Atmospheres of Venus, Earth, and Mars

Planetary Atmospheres

• Mercury and the Moon have basically no atmosphere.
• Venus, Earth, and Mars were able to obtain significant secondary atmospheres.

Venus, Earth, and Mars

• All three were at some point volcanically active and should have been hit by comets.
• These planets differ in an important way: their surface gravity.

Greenhouse Effect

• Incoming sunlight heats the planet.
• Outgoing infrared radiation cools the planet.
• Some gases, especially $CO_2$ and water vapor, block some infrared radiation, preventing the planet from cooling.
• This greenhouse effect means the planet is at a higher temperature.
• Earth would be $35 K$ colder without this, so everything would freeze!
• A huge greenhouse effect makes Venus $400 K$ hotter!

The Atmospheres Evolved Differently

• Mars and Venus both have mainly $CO_2$ atmospheres.
• Mars’ atmosphere is much less dense than Venus’, so no greenhouse effect.
• Venus: hot early, so no water on surface.
• Earth: water removes $CO_2$ from atmosphere, forming limestone.
• Earth: life further removed $CO_2$.
• Two of the most effective greenhouse gasses, water and $CO_2$, were therefore contained on Earth; on Venus, they were not, which meant a runaway greenhouse effect.

Earth’s Atmosphere

• Mostly nitrogen and oxygen.
• Other planets do not have oxygen ($O_2$) in their atmospheres.
• Life is responsible for the oxygen in Earth’s atmosphere.

Earth’s Atmosphere Layers

• Troposphere
• Stratosphere
• Mesosphere
• Thermosphere
• Magnetosphere
• All of the weather we experience happens within the troposphere.
• Ozone layer exists within the stratosphere.
• Each layer is defined by how temperature changes through it.
• Temperature decreases by altitude in the troposphere and mesosphere.
• Thermosphere is where the solar wind dominates.
• Magnetosphere is where the magnetic field is dominant.

Earth’s Magnetic Field

• Earth’s magnetic field, the magnetosphere, extends out into space.
• Blocks much of the solar wind.
• Solar wind = flow of particles from the Sun.
• Particles from the solar wind get trapped by the Earth’s magnetic field.
• They then collide with the atmosphere.
• This creates the northern and southern lights (auroras).

Winds and Circulation

• Parts of the Earth are heated differently.
• Circulation of air (convection) distributes surface heating.
• Global winds carry heat from hot to cool regions.

Weather

• Convection creates violent weather patterns on Earth.
• Thunderstorms
• Tornadoes
• Hurricanes
• Coriolis effect makes large storms spin.

The Coriolis Effect

• Imagine standing at the North Pole and throwing a rock toward the equator.
• If you could throw it hard enough, you would see that it doesn’t move in a straight line!
• The earth rotates under the rock as it travels, making its path appear curved.
• This phenomenon is called the Coriolis effect.
• Atmosphere experiences the Coriolis Effect in the same way.
• Direction of rotation is opposite in the southern hemisphere, but doesn’t affect toilets!
• Earth isn’t only planet with weather; Dust Devils can occur on both Mars and Earth

Weather on other planets

• Lightning has been observed on Earth, Venus, and all four giant planets.

Venus

• Hot, dense atmosphere, completely cloud-covered.
• Surface temperature about $737 K$ ($870°F$).
• Thick atmosphere means nearly uniform temperatures over the entire planet.
• Rotates on its axis very slowly, and in the opposite direction, probably because of a big impact.
• Can “see” surface of Venus with radar.
• The surface of Venus seems to be relatively young.
• Lots of volcanic activity resurfaces the planet rapidly.
• The Russian Venera spacecraft have visited the surface of Venus.
• Surface temperatures hot enough to melt lead!
• The spacecraft only lasted up to two hours due to the extreme temperatures and pressure.

Mars

• Cold, thin atmosphere.
• No oxygen, no ozone.
• Thin atmosphere = extreme temperature variations.
• Equator: up to $293 K$ ($68 F$).
• Pole: down to $123 K$ ($-238 F$).
• Consequently large winds, which can make big dust storms.

The Moon and Mercury

• Almost totally airless.
• Combination of temperatures and low escape velocity means any atmosphere is lost.
• No erosion from wind or other weathering, so old, cratered surfaces are retained.