Lack of atmosphere around the Moon is due to—

1. Celestial Bodies: Inner and Outer Planets (basic)

To understand our solar system, we first categorize the eight planets into two distinct families based on their location relative to the Asteroid Belt: the Inner Planets and the Outer Planets. The Inner Planets (Mercury, Venus, Earth, and Mars) are also called Terrestrial planets because they are 'Earth-like' — small, dense, and composed primarily of rocks and metals like iron and nickel Physical Geography by PMF IAS, The Solar System, p.25. In contrast, the Outer Planets (Jupiter, Saturn, Uranus, and Neptune) are known as Jovian planets or 'Gas Giants,' meaning they are 'Jupiter-like' and consist mostly of hydrogen and helium, resembling the composition of the Sun more than the Earth.

The stark difference between these two groups is rooted in their formation. The Terrestrial planets formed close to the Sun, where temperatures were too high for gases to condense into solids. Furthermore, the Solar Wind was most intense near the Sun, blowing away huge amounts of gas and dust from these inner bodies. Because these planets are relatively small, their lower gravity was unable to hold onto those escaping gases Physical Geography by PMF IAS, The Solar System, p.31. Conversely, the Jovian planets formed in cooler regions where gases could condense, and the solar winds were too weak to strip away their thick atmospheres.

It is also helpful to distinguish between Inner and Inferior planets. While 'Inner' refers to the four planets before the asteroid belt, 'Inferior' is a term used specifically for Mercury and Venus because their orbits are closer to the Sun than Earth's orbit Physical Geography by PMF IAS, The Solar System, p.27.

Feature	Inner (Terrestrial) Planets	Outer (Jovian) Planets
Composition	Rocks and heavy metals (Silicates, Iron)	Gases (Hydrogen, Helium) and Ices
Density	High Density	Low Density
Size	Smaller diameters	Massive diameters
Atmosphere	Thin or secondary atmospheres	Thick, primary atmospheres

Sources: Physical Geography by PMF IAS, The Solar System, p.25; Physical Geography by PMF IAS, The Solar System, p.27; Physical Geography by PMF IAS, The Solar System, p.31

2. Gravity and Mass: The Governing Forces (basic)

At its simplest, gravity is the 'invisible glue' of the universe—an attractive force that every object with mass exerts on every other object. Two fundamental factors govern how strong this pull is: mass and distance. The more massive a body is, the stronger its gravitational pull. For example, the Sun is so massive that its surface gravity is about 274 m/s², which is 28 times stronger than Earth’s Physical Geography by PMF IAS, The Solar System, p.23. Conversely, the Moon, being much smaller, has a surface gravity of only 1.62 m/s², roughly one-sixth of what we experience here on Earth.

On Earth, gravity is not perfectly uniform everywhere. Because our planet is an oblate spheroid (it bulges at the equator and is flattened at the poles), the distance from the surface to the center of the Earth varies. You would actually weigh slightly more at the North Pole than at the Equator! This is because the poles are closer to the Earth's center of mass, and the outward centrifugal force caused by Earth's rotation is strongest at the equator, acting against gravity FUNDAMENTALS OF PHYSICAL GEOGRAPHY NCERT Class XI, The Origin and Evolution of the Earth, p.19.

Gravity also plays a critical role in determining whether a celestial body can host an atmosphere. To escape a planet's pull, a gas molecule must reach a specific speed known as escape velocity. Because the Moon has low mass and weak gravity, its escape velocity is very low (about 2.38 km/s). Most gas molecules easily exceed this speed and leak into space. In contrast, Earth’s stronger gravity results in a much higher escape velocity (11.2 km/s), allowing us to retain the life-sustaining air we breathe.

Finally, we use gravity to understand what lies beneath our feet. Geologists measure gravity anomalies—differences between the expected gravity and the actual measured gravity at a location. A positive anomaly suggests high-density material (like metal ores) hidden in the crust, while a negative anomaly might indicate a loss of material, such as in deep oceanic trenches where subduction occurs Physical Geography by PMF IAS, Tectonics, p.108.

Celestial Body	Surface Gravity (m/s²)	Relative Strength
Sun	274	~28x Earth
Earth	9.8	1x (Standard)
Moon	1.62	~1/6th Earth

Sources: Physical Geography by PMF IAS, The Solar System, p.23; FUNDAMENTALS OF PHYSICAL GEOGRAPHY NCERT Class XI, The Origin and Evolution of the Earth, p.19; Physical Geography by PMF IAS, Latitudes and Longitudes, p.241; Physical Geography by PMF IAS, Earth's Interior, p.58; Physical Geography by PMF IAS, Tectonics, p.108

3. Evolution of Planetary Atmospheres (intermediate)

The atmosphere surrounding a planet today is not what it started with. The evolution of planetary atmospheres, particularly for terrestrial planets like Earth, occurred in three distinct stages. In the first stage, planets possessed a primordial atmosphere consisting mainly of hydrogen and helium. However, this early layer was stripped away by solar winds—intense streams of energy from the young Sun. This loss occurred across all terrestrial planets, but was most absolute on smaller bodies like the Moon, where the weak gravitational pull and low escape velocity (just 2.38 km/s) meant gas molecules could easily fly off into space NCERT Class XI Fundamentals of Physical Geography, The Origin and Evolution of the Earth, p.15.

The second stage is defined by a process called degassing (or outgassing). As the early Earth cooled and its solid crust formed, gases trapped in the interior were released through massive volcanic eruptions and the Late Heavy Bombardment of asteroids Physical Geography by PMF IAS, Earths Atmosphere, p.270. This created a second atmosphere rich in water vapour, nitrogen, carbon dioxide, methane, and ammonia, but notably lacking in free oxygen. During this time, the cooling of the Earth allowed water vapour to condense, forming the first oceans, which played a crucial role by dissolving much of the atmospheric CO₂ NCERT Class XI Fundamentals of Physical Geography, The Origin and Evolution of the Earth, p.16.

The third and final stage involved the modification of the atmosphere by the living world. Once life emerged in the oceans, the process of photosynthesis began to consume carbon dioxide and release oxygen. Over billions of years, this biological activity shifted the atmospheric composition to the nitrogen-oxygen balance we breathe today. While Earth successfully transitioned through these stages, other bodies failed; for instance, the Moon’s low mass prevented it from retaining the gases released during degassing, leaving it with a negligible exosphere rather than a true atmosphere.

Stage	Primary Process	Key Characteristics
Stage 1	Loss of Primordial Atmosphere	Hydrogen and helium stripped by solar winds.
Stage 2	Degassing / Outgassing	Volcanism releases H₂O, CO₂, and N₂; no free oxygen.
Stage 3	Biological Modification	Photosynthesis increases O₂ levels; CO₂ absorbed by oceans.

Sources: FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), The Origin and Evolution of the Earth, p.15-16; Physical Geography by PMF IAS, Manjunath Thamminidi (1st ed.), Earths Atmosphere, p.270

4. Solar Wind and Magnetospheres (intermediate)

To understand why some planets are lush and others are barren, we must look at the invisible battle between the Solar Wind and a planet's Magnetosphere. The Solar Wind is a constant stream of high-energy charged particles (mostly electrons and protons) ejected from the Sun's corona. If these particles were to hit a planet’s atmosphere directly, they would act like a cosmic sandblaster, gradually knocking gas molecules into deep space—a process known as atmospheric stripping Physical Geography by PMF IAS, Earths Magnetic Field (Geomagnetic Field), p.69.

A planet’s primary defense is its Magnetosphere, the region of space dominated by the planet’s internal magnetic field. For Earth, this field is generated by convection currents of molten iron in the outer core Physical Geography by PMF IAS, Earths Interior, p.57. When the solar wind encounters this magnetic shield, it is deflected around the planet. This interaction creates a distinct shape: the magnetosphere is compressed into a hemisphere on the side facing the Sun, while the solar wind drags the magnetic field lines out into a long magnetotail on the opposite side Physical Geography by PMF IAS, Earths Magnetic Field (Geomagnetic Field), p.65.

The consequences of losing this shield are best seen on Mars. Evidence suggests Mars lost its magnetosphere roughly 4 billion years ago. Without it, the solar wind interacted directly with the Martian ionosphere, stripping away its density and even destroying trace gases like methane Physical Geography by PMF IAS, The Solar System, p.30. However, nature always has exceptions—Venus maintains an incredibly dense atmosphere despite having little to no geomagnetic field, a phenomenon that remains a point of deep scientific study Physical Geography by PMF IAS, Earths Magnetic Field (Geomagnetic Field), p.69.

Body	Magnetosphere	Atmospheric Consequence
Earth	Strong / Well-developed	Protected, thick atmosphere; life-sustaining.
Mars	Lost (4 billion years ago)	Stripped by solar wind; very low density.
Moon	Weak / Non-existent	Negligible exosphere; gases easily escape.

Sources: Physical Geography by PMF IAS, Earths Magnetic Field (Geomagnetic Field), p.69; Physical Geography by PMF IAS, Earths Interior, p.57; Physical Geography by PMF IAS, Earths Magnetic Field (Geomagnetic Field), p.65; Physical Geography by PMF IAS, The Solar System, p.30

5. Comparative Study: Venus, Mars, and Earth (intermediate)

To understand why our neighbor planets look so different from Earth, we must look at their atmospheres and thermal balances. While Earth enjoys a 'temperate' climate, its neighbors represent two extremes. Venus is the most dramatic case; despite being further from the Sun than Mercury, it is the hottest planet in the Solar System Science, Class VIII NCERT, Our Home: Earth, a Unique Life Sustaining Planet, p.213. This is due to a runaway greenhouse effect caused by an atmosphere composed of roughly 96% CO₂. This thick blanket traps solar heat so effectively that surface temperatures soar to nearly 460°C, high enough to melt lead Physical Geography by PMF IAS, The Solar System, p.28. Unlike Earth, where the carbon cycle moves carbon between the atmosphere, oceans, and living organisms to maintain balance Environment, Shankar IAS Academy, Functions of an Ecosystem, p.19, Venus has no such mechanism, leaving its carbon permanently trapped in the air.

Beyond temperature, the atmospheric pressure and magnetism of these planets vary wildly. Venus has the densest atmosphere of all terrestrial planets, with a surface pressure 92 times that of Earth—equivalent to being nearly a kilometer deep in Earth's oceans Physical Geography by PMF IAS, The Solar System, p.28. Interestingly, Venus lacks a magnetic field, yet its dense atmosphere is partially shielded from solar winds by its ionosphere Physical Geography by PMF IAS, Earths Magnetic Field (Geomagnetic Field), p.69. In contrast, Mars (the Red Planet) has an incredibly thin atmosphere, also mostly CO₂, but lacking the density to trap significant heat, making it a frozen desert. Earth sits in the 'Goldilocks zone,' where the greenhouse effect is present enough to prevent freezing but regulated enough to avoid the scorching fate of Venus Science, Class VIII NCERT, Our Home: Earth, a Unique Life Sustaining Planet, p.214.

Feature	Venus	Earth	Mars
Atmosphere	Very Thick (96% CO₂)	Moderate (Nitrogen/Oxygen)	Very Thin (CO₂)
Surface Pressure	92 atm	1 atm	0.006 atm
Magnetic Field	None	Strong (Global)	Weak/Localized
Rotation	Day longer than Year	24 Hours	~24.6 Hours

One final quirk of Venus is its rotation. It rotates so slowly on its axis that a single day on Venus (243 Earth days) actually lasts longer than its orbital year (224 Earth days) Physical Geography by PMF IAS, The Solar System, p.28. This sluggish rotation is one reason it fails to generate a conventional internal magnetic field like Earth does.

Sources: Science, Class VIII NCERT, Our Home: Earth, a Unique Life Sustaining Planet, p.213-214; Physical Geography by PMF IAS, The Solar System, p.28; Physical Geography by PMF IAS, Earths Magnetic Field (Geomagnetic Field), p.69; Environment, Shankar IAS Academy, Functions of an Ecosystem, p.19

6. Concept of Escape Velocity (exam-level)

Escape Velocity is the minimum speed an object must attain to break free from the gravitational pull of a celestial body (like a planet or a moon) and travel into space without any further propulsion. Think of it as a cosmic speed limit: if you throw a ball upward, it usually falls back down because of gravity. However, if you could throw it fast enough, it would never return. This critical speed depends entirely on the mass and the radius of the body you are leaving.

From a first-principles perspective, gravity is a "tug-of-war" between the mass of the planet and the energy of the object trying to leave. A massive planet like Jupiter has an incredibly high escape velocity, while a smaller body like the Moon has a much lower one. Mathematically, it is expressed as vₑ = √(2GM/r), where G is the gravitational constant, M is the mass of the body, and r is the distance from the center of the body. Interestingly, the escape velocity does not depend on the mass of the object escaping; a tiny molecule and a massive rocket both need to reach the same speed to leave the Earth's grasp Physical Geography by PMF IAS, The Solar System, p.39.

One of the most critical applications of this concept in geography is understanding why some planets have thick atmospheres while others are barren. Gas molecules are in constant motion, and their speed (Root Mean Square or thermal velocity) depends on temperature. If a planet's escape velocity is low, gas molecules—especially light ones like Hydrogen and Helium—can easily reach speeds higher than the escape velocity and leak into space. This process is known as atmospheric escape or atmospheric stripping Physical Geography by PMF IAS, Earths Atmosphere, p.280. On the Moon, the escape velocity is so low (~2.38 km/s) that almost all gas molecules escaped long ago, leaving it with no substantial atmosphere.

While gravity is the primary "anchor" for an atmosphere, other factors play a role. For instance, Earth's magnetic field acts as a shield, protecting our atmosphere from being "blasted" away by solar winds, which would otherwise accelerate gas molecules and enhance their escape Physical Geography by PMF IAS, Earths Atmosphere, p.280.

Body	Escape Velocity	Atmospheric Consequence
Earth	~11.2 km/s	High enough to retain Nitrogen and Oxygen; loses lighter Hydrogen over time.
Moon	~2.38 km/s	Too low to hold gas molecules; results in a negligible exosphere.

Sources: Physical Geography by PMF IAS, The Solar System, p.39; Physical Geography by PMF IAS, Earths Atmosphere, p.280

7. Thermal Velocity vs. Gravitational Pull (exam-level)

To understand why some celestial bodies like Earth are wrapped in a thick blanket of air while others like the Moon are barren, we must look at the cosmic tug-of-war between gravitational pull and thermal velocity. Gravity acts as the 'glue' that holds gas molecules down. Every planet or moon has a specific escape velocity—the minimum speed an object (or molecule) must reach to break free from that body's gravitational grip. For Earth, this is a robust 11.2 km/s, but for the Moon, it is a mere 2.38 km/s Physical Geography by PMF IAS, Chapter 20: Earth's Atmosphere, p. 280. On the other side of this tug-of-war is thermal velocity. Gas molecules are never still; they are in constant, chaotic motion fueled by heat from the Sun. The speed at which they move depends on the temperature and their own mass. At any given temperature, lighter gases like Hydrogen (H₂) and Helium (He) move much faster than heavier ones like Nitrogen (N₂) or Oxygen (O₂) Physical Geography by PMF IAS, Earths Atmosphere, p. 271. If the average thermal speed (specifically the Root Mean Square velocity) of these molecules exceeds the escape velocity of the planet, the gas literally leaks out into space. On the Moon, the gravitational pull is so weak and the escape velocity so low that even at moderate temperatures, gas molecules gain enough kinetic energy to fly off into the vacuum. Over billions of years, this process has 'stripped' the Moon of its gases, leaving behind only a negligible exosphere. In contrast, Earth's strong gravity and higher escape velocity ensure that even though our atmosphere is heated by the Sun, most gas molecules remain 'trapped' within our gravitational well.

Feature	Earth	Moon
Gravitational Pull	Strong	Weak (approx. 1/6th of Earth)
Escape Velocity	High (11.2 km/s)	Low (2.38 km/s)
Atmospheric Outcome	Retains heavy and moderate gases	Gases easily escape into space

Sources: Physical Geography by PMF IAS, Chapter 20: Earth's Atmosphere, p.280; Physical Geography by PMF IAS, Earths Atmosphere, p.271

8. Solving the Original PYQ (exam-level)

To solve this question, you must synthesize your knowledge of gravitational pull and kinetic theory of gases. As you learned, the Moon’s mass is significantly smaller than Earth’s, which results in a much weaker gravitational attraction. This weak gravity directly dictates a low escape velocity—the minimum speed required for a molecule to break free from a celestial body's influence. On the Moon, this threshold is only about 2.38 km/s. When the thermal energy of gas molecules (their RMS velocity) exceeds this low threshold, the molecules drift into space rather than being held in place to form a stable atmosphere.

The reasoning follows a clear cause-and-effect chain: low mass leads to low gravity, which in turn leads to a low escape velocity. Because the escape velocity is so low, even common solar heating provides enough kinetic energy for gas molecules to 'outrun' the Moon's pull. Therefore, (A) low escape velocity of air molecule and low gravitational attraction is the most comprehensive answer because it addresses both the fundamental cause and the physical mechanism as detailed in Physical Geography by PMF IAS.

UPSC often uses 'partial truths' to test your depth of understanding. Options (C) and (D) are classic examples of this; while they are technically correct in isolation, they are incomplete because the two concepts are inextricably linked in this physical phenomenon. Option (B) is a factual contradiction designed to catch students who confuse 'high' and 'low' under exam pressure. Always look for the option that explains the full physical relationship rather than just one side of the coin.