Download Lecture #5: Evolution of the Earth`s Atmosphere

Lecture #5: Evolution of the Earth’s Atmosphere
Origin of the Earth:
The source of all energy and matter in the universe is the “Big bang”, which
occurred 15 - 20 billion years ago. Expanding gases from the big bang cooled and began
to condense to the first stars and galaxies. The planets in our solar system (including
Earth) condensed from the solar nebula ~4.5 billion years ago under the influence of
gravity. In the early days of the Earth (the first ½ billion years), bombardment by
meteorites was frequent. The Earth-Sun distance is about 150 million km.
Prebiotic Atmosphere:
Initially, the Earth’s atmosphere likely contained mostly hydrogen (H) and helium
(He), the most abundant elements in the solar nebula. The solar wind (which was
stronger than today) eventually stripped away the first atmosphere. Additional H and He
were also lost through gravitational escape. Only these elements are light enough to
escape Earth’s gravitational field. As the Earth’s core formed, dense compounds such as
iron (Fe) and nickel (Ni) settled to the center of the Earth, while lighter elements such as
silicates (Si), sodium (Na) and calcium (Ca) rose to the surface and formed the Earth’s
crust.
The second atmosphere (after a few hundred million years) evolved as a result of
outgassing (i.e. volcanic eruptions) from the Earth’s mantle (the mantle is the fluid
interior of the Earth between the core and the crust). The atmosphere consisted mainly of
N2, H2O, and CO2. At this point, all molecular oxygen (O2) was tied up in oxidized
molecules such as CO2 and H2O. Most outgassed water vapor (H2O) condensed to form
the oceans. Earth is the only planet in the solar system with surface oceans of water.
Water is essential to the evolution of life on Earth. Evaporation of water from the oceans
and subsequent condensation into clouds and precipitation (i.e. rain and snow) establishes
the hydrological cycle. The hydrological cycle provides energy to drive atmospheric
circulation, moves water to the continents, and causes weathering of soil and rocks. The
oceans also provided the first living organisms protection from UV radiation from the
sun. Without O2, and hence ozone (O3), UV radiation from the sun was much greater
than today.
Biotic Atmosphere Before Oxygen:
Scientists are still debating how life on Earth started. One idea is based on an
experiment by Stanley Miller and Harold Urey in 1953. They sealed H2O, CH4, NH3,
and H2 in a flask and subjected the gaseous mixture to an electrical discharge. This
resulted in a “soup” of organic molecules at the bottom of the flask. This organic “soup”
was not alive, but generated amino acids, the building blocks of proteins. Proteins are
essential components of all living cells. Over hundreds of millions of years, these
organic molecules may have let to the first self-replicating molecules, DNA (the
molecule carrying the genetic code). It has also been argued that life may have been
carried to Earth on comets from other parts of the galaxy. In this scenario, life would
have had to evolve elsewhere.
The oldest organisms did not require molecular oxygen (O2). During early biotic
evolution, the major energy producing process was fermentation:
C6H12O6(aq) Æ 2C2H5OH(aq) + 2CO2(g)
Anaerobic respiration by methanogenic bacteria was a source of methane (CH4) in the
early atmosphere:
2H2(g) + CO2(g) Æ CH4(g) + 2H2O(aq)
At some point, bacteria (phototrophs) developed the ability to obtain their energy from
sunlight. This process is called photosynthesis.
CO2(g) + 2H2S + hv (sunlight) Æ CH2O(aq) + H2O(aq) + 2S·
Since this does not result in the production of O2, it is called anoxygenic photosynthesis.
The Oxygen Age:
The Earth’s atmosphere lacked O2 until the start of oxygen producing
photosynthesis:
CO2(g) + H2O(aq) + hv (sunlight) Æ CH2O(aq) + O2(g)
At first, build up of atmospheric O2 was slow, as most was oxidized, e.g. CH4 + 3O2 Æ
CO2 + 2H2O. O2 was only a few percent of present atmospheric levels (PAL). It took
about 1.5 billion years for oxygen levels to begin their sharp rise to levels present today.
Geological evidence points to a rise in atmospheric O2 roughly 2 billion years ago.
Formation of the ozone (O3) layer:
The rise of O2 in the atmosphere allowed for the formation of the ozone (O3)
layer. Ozone absorbs sunlight in the ultraviolet (UV) region of the radiation spectrum.
Since UV radiation can harm life, the formation of the O3 layer contributed to the further
evolution of life, and hence the atmosphere. The details on how the O3 layer formed will
be covered in week 9.