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

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