Unit 1: Many Planets, One Earth // Section 2: Many Planets, One Earth
Our solar system formed from a solar nebula, or cloud of gas and dust, that collapsed and condensed about 4.56 billion years ago. Most of this matter compacted together to form the sun, while the remainder formed planets, asteroids, and smaller bodies. The outer planets, Jupiter, Saturn, Uranus, and Neptune, condensed at cold temperatures far from the sun. Like the sun, they are made mostly of hydrogen and helium. In contrast, the terrestrial planets, Mercury, Venus, Earth and Mars, formed closer to the sun where temperatures were too high to allow hydrogen and helium to condense. Instead they contain large amounts of iron, silicates (silicon and oxygen), magnesium, and other heavier elements that condense at high temperatures.
The young Earth was anything but habitable. Radioactive elements decaying within its mass and impacts from debris raining down from space generated intense heat—so strong that the first eon of Earth's history, from about 4.5 to 3.8 billion years ago, is named the Hadean after hades, the Greek word for hell. Most original rock from this period was melted and recycled into Earth's crust, so very few samples remain from our planet's formative phase. But by studying meteorites—stony or metallic fragments up to 4.5 billion years old that fall to Earth from space—scientists can see what materials were present when the solar system was formed and how similar materials may have been melted, crystallized, and transformed as Earth took shape (Fig. 2).
Figure 2. The Willamette Meteorite, the largest ever found in the United States (15 tons)
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Source: © Denis Finnin, American Museum of Natural History.
About 4 billion years ago, conditions on Earth gradually began to moderate. The planet's surface cooled, allowing water vapor to condense in the atmosphere and fall back as rain. This early hydrologic cycle promoted rock weathering, a key part of the carbon-silicate cycle that regulates Earth's climate (discussed in section 4). Evidence from ancient sediments indicates that oceans existed on Earth as long ago as 3.5 billion years.
Conditions evolved very differently on adjoining planets. Venus, which has nearly the same size and density as Earth and is only about 30 percent closer to the sun, is sometimes referred to as our "sister planet." Scientists once thought that conditions on Venus were much like those on Earth, just a little bit warmer. But in reality Venus is a stifling inferno with an average surface temperature greater than 460°C (860°F). This superheated climate is produced by Venus's dense atmosphere, which is about 100 times thicker than Earth’s atmosphere and is made up almost entirely of carbon dioxide (CO2) (Fig. 3). As we will see in Unit 2, "Atmosphere," CO2 is a greenhouse gas that traps heat reflected back from planetary surfaces, warming the planet. To make conditions even more toxic, clouds on Venus consists mainly of sulfuric acid droplets.
Paradoxically, if Venus had an atmosphere with the same composition as Earth's, Venus would be colder even though it is closer to the sun and receives approximately twice as much solar radiation as Earth does. This is because Venus has a higher albedo (its surface is brighter than Earth's surface), so it reflects a larger fraction of incoming sunlight back to space. Venus is hot because its dense atmosphere functions like a thick blanket and traps this outgoing radiation. An atmosphere with the same makeup as Earth's would function like a thinner blanket, allowing more radiation to escape back to space (Fig. 3).
Figure 3. Comparison of Venus and Earth
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Source: Courtesy NASA/JPL-Caltech.
Mars is not much farther from the sun than Earth, but it is much colder. Clouds of ice and frozen CO2 (dry ice) drift over its surface. Frozen ice caps at the poles, which can be seen from Earth with a telescope, reflect sunlight. Although Mars's atmosphere consists mainly of CO2, it is 100 times thinner than Earth's atmosphere, so it provides only a small warming effect. Early in its history, the "Red Planet" had an atmosphere dense and warm enough to sustain liquid water, and it may even have had an ocean throughout its northern hemisphere. Today, however, all water on Mars is frozen.
Why is Venus so hot? Why is Mars so cold? And why has the Earth remained habitable instead of phasing into a more extreme state like Mars or Venus? The key difference is that an active carbon cycle has kept Earth's temperature within a habitable range for the past 4 billion years, despite changes in the brightness of the sun during that time. This process is described in detail in section 4, "Carbon Cycling and Earth's Climate." Moderate surface temperatures on Earth have created other important conditions for life, such as a hydrologic cycle that provides liquid water.
How unique are the conditions that allowed life to develop and diversify on Earth? Some scientists contend that circumstances on Earth were extremely unusual and that complex life is very unlikely to find such favorable conditions elsewhere in our universe, although simple life forms like microbes may be very common (footnote 2). Other scientists believe that Earth's history may not be the only environment in which life could develop, and that other planets with very different sets of conditions could foster complex life. What is generally agreed, however, is that no other planet in our solar system has developed along the same geologic and biologic path as Earth. Life as we know it is a direct result of specific conditions that appear thus far to be unique to our planet.