Intrinsic Magnetic Fields

Both Jupiter and Saturn are the seat of strong intrinsic magnetic fields. The Jupiter field was first discovered before the first spacecraft visited the planet by Earth-based observatories through the reception of decimetre and decametre radio waves. These were interpreted as synchronous radiation in a strong planetary intrinsic magnetic field. The various details are collected in Table 14.3. It is seen immediately that the Jovian field is very strong but the Saturnian field is more moderate,...

The Oort Cloud

New comets appear at a constant rate of a little less than 1 per year and it is necessary to find where these come from. A significant step was taken by Jan Oort in 1948 who proposed a reservoir of potential comets beyond the normal scale of the Solar System, including the Edgeworth-Kuiper belt. New comets can come from one (or I suppose all) of only three sources. They could appear from outside the Solar System. This would imply that the solar neighbourhood is full of small objects but there...

The Roche Limit

Tidal forces have an extreme effect for the near approach of a body of appropriate size which can be disrupted by the gravitational forces. The smallest orbit that such a body can follow with stability is called the Roche limit. This comes about as follows. In Sec. 3.5a it was seen that the effect of tidal forces on a companion body is to stretch the material along the line joining the centres of masses of the two bodies, there being induced a force in the direction of the larger mass and away...

Separation into a Dipole and Nondipole Fields

The analysis of the details of the magnetic field has occupied many people over the last two centuries drafting maps and collecting data. The original approach was made by Gauss who showed mathematically that the observed Fig. 17.5. Lines of equal inclination and of declination for the northern hemisphere for the epoch 1835. The units are degrees (after Airy). Fig. 17.5. Lines of equal inclination and of declination for the northern hemisphere for the epoch 1835. The units are degrees (after...

The Magnetic Elements

The strength and orientation of the magnetic field at any point is described by the magnetic elements, shown in Fig. 17.2. Three reference axes are defined X pointing to the geographical north, Y to the east and Z pointing vertically downwards. The suspended magnet makes an angle with the Fig. 17.2. The magnetic elements in relation to the geographical axes, north, east and vertically downwards. Fig. 17.2. The magnetic elements in relation to the geographical axes, north, east and vertically...

Magnetic Storms arid Transient Disturbances

It has been known for a century or more that the intrinsic magnetic field has an underlying weaker field of apparently random transient components. The magnitude of this secondary field can be as small as a few gammas (1-y 10-9 T) but at other times of strong disturbance in may be as large as 10 of the main intrinsic field. The secondary field is variable over a wide range of time scales. The longest is 11 years, the period of the solar cycle, but the shortest can be hours or even minutes. The...

The Moon

The Moon has been a mystic in the sky since antiquity. Its motions were used as a clock for agriculture and for religion very early on and it has been the source of many myths and stories over the centuries. It is not easy to see clearly by eye but this changed with the introduction of the telescope by Galileo in 1609. The first map appeared in 1645, the work of Langrenus who was Astronomer to the Court of King Phillip II of Spain. It included some 300 features including 250 prominent craters....

The Venus Interior

The surface shows a range of volcanoes distributed widely over its surface and this gives the clue to the internal structure and how it differs from Earth. Like Earth, it will have a crust, a mantle and a core. The basic composition will be similar except for a lack of water in the crustal regions. Venus is, however, a slightly smaller version of Earth and the internal pressure will be lower, at any depth, by a factor of about 0.94. This small difference can have important consequences here as...

The Solar Cosmic Abundance of the Elements

Observations show a universal distribution of the chemical elements. The observations are of limited extent (for instance, they cannot be made directly inside a body) but their general occurrence is significant. The broad distribution is shown in Fig. 8.1. There are some strange apparent anomalies but generally the curve shows that the lightest elements are the most common. Fig. 8.1. The cosmic distribution of the chemical elements. Table 8.1. The cosmic distribution of the most abundant 14...

Above the Surface The Chromosphere and Corona

Figure 24.1 shows that the photosphere is enclosed by a thin atmosphere involving high temperature with the emission of high energy radiation. This is the chromosphere. This relatively thin region is encased in a more extensive region called the corona. Before observations with spacecraft were possible, which have allowed full photographs of the Sun such as Figs. 24.1 and 24.5 to be obtained, details of the outer regions enclosing the visible surface could only be detected during a short time...

The Sun and Its Interior

Density Sun

The Sun is typical of over 90 of the stars in the Universe and it is there for us to observe only 149 million kilometres away (which is 1 astronomical unit). It is composed very largely of hydrogen and derives its energy from thermonuclear processes which convert hydrogen nuclei into helium nuclei, Fig. 23.1. The Sun the typical main sequence star. (NASA ESA SOHO) Fig. 23.1. The Sun the typical main sequence star. (NASA ESA SOHO) Table 23.1. Some bulk data for the Sun with representative values...

Intrinsic Magnetism of the Earth

We explore now what is known of magnetism of the condensed Earth. The magnetic field was known to antiquity and was used very early on as a means of navigation. Only more recently has it been possible to view it on a truly global scale and to begin to relate the Earth's field to those of other planets. The study of the Earth's field has defined the parameters of the subject. The field is seen most easily by suspending a magnetic needle (originally made from lodestone, a magnetised rock, but it...

Not to be viewed either directly or through optical instruments

The mean surface temperature is 5800 K. It radiates in wavelengths from the longest to the shortest but those in the broadly visible range (between the infrared and ultra-violet) the emission is very similar to that of a black body. A solar spectral distribution is shown in Fig. 23.4. There, the dotted line is the theoretical Planck curve for a black body at 5800 K. There are components in the radio and X-ray regions but these are not black body more will be said of them later. The visible...

The Role of an Atmosphere Planetary Mass

Even a low mass star emits considerable quantities of ultraviolet and X-radiation so it might seem that living material will not be able to survive a stellar source of radiation. There is a second line of defence the planetary atmosphere. Among other things, this presents a protective layer for the surface but its effect depends on its composition. One effect is to filter out certain of the incoming radiation another effect is to restrict the wavelengths of the radiation able to escape to...

Energy from Fusion

Hydrogen is the most abundant element in the Universe and is the starting point of the energy production in stars. Four nucleons (two protons and two neutrons) combine to form one helium nucleus. The He nucleus consists of two protons and two neutrons and the total mass of all the constituents is 2 x (1.007825 + 1.008665) 4.032980 amu. The measured mass of helium is 4He 4.002603 amu, which is less than the masses of the constituents.3 The difference is 4.032980 - 4.002603 0.030377 Am. This is a...

The Formation of Molecules

One or two important molecules can be guessed at once. The first and second most abundant chemically active elements are hydrogen and oxygen (helium is chemically inactive). An immediate outcome must be water and we can expect this to be a common molecule. The occurrence of carbon and nitrogen suggests the presence of such molecules as NH3 and CH4. More complicated molecules are based on oxygen. These, in fact, form the silicate materials to be considered later. Iron is surprisingly abundant....

The Satellites of Mars

Mars has two satellites, both very small bodies probably captured long ago from the asteroid belt. Each has a density below 2,000 kg m3 and they appear to be composed of silicate ferrous material, probably very porously packed. Images of the satellites are shown in Figs. 16.6 and 16.7. The bodies have masses of 1.06 x 1016 kg Phobos and 2.4 x 1015 kg Deimos and are really large rock material. Deimos has an orbital period of 1.26 days and so will pass though all its phases each Mars day. Phobos...