CDS (II) 2022 General Knowledge · Question 60 of 120

Correct — B, Sulphuric acid. NASA's own description of Venus is that the planet is wrapped in dense clouds composed of sulfuric acid, sitting above an atmosphere that is mostly carbon dioxide. The chemistry runs like this: sulphur dioxide, most likely from volcanic activity, is carried up to the cloud deck where ultraviolet sunlight and traces of water vapour convert it to sulphuric acid, which condenses into droplets. The deck is unbroken, roughly 50 to 70 kilometres above the surface, and it is what gives Venus both its pale yellowish-white colour and its extreme reflectivity — the reason Venus is the brightest object in our sky after the Sun and the Moon. The same clouds hide the surface completely from ordinary cameras, which is why the surface had to be mapped by radar.

• (a)Chlorine — Chlorine is a greenish-yellow gas. The colour is close enough to make the option tempting, but chlorine is not a significant component of Venus's atmosphere and does not form its cloud droplets.
• (c)Nitric acid — Nitric acid is not a cloud-forming species on Venus. Nitrogen is present, at about three and a half per cent of the atmosphere, but it stays as molecular nitrogen rather than oxidising to acid.
• (d)Bromine — Bromine is a reddish-brown liquid at ordinary temperatures and is vanishingly rare in planetary atmospheres. It plays no part in Venus's clouds.

Venus is Earth's near twin in size and mass and nothing like it in condition. Its atmosphere is about 96·5 per cent carbon dioxide with roughly 3·5 per cent nitrogen, exerts about 93 times Earth's sea-level pressure at the surface, and holds a mean surface temperature near 467 degrees Celsius — hotter than Mercury, which is closer to the Sun. The heat is a runaway greenhouse effect: the thick carbon dioxide traps outgoing infrared radiation, and the sulphuric acid cloud deck reflects much of the incoming sunlight while sealing the heat that does get through.

The distractors are all chosen for their colour, so a candidate who has only stored 'yellowish clouds' can be pulled towards chlorine or bromine. The way in is the chemistry of the planet rather than the palette: Venus is a volcanic, sulphur-rich, carbon dioxide world, so the acid it makes is the acid of sulphur. It is also worth separating two facts that are often merged — the atmosphere is carbon dioxide, the clouds are sulphuric acid. A question can ask either, and CDS has asked both: this paper asks the clouds, and the 2023 paper asks the carbon dioxide share.

• Venus's clouds are droplets of sulphuric acid; the atmosphere below them is about 96·5 per cent carbon dioxide.
• Surface pressure is about 93 times that at Earth's sea level, and the mean surface temperature is about 467 degrees Celsius.
• Venus is the hottest planet in the solar system, hotter than Mercury, because of a runaway greenhouse effect.
• A Venusian day lasts about 243 Earth days and the planet rotates in the opposite sense to Earth, so the Sun rises in the west.
• The reflective cloud deck is why Venus is the brightest natural object in the night sky after the Moon, and why its surface had to be mapped by radar.

• Answering with the atmosphere's main gas when the question asks about the clouds, or the reverse.
• Picking a substance because its colour matches the description; three of the four options here are coloured.
• Assuming the planet closest to the Sun must be the hottest — Venus, not Mercury, holds that record.

As a direct cloud-composition item, as a percentage item on carbon dioxide, or as a statements item comparing Venus with Earth and Mars.

• practice — not a real PYQ

  Venus is hotter than Mercury even though it is farther from the Sun mainly because

  • (a)it rotates faster
  • (b)its thick carbon dioxide atmosphere traps outgoing heat
  • (c)it has an internal nuclear heat source
  • (d)it lies closer to the asteroid belt

  Answer(b) its thick carbon dioxide atmosphere traps outgoing heat — a runaway greenhouse effect holds the surface near 467 degrees Celsius.

• practice — not a real PYQ

  The surface of Venus had to be mapped by radar rather than by ordinary photography because

  • (a)the surface is too dark
  • (b)the planet rotates too fast
  • (c)an unbroken cloud deck hides the surface from visible light
  • (d)the surface is covered by liquid

  Answer(c) an unbroken cloud deck hides the surface from visible light — radar passes through the sulphuric acid clouds, cameras do not.