Planetary atmospheres vary widely in composition, pressure, temperature, and weather. This cheat sheet helps students compare the atmospheres of terrestrial planets and gas giants using the same science ideas. It is useful for understanding why Venus is extremely hot, why Mars has little air, and why Jupiter and Saturn have powerful storms.
Comparing planets also shows how gravity, distance from the Sun, and atmospheric chemistry shape surface conditions.
Key Facts
- Mercury has almost no true atmosphere because its low gravity and high temperatures allow most gases to escape into space.
- Venus has a thick atmosphere made mostly of carbon dioxide, with a surface pressure about 92 times Earth's and a strong greenhouse effect.
- Earth's atmosphere is about 78% nitrogen, 21% oxygen, and 1% other gases, with an average surface pressure of about 1 atm.
- Mars has a thin atmosphere made mostly of carbon dioxide, with surface pressure less than 1% of Earth's, so liquid water is unstable on the surface.
- Jupiter and Saturn are gas giants with atmospheres made mostly of hydrogen and helium, plus trace gases such as methane, ammonia, and water vapor.
- Uranus and Neptune contain hydrogen, helium, and methane, and methane absorbs red light, giving these planets a blue or blue-green color.
- Atmospheric pressure follows the rule pressure = force / area, so more gas above a surface creates greater pressure.
- A planet is better at keeping an atmosphere when its escape velocity is high and its atmospheric gas particles move slowly compared with that speed.
Vocabulary
- Atmosphere
- An atmosphere is the layer of gases held around a planet or moon by gravity.
- Greenhouse effect
- The greenhouse effect is the warming that happens when atmospheric gases absorb and re-radiate infrared energy from a planet's surface.
- Atmospheric pressure
- Atmospheric pressure is the force per unit area caused by the weight and motion of gas particles in an atmosphere.
- Escape velocity
- Escape velocity is the minimum speed an object or gas particle needs to leave a planet's gravity without returning.
- Gas giant
- A gas giant is a large planet, such as Jupiter or Saturn, made mostly of hydrogen and helium with no solid surface like Earth.
- Trace gas
- A trace gas is a gas that makes up only a small percentage of an atmosphere but can still strongly affect temperature, color, or chemistry.
Common Mistakes to Avoid
- Confusing atmosphere thickness with oxygen amount is wrong because a planet can have a thick atmosphere with almost no oxygen, such as Venus.
- Assuming the closest planet to the Sun is always the hottest is wrong because Venus is hotter than Mercury due to its dense carbon dioxide atmosphere and greenhouse effect.
- Treating all gas giants as identical is wrong because Jupiter, Saturn, Uranus, and Neptune differ in temperature, methane content, storm systems, and internal heat.
- Ignoring gravity when comparing atmospheres is wrong because stronger gravity helps a planet hold gases for longer periods of time.
- Thinking a thin atmosphere means no weather is wrong because Mars has dust storms and clouds even though its atmosphere is much thinner than Earth's.
Practice Questions
- 1 Earth's surface pressure is about 1 atm, while Venus has about 92 atm. How many times greater is Venus's surface pressure than Earth's?
- 2 Mars has surface pressure less than 1% of Earth's. If Earth's pressure is 101 kPa, estimate the maximum typical surface pressure on Mars in kPa.
- 3 A planet's atmosphere is 78% nitrogen, 21% oxygen, and 1% other gases. Which planet is this, and what feature makes it unusual among the rocky planets?
- 4 Explain why a small, hot planet is less likely to keep a thick atmosphere than a large, cooler planet.