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Elements of LifePractice
    1. All chemical elements in the universe except lithium
    2. About 25 different elements
    3. About 96 percent of all known elements
    4. Ninety-two naturally occurring elements
    1. To explain how it is that the elements required for life can be found everywhere.
    2. To provide evidence that our solar system is relatively young.
    3. To argue that some solar systems are more likely to support life than others.
    4. To explain why heavy elements have greater mass than hydrogen and helium.
    1. Older star systems are likely to have fewer planets, moons, asteroids, and comets than newer star systems.
    2. Newer star systems probably contain more hydrogen and helium than older star systems.
    3. Newer star systems probably contain more heavy elements than older star systems.
    4. The process of solar-system formation may have been fundamentally different in older star systems than in newer star systems.
    1. additionally
    2. however
    3. in particular
    4. on the contrary
    1. typically
    2. unsurprisingly
    3. necessarily
    4. naturally
    1. Planetesimals may remain in star systems when hydrogen and helium combine with less common heavier elements.
    2. Planetesimals are composed of heavy elements because hydrogen and helium stay in the form of gases.
    3. Planetesimals are small, solid objects that condense within a forming star system and may become planets.
    4. When planetesimals accumulate to form planets, they inevitably contain gaseous as well as heavy elements.
    1. It cannot be based on silicon or nitrogen.
    2. It could not survive on Earth.
    3. It probably would not be made of elements produced by stars.
    4. It is likely to have carbon as its chemical basis.
    1. Some of them were probably brought to Earth by asteroids or comets.
    2. Some of them probably formed in the atmosphere and oceans.
    3. They were probably significantly different from the organic molecules present on other planets in the solar system.
    4. They included complex molecules.
    1. Organic molecules must be protected from solar radiation by a surface layer of liquid.
    2. Planets that lack both of these features are probably too small to have been hit by many asteroids or comets carrying organic matter.
    3. Organic molecules need a liquid or gaseous environment to bring them together so they can interact.
    4. An atmosphere is needed to protect organic molecules from being destroyed by asteroids and comets.
  1. To answer the question “Could life exist on other planets?” we must first look at the necessary preconditions for life.
  2. The chemical elements that make up cells are likely to be available on just about any planet.
    1. Somewhere between 4 and 25 naturally occurring elements are necessary for life, depending on the complexity of the organism.
    2. Life is most likely to be found in the oldest star systems, where heavy elements have been continually produced since those systems were formed.
    3. Life is most likely to exist on those bodies that were not heavily bombarded with asteroids and comets during the formation of the solar system.
    4. The most common elements used by life-oxygen, carbon, hydrogen, and nitrogen—are also some of the most widely distributed elements in the universe.
    5. Planets, moons, asteroids, and comets are all composed of heavy elements, which means they contain the basis for any life form, carbon based or otherwise.
    6. Organic molecules are widely available, but chemical reactions among these molecules probably require either an atmosphere or a liquid medium.