TOEFL Reading: Negative Factual Information

Life on Mars

The possibility that Mars hosts, or has hosted, life has a rich history dating back to the “canals” that some people claimed to see on the martian surface toward the end of the nineteenth century and the beginning of the twentieth. With the dawn of the space age came the possibility to address this question up close through a progression of missions to Mars that began with the first successful flyby of a robotic spacecraft in 1964 and have led to the deployment of capable rovers, like Curiosity and Perseverance, with instruments to look for organic chemistry.

The earliest missions to Mars provided some hints that liquid water—one of life’s primary requirements—may once have flowed on the surface, and later missions have strengthened this conclusion. The NASA Viking landers, whose purpose was to search directly for evidence of life on Mars, arrived on Mars 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.

Since the visit of Viking, our understanding of Mars has deepened spectacularly. Orbiting spacecraft have provided ever-more detailed images of the surface and detected the presence of minerals that could have formed only in the presence of liquid water. Two bold surface missions, the Mars Exploration Rovers Spirit and Opportunity (2004), followed by the much larger Curiosity Rover (2012), confirmed these remote-sensing data. All three rovers found abundant evidence for a past history of liquid water, revealed not only from the mineralogy of rocks they analyzed, but also from the unique layering of rock formations.

Curiosity has gone a step beyond evidence for water and confirmed the existence of habitable environments on ancient Mars. “Habitable” means not only that liquid water was present, but that life’s requirements for energy and elemental raw materials could also have been met. The strongest evidence of an ancient habitable environment came from analyzing a very fine-grained rock called a mudstone—a rock type that is widespread on Earth but was unknown on Mars until Curiosity found it. The mudstone can tell us a great deal about the wet environments in which they formed. The Perseverance rover is collecting samples of sedimentary rock in a former lakebed, to later return to Earth for laboratory analysis.

Five decades of robotic exploration have allowed us to develop a picture of how Mars evolved through time. Early Mars had epochs of warmer and wetter conditions that would have been conducive to life at the surface. However, Mars eventually lost much of its early atmosphere and the surface water began to dry up. As that happened, the ever-shrinking reservoirs of liquid water on the martian surface became saltier and more acidic, until the surface finally had no significant liquid water and was bathed in harsh solar radiation. The surface thus became uninhabitable, but this might not be the case for the planet overall.

Reservoirs of ice and liquid water could still exist underground, where pressure and temperature conditions make it stable. There is recent evidence to suggest that liquid water can occasionally flow on the surface even today. Thus, Mars might even have habitable conditions in the present day, but of a much different sort than we normally think of on Earth.

“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.