Composition and Structure of the Atmosphere Questions and Answers

Answer: B

Venus has a surface pressure of about 92 bars and a surface temperature near 740 kelvin, maintained by an extreme carbon dioxide greenhouse effect beneath a global sulphuric acid cloud deck. Its rotation is extremely slow rather than fast, taking 243 Earth days, which is what makes the superrotation of its atmosphere so striking. The cloud deck reflects most incoming sunlight yet the surface remains the hottest in the solar system. Most errors here come from carrying a result across from a neighbouring case where it does not hold.

Answer: C

Venus demonstrates that a sufficiently large greenhouse effect can raise a surface temperature far above the value that solar heating alone would give. Whether the terrestrial climate could reach such a state is a separate question, and the consensus is that it could not under any plausible emission scenario. The Venusian clouds reflect strongly yet the surface is still the hottest in the solar system, which is why the third option is instructive.

Answer: D

Titan's surface pressure is about 1.5 bars, the closest to Earth's of any body in the solar system, and like Earth its atmosphere is predominantly nitrogen. The question deliberately invites the answer Venus or Mars, both of which are extreme in opposite directions. Titan also has a hydrological cycle, though with methane in place of water, which makes it the most Earth-like atmosphere we know of despite its temperature of about 94 kelvin. Its study is a valuable check on ideas developed for the terrestrial case.

Answer: A

Venus, Earth and Mars began with broadly similar compositions and diverged because of differences in mass, distance and the presence of liquid water, which allowed carbon to be locked into rocks on Earth and not on Venus. The comparison shows how sensitive the outcome is to initial conditions. Layered structure depends on whether an absorbing constituent is present, and surface temperature depends as much on atmospheric composition as on distance from the sun, as Venus demonstrates emphatically.

Answer: B

Under hydrostatic balance the surface pressure equals the weight per unit area of the whole column, so multiplying the mean surface pressure by the surface area and dividing by gravity gives the total mass, about \( 5.1 \times 10^{18} \) kilograms. No knowledge of the vertical structure is required, which is the elegance of the calculation. Using surface density and a nominal depth would require an assumption about how density varies with height, and counting molecules directly is not a practical measurement.

Answer: C

The atmosphere weighs about \( 5.1 \times 10^{18} \) kilograms, which is roughly a millionth of the mass of the Earth and about a three hundredth of the mass of the oceans. That the oceans outweigh the atmosphere by such a margin is what gives them their dominant role in the heat capacity of the climate system. Placing these masses alongside one another puts many other quantities into proportion. Most errors here come from carrying a result across from a neighbouring case where it does not hold.