Uranus and Neptune

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

They contain, by weight, only about 15-20% hydrogen and helium. The greater part of the planetary mass consists of rocky material and water ice (a mixture 0fH2O,NH3 and CII4). 

Uranus The temperature in the Uranus atmosphere, which consists of molecular hydrogen containing around 12% helium, is close to 60 K. A methane cloud layer has been detected in the lower layers of this atmosphere. The planet is surrounded by a magnetosphere which extends into space for about ten times the diameter of Uranus. The planet has 27 moons of various sizes and is surrounded by a ring system which consists of thin dark rings. The planet is unusual in two respects its tilted axis and retrograde rotation. 

In August 2006, the International Astronomical Union redefined the term planet and decided that the former ninth planet in the solar system should be referred to as a dwarf planet with the number 134340. The dwarf planet Pluto and its moon, Charon, are the brightest heavenly bodies in the Kuiper belt (Young, 2000). The ratio of the mass of the planet to that of its moon is 11 1, so the two can almost be considered as a double planet system. They are, however, quite disparate in their composition while Pluto consists of about 75% rocky material and 25% ice, Charon probably contains only water ice with a small amount of rocky material. The ice on Pluto is probably made up mainly of N2 ice with some CH4 ice and traces of NH3 ice. The fact that Pluto and Charon are quite similar in some respects may indicate that they have a common origin. Brown and Calvin (2000), as well as others, were able to obtain separate spectra of the dwarf planet and its moon, although the distance between the two is only about 19,000 kilometres. Crystalline water and ammonia ice were identified on Charon it seems likely that ammonia hydrates are present. 

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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. 

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 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. 

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). 

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. 

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... 

It is, therefore, noteworthy that almost immediately upon Welsh and associates discovery of collision-induced absorption in hydrogen [128, 129, 420], Herzberg found the first direct evidence of the H2 molecule in the atmospheres of the outer planets [181, 182], He was able to reproduce in the laboratory the unidentified diffuse feature at 827.0 nm observed by Kuiper in the spectra of Uranus and Neptune, using an 80 m path of hydrogen at 100 atmospheres pressure and a temperature of 78 K. The feature is the S3(0) line of the 3 — 0 collision-induced rotovibrational band of the H2 molecule [182]. 

G. Herzberg. Spectroscopic evidence of molecular hydrogen in the atmospheres of uranus and neptune. Astrophys. J., 115 337, 1952. 

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. 

The orbits from Venus to Ceres are represented by the unimodular series 4. In the outer system the Ford circles of only Uranus and Neptune are tangent, but the likeness to Farey sequences in atomic systems is sufficient to support the self-similarity conjecture. 

When thinking about how our solar system may have evolved from proplyds (protoplanetary disks), we must remember that the violence of the early Solar System was tremendous as huge chunks of matter bombarded each other. In the inner Solar System, the Sun s heat drove away the lighter-weight elements and materials, leaving Mercury, Venus, Earth, and Mars behind. In the outer part of the system, the solar nebulas (gas and dust) survived for some time and were accumulated by Jupiter, Saturn, Uranus, and Neptune. 

For Saturn, Uranus, and Neptune, the habitable zone is broader relative to the planetary radius. On Saturn, the temperature is about 300 K when dihydrogen becomes supercritical. On Uranus and Neptune, the temperature when dihydrogen becomes supercritical is only 160 K, a temperature at which organic molecules are stable. 

But what was there, in addition to water, on the primitive Earth The four outer planets of the solar system (Jupiter, Saturn, Uranus and Neptune) are still made up mainly of hydrogen, helium, methane, ammonia and water, and it is likely that those same chemicals were abundant everywhere else in the solar system, and therefore even in its four inner planets (Mercury, Venus, Earth and Mars). These were too small to trap light chemicals, such as hydrogen and helium, but the Earth had a large enough mass to keep all the others. It is likely therefore that the Earth s first atmosphere had great amounts of methane (CH4), ammonia (NHJ and water, and was, as a result, heavy and reducing, like Jupiter s. 

Attempts to model the accretion of Uranus and Neptune from planetesimals orbiting 20-30 AU from the Sun (the current locations of these planets) have met with severe difficulties. Long orbital periods in the outer solar system mean that accretion occurs very slowly. In addition, solar gravity is sufficiently weak here that gravitational interactions between planetary embryos would have ejected a substantial amount of mass from this region of the disk (Levison and Stewart, 2001). Numerical simulations show that it is unlikely that bodies larger than Earth could have accreted in situ at the locations of Uranus and Neptune, even if the nebula was substantially more massive than the minimum-mass nebula (Thommes et al., 2003). 

A more likely scenario is that Uranus and Neptune, along with the cores of Jupiter and Saturn, formed in the region 5-10 AU from the Sun. Such a system would have remained dynamically stable until one object (Jupiter) accreted a large H/He-rich atmosphere. At this point at least two of the other bodies would have been permrbed into the region beyond 15 AU. Gravitational interactions with planetesimals in the outer solar system would then have circularized the orbits of Uranus and Neptune by dynamical friction, while at the same time scattering most of these planetesimals onto... 

Thommes E. W., Duncan M. J., and Levison H. F. (1999) The formation of Uranus and Neptune in the Jupiter-Saturn region of the solar system. Nature 402, 635-638. 

Modern telescopic and spacecraft study of Jupiter, Saturn, Uranus, and Neptune, their properties, and their systems of rings, moons, and magnetospheres, has been the purview of the planetary scientist with little connection to the universe beyond until 1995, when the first extrasolar giant planet was discovered. Now the solar system s giants are the best-studied example of a class of some 100 objects which—while only one has been measured for size and hence density—may be present 10% of Sun-like stars. 

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