ACT Reading Practice Test 8

NATURAL SCIENCE:

This passage is adapted from the article "The Pioneer Mission to Venus" by Janet G. Luhmann, James B. Pollack, and Lawrence Colin (©1994, Scientific American).

Venus is sometimes referred to as the Earth's "twin because it resembles the Earth in size and in distance from the Sun. Over its 14 years of operation, the National Aeronautics and Space Administration’s Pioneer Venus mission revealed that the
Line 5 relation between the two worlds is more analogous to Dr. Jekyll and Mr. Hyde. The surface of Venus bakes under a dense carbon dioxide atmosphere, the overlying clouds consist of noxious sulfuric acid, and the planet's lack of a magnetic field exposes the upper atmosphere to the continuous hail of charged particles
10 from the sun. Our opportunity to explore the hostile Venusian environment came to an abrupt close in October 1992, when the Pioneer Venus Orbiter burned up like a meteor in the thick Venusian atmosphere. The craft's demise marked the end of an era for the U.S. space program; in the present climate of
15 fiscal austerity, there is no telling when humans will next get a good look at the earth's nearest planetary neighbor.The information gleaned by Pioneer Venus complements the well-publicized radar images recently sent back by the Magellan spacecraft. Magellan concentrated on studies of Venus's surface
20 geology and interior structure. Pioneer Venus, in comparison, gathered data on the composition and dynamics of the planet's atmosphere and interplanetary surroundings. These findings illustrate how seemingly small differences in physical conditions have sent Venus and the Earth hurtling down very different
25 evolutionary paths. Such knowledge will help scientists intelligently evaluate how human activity may be changing the environment on the Earth.Well before the arrival of Pioneer Venus, astronomers had learned that Venus does not live up to its image as Earth's
30 near twin. Whereas Earth maintains conditions ideal for liquid water and life, Venus's surface temperature of 450 degrees Celsius is hotter than the melting point of lead. Atmospheric pressure at the ground is some 93 times that at sea level on Earth. Even aside from the heat and the pressure, the air on Venus
35 would be utterly unbreathable to humans. The Earth's atmosphere is about 78 percent nitrogen and 21 percent oxygen.Venus's much thicker atmosphere, in contrast, is composed almost entirely of carbon dioxide. Nitrogen, the next most abundant gas makes up only about 3.5 percent of the gas molecules. Both planets
40 possess about the same amount of gaseous nitrogen, but Venus's atmosphere contains some 30,000 times as much carbon dioxide as does Earth's. In fact, Earth does hold a quantity of carbon dioxide comparable to that in the Venusian atmosphere. On Earth, however, the carbon dioxide is locked away in carbonate
45 rocks, not in gaseous form in the air. The crucial distinction is responsible for many of the drastic environmental differences that exist between the two planets.
The large Pioneer Venus atmospheric probe carried a mass spectrometer and gas chromatograph, devices that measured
50 the exact composition of the atmosphere of Venus. One of the most stunning aspects of the Venusian atmosphere is that it is extremely dry. It possesses only a hundred thousandth as much water as Earth has in its oceans. If all of Venus's Water could somehow be condensed onto the surface, it would make a global
55 puddle only a couple of centimeters deep.Unlike the Earth, Venus harbors little if any molecular oxygen in its lower atmosphere. The abundant oxygen in the earth's atmosphere is a by-product of photosynthesis by plants; if not for the activity of living things, Earth's atmosphere also
60 would be oxygen poor. The atmosphere of Venus is far richer than the earth's in sulfur-containing gases, primarily sulfur dioxide. On Earth, rain efficiently removes similar sulfur gases from the atmosphere.Pioneer Venus revealed other ways in which Venus is more
65 primordial than Earth. Venus's atmosphere contains higher concentrations of inert, or noble, gases-especially neon and isotopes of argon-that have been present since the time the planets were born. This difference suggests that Venus has held on to a far greater fraction of its earliest atmosphere. Much of
70 Earth's primitive atmosphere may have been stripped away and lost into space when our World was struck by a Mars-size body. Many planetary scientists now think the moon formed out of the cloud of debris that resulted from such a gigantic impact.