TOEFL Reading: Negative Factual Information

Water on Mars

Early missions to Mars identified geological features resembling dried-up riverbeds and lakebeds, suggesting that liquid water once flowed on the surface. This evidence helped motivate the Viking landers’ search for life in 1976. Viking’s onboard instruments found no organic molecules and no evidence of biological activity in the martian soils it analyzed.

This result is not particularly surprising because, despite the evidence of flowing liquid water in the past, liquid water on the surface of Mars is not stable today. Over much of Mars, temperatures and pressures at the surface are so low that liquid water would freeze or boil away. (Under very low pressures, water will boil at a much lower temperature than usual.) To make matters worse, unlike Earth, Mars does not have a magnetic field and ozone layer to protect the surface from harmful solar ultraviolet radiation and energetic particles. However, Viking’s analyses of the soil said nothing about whether life may have existed in Mars’ distant past, when liquid water was more abundant. We do know that water in the form of ice exists in abundance on Mars, not so deep beneath its surface. Water vapor is also a constituent of the atmosphere of Mars.

“Astronomy 2e” by OpenStax is licensed under CC BY 4.0. A small excerpt was extracted from Chapter 30 and edited. This textbook can be downloaded free from https://openstax.org/details/books/astronomy-2e.