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2000 1 1010

' 104 8000

2 The densities are the estimated values in situ, i.e. they are compressed densities.

Jupiter

To explore the Jovian interior, we shall take an imaginary journey to its centre. We start in the atmosphere, which is observed to consist by mass, of about 23% helium, nearly all of the rest being hydrogen. Hydrogen is in the molecular form H2, and helium, an unreactive inert gas, is in atomic form He. This helium mass fraction is significantly less than the primordial solar value of around 27% (Section 1.1.3). Though the Sun and Jupiter formed from the same nebula, it is not difficult to understand why the helium fractions differ. Some downward settling of helium would occur, with the result that there is a greater fraction deeper down. Settling has also occurred in the Sun - in the Sun's atmosphere the helium mass fraction today is about 24%, by coincidence the same as in the Jovian atmosphere. The helium fraction in the metallic mantle (see below) in Jovian models is about 27%, and so, with nearly all of the mass of Jupiter in this mantle, the overall helium mass fraction in Jupiter is about that of the Sun at birth (and outside the fusion core today).

Beneath the Jovian atmosphere we pass quickly through various cloud layers, the temperature, pressure, and density gradually increasing as we descend. At several thousand km below the cloud tops the density is several hundred m-3. This is not much less than the 1000 kg m-3 density of liquid water at the Earth's surface. Therefore, we have arrived in what we can regard as a hydrogen-helium ocean - and yet we crossed no surface. Figure 5.10 shows the phase diagram of molecular hydrogen (the presence of helium will modify it somewhat, but the story is still the same in all its essentials). Our journey took us along the path shown as a dashed line. □ Why was there no sudden transition from the relatively low-density gas phase to the much higher density liquid phase? The path stayed well to the right of the critical point (Cr), and so the density gradually increased.

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