CDS (II) 2023 General Knowledge · Question 7 of 120

Correct — D, 95 – 97%. Both of Earth's neighbours have atmospheres that are overwhelmingly carbon dioxide, and both land inside the same narrow band. Venus is about 96·5 per cent carbon dioxide, with almost all of the remaining 3·5 per cent nitrogen, under a surface pressure some 93 times that at Earth's sea level and a surface temperature near 467 °C. Mars is about 95 per cent carbon dioxide, with roughly 2·8 per cent nitrogen and 2 per cent argon, but its atmosphere is so thin that the mean surface pressure is only about 610 pascals — under a hundredth of Earth's. That contrast is what makes the pairing instructive: the same gas dominates both, yet one planet is crushed and roasted by it and the other is frozen, because the amount of atmosphere differs enormously even though its composition barely does. Earth, for comparison, is about 78 per cent nitrogen and 21 per cent oxygen, with carbon dioxide only around 0·04 per cent.

• (a)45 – 47% — Far too low. No measured planetary atmosphere in the inner solar system sits near half carbon dioxide; on both Venus and Mars every other gas together accounts for less than a twentieth of the whole.
• (b)52 – 55% — A little over half, which would leave room for a comparable quantity of something else. Nitrogen on Venus is only about 3·5 per cent, and on Mars nitrogen and argon together come to under 5 per cent.
• (c)74 – 77% — The trap for anyone half-remembering Earth's figures — about 78 per cent is the nitrogen share of Earth's atmosphere, not the carbon-dioxide share of anyone's.

A planet keeps an atmosphere only if its gravity is strong enough to hold the gas against escape and its magnetic and thermal history let it. Venus and Mars both ended up carbon-dioxide worlds, but by opposite routes: Venus, closer to the Sun, lost its water to space and could never lock its carbon into carbonate rock, so a runaway greenhouse built a dense, hot blanket; Mars, small and long without a global magnetic field, lost most of its atmosphere to the solar wind and kept only a thin remnant. Earth escaped both fates because liquid water and living things buried most of its carbon.

The item is worth a mark to anyone who remembers a single fact — that Venus and Mars are both 'almost all carbon dioxide' — because the four bands are widely spaced and only one is anywhere near that. If the figure will not come, work by elimination: option (c) is Earth's nitrogen number wearing a disguise, and the other two are simply too low to describe an atmosphere anyone calls carbon-dioxide dominated. On the sourcing, note that the two planets are not identical — Venus at about 96·5 per cent sits comfortably inside the printed band while Mars at about 95 per cent sits right on its lower edge, and different reference works round the Martian figure between 95·0 and 95·3 per cent. The stem asks for one band covering both, and 95–97 per cent is the only option that does. Nothing here has changed since the 2023 exam; these are long-settled spacecraft measurements, not current affairs.

• Venus: about 96·5 per cent carbon dioxide and 3·5 per cent nitrogen, surface pressure about 93 bar, mean surface temperature about 467 °C — the hottest planet in the solar system.
• Mars: about 95 per cent carbon dioxide, 2·8 per cent nitrogen and 2 per cent argon, with a mean surface pressure of only about 610 pascals.
• Earth: about 78 per cent nitrogen, 21 per cent oxygen and roughly 0·04 per cent carbon dioxide.
• Venus's thick yellowish cloud deck is made of sulphuric acid droplets, not of carbon dioxide itself.
• Venus is hotter than Mercury despite being farther from the Sun, because of the greenhouse effect of its dense carbon-dioxide atmosphere.

• Reaching for 78 per cent, which is Earth's nitrogen figure rather than anyone's carbon-dioxide figure.
• Assuming Venus's clouds are made of carbon dioxide; the clouds are sulphuric acid, while the gas of the atmosphere is carbon dioxide.
• Treating a similar composition as meaning a similar atmosphere — Venus's air is dense enough to crush a submarine, Mars's is thinner than a good laboratory vacuum.

As a figure to recall, as an assertion-and-reason pair on why Venus is inhospitable, or as a comparison of the atmospheres of the terrestrial planets.

• practice — not a real PYQ

  Which one of the following is the principal reason why Venus has a much higher surface temperature than Mercury, although it is farther from the Sun?

  • (a)Venus rotates in the opposite direction to most planets
  • (b)Venus has a dense carbon-dioxide atmosphere that traps outgoing heat
  • (c)Venus is larger than Mercury
  • (d)Venus has no natural satellite

  Answer(b) Venus has a dense carbon-dioxide atmosphere that traps outgoing heat — a runaway greenhouse effect holds its surface near 467 °C, hotter than Mercury's day side.

• practice — not a real PYQ

  The mean atmospheric pressure at the surface of Mars is closest to which one of the following?

  • (a)About the same as Earth's sea-level pressure
  • (b)About 93 times Earth's sea-level pressure
  • (c)Less than one per cent of Earth's sea-level pressure
  • (d)About one-fifth of Earth's sea-level pressure

  Answer(c) Less than one per cent of Earth's sea-level pressure — roughly 610 pascals against about 101,325 pascals on Earth, which is why the Martian atmosphere gives almost no protection or lift.