CBSE Class 9 Science Chapter 14: Natural Resources NCERT Solutions

Our atmosphere is significantly different from those of Venus and Mars primarily due to its composition. Earth's atmosphere is a vital mixture composed of approximately 79% nitrogen, 20% oxygen, and a small fraction of other gases like carbon dioxide, water vapor, and noble gases. This specific composition is essential for supporting the diverse life forms found on Earth.

In contrast, the atmospheres of Venus and Mars are overwhelmingly composed of carbon dioxide. On Venus, carbon dioxide constitutes about 97% of the atmosphere, while on Mars, it makes up around 95%. These compositions create vastly different environmental conditions, making life as we know it impossible on those planets.

The atmosphere acts as a protective blanket for the Earth in several ways, regulating its temperature and making it habitable:

1. Temperature Regulation: It helps maintain a relatively stable average temperature across the Earth's surface, preventing extreme heat during the day and extreme cold during the night.
2. Moderating Daytime Heat: During the day, the atmosphere absorbs some of the incoming solar radiation, preventing the Earth's surface from overheating rapidly.
3. Preventing Heat Loss at Night: At night, the atmosphere slows down the rate at which heat escapes from the Earth's surface into outer space. This trapped heat keeps the planet warm enough for life to survive.

Winds are primarily caused by the uneven heating of the Earth's surface by the sun. Here's how this process leads to wind:

1. Heating and Rising Air: When a part of the Earth's surface is heated more than another, the air above it gets heated, becomes less dense, and rises.
2. Pressure Difference Creation: As the warm air rises, it creates a region of lower atmospheric pressure near the surface. Areas that are not heated as much have cooler, denser air, resulting in higher atmospheric pressure.
3. Air Movement: Air naturally flows from regions of high pressure to regions of low pressure. This horizontal movement of air is what we experience as wind.

Clouds are formed through a process involving evaporation, rising air, cooling, and condensation:

1. Evaporation: During the day, heat from the sun causes water from oceans, rivers, lakes, and other water bodies to evaporate, turning into water vapor in the air. Water vapor also enters the atmosphere through biological processes like transpiration from plants and respiration from animals.
2. Rising and Cooling Air: This warm, moist air, being less dense, rises into the atmosphere. As it ascends to higher altitudes, the atmospheric pressure decreases, causing the air to expand and cool down.
3. Condensation: When the air cools sufficiently, it can no longer hold all the water vapor. This excess water vapor condenses into tiny liquid water droplets or ice crystals. The presence of microscopic dust particles, salt, or smoke in the air acts as nuclei, facilitating this condensation process.
4. Cloud Formation: These numerous tiny water droplets or ice crystals suspended in the atmosphere collectively form clouds.

Several human activities contribute significantly to air pollution. Here are three prominent examples:

1. Burning of Fossil Fuels: The combustion of fossil fuels like coal, petroleum (petrol, diesel), and natural gas in vehicles, power plants, and industries releases harmful gases (such as carbon monoxide, sulfur dioxide, nitrogen oxides) and particulate matter into the atmosphere.
2. Industrialization: Many industrial processes, including manufacturing, chemical production, and smelting, release pollutants into the air. These can include toxic gases, volatile organic compounds (VOCs), and fine particles.
3. Deforestation: While not a direct emission source, deforestation reduces the number of trees that absorb carbon dioxide and release oxygen. This imbalance can contribute to increased levels of greenhouse gases and reduced air quality over time. Additionally, the burning of cleared vegetation also causes immediate air pollution.