Info

distance from the centre the aerosol became more and more dilute until finally it became pure gaseous phase.

In space, including the solar system, the abundance of the elements decreases with increasing atomic weight. The second column of Table 3 gives the cosmic abundance of the elements expressed relative to silicon as 104 (Urey, 1952). The last two columns of this table contain the stable compounds of the elements for a pressure of 10 _ 3 atm and for two different temperatures. On the basis of these data it can be assumed that the gas phase of the protoplanet consisted mainly of H2 and He with lesser amounts of CH4, HzO, NH3 and H2S. In the case of the inner planets the development of the gravitation field and the temperature (due to the radioactive heating and solar radiation) promoted the dissipation of the cosmic gases, which means that these planets lost their original gaseous materials rather quickly. According to model calculations (e.g. Szadeczky-Kardoss, 1968) the mass of the Earth before dissipation was at least 190 times greater than the present value. The duration of this dissipation is estimated by Kuiper to be around 108 years (see in Szadeczky-Kardoss, 1968). It is possible, however, that this time was even longer since, in the original system, the dissipation was somewhat hindered by the presence of dispersed particles.

It seems evident that the degree of dissipation was small in the case of the outer giant planets. This explains the fact that the atmosphere of these planets is composed of the above mentioned substances. Thus, according to Opik (see Cadle, 1966),

Table 3

Cosmic and terrestrial abundance of elements and their stable compounds under different conditions of temperature and pressure (Urey, 1952). The values of the dissipation losses are also given

Stable components

Element Cosmic abundance Present Earth Loss, _ IP-10-atm)'

H 3.5 xio8 3.0xl04 9.4 xlO"4 H2, CH4, H2, H20,

NHj, H20 H2S

-2.4 xlO5) 6.5 x 10"" 1.6x10"7 Ne Ne

Si 1.0 xlO4 SiOj Si02

S 3.5 xlO3 1.5x10 4.5 xlO"2 FeS H2S

Fe 1.8 xlO4 FeS, Fe304 Fe

Fe2Si04

Not* . 1 Relative to oxygen;1 For P - I atm and T=298 K the stable compounds are the same as for /*=■= 10 3 atm and

99.5% of the Jupiter's atmosphere consists of hydrogen and helium, while the clouds of the planet probably consist of ammonia crystals.

In the case of the inner planets, there was no dissipation of those gaseous compounds that either condensed on (depeding on temperature and pressure conditions) or chemically reacted with the solid and liquid phases. Under the conditions of the proto-Earth water accumulated, an event of major importance. It was this accumulation that made the formation of the biosphere possible, the evolution of which led to the appearance of a thinking species, man.

It can be seen from Table 3 that the dissipation of noble gases was particularly intense. This is reasonable, since these gases did not react with the solid or liquid materials forming the planet Earth. On the other hand the loss in the case of hydrogen was relatively small, a significant portion of this element remained in a condensing compound (water).

The process of dissipation can be well demonstrated by the so-called deficiency factor of rare gases. This factor gives the ratio between terrestrial and solar abundances. The deficiency factors of rare gases in meteorites and in the atmosphere as a function of the atomic weight are plotted in Fig. 2 (Suess, 1966). One can see that

Rare Gases The Atmosphere

The deficiency factor of rare gases on the Earth (solid line) and in some meteorites according to Suess (1966), where M„ is the molecular weight (By courtesy of Tellus)

The deficiency factor of rare gases on the Earth (solid line) and in some meteorites according to Suess (1966), where M„ is the molecular weight (By courtesy of Tellus)

even the quantity of xenon with an atomic weight of 130 decreased by a factor of 107. It can also be seen that the abundance of argon on the protoplanet must have been some hundred millionfold greater than in the present atmosphere. Thus the present quantity of atmospheric argon is only a small residue of this huge mass.

0 0

Post a comment