The Earth formed in the midplane dust layer of the circumsolar disk. Proposed planetesimal formation mechanisms are streaming instability, disk fragmentation, and accretion. Scientists have thought that Jupiter formed prior to the inner planets, but new research might change the order. In addition to its place in the solar system, this chapter reviews several factors that led to the habitability of Earth: low percentage of water, the moon, plate tectonics, Earth's magnetic field, and the size and composition of the atmosphere.
Section 5-2 describes research on meteorites that indicates that the inner terrestrial planets formed quickly in the solar system, prior to Jupiter. If so, then the inner solar system planetesimals formed in a wet and frozen section of the disk. The planetesimals then dried out due to radioactive decay of aluminum 26 and associated heating. This research also indicates that the inner terrestrial planets formed from materials in the inner solar system and not from material that moved inward from the outer solar system.
Our single large moon has been an important factor in the habitability of the earth. Section 5-3 described two popular moon formation hypotheses. The most popular moon formation hypothesis has been the giant impact hypothesis. Recently, planetary scientist Sarah Stewart modified the giant impact hypothesis and proposed the synestia hypothesis of moon formation: the moon formed when a planetary embryo crashed into the earth and turned the earth into a rotating disk that eventually formed the moon and earth.
The Hadean Eon was the first eon in Earth's history. It may have been hellish, as the name implies, but it was an important first step in the habitability of earth. First, aluminum 26 dried out planetesimals and led to Earth having a low fraction of water. Second, the moon formed. Third, radiative heating from aluminum 26 also enabled differentiation of earth's crust. The primary atmosphere of the earth was hydrogen and helium, and this was replaced by carbon dioxide, methane, water, and nitrogen from volcanoes (Section 5-4).
The origin of life required a strongly reducing methane atmosphere that was devoid of oxygen. When astronauts brought back rocks from the moon, they discovered a period of intense asteroid bombardment, 3.9 Ga, the Late Heavy Bombardment (LHB). Section 5-5 describes how the LHB released methane into the atmosphere and was apparently the trigger for life since there is probably evidence of life within 20 million years of the end of the LHB. Scientists recently discovered the chemistry that forms the organic compounds needed for the origin of life. It begins with hydrogen cyanide and hydrogen sulfide, both of which were released by asteroids during the LHB.
Section 5-6 describes two of the keys to life on earth, plate tectonics and Earth’s magnetic field. Plate tectonics causes volcanoes, which are essential to life because they emit life supporting gases into the atmosphere. The earth’s magnetic field prevents solar radiation from stripping these gases out of the atmosphere. This section describes the processes in the Earth’s core that cause plate tectonics and the magnetic field.
Section 5-7 compares Earth’s atmosphere to Biosphere 2, Venus, and Mars. The mass of earth’s atmosphere, composition of gases, and 1-day rotation controls the average temperature and diurnal temperature range. Biosphere 2 requires enormous energy input to control temperature in its tiny atmosphere. Venus, with a massive atmosphere, and Mars, with a minimal atmosphere, are uninhabitable.
For the last few decades, there has been an ongoing debate over the existence of intelligent life on other planets. Ward and Brownlee triggered this debate with the publication of The Rare Earth Hypothesis. Section 5-8 includes a discussion between two modern astronomers (Silk and Livio), who discuss the fine tuning (abnormality) or mediocrity (commonality) of the solar system and earth, the possibility of other civilizations in the universe, the views of the ancient Greeks, and the search for extraterrestrials in the late 20th century, and the modern debate instigated by the publication of The Rare Earth Hypothesis by Ward and Brownlee.
Section 5E (optional) compares the Mosaic account of the third day to the formation of our habitable planet in the solar system
Earthrise. Credit: William Anders from Apollo 8 mission. Credit: NASA