The atmosphere of Saturn contains which one of the following gas in large quantity ?

Question

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Correct — D, Methane. Saturn is a gas giant, and like Jupiter it is made overwhelmingly of the two lightest elements: its atmosphere is roughly 96 per cent molecular hydrogen and about 3 per cent helium by volume. Everything else is a minor constituent — and among those minor constituents methane is the principal one, alongside ammonia, ethane, phosphine and water ice. That is why methane is the answer here: read the question as 'which of these four is actually present in Saturn's atmosphere in any appreciable quantity', because the other three are not there in any meaningful amount at all. Methane also does visible work on the outer planets. It absorbs red light strongly, so a methane-bearing atmosphere reflects the blue end of the spectrum — the reason Uranus and Neptune, where methane is proportionally far more abundant, appear blue-green, while on Saturn the ammonia haze dominates the pale-gold colour we see.

Why the other options are wrong — (a) Carbon monoxide has been detected in the outer planets only in traces. In a hydrogen-rich, chemically reducing atmosphere such as Saturn's, carbon is held overwhelmingly in its fully hydrogenated form — methane — rather than as an oxide, so carbon monoxide cannot be a bulk constituent.  (b) The most tempting wrong answer, because nitrogen is indeed the dominant gas in a nearby atmosphere — that of Titan, Saturn's largest moon, whose thick nitrogen envelope with a few per cent methane is one of the most studied in the solar system. But Titan is a moon, not the planet. On Saturn itself the nitrogen present is bound up mainly as ammonia, and free nitrogen is not a major component.  (c) Sulphur dioxide belongs to volcanic and oxidising settings — the clouds of Venus and the plumes of Jupiter's moon Io are the standard examples. It is not a constituent of a cold, hydrogen-dominated giant-planet atmosphere, where sulphur appears in reduced forms if at all.

Concept: The solar system splits into two families of atmosphere. The inner rocky planets could not hold on to hydrogen and helium, so their atmospheres are secondary and heavy — carbon dioxide on Venus and Mars, nitrogen and oxygen on Earth. The outer giants, Jupiter and Saturn, were massive enough and cold enough to keep the nebular gas they formed from, so their atmospheres are still overwhelmingly hydrogen and helium, essentially the composition of the Sun. In such a reducing, hydrogen-saturated environment the lighter elements combine with hydrogen rather than with oxygen: carbon becomes methane, nitrogen becomes ammonia, oxygen becomes water. Naming the minor gases of a giant planet is therefore not memorisation — it follows from the chemistry.

State PCS 2022 Q36 — Science & Technology PYQ with Answer

State PCS 2022 Q36 — Science & Technology PYQ with Answer