Neptune

Neptune is the eighth and farthest known planet from the Sun in the Solar System. In the Solar System, it is the fourth-largest planet by diameter, the third-most-massive planet, and the densest giant planet. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times the mass of Earth but not as dense. Neptune orbits the Sun once every 164.8 years at an average distance of 30.1 astronomical units (4.50×10^9 km). It has an eccentricity of 0.01 and an orbital inclination of 1.8° relative to the ecliptic (but direct observations reveal a 3° tilt with respect to reality). Its equatorial radius is 24,622 kilometers (15,299 miles), making it approximately 3/4ths as wide as Earth. Neptune has a temperature at its cloud tops of approximately −218 °C (−361 °F) and a mean temperature near its core of about 5400 K (5127 °C; 9180 °F). It has only 17% land area compared to Earth’s 75%,[note 4] making it very similar in composition to Jupiter and Saturn. The interior consists primarily of ices and rock.

Like Jupiter and Saturn, Neptune’s atmosphere is composed mostly of hydrogen gas (80%) with some helium (19%), methane (1%). Trace amounts of acetylene,, ethane,, propane, phosphine,, dinitrogen tetroxide,, monoacetylenetrihydride, carbon monoxide, nitrogen sulfide, hydrogen sulfide, argon 35 are also present in small quantities.:209–210 The outermost layers are distinguished based on their visible appearance: below about 20 bar pressure, where light can penetrate into clouds below this level, there is a zone called “the troposphere”, characterized by flat stratocumulus clouds; above that lies “the stratosphere” with cirrus clouds; finally there are detached high cumulonimbus thunderhead clouds above 60 bar pressure comprising “the exosphere”.:224 There may be an additional layer between 40–60 bar containing high altitude hazes such as C2H6S or CH3SH which absorbs ultraviolet light but allows red light through – this region has been tentatively called “the mesosphere”.:226

The extreme cold temperatures found at great depths cause most molecules to dissociate into their atomic constituents; thus molecular hydrogen makes up most of Neptune’s interior by mass.:317 At pressures greater than 10 GPa (100 kbar), where electrons become delocalized due to quantum effects creating metallic hydrogen,:785–786 conditions become increasingly conducive for nuclear fusion reactions – therefore it is thought that some energy generation via deuterium fusion could be taking place in Neptune’s deep interior although no direct evidence currently exists for this hypothesis.- As on Jupiter and Saturn, compression heats up material deeper down so that heat flows outwards from the hot interior towards cooler regions causing convection currents which mix material around so that heavier elements sink while lighter elements rise leading to differential mixing ratios depending on depth:- For example abundances vary significantly between methane clathrates found close to surface ice layers and those found deeper down where they act as barriers impeding further downward mixing:- – ch4 abundance decreases from 7% near surface layers to 2% at 100 bars whereas h2s increases from 2% to 30%.