Learning Objectives
Explain how Earth's atmosphere was initially developed. Summarize the composition of Earth's early atmosphere.
Explain the sources of water for Earth's early ocean and explain the source of salts dissolved in it.
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Outgassing The
Process of releasing gases, including water vapor, from magma.
He early atmosphere must have formed after Earth had coalesced and was large enough to retain gases in its gravitational field. The gases comprising Earth’s original atmosphere were lost during the time that the Solar System was forming. After this, and while Earth was in the molten state, volatiles easily escaped to the surface in a process called outgassing (or degassing). The outgassing process was responsible for Earth’s second atmosphere. Magma is composed not just of liquefied rock but also of a large proportion of compounds and elements in the gaseous state. Most outgassing of Earth accompanied early differentiation of the surface and interior, but degassing, on a much smaller scale, still continues to the present day. Active volcanoes and deep-sea vents constantly emit atmospheric gases to the atmosphere.
By applying uniformitarianism, we can use the composition of gases from modern volcanic eruptions to determine what gases were released to the primitive atmosphere during the Archean Eon. Most of the gas released during a volcanic eruption is water vapor (HgO). Smaller quantities of hydrogen (H2), hydrogen chloride (HCl), carbon dioxide (CO2), carbon monoxide (CO), nitrogen (N2), and other gases are also emitted. The early atmosphere was probably anoxic, but it may not have been anoxic for long.
What is the origin of the world ocean? For many years, it has been assumed that water vapor released during the major phases of Earth’s outgassing condensed to form the liquid water of the ocean. This seems reasonable, based on the fact that more water vapor is released from volcanoes than from any other source. This view is now changing, however. Some astronomers contend that the largest share of water in the early ocean was derived from comets passing through Earth’s atmosphere. Comets, which come from outside the Solar System, consist of ice and cosmic dust (Figure S. S). When their orbital paths bring them into the Solar System, heat from the Sun causes some of the ice to melt. Today, as many as 15 million comets, most of them small, pass through our planet’s upper atmosphere per year. As they melt, they deliver a nearly continual supply of water to the surface.
Comets and volcanoes release water that is virtually salt-free, so the ocean’s salt must have another
Comets and the ocean figure s. s
Comets like this one enter the Solar System and undergo partial melting because of heat from the Sun. The comet's tail is mostly composed of water and dust released by melting. Comets frequently enter Earth's upper atmosphere and are a nearly constant source of water raining to the surface. They may have been a major source of water for the world ocean.
Source. The most likely source is chemical weathering of rocks, particularly on land and at the shoreline. Rivers began carrying dissolved ions to the oceans early in the Archean Eon, and it is hypothesized that the ocean reached its current salinity well before the end of the eon. Since the Archean, the salinity flux of the world ocean has remained relatively constant. Salts have been added to the ocean by rivers at about the same rate that they have been precipitated from seawater as sedimentary deposits.
CONCEPT CHECK
How was Earth's early atmosphere developed?
What was the gaseous composition of Earth's early atmosphere?
What are the most likely sources of water for
Earth's early ocean? What is thought to be the most important source of water?
Why, if the likely sources of water for the ocean are essentially freshwater sources, is the ocean salty?
World History
