UPSC CSE Prelims
Earth and Universe Previous Year Questions (PYQs)

Detailed Explanation:

Correct Answer: ✅ Option 4 (Statement-I is incorrect, but Statement-II is correct)

Stars live for different lengths of time depending mainly on how fast they consume their nuclear fuel. Although giant stars contain more fuel, they burn it much more rapidly than dwarf stars.

❌ Statement I is Incorrect: Giant stars do not live longer than dwarf stars. Due to their high energy output, they exhaust their fuel quickly and have shorter lifespans.

✅ Statement II is Correct: Giant stars have much higher core temperatures and pressures, leading to a faster rate of nuclear fusion reactions than dwarf stars.

Relationship: Statement II actually explains why Statement I is incorrect. Faster nuclear reactions mean faster fuel consumption, resulting in a shorter lifespan.

Short Notes: Stellar Life Cycle

• The lifespan of a star depends mainly on its mass.

• More massive stars have higher core temperatures and faster fusion rates.

• Giant stars burn fuel much faster and may live only millions of years.

• Small dwarf stars can survive for billions to trillions of years.

• The Sun is a medium-sized star with an expected lifespan of about 10 billion years.

• Red dwarfs are the longest-living stars in the universe.

• Stars generate energy through nuclear fusion, mainly converting hydrogen into helium.

• After exhausting fuel, stars evolve into white dwarfs, neutron stars, or black holes depending on their mass.

Detailed Explanation:

Answer: Option 2 — Second half of the month of June

The longest day of the year in the northern hemisphere is known as the summer solstice, which typically occurs on June 20th or 21st, falling in the second half of June. On this day, the sun is directly overhead at the Tropic of Cancer (23.5°N latitude), resulting in maximum daylight hours in the northern hemisphere. The exact date may vary slightly from year to year due to the calendar system and Earth's orbital mechanics.

📝 Short Notes: Solstices and Equinoxes

• During solstices, the sun is directly overhead at either the Tropic of Cancer (June) or Tropic of Capricorn (December)
• During equinoxes, the sun is directly overhead at the Equator, resulting in equal day and night globally
• The tilt of Earth's axis (23.5°) is responsible for seasonal variations and solstices
• The term 'solstice' comes from Latin 'sol' (sun) and 'sistere' (to stand still), as the sun appears to pause at its northernmost or southernmost position

Detailed Explanation:

Answer: Option 3 — 1 and 3 only

This question tests knowledge about Earth's magnetic field reversals and the evolution of Earth's atmosphere. Statements 1 and 3 are scientifically accurate, while Statement 2 contains factual errors about the early Earth's atmospheric composition.

✅ Statement 1 – Correct: The Earth's magnetic field undergoes geomagnetic reversals approximately every few hundred thousand years, a phenomenon recorded in volcanic rocks and ocean floor sediments.

❌ Statement 2 – Incorrect: The early Earth's atmosphere (over 4 billion years ago) had virtually no free oxygen and was dominated by carbon dioxide, water vapor, nitrogen, and methane—not 54% oxygen.

✅ Statement 3 – Correct: Living organisms, particularly cyanobacteria through photosynthesis during the Great Oxygenation Event (~2.4 billion years ago), fundamentally transformed Earth's atmosphere by adding oxygen.

📝 Short Notes: Earth's Atmospheric Evolution

• Primordial Atmosphere: Early Earth (4+ billion years ago) had no free oxygen; atmosphere consisted mainly of hydrogen, helium, water vapor, methane, ammonia, and carbon dioxide.
• Secondary Atmosphere: Formed through volcanic outgassing, releasing CO₂, nitrogen, water vapor, and sulfur compounds.
• Great Oxygenation Event (GOE): Around 2.4 billion years ago, cyanobacteria began photosynthesis, gradually increasing atmospheric oxygen from ~0% to ~21% today.
• Geomagnetic Reversals: Earth's magnetic poles switch positions irregularly; last reversal occurred ~780,000 years ago (Brunhes-Matuyama reversal).
• Evidence of Reversals: Preserved in magnetized minerals in volcanic rocks (paleomagnetism) and oceanic crust spreading from mid-ocean ridges.
• Impact on Life: The oxygen revolution caused mass extinction of anaerobic organisms but enabled evolution of complex aerobic life forms.

Detailed Explanation:

Carbon, Hydrogen, and Nitrogen are the primary elements responsible for the origin of life on Earth.

Carbon forms the backbone of all organic molecules, while Hydrogen and Nitrogen are essential for creating amino acids, nucleic acids (DNA/RNA), and proteins — the fundamental building blocks of life.

Detailed Explanation:

Earth's magnetic field (magnetosphere) acts as a protective shield, deflecting charged particles (solar wind, cosmic rays) toward the polar regions.

When these particles collide with atmospheric gases near the poles, they create auroras (Aurora Borealis and Aurora Australis), preventing direct surface exposure that would harm living organisms.