Visions of the Cosmos

Planetary Science

The Solar System

Uranus and Neptune

Ice Giants of the Frozen Zone - Uranus and Neptune

Introduction

For an observer on Earth the Sun presents a large disc too resplendent to gaze upon.   One day when standing upon one of the airless low gravity moons of Uranus, an astronaut of the future may behold our Sun as an extremely brilliant point of light, the brightest star in the sky but still just a ‘star’ among all the other stars..  Although the Sun will give 1,000 times as much light as the full Moon seen from Earth it will give little warmth to the surface of that tiny world.    At its nearest Saturn will be quite bright but Neptune the next planet out would only be visible with the naked eye when at its closest.   Our Earth and the other planets of the Inner Solar System would forever remain invisible to the naked eye.   

In 1871, William Herschel discovered Uranus and six years later in 1787 its two largest moons Oberon and Titania.  Uranus itself cannot be seen from Earth except with the aid of a telescope.  At its nearest to the Sun it is over 2,700 million kilometres and at its most distant it is just over 3,000 million.   Uranus is a great big sphere of gas – like Jupiter it has no solid surface but unlike Jupiter it contains more methane, ammonia and water and the pressures in its interior is not believed to be high enough to turn hydrogen into a metal.  For these reasons Uranus is not really a 'Gas Giant' like Jupiter and Saturn.  The true Gas Giant planets have roughly speaking the same composition as stars.  Uranus and Neptune on the other hand have much greater proportion of the ice forming compounds such as water, ammonia, and methane.  These two distant worlds hold many surprises.  The internal structures of these two planets are believed to be quite different to the Jovian Giants. They are in fact totally different to Jupiter and Saturn; they are not tiny Sun-like planets and are comparatively deficient in hydrogen.   Under the thick atmosphere of hydrogen and helium there are regions occupied by ices, which overlay the rocky cores.

Because of the presence of methane in its upper atmosphere Uranus is a blue green colour.  It has a fairly well defined and complicated ring system but nothing like as complex or extensive as Saturn.  One of its most peculiar features is that it is inclined at an angle of 98 degrees.  Twice during its long journey round the Sun – it’s year is just over 84 Earth years – it is actually at right angles to the Sun so that the poles take turns at being exactly beneath it.  The reason for this strange situation is not known but it believed that at some time in the past it may have undergone a gigantic collision.

It is very cold and the temperature of the top of the atmosphere at the 1.3 bar pressure level was measured as -192º Celsius (81ºKelvin) by the instruments on board the Voyager spacecraft.   The daytime temperature on the surfaces of its tiny moons will be around -187º Celsius (86 degrees above absolute zero).

Nearly half as far again as Uranus from the celestial fires of our star, Neptune and its satellite Triton pursue a 165-year orbit round the Sun.  Triton, the large planet-sized moon of Neptune is undoubtedly one of the coldest spots in the Solar System.   The Sun will be even less brilliant than it is from Miranda and the temperature during a warm summer's day may hit around -233º Celsius (40 degrees above absolute zero).   During summer nights in the warmest time of the long year, the thin nitrogen atmosphere will deposit solid nitrogen frost upon the surface of Triton.  Occasionally geysers of liquid nitrogen under high pressure will erupt into the thin atmosphere before the nitrogen changes to a gas and then sublimes to scatter nitrogen snow upon the frozen ground

As for Neptune itself, although it is possessed of a certain amount of internal heat, the temperature at the top of the troposphere is so cold that even gases like ethane freeze to the solid state. At about -208ºCelsius (65ºKelvin) it is even colder than Uranus. At this temperature methane condenses and it is possible that for Neptune there is a haze of droplets of liquid Argon at about the 5-millibar level.

Neptune and Triton are so remote from our warm and beautiful planet that even when they are at their nearest radio waves take about 4 hours to reach us, a matter of importance during space missions. 

Table 14.1 lists some of the most important properties of the two large planets that orbit in the frozen zone.

Statistical Data on Uranus and Neptune

Planet

Distance from Sun in Kilometres

Mass compared to Earth

Density grams per cc

Equatorial Diameter Kms

Polar Diameter Kms

Uranus

2,735,000,000            Perihelion       3,004,000,000            Aphelion

approximately

14.5

1.267

51,320

50,100

Neptune

4,537,000,000             Perihelion

4,456,000,000            Aphelion

approximately

17.26

1.640

49,528

48,680

 

Often distances of the planets from the Sun are stated in Astronomical Units.  An astronomical unit is taken as the mean distance of the Earth from the Sun  (149,597,892 kilometres).  The approximate mean distances of Uranus and Neptune from the Sun in astronomical units are 19.18 and 30.06 respectively.

Although Uranus has a somewhat lower mass compared to Neptune, it has a slightly larger diameter.  This is because it has a considerably lower density, which is a reflection of the fact that the two planets have somewhat different compositions.

The outer atmospheres of these planets contain mostly hydrogen and helium but in both cases relatively large proportions of methane are present.  This substance gives Uranus its bluish-green colour and Neptune its blue colour.  This is shown in the colour photographs.  Estimations of the ratio of carbon to hydrogen in the outer atmospheres are very much higher than in the Sun and Jupiter and therefore very much greater than the assumed average in the original solar nebula.  It is for this reason that the outer atmospheres are so enriched in methane.

Uranus seems a much quieter planet than Neptune.   A few spots and streaks seemed to move zonally from east to west.  No lightning or auroral display were observed by the Voyager spacecraft. 

Neptune proved a much more interesting subject for observation.  The planet had a Great Dark Spot (GDS) about 20 degrees South.   The GDS rotated in a counter-clockwise direction and had streamers and a high altitude bright haze associated with it.  The main cloud level was at a 3 bar (about three Earth atmospheres) but thin methane cirrus clouds overlay most of the planet at 1.5 bar.  A pinkish colour instead of blue occurred in a few places and it has been suggested that this may be due to clouds of solid methane.  Several other markings could be seen on Neptune such as the Small White Spot at 55 degrees South and an irregular shaped cloud called the Scooter at 42 degrees South.   Very strong winds were detected in the upper atmosphere of Neptune.

Although the two planets only receive relatively small amounts of radiation from the Sun there is some evidence that slow rates of photolysis by ultraviolet radiation does occur in the upper layers of their atmospheres. Small traces of acetylene have been detected in the upper atmosphere of Uranus.   Ethane has been detected in the upper atmosphere of Neptune.  In late 1998 the European Space Infrared Space Observatory (ISO) discovered the presence of the methyl radical in the upper atmosphere of Neptune.  This was the first time that this highly reactive free radical had been detected in the atmosphere of any of the outer giants and may account for the presence of ethane in the atmosphere of Neptune.  It was present in one part in 30 million in the upper atmosphere, which was about two to three times higher than its concentration in the atmosphere of Saturn.

Hydrogen (H2), methane, helium and the HD molecule have been detected in the lower parts of the atmospheres of Uranus and Neptune.   The isotopic ratio of deuterium to hydrogen lies between 2 x 10-5 and 4 x10-4 which is only slightly above the value believed to have been present in the original solar nebula (see details under Jupiter, Earth and Venus).

It is suggested that at lower levels there may be massive clouds of water mixed with ammonia above a deep hot ocean of the material that makes up the major parts of these two planets.

Most of the hydrogen will form a deep ocean of supercritical hydrogen in the molecular state.  It is thought that the pressure never rises high enough to bring about the phase change to metallic hydrogen as it does on Jupiter and Saturn. layer of lightest material overlying the ices.

Both are relatively poor in hydrogen and helium and enriched in other chemical elements by factors of around 200 relative to the Sun.  The ices represent a region rich in molecules of H2O, NH3 and CH4.    The picture of the 'ices' as solid entities may not in fact represent the true state of affairs.   The pressures and temperatures that exist in the interiors of these planets must lie beyond the values at which ‘ices’ of H2O, NH3 and CH4 can exist as solids.   In order to comprehend the nature of these substances under extremes of temperature and pressure it is necessary to understand the nature of supercritical fluids.

The Supercritical Fluid State and the Planetary Interiors.

Planetary geologists are interested in the way in which matter behaves at the extremely high pressures and temperatures met with in the interiors of planets.

We have already seen that the outer atmospheres of Jupiter and Saturn merge into what is often described as an ocean of  'liquid molecular hydrogen'.  This should be more accurately called an ocean of supercritical molecular hydrogen.

Consider the so-called 'ice layer' and 'rock layer' in the interiors of Uranus and Neptune.   The temperatures and pressures of the simple molecular substances such as water, ammonia and methane are well above the critical values.   These substances are in a supercritical state.

It is possible that the concept of distinct layers in the internal structure of Uranus and Neptune is an incorrect model and that there are no sharply defined discontinuities.  The low molecular weight substances such as hydrogen and helium, which compose the outer envelopes of Uranus and Neptune, may grade into a supercritical ocean of complex composition which grades further down into a dense ocean rich in minerals.

The solubility of many ionic substances such as sodium chloride in ordinary liquid water is well known. What is not generally known however is that the minerals of which rocks are composed are appreciably soluble in supercritical water particularly if ammonia is also present.    Rocky minerals such as enstatite  (magnesium silicate MgSiO3 ) are not soluble in water and not appreciably soluble in solutions of ammonia in water.   However, enstatite is known to be soluble in a supercritical layer rich in water and ammonia above the critical point of water: -

H2O   +   NH3  +    MgSiO3    ®        SiO44-   +   2NH4+   +   Mg2+

It may be postulated that many other combinations of minerals containing the silicate ions, oxides ions, sulphide ions and metal ions such as Fe2+, Ca2+, Na+, K+ may well dissolve in the supercritical fluid systems that may exist in the interiors of Uranus and Neptune.

Illustration 14.1. Uranus from Voyager

 

Two images of Uranus taken by Voyager 2 at a distance of 9.1 million km. 17 January 1986. Acknowledgement NASA/JPL phot-library indexP-29478

The picture on the left is a composite using images from the blue, green, and orange, filters, processed to show Uranus more or less as the human eye would see it. The atmosphere is very clear, the blue-green color coming from absorption of red light by methane. The image on the right was produced using ultraviolet, violet, and orange filters to exaggerate the contrast. The dark polar hood is over the south pole of the planet. The doughnut shapes are camera blemishes.
 

Illustration 14.2. Neptune.


In the summer of 1989, NASA's Voyager 2 became the first spacecraft to observe the planet Neptune, its final planetary target. Passing about 4,950 kilometers (3,000 miles) above Neptune's north pole, Voyager 2 made its closest approach to any planet since leaving Earth 12 years previously. Five hours later, Voyager 2 passed about 40,000 kilometers (25,000 miles) from Neptune's largest moon, Triton, the last solid body the spacecraft will have an opportunity to study.                       

Acknowledgements Photo Courtesy NASA/JPL

 Solar System