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For one, Mars‘ average temperature is -60° Celsius (C) [-76° Fahrenheit (F)]. The atmospheric pressure is less than one-hundredth that on Earth‘s surface, and Mars‘ very thin atmosphere is almost entirely carbon dioxide (CO2), with a pinch of nitrogen (N) and very little water (H2O) vapor or oxygen (O2). In addition, Mars doesn‘t have an ozone (O3) layer and thus is bombarded by three times the amount of ultraviolet (UV) radiation as on Earth. This UV pounding effectively shreds any of the basic carbon-based building blocks of life, preventing their aggregation into the increasingly complex molecules associated with even such single-celled prokaryotes as bacteria.

Yet, just as Earth has experienced major climatic changes through geologic time, so has Mars. At brief periods of its history, Mars has been comparatively warm and humid, and astrobiologists believe that these periods may have been conducive to life. Interestingly, scientists have found two very different earthly environments that may be analogous to Mars during a warm period and the frozen Mars of today: The hot-vent systems of Yellowstone National Park in Wyoming and the icy deserts of Antarctica.

Yellowstone‘s steaming vents are 90°C (193°F) and provide an extremely acidic environment for any life-forms hardy enough to survive there. The pH, which is 1, can dissolve metals. The primary organisms living there are a bacterium associated with diseases such as leprosy and a photosynthetic alga that is one of the most acid-tolerant organisms of its kind. The alga probably obtains energy through photosynthesis, but the bacterium may get its energy from the surrounding metal-rich environment. Astrobiologists believe that this hot-vent ecosystem may be a good model of Mars during its warmer period: acidic, metallic, and hot. In addition, these organisms fossilize, holding a hope that even if life on Mars has died out, evidence in the form of fossils might remain.

But during the more-prevalent cool periods, Mars is more like the icy deserts of Antarctica, where the very salty environments, averaging about -32°C (-25°F), harbor a common type of Penicillium bacterium, as well as a fungus that usually thrives on insects not present in Antarctica‘s dry valleys. How these organisms arrived is a mystery, as is how they survive. The extreme saltiness of the environment lowers the freezing point of the water to as low as -56°C (-69°F), so that these organisms are in liquid water even in the extreme cold. This environment may be similar to the habitat under the Martian ice caps.

Everyone must be aware of the possibility that the rovers and probes have already inadvertently brought earthly life to Mars. A scientific committee recently reported that there are some organisms capable of surviving the flight to Mars in the rovers‘ circuitry. Once there, these organisms might even be able to survive the extreme UV pounding if concealed under as little as 1 millimeter of soil. Thus, scientists must be extremely careful both in sterilization techniques here at home and in interpretations of any signs of life found on the Red Planet.

1. Which of the following shows that there may be life on Mars?

A. Past water flow.

B. Ozone layer.

C. Atmosphere.

D. Temperature.

2. Which of the following is true?