The relationship between depth and seismic velocity. The chemistry of the interstellar medium

The times at which signals from the same earthquake arrive at different seismic observatories can be recorded, and thus it is possible to determine the travel time of the disturbance in terms of distance. Figure 3.5 illustrates travel time curves for P. S and surface waves plotted against distance from the earthquake.

The graph for the first arrival of surface waves is a straight line as it depends simply on geographical distance. The graphs for P and S waves, however, are curves showing that travel times do not increase proportionately with distance. The explanation is that their velocity is greater at depth within the Earth. The travel time depends on how the velocities of the different seismic waves change as they pass through materials having different elastic properties.

From the travel time curves it is thus possible to obtain velocity depth curves (Figure 3.9) which provide the basic information for estimating the physical properties of the Earth's interior, starting with the density. Although velocity generally increases with depth it is possible to predict the effects of a zone in which the velocity decreases with depth.

3.9: The variation of P and S wave velocity and density within the Earth. The width of the narrow bands indicate the uncertainty in the data

This will cause refraction, as shown in Figure 3.10. and there will be a range of distances from the source of waves in which arrivals are not observed (a shadow zone). There is a discontinuous drop in the velocity of P waves across the core-mantle boundary leading to a shadow zone, which was predicted by Oldham in 1906 and verified by Gutenberg in 1912.

3.10: The effect seismic waves of a layer in which the velocity (a) decreases with depth and (b) increases with depth in an otherwise ‘normal’ Earth, (c) P. PKP and PKIKP waves and the shadow zone

The shadow zone applies to P waves in the range 105° to 143° (ie 11 600 km to 16 000 km) away from the source. However, some waves are recorded in this range, and so it cannot be regarded as a true shadow zone. The amplitudes of such waves are, nevertheless, much reduced and for many years their presence was attributed to diffraction around the boundary of the core. Lehmann suggested in 1935 that the waves recorded in the shadow zone had passed through an inner core in which the P velocity is significantly greater than that in the outer core. Later work has corroborated her hypothesis and the existence of a solid inner core is now well established.

The chemistry of the interstellar medium.Space between stars is not empty but is sparsely occupied by diffuse matter, both gaseous and particulate. Locally this matter is gathered into clouds, and in our galaxy such clouds create dark patches in the Milky Way by obscuring starlight.

This interstellar medium is composed of primordial gaseous hydrogen and helium synthesized in the big bang, together with a mix of heavier elements, created in the envelopes of red giant stars and blownout into space by the pressure of the solar wind, and others spewed into space during periodic supernovae. This is the material out of which new stars such as our Sun and planets are born when the clouds undergo local condensation into a hot, denser cloud, or solar nebula.

Knowledge of the physical and chemical properties of the interstellar medium comes from spectral studies of starlight which is modified by both absorption and scattering in its passage through the clouds. Hydrogen and helium are the predominant gaseous elements. Gaseous carbon, oxygen and nitrogen do not occur in the same abundance as observed in the stars, but instead are believed to be present as solid chemical compounds.

Thus the particles of interstellar dust are composed of mineral cores about 0.1 micrometre (um) in size, surrounded by a mantle of dirty ice 0.3 um in diameter. Minerals that have been tentatively identified are quartz, magnetite, and magnesium silicate; water, ice. metallic iron and graphite are also present. There is a growing list of more complex molecules that have been identified, especially organic molecules composed of the life-forming elements carbon, oxygen and nitrogen. For instance ammonia (NH₃). carbon monoxide (CO), thioformaldehyde (H₂CS) and even complex species like HC-N (cyanotriacetylene) have been detected.

4.4: The periodic table of the elements, showing their geochemical classification

Stars born early in the evolution of the galaxy (called first generation stars) are composed principally of primordial hydrogen and helium. However, second and later generation stars condense out of interstellar gas enriched in all the heavier elements and compounds mentioned above, due to the course of nucleosynthesis. Compared to the universe as a whole, the solar system, and particularly the Earth, is enriched in heavier elements and life-essential elements (C. 0. N, P). All the heavier elements on Earth were synthesized inside a generation of stars that evolved and died prior to the birth of the Sun and planets from a second generation solar nebula.