Carbon dioxide accounts for about \(95 \%\) of the present-day atmospheres of both Mars and Venus. Why, then, is there a strong greenhouse effect on Venus but only a weak greenhouse effect on Mars?

A planetary atmosphere is a layer of gases surrounding a planet, held in place by the planet's gravity. These gases can vary widely in composition, density, temperature, and depth, influencing a range of planetary characteristics, including climate and the potential for life.

Earth's atmosphere contains a mixture of nitrogen, oxygen, and trace amounts of other gases, including carbon dioxide (CO2), which is a crucial component of the greenhouse effect. A planet's atmosphere acts as a blanket, trapping some solar radiation, which warms the surface. However, too much CO2 can lead to excessive warming, as seen in the runaway greenhouse effect on Venus. Contrastingly, Mars, with its thin atmosphere high in CO2, is less able to retain heat, leading to a much milder greenhouse effect and a cooler climate.

The atmospheric composition plays a key role in temperature regulation through various physical processes such as convection, conduction, and radiation, and determines the efficiency of the greenhouse effect for each planet.

Carbon dioxide (CO2) plays a significant role in shaping the climate on Mars and Venus because it is a potent greenhouse gas. Both planets have atmospheres made up of roughly 95% CO2. However, the impact of CO2 on each planet is dramatically different due to several other factors.

Venus's proximity to the sun and its dense atmosphere result in a surface temperature high enough to melt lead, despite the similarities in atmospheric composition with Mars. This is largely because the sheer amount of CO2 creates an incredibly dense atmosphere that can absorb and re-emit significant amounts of thermal energy.

In contrast, Mars' atmosphere is much thinner, with an atmospheric pressure so low that the CO2 present is inadequate to sustain a strong greenhouse effect, leading to a much cooler surface temperature. This exemplifies how the same gas can have varying impacts depending on the atmospheric context.

Atmospheric pressure, the force exerted by the weight of the atmosphere above a point, can greatly influence a planet's ability to retain heat and sustain a greenhouse effect. Higher atmospheric pressure allows an atmosphere to hold more gas molecules, including greenhouse gases which can absorb and re-emit infrared radiation.

On Venus, the atmospheric pressure is extremely high, about 92 times that of Earth's, which significantly enhances its capability to trap heat through the greenhouse effect. In contrast, Mars has a very low atmospheric pressure, less than 1% of Earth's, so despite its CO2-rich atmosphere, the pressure is too low to trap heat effectively. This illustrates that the greenhouse effect is not solely dependent on the composition of greenhouse gases, but also on the quantity and density of the atmosphere that exerts the pressure.

Solar radiation, the emission of energy from the sun, plays an essential role in determining a planet's climate and the strength of the greenhouse effect. The amount of solar radiation a planet receives is determined by its distance from the sun and impacts how much energy is available to be trapped by the planet's atmosphere.

Venus, being closer to the sun than Mars, receives more solar radiation. This abundant energy contributes to the planet's extreme temperatures through the greenhouse effect. Mars, situated further from the sun, receives less solar radiation, which means that even with its CO2-rich atmosphere, there is less energy available to be trapped and re-emitted as heat.

The impact of solar radiation is a critical factor in the greenhouse effect as it establishes the baseline temperature around which greenhouse gases can act, demonstrating the importance of a planet's position within its solar system in dictating climatic conditions.