Lectures - Monday and Wednesday, 11:00 AM - 12:15 PM
Lab - Tuesday, 4:10 PM -7 PM
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 11 km/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.
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.
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.
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.
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.
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 (Figure 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.
Text by James D. Hays, Spring 2004