According to the passage, oxygen has been regarded as an especially promising sign of life because

it is the most abundant gas in Earth's atmosphere.

it can be detected across greater distances than other atmospheric gases.

without continuous biological production it would vanish from an atmosphere within a few million years.

it is produced by every known form of life.

The passage indicates that oxygen can accumulate on a lifeless planet when

the planet's atmosphere is unusually thin.

the planet loses a deep ocean to space, or stellar ultraviolet light breaks carbon dioxide apart.

the planet orbits a star considerably older than the Sun.

methane is entirely absent from the atmosphere.

Oxygen found together with methane is difficult to explain without biology because

methane is easier to detect than oxygen at interstellar distances.

methane is produced only by living organisms.

the two gases are generated by the same underlying process.

the two gases react with each other, so their coexistence implies that both are being replenished.

It can be inferred that the approach described in the final paragraph is

more demanding to carry out than the earlier approach, but less easily explained away.

less reliable than searching for oxygen on its own.

already routine with the telescopes currently in operation.

intended to remove oxygen from consideration as a sign of life.

The author's attitude toward the abiotic explanations described in the second paragraph is best described as

dismissive of them as speculative modelling unsupported by observation.

convinced that they account for most of the oxygen that telescopes will detect.

accepting of them as genuine possibilities that a single-gas test cannot rule out.

indifferent to their bearing on the search for life beyond Earth.

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Oxygen and the Search for Biosignatures

PRACTICE

100:00+4 −1MCQ Single Answer

Reading Passage

Of all the gases in Earth's atmosphere, oxygen is the one most closely tied to the presence of life. It is chemically reactive, combining readily with rock and with other gases, and if biological production were to stop it would disappear from the air within a few million years — an instant in geological terms. Because the oxygen we breathe is continuously replaced by photosynthesis, its abundance has long been treated as the single most promising sign of life on a distant world, and the next generation of space telescopes has been designed with the detection of oxygen in exoplanet atmospheres among its central objectives. The reasoning is sound as far as it goes, but modelling over the past decade has identified routes by which oxygen can accumulate on a lifeless planet. A world that begins with a deep ocean and orbits close to its star may lose that water to space; hydrogen, being light, escapes first, and the oxygen left behind can build up in the atmosphere over time. A different route runs through stellar ultraviolet light, which in some conditions breaks carbon dioxide apart faster than the resulting oxygen can recombine. Neither process requires an organism, and either would produce the signal that telescopes are being built to find. Rather than abandoning oxygen as an indicator, researchers have moved toward reading it in context. What matters is not oxygen alone but the company it keeps. Oxygen detected alongside methane is difficult to explain without biology, because the two gases react with one another and cannot persist together unless something is replenishing both. Oxygen accompanied by large quantities of carbon monoxide and no water vapour points instead toward the photochemical route. The single-gas biosignature has given way to a search for a pattern of gases — a considerably harder observation to make, and a considerably harder one to counterfeit.

The passage is primarily concerned with