Download Week 11 Wednesday session

Week 11 Wednesday session
I Comparison of Earth with its neighbors
A. Earth (Figure 1) has much in common with its two nearest
neighbors Venus (Figure 2) and Mars (Figure 3) yet it is
also very different. All three planets are dominantly rocky
objects with atmospheres dominated by elements other
than hydrogen and helium. The inner planets are
sufficiently small and consequently their low escape
velocities (Earth 11km /sec.) are insufficient to retain the
abundant light elements of hydrogen and helium. Venus
and the Earth are nearly the same size while Mars is less
than half the size of Earth. Venus and Mars have
atmospheres dominated by CO2. Earth on the other hand
has an atmosphere dominated by N2 and O2. Large
amounts of oxygen in the atmosphere of earth are unusual
for oxygen is a very reactive gas and readily combines with
metals and other elements. Earth’s atmosphere with
abundant free molecular O2 would mark it as a very special
planet for any intelligent beings visiting our solar system
for the first time. Earth also has large amounts of water in
its great oceans.
B. Venus carbon dioxide rich atmosphere creates a very strong
green house effect so the planet’s surface temperature is
hundreds of degrees Celsius, even with an albedo of .75.
Unlike Earth, Venus has little water in its atmosphere or on
its surface. Venus atmosphere does have nitrogen in fact by
mass there is about as much nitrogen in the atmosphere of
Venus as there is in the Earth’s atmosphere.
C. Mars like Venus has little water and an atmosphere
dominated by carbon dioxide. Because of its smaller size its
escape velocity is low so it has not been able to retain all its
nitrogen.
D. The larger planets Jupiter (Figure 4), Saturn (Figure 5),
Uranus and Neptune are far enough from the sun
(temperatures sufficiently cold) so that they retain the
original gasses of the nebula from which the solar system
formed. Consequently their atmospheres are very much like
the sun’s atmosphere. These planets are so massive that
their escape velocities are many times that of Earth and
they retain the light gasses such as Hydrogen and Helium.
E. It is likely that the gasses in the atmosphere of Venus came
from the interior of the planet as did our ocean and
atmosphere. The atmospheres of the outer giant planets
Jupiter, Saturn, Uranus and Neptune are very much like the
atmosphere of the sun and probably are residuals of the
original solar nebula from which the solar system was
made. However, the very different ratios of noble gasses in
the atmosphere of earth from that of the sun suggests that
the earth’s atmosphere is not a residual of the original solar
nebula. Consequently the earth’s atmosphere probably
originated from the expulsion of gasses from its interior.
Since Venus has a similar size and it probably scavenged
similar kinds of material as it was forming we expect that
the amounts of gasses expelled from its interior should be
similar to the amounts expelled from the earth’s interior.
F. The questions that need to be answered then are:
1. Where did all the water go that must have been
ejected into the atmosphere of Venus?
2. Where did all the Carbon Dioxide go that must have
been ejected into the atmosphere of Earth?
3. Why does the Earth have so much Oxygen in its
atmosphere and Venus has none?
II. The Early Earth and the Evolution of the Atmosphere
A. Early Earth probably had an atmosphere dominated by
carbon dioxide similar to the atmosphere of Venus today.
B. There are a group of one-celled organisms that can live in
an oxygen free environment. These are the bacteria or
prokaryotes. They do not have a nucleus and reproduce
only by cell division. These creatures are the earliest
evidence of life on earth. They were the first organisms to
develop photosynthesis. Photosynthesis today is balanced
by oxygen using respiration.
1) Hypothesis Oxygen was nearly absent in the
atmosphere of early Earth so photosynthesis would have
created a net gain of oxygen first in the ocean and later
in the atmosphere. Eventually with sufficient oxygen in
the atmosphere respiration would have balanced
photosynthesis except when burial removed the organic
material from the oxygenated water or air. Before oxygen
could build up in the atmosphere it must have oxidized
reduced ions in seawater.
a) Evidence to support the above hypothesis
Iron (Fe) is a very abundant element in the earth’s
crust so much is released by the chemical
disintegration of minerals contained in rocks. Fe++ is
slightly soluble in seawater while Fe+++ is insoluble
(Fitgure 6). During the time when the earth had a
reducing atmosphere Fe++ should have accumulated as
dissolved ions in seawater. However at some point the
oxygen build-up in the ocean from prokaryote
photosynthesis should have oxidized the Fe++ to Fe+++
resulting in the precipitation of insoluble iron
compounds. Are such ancient iron rich compounds
preserved? Yes there are, in fact the bulk of the iron
ore mined to produce steel comes from iron deposits
that are about two billion years old (Figure 7). Such
deposits are found on all continents and all look
much the same (Figure 8). They are reddish and have
clearly visible bands hence they are called Banded
Iron Formations. The Messabi range of Minnesota is
an example of such a deposit. It was for much of US
history the primary source of iron ore for the steel
mills of Pittsburg, Pennsylvania and Gary, Indiana.
If we know the mass of these banded iron formations
and the rate at which we mine them we can calculate
their residence time and determine how long they will
last, or when we will run out of this kind of iron ore
(Figure 9).
A second line of evidence, to suggest that the early
earth had a reducing atmosphere like Venus and
Mars, is the presence of detrital (formed from the
products of erosion of pre-existing rocks) pyrite in
sedimentary deposits older than two billion years old.
Iron pyrite forms in reducing environment and is
quickly chemically decomposed in the presence of
oxygen. Today such minerals are only preserved in
rocks that formed in reducing environments such as
swamps etc. However, in rocks older than two billion
years old this mineral (iron pyrite) is found in rocks
that were probably formed in streambeds.
C. The possible changing composition of the Earth’s
atmosphere during its early history is shown in Fig 10. All nucleated
cells (Eucaryote cells) require oxygen for metabolism. We and all
other plants and animals are built of eukaryotic cells so we all
require oxygen. Hence early primitive life (procaryote cells)
modified our planet by converting CO2 and H2O to organic matter
and releasing oxygen to the environment. As a consequence these
organisms moved carbon from the atmosphere to the rocks (Figure
11) and broke down water molecules releasing oxygen to the ocean
and eventually to the atmosphere. Life therefore is a powerful force
controlling the composition of the Earth’s atmosphere which in turn
exerts a powerful control on our planet’s climate.