<Table> Neptune <Tr> <Td_colspan="2"> The Great Dark Spot and its companion bright smudge; on the west limb the fast moving bright feature called Scooter and the little dark spot are visible . </Td> </Tr> <Tr> <Th_colspan="2"> Discovery </Th> </Tr> <Tr> <Th> Discovered by </Th> <Td> <Ul> <Li> Johann Galle </Li> <Li> Urbain Le Verrier </Li> </Ul> </Td> </Tr> <Tr> <Th> Discovery date </Th> <Td> 23 September 1846 </Td> </Tr> <Tr> <Th_colspan="2"> Designations </Th> </Tr> <Tr> <Th> Pronunciation </Th> <Td> / ˈnɛptjuːn / (listen) </Td> </Tr> <Tr> <Th> Adjectives </Th> <Td> Neptunian </Td> </Tr> <Tr> <Th_colspan="2"> Orbital characteristics </Th> </Tr> <Tr> <Td_colspan="2"> Epoch J2000 </Td> </Tr> <Tr> <Th> Aphelion </Th> <Td> 30.33 AU (4.54 billion km) </Td> </Tr> <Tr> <Th> Perihelion </Th> <Td> 29.81 AU (4.46 billion km) </Td> </Tr> <Tr> <Th> Semi-major axis </Th> <Td> 30.11 AU (4.50 billion km) </Td> </Tr> <Tr> <Th> Eccentricity </Th> <Td> 6997945600000000000 ♠ 0.009 456 </Td> </Tr> <Tr> <Th> Orbital period </Th> <Td> <Ul> <Li> 164.8 yr </Li> <Li> 60,182 days </Li> <Li> 89,666 Neptunian solar days </Li> </Ul> </Td> </Tr> <Tr> <Th> Synodic period </Th> <Td> 367.49 days </Td> </Tr> <Tr> <Th> Average orbital speed </Th> <Td> 7003543000000000000 ♠ 5.43 km / s </Td> </Tr> <Tr> <Th> Mean anomaly </Th> <Td> 7000447202223580004 ♠ 256.228 ° </Td> </Tr> <Tr> <Th> Inclination </Th> <Td> 6998308569848429468 ♠ 1.767 975 ° to ecliptic 6.43 ° to Sun's equator 0.72 ° to invariable plane </Td> </Tr> <Tr> <Th> Longitude of ascending node </Th> <Td> 7000230006470144821 ♠ 131.784 ° </Td> </Tr> <Tr> <Th> Argument of perihelion </Th> <Td> 7000482297304179106 ♠ 276.336 ° </Td> </Tr> <Tr> <Th> Known satellites </Th> <Td> 14 </Td> </Tr> <Tr> <Th_colspan="2"> Physical characteristics </Th> </Tr> <Tr> <Th> Mean radius </Th> <Td> 7007246220000000000 ♠ 24,622 ± 19 km </Td> </Tr> <Tr> <Th> Equatorial radius </Th> <Td> 7007247640000000000 ♠ 24,764 ± 15 km 3.883 Earths </Td> </Tr> <Tr> <Th> Polar radius </Th> <Td> 7007243410000000000 ♠ 24,341 ± 30 km 3.829 Earths </Td> </Tr> <Tr> <Th> Flattening </Th> <Td> 6998171000000000000 ♠ 0.0171 ± 0.0013 </Td> </Tr> <Tr> <Th> Surface area </Th> <Td> 7015761830000000000 ♠ 7.6183 × 10 km 14.98 Earths </Td> </Tr> <Tr> <Th> Volume </Th> <Td> 7022625399999999999 ♠ 6.254 × 10 km 57.74 Earths </Td> </Tr> <Tr> <Th> Mass </Th> <Td> 7026102413000000000 ♠ 1.024 13 × 10 kg 17.147 Earths 5.15 × 10 Suns </Td> </Tr> <Tr> <Th> Mean density </Th> <Td> 7003163800000000000 ♠ 1.638 g / cm </Td> </Tr> <Tr> <Th> Surface gravity </Th> <Td> 11.15 m / s 1.14 g </Td> </Tr> <Tr> <Th> Moment of inertia factor </Th> <Td> 6999230000000000000 ♠ 0.23 (estimate) </Td> </Tr> <Tr> <Th> Escape velocity </Th> <Td> 23.5 km / s </Td> </Tr> <Tr> <Th> Sidereal rotation period </Th> <Td> 0.6713 day 16 h 6 min 36 s </Td> </Tr> <Tr> <Th> Equatorial rotation velocity </Th> <Td> 2.68 km / s (9,650 km / h) </Td> </Tr> <Tr> <Th> Axial tilt </Th> <Td> 28.32 ° (to orbit) </Td> </Tr> <Tr> <Th> North pole right ascension </Th> <Td> 19 57 20 299.3 ° </Td> </Tr> <Tr> <Th> North pole declination </Th> <Td> 42.950 ° </Td> </Tr> <Tr> <Th> Albedo </Th> <Td> 0.290 (bond) 0.442 (geom .) </Td> </Tr> <Tr> <Td_colspan="2"> <Table> <Tr> <Th> Surface temp . </Th> <Th> min </Th> <Th> mean </Th> <Th> max </Th> </Tr> <Tr> <Th> 1 bar level </Th> <Td> </Td> <Td> 72 K (− 201 ° C) </Td> <Td> </Td> </Tr> <Tr> <Th> 0.1 bar (10 kPa) </Th> <Td> </Td> <Td> 55 K (− 218 ° C) </Td> <Td> </Td> </Tr> </Table> </Td> </Tr> <Tr> <Th> Apparent magnitude </Th> <Td> 7.67 to 8.00 </Td> </Tr> <Tr> <Th> Angular diameter </Th> <Td> 2.2--2.4" </Td> </Tr> <Tr> <Th_colspan="2"> Atmosphere </Th> </Tr> <Tr> <Th> Scale height </Th> <Td> 7004197000000000000 ♠ 19.7 ± 0.6 km </Td> </Tr> <Tr> <Th> Composition by volume </Th> <Td> <Ul> <Li> Gases: </Li> <Li> 6999800000000000000 ♠ 80% ± 3.2% hydrogen (H 2) </Li> <Li> 6999190000000000000 ♠ 19% ± 3.2% helium (He) </Li> <Li> 6998150000000000000 ♠ 1.5% ± 0.5% methane (CH 4) </Li> <Li> ~ 0.019% hydrogen deuteride (HD) </Li> <Li> ~ 0.00015% ethane (C 6) </Li> <Li> Ices: </Li> <Li> ammonia (NH 3) </Li> <Li> water (H 2O) </Li> <Li> ammonium hydrosulfide (NH 4SH) </Li> <Li> methane ice (?) (CH 4 5.75 H O) </Li> </Ul> </Td> </Tr> <Tr> <Td_colspan="2"> </Td> </Tr> </Table> <Tr> <Td_colspan="2"> The Great Dark Spot and its companion bright smudge; on the west limb the fast moving bright feature called Scooter and the little dark spot are visible . </Td> </Tr>

The main constituent of neptune's atmosphere is
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