Some scientists speculated that the absence of organic material in samples from Martian soil collected by the Viking landers in the 1970s was the result of the intense ultraviolet radiation that penetrates the atmosphere of Mars. Billions of years ago, the atmosphere of Mars had a greater density of carbon dioxide, which protected the surface from harmful rays of the sun. These scientists thus argued that even if life had gotten a start on early Mars, it could not have survived the exposure to ultraviolet radiation when the atmosphere thinned, given that Mars never developed a protective layer of ozone—a gas composed entirely of oxygen molecules—as Earth did.
Which choice best describes data from the graph that support the scientists' argument?
Correct
Incorrect
The question asks you to use data from the graph to support the scientists' argument, so begin by identifying this argument and restating it in your own words. The last sentence of the passage mentions the argument: life on Mars would have died due to radiation because Mars doesn't have an ozone layer like Earth does.
Compare this to the information displayed in the graph. The y-axis displays the percentages of three types of gases in the atmospheres of the planets identified on the x-axis: Venus, Mars, and Earth. The scientists' argument mentions Mars and Earth; use the passage to determine which gases in the graph are relevant to the argument that life on Mars would have died due to the lack of an ozone layer. The second sentence explains that carbon dioxide in the atmosphere used to protect the surface of Mars. The last sentence identifies oxygen as the gas that makes up ozone. Look back at the argument you need to support: life on Mars would have died due to radiation because Mars doesn't have an ozone layer like Earth does. Although changes in carbon dioxide levels help explain why radiation impacts Mars, oxygen levels are relevant to the specific argument that Mars didn't develop an ozone layer. Indeed, the graph shows that Mars's atmosphere is composed of a very low percentage of oxygen, while Earth's atmosphere is about 20% oxygen. Use this data as your prediction; (C) matches and is correct.
Choices (A) and (B) are incorrect because neither Venus nor nitrogen levels are relevant to the argument about ozone. Choice (D) is incorrect because the comparative percentages of carbon dioxide on Venus and Mars are irrelevant to the development of ozone on Mars.
