Composition

Abstract

It is generally believed that the planets in our solar system formed following the cooling and condensation of the primordial solar nebula, a large, dense cloud of dust and gas surrounding the juvenile Sun. The Sun and its planetary system represent perhaps only one of the thousands of fragments which may have resulted following the collapse of a large interstellar cloud. According to the above hypothesis of solar system formation, the constituents of planetary atmospheres would be in solar proportions provided that condensation, selective escape, fractionation, or evolution has not occurred. The massive major planets of our solar system — Jupiter, Saturn, Uranus, and Neptune — have large escape velocities which would prevent even the lightest of gases, hydrogen, from escaping. For example, the equatorial gravitational escape velocity (Appendix A1.1) at Jupiter is 57 kms⁻¹, whereas the mean thermal velocity (Appendix A1.1) of hydrogen atoms at Jupiter’s exospheric temperature (∼1000K) is only 4 kms⁻¹. The time constant (also known as the e-folding time) for hydrogen atoms, i.e., the time required for escape of ∼70% of the total atmospheric hydrogen, turns out to be 10³⁰⁰ years! Nonthermal escape is likely, but is not very significant at Jupiter and Saturn. On Uranus, nearly half of the hot hydrogen atoms produced on dissociation of molecular hydrogen by soft electrons are capable of overcoming the gravitational energy barrier (additional discussion of this phenomena on Uranus can be found in Chapter 2 on Thermal Structure and Chapter 4 on Vertical Mixing).