Uranus and Neptune are difficult bodies to study. The radiation received at the Earth from Uranus is less than one-thousandth of that received from Jupiter and from Neptune it is even less. The greater distances of the two planets make surface details more difficult to detect than on either Jupiter or Saturn. No planets are exactly the same but Uranus and Neptune are more nearly alike than any other pair. Neptune was once thought to be the larger of the two but modern measurements show that the equatorial radius of Uranus is just five per cent greater than that of Neptune although Uranus has 15 per cent less mass. Because of their similarities it is convenient to consider the two planets together.

Molecular hydrogen has been detected on both Uranus and Neptune. A feature seen by G.P..Kuiper in the spectra of both planets was shown by G.Herzberg in. 1952 to be a molecular hydrogen line at a wavelength of 827 nm. Other lines have subsequently been detected in the spectrum of each planet.

The detection of methane in the atmosphere of Uranus followed the same pattern as for Jupiter and Saturn. In 1869 Angelo Secchi was the first person to observe the absorption lines in the spectrum of Uranus. Rupert Wildt suggested in 1932 that these might be caused by methane and in 1934 Adel and V.M.Slipher confirmed that this was indeed the case. In the first decade of the twentieth century V.M.Slipher had shown that the spectrum of Neptune was very like that of Uranus, except that the absorption was even stronger. It therefore followed that methane was also present on Neptune. No other atoms or molecules have been identified on either planet. However it is expected that ammonia and helium are both present. Unlike Jupiter and Saturn, any ammonia is only expected deep in their atmospheres.

The effective temperatures of Uranus and Neptune are calculated to be 57 and 45 K respectively if the only energy source is the absorbed sunlight. Temperature measurements agree with these values, which suggest that these planets do not have appreciable internal energy sources (which would raise the effective tempera¬tures). In the case of Neptune, however, it is possible that there is such a source, which may be caused by tidal friction produced by Triton, Neptune’s very large and close satellite.

All but one of the planets have rotation axes roughly perpendicular to the planes of their orbits. The exception is Uranus, which has an axial inclination of 98° so that the axis is more or less in the plane of the orbit. Twice hi each orbit of the Sun, the axis is at right angles to the direction to the Sun; this last occurred in 1966. In between these times, one or other of the poles points more or less directly at the Sun. If north and south are defined by the direction of the rotation the north pole will face the Sun continuously in 1985. It is believed that there is very little effective heat exchange between the two hemispheres. The poles will there¬fore heat up when they face the Sun, possibly by as much as 20 per cent. The poles actually receive more heat per orbit of the Sun than does the equator and hence are the hottest parts of Uranus.

Telescopically, Uranus appears as a small bluish-green disc about four seconds of arc m diameter. The best modern observations, even with large telescopes or from balloons, show no surface features. Yet. at times of superior seeing, the best classic visual observers have usually reported two faint equatorial belts on either side of a bright zone and darker poles. Certainly these belts, if they exist, must be right on the limit of what can be detected. Neptuue shows a disc of the same bluish-green colour but just 2.5 seconds of arc in diameter. There are a few reports of some very weak spots of irregular shape, but none of any band structure.

Certainly neither planet has any surface features that can be used to determine the rotation period. The currently-used rotation periods of 10.8 hours for Uranus and 1EL8 hours for Neptune were derived from the variation across the disc of the Doppler shifts of the spectral lines. The measurements are somewhat uncertain and could be in error by as much as half an hour or an hour respectively. An alternative way to obtain the period is to observe any changes in the light from the planet caused by regions of different brightness passing across the visible disc as the planet rotates. Observations of Uranus are in disagreement as to whether such fluctuations do take place and it is not known whether any of those reported are real. The situation for Neptune is similar.

The greenish-blue colour of the two planets is caused by the tremendous absorption of red light, for which the methane in the atmospheres is largely responsible. Uranus reflects about 50 per cent of the incident short wavelength (blue) light but only 10 to 20 per cent of the long wavelength (red) light. The albedo of Neptune is much the same as that of Uranus.

Much less is known about the atmospheric structures of Uranus and Neptune than for Jupiter and Saturn. The visible layers in the atmospheres must have temperatures roughly equal to the effective temperatures quoted above. Deeper in the atmosphere the temperatures will be higher. Radio emission from both Uranus and Neptune has been detected over a wide wavelength range. This is thermal emission coming from below the visible layers of the atmospheres where the temperature is between 130 and 200K. If, as expected, ammonia is present on Uranus then a cloud layer of solid ammonia particles must form where the temperature is about 170 K and the pressure about eight atmospheres. This layer is much deeper in the atmosphere than are the corresponding layers on Jupiter and Saturn and it is well below the visible part of the atmosphere. There should also be a thin methane haze with its base at 60 K and 0.4 atmospheres. On Neptune the ammonia cloud layer must be deeper in the atmosphere where the pressure is higher than it is on Uranus. There may also be thin argon clouds high in Neptune’s atmosphere.

Like Jupiter and Saturn, Uranus and Neptune are believed to contain large amounts of lighter elements, but are not thought to be as rich in hydrogen. In recent years the radii of both Uranus and Neptune have been remeasured and found to differ by around ten Per cent from previous measurements. This means that the calculated mean densities have altered by 30 per cent. A consequence of this is that theoretical models of the internal structure of Uranus and Neptune calculated during the 1960s using the old, erroneous values are now obsolete. It is not known whether the two planets have hot or cold interiors or whether they have a solid surface. One possibility, shown in figure 11.20, is that each has a rocky core 16000km in diameter surrounded by an 8000km layer of ice. The rest of the planet consists of molecular hydrogen.

Comments are closed.