|
The Planet
Neptune
The
blue clouds of Neptune are mostly frozen methane, the main chemical in natural
gas -- a fuel for heating and cooking on Earth. The other object shown is
Neptune's moon Triton. Image credit: NASA/JPL
Neptune is the fourth largest
planet in our solar system. About 60 Earths could fit inside. Neptune is named
for the Roman god of the sea.
The eighth planet from the Sun,
Neptune was the first planet located through mathematical predictions rather
than through regular observations of the sky. (Galileo had recorded it as a
fixed star during observations with his small telescope in 1612 and 1613.) When
Uranus didn't travel exactly as astronomers expected it to, a French
mathematician, Urbain Joseph Le Verrier, proposed the position and mass of
another as yet unknown planet that could cause the observed changes to Uranus'
orbit. After being ignored by French astronomers, Le Verrier sent his
predictions to Johann Gottfried Galle at the Berlin Observatory, who found
Neptune on his first night of searching in 1846. Seventeen days later, its
largest moon, Triton, was also discovered.
Nearly 4.5 billion kilometers
(2.8 billion miles) from the Sun, Neptune orbits the Sun once every 165 years.
It is invisible to the naked eye because of its extreme distance from Earth.
Interestingly, due to Pluto's unusual elliptical orbit, Neptune is actually the
farthest planet from the Sun for a 20-year period out of every 248 Earth years.
The main axis of Neptune's
magnetic field is 'tipped over' by about 47 degrees compared with the planet's
rotation axis. Like Uranus, whose magnetic axis is tilted about 60 degrees from
the axis of rotation, Neptune's magnetosphere undergoes wild variations during
each rotation because of this misalignment. The magnetic field of Neptune is
about 27 times more powerful than that of Earth.
Neptune's atmosphere extends to
great depths, gradually merging into water and other 'melted ices' over a
heavier, approximately Earth-sized solid core. Neptune's blue color is the
result of methane in the atmosphere. Uranus' blue-green color is also the result
of atmospheric methane, but Neptune is a more vivid, brighter blue, so there
must be an unknown component that causes the more intense color that we see. The
cause of Neptune's bluish tinge remains a mystery.
Despite its great distance from
the Sun and lower energy input, Neptune's winds are three times stronger than
Jupiter's and nine times stronger than Earth's. In 1989, Voyager 2 tracked a
large oval dark storm in Neptune's southern hemisphere. This hurricane-like
'Great Dark Spot' was observed to be large enough to contain the entire Earth,
spun counterclockwise, and moved westward at almost 1,200 kilometers (750 miles)
per hour. (Subsequent images from the Hubble Space Telescope showed no sign of
the Great Dark Spot photographed by Voyager. A comparable spot appeared in 1994
in Neptune's northern hemisphere but had disappeared by 1997.) Voyager 2 also
photographed clouds casting shadows on a lower cloud deck, enabling scientists
to visually measure the altitude differences between the upper and lower cloud
decks.
This
photograph shows the last face-on view of the Great Dark Spot that Voyager will
make with the narrow-angle camera. The image was shuttered 45 hours before
closest approach at a distance of 2.8 million kilometers (1.7 million miles).
The smallest structures that can be seen are of an order of 50 kilometers (31
miles). The image shows feathery white clouds that overlie the boundary of the
dark and light blue regions. The pinwheel (spiral) structure of both the dark
boundary and the white cirrus suggest a storm system rotating counterclockwise.
Periodic small-scale patterns in the white cloud, possibly waves, are
short-lived and do not persist from one Neptunian rotation to the next. This
color composite was made from the clear and green filters of the narrow-angle
camera.
The planet has six rings of
varying thicknesses, confirmed by Voyager 2's observations in 1989. Neptune's
rings are believed to be relatively young and relatively short-lived.
Neptune has 13 known moons, six
of which were discovered by Voyager 2. The largest, Triton, orbits Neptune in a
direction opposite to the direction of the planet's rotation. Triton is the
coldest body yet visited in our solar system - temperatures on its surface are
about -235 degrees Celsius (-391 degrees Fahrenheit). Despite this deep freeze
at Triton, Voyager 2 discovered geysers spewing icy material upward more than 8
kilometers (5 miles). Triton's thin atmosphere, also discovered by Voyager, has
been seen from Earth several times since, and is growing warmer - although
scientists do not yet know why.
Neptune: Facts &
Figures
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Average Distance from the Sun
|
|
|
|
Metric: 4,498,252,900 km
|
|
English: 2,795,084,800
miles
|
|
Scientific Notation:
4.4982529 x 109 km (30.069 A.U.)
|
|
By Comparison: 30.069 x
Earth
|
|
|
|
|
|
|
|
|
Metric: 4,459,630,000 km
|
|
English: 2,771,087,000
miles
|
|
Scientific Notation:
4.45963 x 109 km (29.811 A.U.)
|
|
By Comparison: 29.820 x
Earth
|
|
|
|
|
|
|
|
|
Metric: 4,536,870,000 km
|
|
English: 2,819,080,000
miles
|
|
Scientific Notation:
4.53687 x 109 km (30.327 A.U.)
|
|
By Comparison: 30.326 x
Earth
|
|
|
|
|
|
|
|
|
Metric: 24,764 km
|
|
English: 15,388 miles
|
|
Scientific Notation:
2.4764 x 105 km
|
|
By Comparison: 3.883 x
Earth
|
|
|
|
|
|
|
|
|
Metric: 155,597 km
|
|
English: 96,683 miles
|
|
Scientific Notation:
1.55597 x 105 km
|
|
|
|
|
|
|
|
|
Metric:
62,526,000,000,000 km3
|
|
Scientific Notation:
6.2526 x 1013 km3
|
|
By Comparison: 57.7 x
Earth's
|
|
|
|
|
|
|
|
|
Metric:
102,440,000,000,000,000,000,000,000 kg
|
|
Scientific Notation:
1.0244 x 1026 kg
|
|
By Comparison: 17.147 x
Earth's
|
|
|
|
|
|
|
|
|
Metric: 1.76 g/cm3
|
|
By Comparison: 0.317 x
Earth
|
|
|
|
|
|
|
|
|
Metric: 7,640,800,000 km2
|
|
English: 2,950,100,000
square miles
|
|
Scientific Notation:
7.6408 x 109 km2
|
|
By Comparison: 14.980 x
Earth
|
|
|
|
|
|
Equatorial Surface Gravity
|
|
|
|
Metric: 10.71 m/s2
|
|
English: 35.14 ft/s2
|
|
By Comparison: If you
weigh 100 pounds on Earth, you would weigh 110 pounds
on Neptune.
|
|
|
|
|
|
|
|
|
Metric: 85,356 km/h
|
|
English: 53,038 mph
|
|
Scientific Notation:
23,710 m/s
|
|
By Comparison: Escape
velocity of Earth is 25,022 mph.
|
|
|
|
|
|
Sidereal Rotation Period (Length
of Day)
|
|
|
|
0.67125 Earth days
|
|
16.11 hours
|
|
By Comparison: 0.673 x
Earth
|
|
|
|
|
|
Sidereal Orbit Period (Length of
Year)
|
|
|
|
164.79 Earth years
|
|
60,190 Earth days
|
|
|
|
|
|
|
|
|
Metric: 19,720 km/h
|
|
English: 12,253 mph
|
|
Scientific Notation:
5,477.8 m/s
|
|
By Comparison: 0.490 x
Earth
|
|
|
|
|
|
|
|
|
.00859
|
|
By Comparison: 0.514 x
Earth
|
|
|
|
|
|
Orbital Inclination to Ecliptic
|
|
|
|
|
|
|
|
Equatorial Inclination to Orbit
|
|
|
|
29.58 degrees
|
|
By Comparison: 1.261 x
Earth
|
|
|
|
|
|
|
|
|
Metric: 28,142,000,000 km
|
|
English: 17,487,000,000
miles
|
|
Scientific Notation:
2.8142 x 1010 km
|
|
By Comparison: 30.44 x
Earth
|
|
|
|
|
|
|
|
|
Metric: -214 °C
|
|
English: -353 °F
|
|
Scientific Notation: 59 K
|
|
|
|
|
|
|
|
|
Hydrogen, Helium, Methane
|
|
Scientific Notation: H2,
He, CH4
|
|
By Comparison: Earth's
atmosphere consists mostly of N2 and O2.
|
|
|
Neptune/Earth
Comparison
Bulk parameters
Neptune Earth Ratio (Neptune/Earth)
Mass (1024 kg) 102.43 5.9736 17.147
Volume (1010 km3) 6,254 108.321 57.74
Radius (1 bar level) (km)
Equatorial 24,764 6,378.1 3.883
Polar 24,341 6,356.8 3.829
Volumetric mean radius (km) 24,622 6,371.0 3.865
Ellipticity (Flattening) 0.01708 0.00335 5.10
Mean density (kg/m3) 1,638 5,515 0.297
Gravity (eq., 1 bar) (m/s2) 11.15 9.80 1.14
Acceleration (eq., 1 bar) (m/s2) 11.00 9.78 1.12
Escape velocity (km/s) 23.5 11.19 2.10
GM (x 106 km3/s2) 6.8351 0.3986 17.15
Bond albedo 0.290 0.306 0.95
Visual geometric albedo 0.41 0.367 1.12
Visual magnitude V(1,0) -6.87 -3.86 -
Solar irradiance (W/m2) 1.51 1,367.6 0.0011
Black-body temperature (K) 46.6 254.3 0.183
J2 (x 10-6) 3411. 1082.63 3.151
Number of natural satellites 13 1
Planetary ring system Yes No
Orbital parameters
Neptune Earth Ratio (Neptune/Earth)
Semimajor axis (106 km) 4,495.06 149.60 30.047
Sidereal orbit period (days) 60,189. 365.256 164.79
Tropical orbit period (days) 59,799.9 365.242 163.73
Perihelion (106 km) 4,444.45 147.09 30.216
Aphelion (106 km) 4,545.67 152.10 29.886
Synodic period (days) 367.49 - -
Mean orbital velocity (km/s) 5.43 29.78 0.182
Max. orbital velocity (km/s) 5.50 30.29 0.182
Min. orbital velocity (km/s) 5.37 29.29 0.183
Orbit inclination (deg) 1.769 0.000 -
Orbit eccentricity 0.0113 0.0167 0.677
Sidereal rotation period (hours) 16.11* 23.9345 0.673
Length of day (hrs) 16.11 24.0000 0.671
Obliquity to orbit (deg) 28.32 23.45 1.208
* Magnetic coordinates (as
determined by the Voyager 2 Radio Science experiment)
Neptune Observational Parameters
Discoverer: Johann Gottfried Galle (based on predictions by
John Couch Adams and Urbain Leverrier)
Discovery Date: 23 September 1846
Distance from Earth
Minimum (106 km) 4305.9
Maximum (106 km) 4687.3
Apparent diameter from Earth
Maximum (seconds of arc) 2.4
Minimum (seconds of arc) 2.2
Mean values at opposition from Earth
Distance from Earth (106 km) 4347.31
Apparent diameter (seconds of arc) 2.3
Apparent visual magnitude 7.8
Maximum apparent visual magnitude 7.78
Neptune Mean Orbital Elements
(J2000)
Semimajor axis (AU) 30.06896348
Orbital eccentricity 0.00858587
Orbital inclination (deg) 1.76917
Longitude of ascending node (deg) 131.72169
Longitude of perihelion (deg) 44.97135
Mean Longitude (deg) 304.88003
North Pole of Rotation
Right Ascension: 299.36 + 0.70 sin N
Declination : 43.46 - 0.51 cos N
Reference Date : 12:00 UT 1 Jan 2000 (JD 2451545.0)
N = 359.28 + 549.308T degrees
T = Julian centuries from reference date
Neptunian Magnetosphere
Goddard Space Flight Center OTD (O8)
Model
Dipole field strength: 0.142 gauss-Rn3
Dipole tilt to rotational axis: 46.9 degrees
Longitude of tilt: 288 degrees (IAU convention)
Dipole offset (planet center to dipole center) distance: 0.55 Rn
Note: Rn denotes Neptunian
radii, here defined to be 24,765 km
Neptunian Atmosphere
Surface Pressure: >>1000 bars
Temperature at 1 bar: 72 K (-201 C)
Temperature at 0.1 bar: 55 K (-218 C)
Density at 1 bar: 0.45 kg/m3
Wind speeds: 0-200 m/s
Scale height: 19.1 - 20.3 km
Mean molecular weight: 2.53 - 2.69 g/mole
Atmospheric composition (by volume, uncertainty in parentheses)
Major: Molecular hydrogen (H2) - 80.0% (3.2%); Helium (He) - 19.0% (3.2%);
Methane (CH4) 1.5% (0.5%)
Minor (ppm): Hydrogen Deuteride (HD) - 192; Ethane (C2H6) - 1.5
Aerosols: Ammonia ice, water ice, ammonia hydrosulfide, methane ice(?)
Credit:NASA
|
