Helium mantle structure

Moreira M. and Allegre C. J. (1998) Helium-neon systematics and the structure of the mantle. Chem. Geol 147, 53—59. 

Neptune s upper atmosphere (what we see) is a mixture of hydrogen, helinm, methane and traces of acetylene (C2H2), carbon dioxide, and other gasses. Orrly 10% of the planet s mass is in this outermost layer (approximately 3,100 mi or 5,000 km thick). Under the upper atmophere lies a lower atmophere of molecnlar (gaseous) hydrogen and helium, plus some ices (approximately 6,200 mi or 10,000 km thick). Below the atmosphere lies the mantle, a water ice and rock mixture that perhaps contains methane ice and aitrmonia ice mixed in. A core is at the center of the planet s mass, and it is likely a body with a 6,200-mi radios and represents 45% of the planet s mass that is conposed of silicate rock and water ice. Like the other Jovian planets (Jupiter, Saturn, and Uranns), Neptune has a distinctive structure quite different from the terrestrial planets like Earth. 

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