Position and Size of the Earth (13.3.1–13.3.2) — "Our Home: Earth, a Unique Life Sustaining Planet"

1. 13 marks

   Define the habitable zone (or Goldilocks zone), and explain why it is the single most important reason Earth supports life.

   Hint. The chapter states this is THE most important reason, tied to one specific substance staying in one specific state.

   ✦ 'The most important reason why the Earth can support life is its distance from the Sun. It is just at the right distance, where the temperature allows water to exist in a liquid form.' The range of distances from a star over which water remains liquid is called the habitable zone, or Goldilocks zone. If Earth were closer to the Sun, all its water would evaporate; if farther, it would freeze — either extreme would make it impossible for most life forms to grow and thrive.

2. 22 marks

   Why is Earth called the 'Blue Planet,' according to the chapter?

   Hint. The colour comes from what covers most of the surface, seen from a specific vantage point.

   ✦ Most of Earth's surface is covered with water, so when seen from space, the Earth looks blue because of this vast amount of water — hence the name Blue Planet.

3. 33 marks

   Did Mars ever support life? Summarise what the chapter says scientists currently believe, and why this question matters to science.

   Hint. The chapter's answer is deliberately uncertain, and it uses that uncertainty to make a broader point about how science works.

   ✦ Mars lies at the edge of the Sun's habitable zone, and although several spacecraft and rovers have explored its surface, no proof of life has been found yet. However, scientists think Mars may have had liquid water — maybe even lakes — in the past, with conditions that could have supported simple life forms. The chapter uses this open question to make a broader point: 'science doesn't always have final answers... science stays open to change when we learn more,' since new evidence could still reshape what we currently believe about Mars.

4. 42 marks

   Besides temperature and distance from the Sun, what other factor does the chapter say helps keep Earth's climate from having extreme summers and winters?

   Hint. The shape of Earth's path around the Sun matters, not just its average distance.

   ✦ The Earth's nearly circular orbit around the Sun keeps the amount of sunlight and heat nearly steady throughout the year, since a steady orbit prevents extreme summers and winters at most places — an additional factor beyond simply having the right average distance from the Sun.

5. 54 marks

   Explain what would happen to Earth's atmosphere if Earth were much smaller (with the same density), and what would happen if it were much larger, according to the chapter.

   Hint. Two opposite failure modes, both tied to how strong gravity would be at each extreme.

   ✦ If Earth were much smaller (with the same average density), its gravity would be too weak to hold on to the gases in the atmosphere, and they would escape into space — this is why Mars, a smaller planet, has an atmosphere 100 times thinner than Earth's, and Mercury has no atmosphere at all. If the planet were too large instead, gravity would be much stronger, pulling everything down with such a large force that living things' bones could get crushed. Earth's right size avoids both extremes, letting it support an atmosphere essential for life without crushing anything living on it.

6. 63 marks

   What is ozone, where does it form, and what specific danger does it protect life from?

   Hint. Ozone is made from an element already familiar from earlier in the chapter, just rearranged into a different molecule.

   ✦ Some of the oxygen in Earth's atmosphere gets converted to another form called ozone (a three-atom oxygen molecule), forming an important part of the atmosphere called the ozone layer. This layer acts like a shield, blocking harmful ultraviolet (UV) rays from the Sun, since those rays can damage living cells.

7. 73 marks

   Our scientific heritage: what did India's Mangalyaan mission set out to study, and what did it demonstrate about Indian space science?

   Hint. The mission carried tools aimed at three specific aspects of Mars, and its significance went beyond just the science collected.

   ✦ India's Mangalyaan (Mars Orbiter Mission), launched in 2013 by ISRO, carried tools to study Mars's atmosphere, surface, and signs of past water — helping scientists ask big questions like whether Mars was ever suitable for life. Beyond the science itself, Mangalyaan showed the world that India could do space science with smart, low-cost technology, which is why it helped bring Mars closer to people everywhere.

8. 82 marks

   Give one real difference between Mars's atmosphere and Earth's atmosphere, as stated directly in the chapter.

   Hint. A specific numerical comparison is given for how much thinner Mars's atmosphere is.

   ✦ Mars's atmosphere is 100 times thinner than Earth's atmosphere — a direct consequence of Mars's smaller size and weaker gravity being less able to hold gases close to the planet.

Solutions written by the tuition.in editorial team and checked against NCERT Curiosity — Textbook of Science for Grade 8, Chapter 13 (hecu113.pdf), Reprint 2026-27, pages 210-228. Questions are referenced from the NCERT textbook for identification.