Triton

             Voyager 2 obtained this colour image of Neptune's large satellite Triton during its close flyby on Aug. 25, 1989. The large south polar cap at the bottom of the image is highly reflective and slightly pink in colour; it might consist of a slowly evaporating layer of nitrogen ice deposited during the previous winter. From the ragged edge of the polar cap northward, the satellite's face is generally darker and redder in colour. This colouring may be produced by the action of ultraviolet light and magnetospheric radiation upon methane in the atmosphere and surface. Running across this darker region, approximately parallel to the edge of the polar cap, is a band of brighter white material that is almost bluish in colour. The underlying topography in this bright band is similar; however, to that in the darker, redder regions surrounding it. (Courtesy NASA/JPL)

  Neptune on Triton's Horizon

Composite view showing Neptune on Triton's horizon. Neptune's south pole is to the left; clearly visible in the planets' southern hemisphere is a Great Dark Spot, a large anticyclonic storm system located about 20 degrees South. The foreground is a computer-generated view of Triton's maria as they would appear from a point approximately 45 km above the surface. The terraces visible in this image indicate multiple episodes of 'cryovolcanic' flooding. This three-dimensional view was created from a Voyager image by using a two-dimensional photoclinometric model. Relief has been exaggerated roughly 30-fold, the actual range of the relief is about 1 km. Would Neptune appear to be rising or setting? Neither, due to the motion of Triton relative to Neptune, it would appear to move laterally along the horizon, eventually rising and setting at high latitudes. (Courtesy NASA/JPL)

           

            7) Nereid

Nereid [NEER-ee-ed] was discovered in 1949 by astronomer Gerard Kuiper. Nereid is about 340 kilometers (210 miles) in diameter and is so far from Neptune that it requires 360 days to make one orbit. Voyager's best photos of Nereid were taken from about 4.7 million kilometers (2.9 million miles). The photos show that the moon's surface reflects about 14 percent of the sunlight that strikes it, making it somewhat more reflective than Earth's Moon, and more than twice as reflective as Proteus . Nereid's orbit is the most eccentric in the solar system. Its distance to Neptune ranges from about 1,353,600 kilometers (841,100 miles) to 9,623,700 kilometers (5,980,200 miles).

Nereid Statistics

 Discovered by

Gerard Kuiper 

 Date of discovery

1949 

 Mass (kg)

 Equatorial radius (km)

170 

 Mean distance from Neptune (km)

5,513,400 

 Orbital period (days)

360.1362 

 Mean orbital velocity (km/sec)

1.12 

 

           

8) Proteus

             Proteus [PROH-tee-us], like all six of Neptune's newly discovered small satellites, is one of the darkest objects in the solar system -- "as dark as soot" is not too strong of a description. Like Saturn's satellite, Phoebe , it reflects only 6 percent of the sunlight that strikes it. Proteus is about 400 kilometers (250 miles) in diameter, larger than Nereid . It wasn't discovered from Earth because it is so close to Neptune that it is lost in the glare of reflected sunlight. Proteus circles Neptune at a distance of about 92,800 kilometers (57,700 miles) above the cloud tops, and completes one orbit in 26 hours, 54 minutes. Scientists say it is about as large as a satellite can be without being pulled into a spherical shape by its own gravity. Proteus is irregularly shaped and shows no sign of any geological modification. It circles the planet in the same direction as Neptune rotates, and remains close to Neptune's equatorial plane.

Proteus Statistics

 Discovered by

Stephen Synnott 

 Date of discovery

1989 

 Mass (kg)

 Equatorial radius (km)

200 

 Mean distance from Neptune (km)

117,600 

 Orbital period (days)

1.122315 

 Mean orbital velocity (km/sec)

7.63 

Exercise

            In the solar system, four moons circle their planets at the same distance :

d = 400,000 km. These moons are the Moon (Earth), Io (Jupiter), Dione (Saturn) and Triton (Neptune).

Their orbital periods are respectively of 28 days, 1,8 day, 2,7 days and 5,8 days.

1)       Calculate the mean orbital velocity of these four moons.

2)       Find a relation between the mean orbital velocity of a moon, and the mass of the planet that it circles.

Data : Mass of the Earth = 1

                        Mass of Jupiter = 317

                        Mass of Saturn = 95

                        Mass of Neptune = 17

Answers

1)       Velocities : V Moon = (2× p ×D) / 28 ×(1 000/86 400) = 1040 m/s

V Io = 16 161 m/s

V Dione = 10 774 m/s

V Triton = 5015 m/s

            2) The more important the mass of the planet is, the higher the orbital velocity of its moon is.

IV. Comparison between Jupiter and Neptune

            Though they are both gaseous planets, Jupiter and Neptune show many differences. Their atmospheres have almost the same composition, but there is more methane on Neptune than on Jupiter. Besides, their sizes can not be compared : Jupiter is more than three times larger than Neptune. Jupiter’s orbital period is of 11,86 years, whereas Neptune’s one is of 164,7 years. And Jupiter has got sixteen moons, that is to say more than Neptune.

            In fact, the main similarity they have is their dynamic. We can observe many spots on Neptune, which largest one is called the Great Dark Spot, and these huge storms in Neptune’s atmosphere remind us of Jupiter’s Big Red Spot.

JUPITER

BIBLIOGRAPHY:

-          Encyclopédie Microsoft Encarta 1999

-          Encyclopédie Universalis

-          Websites : -www.mper.chez.tiscali.com

o              - www.solarviews.com

o              - www.astro.vision.free.fr

o              - www.chez.com