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16 July, 10:45

Which factors allow for life on Earth? Question 3 options: A Protective BAtmosphere Liquid Water C Livable Temperature D All of the above

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  1. 16 July, 13:29
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    What Makes a World Habitable?

    Use this table to identify the factors (and the appropriate levels) that will enable you to design your habitable worlds.

    Factors that make a

    Planet Habitable Not Enough of the Factor Just Right Too Much of the Factor Situation in the Solar System

    Temperature

    Influences how quickly

    atoms & molecules

    move

    Low temperatures cause chemicals

    to react slowly, which interferes

    with the reactions necessary for

    life. Also low temperatures freeze

    water, making liquid water

    unavailable.

    Life seems limited to a

    temperature range of minus 15oC

    to 115oC. In this range, liquid

    water can still exist under certain

    conditions.

    At about 125oC, protein and

    carbohydrate molecules and

    genetic material (e. g., DNA and

    RNA) start to break apart. Also,

    high temperatures quickly

    evaporate water.

    Surface: Only Earth's surface is in

    this temperature range.

    Sub-surface: The interior of the

    solid planets & moons may be in

    this temperature range.

    Water

    Dissolves & transports

    chemicals within and to

    and from a cell

    The chemicals a cell needs for

    energy & growth are not dissolved

    or transported to the cell

    Water is regularly available. Life

    can go dormant between wet

    periods, but, eventually, water

    needs to be available.

    Too much water is not a

    problem, as long as it is not so

    toxic that it interferes with the

    chemistry of life

    Surface: Only Earth's surface has

    water, though Mars once had

    surface water and still has water ice

    in its polar ice caps. Saturn's moon,

    Titan, seems to be covered with

    liquid methane.

    Sub-surface: Mars & some moons

    have deposits of underground ice,

    which might melt to produce water.

    Europa, has a vast oceans beneath

    its outer shell if ice.

    Atmosphere

    Traps heat, shields the

    surface from harmful

    radiation, and provides

    chemicals needed for

    life, such as nitrogen

    and carbon dioxide.

    Small planets and moons have

    insufficient gravity to hold an

    atmosphere. The gas molecules

    escape to space, leaving the planet

    or moon without an insulating

    blanket or a protective shield.

    Earth & Venus are the right size to

    hold a sufficient-sized atmosphere.

    Earth's atmosphere is about 100

    miles thick. It keeps the surface

    warm & protects it from radiation &

    small - to medium-sized meteorites.

    Venus's atmosphere is 100 times

    thicker than Earth's. It is made

    almost entirely of greenhouse

    gasses, making the surface too

    hot for life. The four giant planets

    are completely made of gas.

    Of the solid planets & moons, only

    Earth, Venus, & Titan have

    significant atmospheres. Mars'

    atmosphere is about 1/100th that of

    Earth's, too small for significant

    insulation or shielding.

    Energy

    Organisms use light or

    chemical energy to run

    their life processes.

    When there is too little sunlight or

    too few of the chemicals that

    provide energy to cells, such as iron

    or sulfur, organisms die.

    With a steady input of either light

    or chemical energy, cells can run

    the chemical reactions necessary

    for life.

    Light energy is a problem if it

    makes a planet too hot or if there

    are too many harmful rays, such

    as ultraviolet. Too many energyrich chemicals is not a problem

    Surface: The inner planets get too

    much sunlight for life. The outer

    planets get too little.

    Sub-surface: Most solid planets &

    moons have energy-rich chemicals.

    Nutrients

    Used to build and

    maintain an organism's

    body.

    Without chemicals to make proteins

    & carbohydrates, organisms cannot

    grow. Planets without systems to

    deliver nutrients to its organisms

    (e. g., a water cycle or volcanic

    activity) cannot support life. Also,

    when nutrients are spread so thin

    that they are hard to obtain, such

    as on a gas planet, life cannot exist.

    All solid planets & moons have the

    same general chemical makeup, so

    nutrients are present. Those with a

    water cycle or volcanic activity can

    transport and replenish the

    chemicals required by living

    organisms.

    Too many nutrients are not a

    problem. However, too active a

    circulation system, such as the

    constant volcanism on Jupiter's

    moon, Io, or the churning

    atmospheres of the gas planets,

    interferes with an organism's

    ability to get enough nutrients.

    Surface: Earth has a water cycle,

    an atmosphere, and volcanoes to

    circulate nutrients. Venus, Titan, Io,

    and Mars have nutrients and ways

    to circulate them to organisms.

    Sub-surface: Any planet or moon

    with sub-surface water or molten

    rock can circulate and replenish

    nutrients for organism
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