Explanation:
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
