ACT reading practice test 46

DIRECTIONS: Each passage is followed by several questions. After reading a passage, choose the best answer to each question and fill in the corresponding oval on your answer document. You may refer to the passages as often as necessary.


Natural Science

This passage is adapted from the article "Could Jupiter Moon Harbor Fish-Sized Life" by Victoria Jaggerd (© 2009 by National Geographic).

Below its icy crust Jupiter's moon Europa is believed to host a global ocean a hundred miles deep, with no land at the surface. And the extraterrestrial ocean is currently being fed more than a hundred times more

5 oxygen than previous models had suggested, according to provocative new research. That amount of oxygen would be enough to support more than just microscopic life-forms: at least three million tons of fishlike creatures could theoretically live and breathe on Europa, said

10 study author Richard Greenberg of the University of Arizona. "There's nothing saying there is life there now," said Greenberg. "But we do know there are the physical conditions to support it."

Based on what we know about the Jovian moon,

15 parts of Europa's seafloor should greatly resemble the environments around Earth's deep-ocean hydrothermal vents, said deep-sea molecular ecologist Timothy Shank. "I'd be shocked if no life existed on Europa," said Shank, of the Woods Hole Oceanographic Institution.

20 Despite the promising new estimates, it's too early to do more than speculate about how Europan life might have evolved. A closer look-perhaps by a NASA orbiter now in development-will be needed to tell exactly how chemicals are distributed on Europa and how the moon's

25 geologic history might have contributed to life's chances.

It wasn't until Galileo, the NASA spacecraft, reached the Jupiter system in 1995 that scientists were able to study the moon in detail. What the Galileo probe

30 found was so exciting that NASA deliberately crashed the spacecraft into Jupiter in 2003 to prevent the craft from contaminating one of its own discoveries: the salty, subsurface ocean on Europa. Although the probe didn't see the ocean directly, scientists are pretty sure it's there,

35 based on the age, composition, and structure of the moon's icy surface.

Europa, like the other planets and moons in our solar system, is more than four billion years old. But a relative lack of impact craters implies that the icy crust is just 50

40 million years old. "It's an entirely different surface now than it was at the time the dinosaurs went extinct on Earth," Greenberg said. Europa's smooth surface is marred only by dark, crisscrossing ridges that suggest the icy shell is being stretched and compressed by tidal

45 forces.

"We're used to thinking of tides on Earth as something seen on the shore," Greenberg explained. But on a larger scale, gravity from the sun and moon constantly squishes and stretches Earth. Europa, which is about as

50 big as our moon, also gets tidally stretched, not by the sun but by the gravity of massive Jupiter. The friction from all this tidal stretching probably heats Europa enough to maintain liquid water, Greenberg said.

The warmer ocean material may be oozing up

55 through cracks in the ice and freezing on the surface at the same rate that older ice sinks and melts into the liquid interior. This cycle of "repaving" would explain the young look of the surface ice-and would open the door for oxygen at the surface to permeate the subsurface

60 ocean. Oxygen is created when charged particles from Jupiter's magnetic field hit the ice. Given his estimates for the moon's rate of repaving, Greenberg thinks it would have taken one to two billion years for the first surface oxygen to reach the ocean below.

65 A few million years after the ice-repaving process had started, oxygen levels in Europan seas reached their current levels-which exceed levels in Earth's oceans-Greenberg speculates. This timeframe actually improves the chances that life as we know it took root on Europa.

70 For starters, the most primitive life-forms need an absence of oxygen to form, Greenberg said. "Oxygen tends to cause other molecules to come apart," he said, so genetic material such as DNA can't freely assemble with oxygen present. "You need the delay so genetic material

75 and structures can take shape," he said. "And then when oxygen arrives, organisms will at least have a fighting chance." Greenberg's generous estimate of oxygen in Europa's ocean-and the resulting speculation that fishlike creatures may exist there-depends on the surface

80 repaving to have happened at a relatively stable rate, in this case, a complete renewal every 50 million years.

But planetary scientist Robert Pappalardo said the process may have been more intermittent, and therefore the oxygen level-and chance for fishlike life-lower.

85 "Maybe 50 million years ago it was churning away, and now it's slowed down," said Pappalardo. Europa is gravitationally locked with its neighboring moon Io, which may be pushing and pulling on Europa in extreme cycles, resulting in periods of high and low tidal friction. If tidal

90 activity on Europa comes in fits and starts, that would change the rates at which heat and nutrients from the rocky mantle become available, he said. "Say there are microbes down there," Pappalardo added. "What would it mean for their evolution if every hundred thousand

95 years there was much more heat and chemicals It might lead to much more hardy organisms"-but not necessarily complex life.

1. Based on the passage as a whole, the main factor that makes some scientists suspect Europan life is the:

A. high oxygen levels of its ocean.
B. unusual warmth of its ocean.
C. fact that NASA kept the Galileo probe from contaminating its ocean.
D. fact that its tidal activity comes in fits and starts.

2. In terms of the passage as a whole, the third paragraph (lines 27-36) mainly serves to:

F. provide evidence that, contrary to popular belief, NASA still does important work.
G. explain why the possibility of an ocean on Europa is so exciting.
H. demonstrate the level of control that NASA has over its exploratory craft.
J. emphasize both the importance and recentness of the discovery of Europa's ocean.

3. The passage suggests that compared to other bodies in our solar system, Europa has:

A. been the focus of much more scientific attention.
B. a unique system of tidal activity.
C. much more resilient microbes.
D. a surface formed much more recently.

4. Lines 71-77 primarily serve to illustrate the point that, to have resulted in advanced life, oxygen levels in Europa's ocean would need to:

F. exceed levels in Earth's oceans.
G. have slowed in the last 50 million years.
H. have risen gradually.
J. have been consistently high all along.

5. It can reasonably be inferred from the passage that which of the following bodies has the LEAST gravitational effect on Europa?

A. The sun
B. The Earth
C. Jupiter
D. Io

6. Which of the following is NOT mentioned in the passage as being something about which scientists are not yet sure?

F. Whether life of any kind exists on Europa
G. Whether Europa is gravitationally locked with Io
H. Whether Europa is warm enough for liquid water
J. Whether Europa's ocean even exists

7. As it is used in line 80, the word stable most nearly means:

A. regular.
B. sturdy.
C. average.
D. safe.

8. The main purpose of the final paragraph (lines 82-97) is to:

F. represent the viewpoint that any life on Europa is impossible.
G. explain the process by which microbes evolve into larger life forms.
H. explore the possibility that evolution on Europa works differently from how it does on Earth.
J. suggest that advanced life on Europa may be unlikely.

9. As it is used in line 90, the expression in fits and starts most nearly means:

A. unpredictably.
B. effectively.
C. intermittently.
D. dangerously.

10. It can most reasonably be inferred from the passage that tidal activity on any celestial body is due to:

F. the moon.
G. surface repaving.
H. the influence of other celestial bodies.
J. oxygen-rich liquid water.