Neptune composition

Effects of condensation are also seen in the bulk compositions of the planets and their satellites. The outer planets, Uranus and Neptune, have overall densities consistent with their formation from icy and stony solids. The satellites of Uranus have typical densities of 1.3g/cm which would tend to indicate a large ice com-... 

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. 

Bouman C, Vroon PZ, Elliott TR, Schwieters JB, Hamester M (2002) Determination of lithium isotope compositions by MC-ICPMS (Thermo Finnigan MAT Neptune). Geochim Cosmochim Acta 66 A97 Bray AM (2001) The geochemistry of boron and lithium in mid-ocean ridge hydrothermal vent fluids. PhD thesis. University of New Hampshire, 125 p... 

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). 

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. 

The temperature profiles within Jupiter and Saturn are thought to be essentially adiabatic, reflecting the high central temperatures and the dominant role of convection below the observable atmosphere where radiative processes become important. There may be deeper layers restricted in radial extent where the temperature profile becomes subadiabatic, due to a decrease in the total opacity, or by virtue of the behavior of the equation of state of hydrogen and helium. The same may hold for Uranus and Neptune, although with less certainty, because of the possibility that stable compositional gradients could exist and dominate the heat flow regime. In particular, Uranus small heat flow, if primordial and not a function of seasonal insolation, could be the result of a stable compositional stratification and hence subadiabatic temperature profile in the interior (Podolak et al., 1991). 

How do giant planets form Two different models, disk instability versus core accretion followed by gas collapse, are viable. They require very different timescales, have very different implications for satellite formation and internal composition, and may have implications for the ubiquity of giant planets and terrestrial planets around other stars. The formation of Uranus and Neptune is even less well understood, and no agreement exists as to whether these are stillborn Jupiters or the product of a distinct kind of formation process. 

Figure 18 D/H ratios of several comets compared to the oceans (SMOW), planets, the solar nehula (PSN), and the interstellar medium. Low-temperature fractionation processes increase D/H. Jupiter and Saturn have compositions close to the original nehular composition, hut low-temperature formation of ice caused the enhancements seen in Uranus and Neptune (the ice giants) and comets. The discrepancy between the plotted LP comets and SMOW argues against these comets providing Earth with a major fraction of its water. Other comets, formed in warmer environments, near Jupiter, could he more similar to SMOW (source Huehner, 2002).

To account for these compositional features of comets, Bar-Nun et al. [23] experimentally showed a relation of the relative abundances of CO, Ar, N2, and CHj trapped in ice, versus temperatures, and inferred tliat the abundances of N2 and CO in comets were inherent from the gases trapped during ice forming in die Uranus-Neptune regions [24]. Notesco et al. [25] have performed a similar analysis for CH and C2H, successfully accounting for the proportions of these two gases observed in comet Hyakutake by Mumma et al. [22]. 

S4 [90,91] (Table 16.2). A typical comet composition is based on the relative amounts of the different gas species measured in the coma of comets from the Oort cloud, an immense spherical cloud surrounding the solar system and extending from approximately 5000 to 100,000 AU. About half of the short-period comets arc from the Kuiper Belt, a disk-shaped region containing many small icy bodies extending beyond Neptune s orbit from 30 to 50 AU, which are depleted in carbon-chain molecules. The other short-period comets have the typical composition of Oort cloud comets. So far, carbyne molecules detected in the coma of active comets are very short chains unlike the longer chains characteristic of natural carbyne crystals such as chaoite in the Murchison meteorite [20,67]. 

The composition of the outer planets is also very different from that of the inner planets. Mercury, Venus, Earth, and Mars are all made of rocky-like material with a density of about 5.5 g/cm3. By contrast, the outer planets seem to consist largely of gases (which accounts for their sometimes being called the gas giants) with densities of about 0.69 g/cm3 for Saturn to 1.54 g/cm3 for Neptune. These... 

Researchers have learned a vast amount of new information about Jupiter, Saturn, Uranus, Neptune, Pluto and the Kuiper Belt Objects in the last century. Improved terrestrial telescopes, the Hubble Space Telescope, and space explorations such as Voyager 1 and 2, Galileo, and Cassini have produced new data that will take astrochemists years to analyze and interpret, providing them with even more detailed information about the chemical composition of the atmospheres, satellites, surfaces, and other features of the outer planets and their associated bodies. 

The four giant planets have hydrogen- and helium-rich compositions reminiscent of the Sun, but all of them clearly depart from strict solar composition in that their densities are too high and the few heavier elements whose tropospheric abundances can be measured all show clear evidence of enrichment. For all four giant planets we have spectroscopic compositional data on the few compounds that remain uncondensed in the visible portion of their atmospheres, above their main cloud layers. These include ammonia, methane, phosphine, and germane. For Jupiter, these volatile elements (C, N, S, P and Ge) are enriched relative to their solar abundances by about a factor of five. For Saturn, with no detection of germane, the enhancement of C, N, and P is about a factor of 10. For Uranus and Neptune the methane enrichment factor is at least 60, consonant with their much higher uncompressed densities. 

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