When, How Did Earth Become Life Friendly?
Art by Karen Teramura
On March 31, a Frontiers of Astronomy Community Lecture presented by the IfA and the UH NASA Astrobiology Institute (NAI) attempted to answer the question, "When and how did our planet become conducive to life?"
A panel consisting of Jeff Taylor, a geologist from the Hawaii Institute of Geophysics and Planetology and a member of the UH NAI team; Karen Meech, IfA astronomer and principal investigator for UH NAI; and Steve Mojzsis from the Department of Geological Sciences at the University of Colorado explained what we know so far about this process. Steve Freeland, an evolutionary biologist who is now the project manager of UH NAI, served as moderator.
Freeland explained that NASA organized the NASA Astrobiology Institute (NAI) in 1998 to study the origins, evolution, distribution, and future of life in the Universe and that UH NAI is one of 14 NAI centers located throughout the United States. Life on Earth, he said, is amazingly diverse, with only a small fraction of it visible to the unaided eye.
So how did Earth, at first a ball of molten rock, evolve into a planet with a surface of blue oceans and green lands filled with life? First, it was necessary to build a planet. Taylor spoke about what he termed the "haphazard" construction of our planet. In the solar nebula, dust grains collided and stuck together. Collisions led to larger and larger bodies. Less volatile elements and compounds, such as ferrous oxide (FeO), condensed out of the solar nebula at the higher temperatures prevalent in the inner solar system, so early Earth was mostly metallic and rocky.
But what about water, which condenses at a much lower temperature? Liquid water is required for life as we know it, so next Meech discussed the origin of Earth's water. There is evidence of some water in many locations in the solar system--on Saturn's moon Enceladus, on Mars, and on the Moon--but generally speaking, the inner planets are dry. Even on Earth, the oceans account for only 0.023 percent of the planet's mass, and the total amount of water is estimated at no more than a tenth of a percent of Earth's mass. The exact amount of water on our planet is undetermined, because scientists can only estimate how much water is deep beneath Earth's surface.
So where did Earth's water come from? Was it captured from the solar nebula because water molecules stuck to the dust that eventually formed the planet? Did a major impact cause Earth's surface to melt into a magma ocean, after which hydrogen in the atmosphere reacted with oxygen to form water? Or did it arrive when asteroids or comets collided with Earth? Scientists are trying to match the ratio of deuterium (D) to hydrogen (H) in the water from these sources with that on Earth, but such attempts may not be accurate because we do not know the D/H ratio for water inside Earth. Meech concluded by saying Earth's water probably came from many sources.
Mojzsis spoke about how Earth became habitable, and how it is a laboratory in the search for life elsewhere. Evidence shows that about 4.53 billion years ago (about 40 million years after the solar system formed), a large body collided with the proto-Earth, causing the formation of the Moon and vaporizing magma on both bodies. After the collision, Earth cooled rapidly. Two million years later, it had an atmosphere of rock vapor and water vapor. Forty million to 600 million years after the solar system formed, Earth had water and a crust, and was ready for life. We know this because geologists have found rocks that are 4.3 billion years old and include minerals that required abundant water to form.
It appears, then, that the answer to the question, "When and how did our planet become conducive to life?" is "very early, and we still have much to learn."