ACT Science Practice Test 61

PASSAGE I

Earth's habitability is sustained by the sun. Currently, the sun provides enough light and warmth to maintain temperature conditions that can support life on our planet. It is undisputed that the sun is a star. All stars go through phases where they change in size, temperature, and brightness. Two scientists present their views on how long Earth will remain habitable.

Scientist 1

Earth's sun has another 7 billion years before it enters the Red Giant phase. Currently, Earth could not sustain human life during the Red Giant phase. However, it is important not to believe that human life on Earth will immediately cease to exist as we know it in 7 billion years. Technology has played a huge role in helping humans adapt to conditions on this planet. We humans have 7 billion years to advance technology and find solutions to adapt to the atmospheric changes the Red Giant phase would bring. For instance, creating a large sunshade to protect Earth might allow life to continue even when the sun enters the Red Giant phase. Another solution would be to develop technology that would stir the sun and bring new hydrogen to the sun's core. This would greatly extend the current phase that our sun is in. There is enough time and incentive to discover ways to thwart the natural progress of nature. Therefore, I believe that human life on this planet will exist indefinitely.

Scientist 2

The sun will enter its Red Giant phase in about 7 billion years. However, new models suggest that Earth has less than a billion years before atmospheric carbon dioxide levels drop to levels that can no longer support photosynthesis. This would lead to a dramatic temperature increase. Once Earth's average temperature rises to above 70°C, the oceans will evaporate and Earth's water sources will be almost completely eliminated. One billion years is not long enough for humans to evolve in order to meet large atmospheric and environmental changes, or to develop the technology needed to make Earth habitable. In a billion years, atmospheric changes will eliminate all life on Earth as we know it. Humans need to accept the reality that advanced life flourishes for only a limited period of time. Science fiction-inspired plans to create space colonies or massive sunshades are unrealistic and will not likely be developed in the next billion years.

PASSAGE II

Radon is a radioactive gas that occurs naturally in the environment as a result of the decay of uranium. If inhaled into the lungs at high concentrations and over a long period of time, radon gas can increase the chance that an individual will develop lung cancer.

Outdoors, radon levels are rarely high enough to pose a health threat to individuals. Indoors, however, radon is a concern because it can seep into the foundation of a home through the ground and accumulate in areas with little ventilation, where levels can then become threatening. Radon gas can seep from the ground through many different pathways, such as cracks in the basement floor, through drains and sump pumps, or through loose-fitting pipes.

The only way to detect radon levels is through testing, using a specialized sensing device. Radon is colorless and odorless, and the levels are constantly changing from one area to the next and from one day to the next. In addition, radon exposure produces no short-term health symptoms. Therefore, radon levels should be monitored on a regular basis.

Radon potential is an estimate of the radon level of a structure measured in picocuries per liter of air (pCi/L). A picocurie is one-trillionth of a Curie (a measurement unit of radioactivity). The Environmental Protection Agency (EPA) assigns each county in the United States to a zone, based on its radon potential. Radon potential is not used to determine which houses should be tested in an area. Instead, levels are used to determine if radon-resistant features should be installed in new structures being built in an area. Table 1 shows the radon levels in pCi/L for each of 3 zones, with areas in Zone 1 indicating a high radon potential, areas in Zone 2 indicating a moderate radon potential, and areas in Zone 3 indicating a low radon potential.