Neptune Saturn

Earth-based infrared observations of Neptune indicated by 1974 that, unlike Uranus, Neptune emits 2.4 times as much energy at infrared wavelengths as it receives frc n sunlight (insolation). This led to the infer-erKe that, like Jupiter and Saturn, Neptune has a significant internal heat source. This may be produced by cc i-... 

Alkanes have the general molecular formula C H2 +2 The srmplest one methane (CH4) rs also the most abundant Large amounts are present rn our atmosphere rn the ground and rn the oceans Methane has been found on Juprter Saturn Uranus Neptune and Pluto and even on Halley s Comet... 

Diacetylene (HC=C—C=CH) has been identified as a component of the hydrocarbon rich atmospheres of Uranus Neptune and Pluto It is also present m the atmospheres of Titan and Triton satellites of Saturn and Neptune respectively... 

Our solar system consists of the Sun, the planets and their moon satellites, asteroids (small planets), comets, and meteorites. The planets are generally divided into two categories Earth-like (terrestrial) planets—Mercury, Venus, Earth, and Mars and Giant planets—Jupiter, Saturn, Uranus, and Neptune. Little is known about Pluto, the most remote planet from Earth. 

The gas giant planets Jupiter, Saturn, Uranus and Neptune. The planet Pluto has a status of its own, and has recently been renamed a dwarf planet. 

Although Uranus and Neptune also belong to the group of gas giant planets, they are constructed differently from Jupiter and Saturn ... 

The latter group was probably responsible for the early bombardment of the protoplanets. Delsemme believes that the cometary nuclei of the members of the Jupiter family never experienced temperatures greater than 225 K. The values suggested for the others are Saturn family, 150K Uranus family, 75 K Neptune family, 50 K. During many million years, these comets got mixed together in the Oort cloud (which has a diameter of around 50,000 AU). 

The formation of the planets around the proto-sun initially started as a simple accretion process, aggregating small particles to form larger particles. This process was common to all planets, even the gas giants Jupiter and Saturn and to a lesser extent Neptune and Uranus. The planetesimals form at different rates and as soon as Jupiter and Saturn had reached a critical mass they were able to trap large amounts of hydrogen and helium from the solar nebula. The centres of Jupiter... 

The density estimates in Table 7.1 show a distinction between the structures of the planets, with Mercury, Venus, Earth and Mars all having mean densities consistent with a rocky internal structure. The Earth-like nature of their composition, orbital periods and distance from the Sun enable these to be classified as the terrestrial planets. Jupiter, Saturn and Uranus have very low densities and are simple gas giants, perhaps with a very small rocky core. Neptune and Pluto clearly contain more dense materials, perhaps a mixture of gas, rock and ice. 

The dissociation reaction predicted by Umemoto et al. s calculations has important implications for creating good models of planetary formation. At the simplest level, it gives new information about what materials exist inside large planets. The calculations predict, for example, that the center of Uranus or Neptune can contain MgSiC>3, but that the cores of Jupiter or Saturn will not. At a more detailed level, the thermodynamic properties of the materials can be used to model phenomena such as convection inside planets. Umemoto et al. speculated that the dissociation reaction above might severely limit convection inside dense-Satum, a Saturn-like planet that has been discovered outside the solar system with a mass of 67 Earth masses. 

Alkanes are often found in natural systems. They are the main constituents in the atmospheres of the planets Jupiter, Saturn, Uranus, and Neptune. Methane is also thought to have been a major component of the atmosphere of the early Earth. Natural gas and oil are primarily made of alkanes. 

Aspects of the chemical composition of the atmospheres of Jupiter, Saturn, Uranus, and Neptune were measured by the Voyager and Galileo spacecraft in the 1980s and 1990s,... 

Hydrogen isotopic compositions, expressed as molar D/H ratios, of solar system bodies. The relatively low D/H values in the atmospheres of Jupiter and Saturn are similar to those in the early Sun, whereas D/H ratios for Uranus and Neptune are intermediate between the Jupiter-Saturn values and those of comets and chondrites. The Earth s oceans have D/H shown by the horizontal line. Mars values are from SNC meteorites. Modified from Righter et al. (2006) and Lunine (2004). 

To the accuracy of the measurement of molecular weights for the giant planets, only hydrogen and helium have significant abundances. The relative proportions of these elements, expressed as the molar fraction He/H, are 0.068+0.002 for Jupiter, 0.068+0.013 for Saturn, 0.076+0.016 for Uranus, and 0.100+0.016 for Neptune (Lunine, 2004). None of these ratios are like those of the nebula (0.085, Table 4.1). 

Diagram on the left shows the composition of the solar nebula (abundances in wt. %). Diagram on the right expands metals (astronomical jargon) into ices (water, methane, and ammonia) and rock (all other remaining elements). Jupiter and Saturn formed mostly from nebular gases, Uranus and Neptune formed mostly from ices, and the terrestrial planets formed primarily from rock. 

Phase diagram for hydrogen, showing the conditions under which hydrogen changes from molecular (H2) to metallic (H+). Below the gray He saturation curves, He and H are immiscible. Adiabats for Jupiter and Saturn cross the saturation curve once H becomes metallic, but the Uranus (and presumably Neptune) adiabats do not reach such high pressures. 

Models of the interiors of the giant planets depend on assumed temperature-pressure-density relationships that are not very well constrained. Models for Jupiter and Saturn feature concentric layers (from the outside inward) of molecular hydrogen, metallic hydrogen, and ice, perhaps with small cores of rock (rocky cores are permissible but not required by current data). Uranus and Neptune models are similar, except that there is no metallic hydrogen, the interior layers of ice are thicker, and the rocky cores are relatively larger. 

Isotopic abundances for hydrogen have been measured in giant planet atmospheres, as shown in Figure 14.11. The D/H ratios in Jupiter and Saturn are similar to those in the Sun, but lower than those in the Earth s oceans or in comets. D/H ratios in Uranus and Neptune... 

Several applications of IR spectroscopy to astrophysics have been made. Small amounts of methane in the earth s atmosphere have been detected by the observation of weak IR absorption lines in solar radiation that has passed through the earth s atmosphere. Intense IR absorption bands of CH4 have been found in the spectra of the atmospheres of Jupiter, Saturn, Uranus, and Neptune. Bands of ammonia have been observed for Jupiter and Saturn bands of C02 have been observed in the Venusian spectrum and bands of H20 have been observed in the Martian spectrum. 

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