Question

Why do we think that Venus should have outgassed similar amounts of carbon dioxide and water vapor as Earth? Where is Venus's carbon dioxide today? Where is Earth's?

Short answer

Venus's carbon dioxide is mainly in its atmosphere, while Earth's is in carbonates and rocks.

Step-by-step solution

Introduction to Planetary Atmospheres

Venus and Earth are similar in size and composition, leading scientists to hypothesize that they formed from similar materials. As such, it is reasonable to assume that they originally had similar amounts of volatiles, such as carbon dioxide (CO extsubscript{2}) and water vapor (H extsubscript{2}O).

Understanding Outgassing

Outgassing is the process by which gases, trapped within a planet, are released into the atmosphere through volcanic activity. It is believed that both Earth and Venus have undergone significant outgassing since their formation, which would involve releasing similar quantities of CO extsubscript{2} and H extsubscript{2}O due to their similar initial compositions.

Venus's Carbon Dioxide

Today, Venus's atmosphere is composed of about 96.5% carbon dioxide. The thick atmosphere and high surface temperatures of Venus suggest that its carbon dioxide remains predominantly in the atmosphere, contributing to its extreme greenhouse effect.

Earth's Carbon Dioxide Storage

In contrast, Earth's carbon dioxide is stored in several different reservoirs. A substantial amount of CO extsubscript{2} has been absorbed by the oceans and converted into carbonates which form part of the oceanic and terrestrial carbon cycle. Additionally, a significant portion is locked within carbonate rocks, such as limestone, through chemical weathering processes.

Key concepts

Outgassing

Outgassing in planetary contexts refers to the release of gases from a planet’s interior into its atmosphere. This process is fundamental in shaping the atmosphere of a planet. Imagine it as the planet 'breathing out', where volcanic activity plays the main role.
This is crucial for planets like Earth and Venus that have similar sizes and made from similar materials. A lot of the gases we find in a planet's atmosphere today, such as carbon dioxide (CO₂) and water vapor (H₂O), were likely released through this process. These gases were originally trapped inside the planet when it was formed. Key points about outgassing:

• Occurs through volcanic eruptions and openings in the planet's crust.
• Essential for the formation of an atmosphere on terrestrial planets.
• Contributes to the presence of major gases like CO₂ and H₂O.

Because Earth and Venus have similar initial conditions, scientists believe they should have outgassed similar amounts of carbon dioxide and water vapor.
Understanding this process helps us explain the present-day atmosphere of these planets.

Carbon Dioxide in Venus

Venus is often referred to as Earth's "twin" because of its similar size and composition, but its atmosphere is vastly different. Today, about 96.5% of Venus's atmosphere is composed of carbon dioxide. This heavy concentration of CO₂ in the atmosphere contributes to the planet's extremely hot surface temperatures. On Venus:

• Most carbon dioxide remains in the atmosphere.
• High atmospheric pressure traps heat.
• The thick layers of CO₂ are responsible for an intense greenhouse effect.

This composition results in Venus having the hottest surface temperatures of any planet in our solar system, even hotter than Mercury, which is closer to the Sun. The carbon dioxide on Venus contributes to the overall atmospheric pressure that is about 92 times that of Earth. This dense atmosphere not only traps heat making planet-wide temperatures rise, but also leads to atmospheric conditions that are vastly different from those on Earth.
In essence, understanding Venus's atmosphere provides valuable insights into extreme greenhouse effects.

Carbon Dioxide Storage on Earth

Unlike Venus, Earth has managed to store much of its carbon dioxide in places other than its atmosphere. This is one of the main reasons Earth can sustain life as we know it. The storage of carbon dioxide on Earth occurs in several ways, buffering the greenhouse effect that we see on Venus. Here's how Earth stores its carbon dioxide:

• Oceans absorb CO₂ from the atmosphere and convert it into carbonates. These carbonates become part of the ocean and terrestrial carbon cycle.
• Carbonates are also formed through chemical weathering and end up being stored in rocks like limestone.
• Plants absorb CO₂ during photosynthesis, storing it in biomass.

These processes help in maintaining a balance in Earth's atmospheric CO₂ levels. This natural storage system of carbon dioxide has been crucial in keeping Earth's climate stable over geological timescales, unlike Venus, where most CO₂ remains atmospheric.
This balance allows Earth to have a more moderate climate, suitable for supporting diverse ecosystems.

Greenhouse Effect on Venus

The greenhouse effect on Venus is a dramatic demonstration of how atmospheric composition can influence a planet's climate. The abundance of carbon dioxide in Venus's atmosphere creates a thick envelope that traps heat, leading to runaway greenhouse conditions. Some characteristics of Venus's greenhouse effect include:

• Venus’s surface temperature is extremely high, averaging around 467 °C (872 °F).
• The thick CO₂ layer effectively blocks heat from escaping back into space.
• The pressure of Venus's atmosphere is enormous, further contributing to its extreme heat retention.

In comparison, Earth's greenhouse effect is balanced by its ability to store CO₂ away from the atmosphere, in oceans and rocks, unlike Venus where the atmospheric CO₂ contributes significantly to its climate. Exploring the greenhouse effect on Venus gives us insights into the possible future scenarios for Earth, especially with rising CO₂ levels from human activities.
It acts as a cautionary tale of what unchecked greenhouse gas emissions could lead to in our own planetary system.