The Inner and Outer Cores

The densities of the two core components require a major constituent to be ferrous material, that is primarily iron with nickel and cobalt. The density of the out core is rather less and probably contains non-ferrous elements, the most likely being oxygen and or sulphur. This surmise is based partly on the cosmic abundance but also upon the chemistry of elements in the proportions of the chondritic Earth. The outer core is unable to withstand shear (is unable to transmit S-waves) and so must be...

Plates and Plate Tectonics

Surface material is formed at the ridge system but the surface of the Earth remains constant. This can only happen if surface material is being returned to the inner Earth at the same rate that it is being formed. These regions are the subduction zones and are placed substantially away from the mid-ocean ridges. Between them is a layer of surface, some oceanic and some continental, which moves from the regions where the surface is formed to the locations where it is destroyed. These surface...

Internal Conditions Internal Differentiation

It is of interest to attempt an assessment of the possibility of internal differentiation of the interior. By this is meant the separation of the materials according to density, the ferrous material having sunk towards the centre forcing the less dense materials (water ice and silicates) upwards. The criteria chosen is the ratio of the gravitational energy of the material per atom to Table 16.4. The ratio of the self-gravitational energy to an average interaction energy per atom in the icy...

The Mercury Atmosphere

The planet is too small and the temperature is too high to sustain a stable, dense atmosphere. Solar wind particles striking the surface can become trapped by the magnetic field (see III. 19) to form an atmosphere of very low pressure which is continually being renewed. Estimates suggest a pressure of about 1015 bar which is 0.01 picobar. The average temperature has been seen to be 440 K although this can be as high as 725 I< on the sunward side. The main constituents are in this order by...

The Hertztsprung Russell Diagram

A star radiates energy and the Stefan-Boltzmann radiation law states that, in the ideal case to be considered in a moment, the quantity of energy radiated through unit area of the star in unit time (that is the flux of energy through the surface) is proportional to the fourth power of the surface temperature. The constant of proportionality is known as the Stefan-Boltzmann constant. This would be the actual surface temperature were the star to radiate as a black body2 but, although it does so...

Problems and Solutions

Let T be the time for a planet to make one orbit round the Sun (the orbital period) and a the mean radius of its orbit (semi-major axis). Using the data for the planets of the Solar System, calculate T2 and a3 for each planet and derive the ratio T2 a3 constant. What conclusions can you draw (Hint it is convenient to express the orbital period and semi-major axis of each planet in terms of those for the Earth). Problem 1.2. Figure 1.8 shows that the variation of the apparent...

Several Planets The Centre of Mass

The Solar System is composed of nine planets orbiting the Sun with orbits lying in a thin flat disc of radius nearly 40 AU (that is a distance a little less than 6 x 1012 m). Beyond that is a region of small particles which extend the System substantially. Kepler's laws refer to a single planet in orbit about the Sun under the action of gravity, although they can be extended quite generally to apply to any body orbiting another under gravity. Just as the force of gravity acts between the Sun...

The Larger Members of the Solar System

The Solar System includes a wide range of objects from the large to the very small. The planets, satellites and asteroids are the larger and more massive members and their general properties are considered now. The Sun is the biggest body in the Solar System with a mass of 1.99 x 1030 kg. The mean radius is 696,00 km (which is 108 Earth radii) giving it a mean density of 1.41 x 103 kg m3. The surface temperature is 5,800 K making it spectroscopically a dG2 star (the d is for dwarf,1 a...

Stars

About Planets and Exo-Planets An Introductory Notebook Imperial College Press 57 Shelton Street Covent Garden London WC2H9HE World Scientific Publishing Co. Pte. Ltd. 5 Toh Tuck Link, Singapore 596224 USA office 27 Warren Street, Suite 401-402, Hackensack, NJ 07601 UK office 57 Shelton Street, Covent Garden, London WC2H 9HE British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library. About Planets and Exo-PIanets An Introductory...

Short Time Variations The Secular Variation

The fact that the magnetic field changes with time was known to very early observers. Magnetic measurements have been made at several magnetic observatories, including Kew in England and Potsdam in Germany, since the 16th century. One set of data are shown in Fig. 17.7 showing the values of the magnetic declination measured at Kew between 1550 and 1998. The declination has fallen and then risen again during this period although the change is not periodic. The variation of the full non-dipolar...

R ne

This shows that the central temperature varies as M1 4. Surprisingly, the central temperature is insensitive to the mass of the star and to the precise chemical composition. 21.7. The Life Expectancy Dependence on the Mass The total energy locked originally in a star will be of order Me which is radiated away at a rate given by the luminosity, say (21.13). Dividing the total energy available by the rate of radiating it away will give a measure of the characteristic time, r, for the radiation to...

B

The surface waves, (a) Rayleigh wave being a vertical elliptical motion in the plane containing the direction of motion (b) Love wave involving horizontal motion. Rayleigh and Love waves are dispersive, that is their velocities depending on the wavelength. The wave profile changes with the distance, the phase travelling with a phase velocity characte ristic of a particular frequency. The wave energy travels with the greater group velocity which again depends on the frequency of the...

The General Topology of Mars

The topography of the surface is shown in Fig. 12.7. The different shades of blue denote regions below the mean radius, with purple being about 8,000 m below. The yellow regions straddle the mean radius and the shades of red show regions above the mean radius. White regions are the highest with an elevation in excess of 12,000 m. The most striking property is the great difference in elevations between the northern and the southern hemispheres. The north lies below the mean radius whereas the...

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

Isoclinic Line

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

Far Side Hunter

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

Magnetised Sphere

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

Biology Solar Abundance Elements

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...