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 Not Enough of the Factor Just Right Too Much of the Factor Situation in the Solar System Planet Habitable Low temperatures cause chemicals At about 125oC, protein and Life seems limited to a Surface: Only Earth’s surface is in Temperature to react slowly, which interferes carbohydrate molecules and temperature range of minus 15oC this temperature range. with the reactions necessary for genetic material (e.g., DNA and Influences how quickly to 115oC. In this range, liquid Sub-surface: The interior of the life. Also low temperatures freeze RNA) start to break apart. Also, atoms & molecules water can still exist under certain solid planets & moons may be in water, making liquid water high temperatures quickly move conditions. this temperature range. unavailable. evaporate water. 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, Water Water is regularly available. Life Too much water is not a The chemicals a cell needs for Titan, seems to be covered with can go dormant between wet problem, as long as it is not so Dissolves & transports energy & growth are not dissolved liquid methane. periods, but, eventually, water toxic that it interferes with the chemicals within and to or transported to the cell Sub-surface: Mars & some moons needs to be available. chemistry of life and from a cell have deposits of underground ice, which might melt to produce water. Europa, has a vast oceans beneath its outer shell if ice. Atmosphere Small planets and moons have Earth & Venus are the right size to Venus’s atmosphere is 100 times Of the solid planets & moons, only Traps heat, shields the insufficient gravity to hold an hold a sufficient-sized atmosphere. thicker than Earth’s. It is made Earth, Venus, & Titan have surface from harmful atmosphere. The gas molecules Earth’s atmosphere is about 100 almost entirely of greenhouse significant atmospheres. Mars’ radiation, and provides th escape to space, leaving the planet miles thick. It keeps the surface gasses, making the surface too atmosphere is about 1/100 that of chemicals needed for or moon without an insulating warm & protects it from radiation & hot for life. The four giant planets Earth’s, too small for significant life, such as nitrogen blanket or a protective shield. small- to medium-sized meteorites. are completely made of gas. insulation or shielding. and carbon dioxide. Light energy is a problem if it Surface: The inner planets get too Energy When there is too little sunlight or With a steady input of either light makes a planet too hot or if there much sunlight for life. The outer too few of the chemicals that or chemical energy, cells can run Organisms use light or are too many harmful rays, such planets get too little. provide energy to cells, such as iron the chemical reactions necessary chemical energy to run as ultraviolet. Too many energy- Sub-surface: Most solid planets & or sulfur, organisms die. for life. their life processes. rich chemicals is not a problem moons have energy-rich chemicals. Without chemicals to make proteins Surface: Earth has a water cycle, Too many nutrients are not a & carbohydrates, organisms cannot All solid planets & moons have the an atmosphere, and volcanoes to problem. However, too active a grow. Planets without systems to same general chemical makeup, so circulate nutrients. Venus, Titan, Io, Nutrients circulation system, such as the deliver nutrients to its organisms nutrients are present. Those with a and Mars have nutrients and ways constant volcanism on Jupiter’s Used to build and (e.g., a water cycle or volcanic water cycle or volcanic activity can to circulate them to organisms. moon, Io, or the churning maintain an organism’s activity) cannot support life. Also, transport and replenish the Sub-surface: Any planet or moon atmospheres of the gas planets, body. when nutrients are spread so thin chemicals required by living with sub-surface water or molten interferes with an organism’s that they are hard to obtain, such organisms. rock can circulate and replenish ability to get enough nutrients. as on a gas planet, life cannot exist. nutrients for organisms.