What is Residual and Accumulated Atmosphere Hypothesis?

Most of the geo-scientists agree that the earth has originated from the cloud composed of gases and cosmic dust. Some of them believe that the protoplanetary cloud was hot while others think it was cool.

Even though much remains unknown about the way in which the waters and gases accumulated around the earth, they both seem to have originated from the volcanic out gassing which has taken place since the formation of the planet.

On the basis of the assumed nature of the origin of the earth, two groups of hypotheses have been advanced to explain the origin of the earth’s atmosphere and hydrosphere.

1. residual atmosphere hypothesis

Advocates of this hypothesis believe that the earth was originally in a molten state and had a dense vapour saturated atmosphere, which later cooled first to a liquid and then slowly developed a solid crust. Thus, according to this theory, the atmosphere and hydrosphere are residual materials from the primitive atmosphere that enveloped the earth.

Since hydrogen and helium are the most abundant elements in the

sun and because methane (CH4) and ammonia (NH3) are the most abundant gases in the atmosphere of the major planets of the solar system (excepting earth) which are also stable in the presence of art abundance of hydrogen and helium, it is believed that the primitive atmosphere was having hydrogen, helium, nitrogen and water vapour.

Helium being a very light and inert gas does not tend to form com­pounds to be retained by the force of the earth’s gravitational attraction. With the continuation of cooling and chemical reactions, hydrogen com­pounds were formed which helped to retain hydrogen in the earth. Thus the gases were gradually transformed in to liquid and then into solid.

Objections

A.P. Vinogradov has indicated that if the molten planet had been initially surrounded by the heavy atmosphere, the water vapour present in such an atmosphere would have exerted a tremendous pressure. Under such conditions the rocks should have contained more water than it is being currently observed.

Besides, the present day atmosphere should have preserved a large volume of inert cosmic gases (viz. Neon, Helium, Argon, Krypton, Xenon etc.) being the relict of the original atmosphere. On the contrary, there is a very small amount of inert gases in the present-day atmosphere.

2. accumulated atmosphere hypothesis

According to this theory, the atmosphere has accumulated from the de­gassing of the earth’s interior and through chemical reactions that took place when the solid planetesimals accumulated.

The cool terrestrial material was heated due to the adiabatic compression and decay of radioactive elements. Some would suggest that an original atmosphere, derived from cosmic gases, was largely lost as the earth heated up.

The basis for this idea is that the atmosphere now has relatively small proportion”: of neon, argon, kryption and xenon compared with other planets. This event may have led to a new start in which most of the atmospheric constituents came from inside the earth water vapour, carbon dioxide carbon monoxide, nitrogen, chlorine, hydrogen and sulphur dioxide.

On cooling, the water vapour would condense in to the ocean. During a certain period of earth’s history, liquid water was relatively sparse at the earth’s surface.

Volcanoes mostly started producing hydro- spheric water and gave rise to small and shallow oceans which gradually reached its present dimensions. In this connection, it may also be indicated that the present ocean-floor is covered with numerous cones of extinct volcanoes.

Thus, according to most of the researchers, the earth’s atmosphere and hydrosphere could be produced by the volcanic out gassing.

Origin of the Continents

It has not yet been possible to ascertain the actual process by which the continents were formed. The views of various authors regarding the origin of earth are as follows:

(i) Because of the cooling and consolidation of the earth, on its surface wrinkles were developed. The upheaved portions formed the continents while the depressions where water got accumulated gave rise to oceans.

(ii) Some authors believe that unequal pressure of the atmos­phere on the surface of the earth, when it was in a liquid state, is responsible for the formation of continents and oceans.

(iii) Chamberlinc, the proponent of Planetesimal hypothesis, believes the unequal accumulation of planetesimals on the surface of the earth gave rise to continents and oceans. Continents were formed where the accumulation was more and oceans where it was less.

(iv) Others believe that folding of the earth’s surface caused the formation of continents and oceans. The anticlinal portion of the folds is considered to correspond to the continents and the synclines to the ocean basins.

(v) Some believe that continents began by the formation of island arcs and erosion of their volcanic materials exposed above sea-level to form sedimentary rocks. There is no record that this was the ease, but the Precambrian-continents were probably smaller than those of today. Addition to continental areas takes place by the formation of island arcs offshore, or by the collision of an island arc with a continent

The actual mode of origin of the earth is still continuing to be a controversial issue and so far not a single theory is able to confirm the actual mode of its formation.