Solar System and Evolution of Earth: UPSC Previous Year Questions (Geography)

Explanation

Scientific studies confirm that Earth’s rotation (affecting day length) and axis orientation (causing polar motion) are shifting due to climate-driven factors--notably melting polar ice, glacier retreat, and groundwater depletion. These changes lead to mass redistribution, altering Earth’s moment of inertia, which in turn causes a rotational slowing (e.g., 1.33 milliseconds per century) and polar wander (axis shifts of 10-20 cm/year). Since 1980, Earth’s spin axis has shifted nearly 4 meters. The Chandler wobble, a natural axis oscillation, is also amplified by these mass changes.

Statement I is correct: Mass redistribution--e.g., Greenland losing 4,700 Gt and Antarctica 2,720 Gt (1992-2020), and 1.6 trillion tons of groundwater pumped since 1990--shifts mass toward the equator, slowing rotation and shifting the axis.

Statement II is correct: Solar flares and coronal mass ejections (CMEs) release up to 10° joules of energy, impacting the ionosphere and thermosphere (80-1000 km altitude). These cause geomagnetic storms, disrupting satellites, radio, and triggering auroras. However, per NASA and NOAA, they do not affect rotation or axis shifts, since they alter magnetic fields and atmospheric density, not mass distribution.

Statement III is correct: Climate-driven polar ice melt and groundwater depletion (e.g., Punjab & Haryana in India) shift mass equatorward, increasing moment of inertia, thus slowing rotation and causing axis drift, as confirmed by NASA and Nature Geoscience.

Explanation

Statement 1 is correct: About 97.5% of Earth’s water is saltwater found in oceans. The remaining 2.5% is freshwater. Most of it is locked in glaciers, ice caps, or deep underground. Only about 0.3% of total freshwater is readily available in rivers, lakes, and accessible groundwater for human use. Since 0.3% of 2.5% is less than 1% of total water, this confirms that freshwater available for use is indeed less than 1%.

Statement 2 is incorrect: Around 68.7% of all freshwater is stored in ice caps and glaciers, primarily in the Antarctic and Greenland ice sheets. The remaining freshwater is found in groundwater, rivers, and lakes. The volume of water in rivers and lakes combined is only about 0.3% of the total freshwater available.

Explanation

As of 2009, Jupiter had the highest number of known moons (63 at that time). Jupiter’s large number of moons is attributed to its massive size and strong gravitational pull, which allows it to capture a variety of objects into its orbit. However in future years Saturn has since been found to have more moons than Jupiter.

Explanation

Order of Planets in the Solar System (from the Sun) is Mercury> Venus > Earth > Mars > Jupiter > Saturn > Uranus > Neptune. Mars is the 4th planet from the Sun and Uranus is the 7th planet from the Sun. Therefore the planets that lie between Mars (4th) and Uranus (7th) are Jupiter (5th) and Saturn (6th).

Explanation

The average distance from the Earth to the Sun is approximately 150 million kilometers or 150 × 106 km. This distance can vary slightly due to the elliptical shape of Earth’s orbit, but the average distance remains close to 150 million kilometers. 1 AU is approximately 149.6 million kilometers, which is roughly 150 × 106 km.

Explanation

Assertion (A) is False: Mars takes a longer time than Earth to orbit the Sun. The time taken by a planet to orbit around the Sun is called its orbital period or year. Mars has an orbital period of about 687 Earth days which means it takes 687 days to complete one revolution around the Sun. Earth takes 365.25 days (about 365 days) to complete one revolution around the Sun. Reason (R) is True: Mars does have a smaller diameter than Earth. The diameter of Earth is 12,742 km. The diameter of Mars is 6,779 km (almost half of Earth). The time taken by a planet to orbit the Sun depends on its distance from the Sun, not its diameter. Since Mars is farther from the Sun than Earth thus it has a longer orbital period.

Explanation

Assertion (a) is true: The conditions on Venus are extremely harsh for human life, and its atmosphere and surface are inhos-pitable for humans. Venus has an extremely high surface temperature, high atmospheric pressure, and a toxic atmosphere primarily composed of carbon dioxide (96.5%), with clouds of sulfuric acid. Reason (R) is true: The atmosphere of Venus is composed primarily of carbon dioxide (CO₂), with small amounts of nitrogen and trace gases. CO₂ constitutes about 96.5% of the atmosphere, and this contributes to a runaway greenhouse effect that traps heat and causes extreme surface temperatures around 465°C. Assertion A and Reason R are true and R correctly explains A as the inhospitability of Venus for humans is explained by the presence of carbon dioxide in the environment.

Explanation

Assertion (A) is true: The Moon is tidally locked to the Earth, meaning that the same hemisphere of the Moon is always facing Earth. This occurs because the Moon’s rotational period (the time it takes to spin once on its axis) is equal to its orbital period (the time it takes to complete one revolution around the Earth). As a result, only about 59% of the Moon’s surface is ever visible from Earth due to lunar libration (a slight wobbling motion). The remaining 41% is permanently hidden from direct view from Earth and is called the far side of the Moon (often incorrectly referred to as the "dark side"). Reason (R) is false: The actual time the Moon takes to rotate on its own axis is 27.3 days, not 23.5 days. The Moon’s orbital period around the Earth is also 27.3 days (called the sidereal month) not 23 and a half days. Since these two periods are equal, the same face of the Moon always faces Earth, which is a result of synchronous rotation (tidal locking)

Explanation

The time a planet takes to complete one revolution around the Sun depends on its distance from the Sun. Planets farther from the Sun have larger orbits, resulting in longer revolution periods.

Option (b) is correct: Jupiter takes the longest time among the given options to orbit the Sun, requiring approximately 4,333 days(11.86 Earth years). This is due to its significantly larger orbital path compared to the other planets listed. Hence, this is the correct answer. Options (a), (c) and (d) are incorrect:

Earth completes one revolution around the Sun in approximately 365.25 days, which is relatively short compared to the other options provided. Mars requires around 687 days(1.88 Earth years) to complete a revolution. While this is longer than Earth’s revolution time, it is still shorter than that of Jupiter. Venus has the shortest revolution period among the listed planets, taking approximately 224.7 Earth days(0.615 years) to orbit the Sun.

Explanation

Option (a) is correct: Earth has the highest average density among all planets in the Solar System. Earth has a metallic core rich in iron and nickel which makes it denser than gas giants like Jupiter or Saturn. Planet Density (g/cm3) Earth 5.51 Mercury 5.43 Venus 5.24 Mars 3.93 Jupiter 1.33 Saturn 0.69 Uranus 1.27 Neptune 1.64 Options (b), (c) and (d) are incorrect:

The most abundant element in Earth’s composition is iron (Fe), not silicon (Si). Earth’s elemental composition (by mass):

Iron(35%)>Oxygen(30%)>Silicon(15%) The Sun actually contains about 99.8% of the total mass of the Solar System, not just 75%. The remaining 0.2% of the mass is distributed among planets, moons, asteroids, and comets. The diameter of the Sun is 109 times and not 190 times that of Earth. Actual ratio of Sun’s diameter to Earth’s diameter:

Ratio = 1,391,000(Sun) × 12,742(Earth) 109 times