PASSAGE 2 – Questions 11-20

Battles of Lexington

Following resolutions made by the Continental Congress, armed conflict seemed imminent. King George, who declared the colonies in a state of rebellion, had authorized the deliverance of British soldiers to reinforce those who had occupied Boston since 1768. General Thomas Gage was sent to Boston to disarm the colonists and arrest the leaders of the rebellion, namely John Hancock and Samuel Adams. After learning of the existence of a large colonial arsenal in Concord, about 20 miles northwest of Boston, General Gage sent a detachment of 700 troops under Colonel Francis Smith to march to Concord for the purposes of seizing and destroying artillery and ammunition. The Colonists, however, already knew their weapons were in jeopardy and had moved most of them to secret locations before the arrival of the Redcoats.

As British troops mobilized in Boston, Dr. Joseph Warren alerted members of the Sons of Liberty – Paul Revere and William Dawes. Dawes promptly rode to Concord to warn Hancock and Adams, and Revere rode through the countryside yelling “The Regulars are Coming” or “The Regulars are Out.” By dawn, about 70 armed minutemen (Massachusetts militia) had gathered in the Lexington town common awaiting the arrival of the Redcoats. Gage’s own wife, Margaret Kemble Gage, is thought to have given Warren information about her husband’s planned raid in sympathy to the Revolution.

When the British arrived on the scene, someone fired a shot that would become known as “The Shot Heard Round the World.” To this day, historians disagree on which side fired first, or if the shot came from a spectator. Nevertheless, the Redcoats subsequently unleashed devastating volleys before charging the minutemen with their bayonets. As the Redcoats marched on to Concord, eight minutemen were killed and ten wounded. As the march continued, the numbers of minutemen arriving from surrounding towns swelled and the substantial militia retreated to a ridge about a mile from Concord across the North Bridge. Meanwhile, British troops had occupied Concord, destroyed several cannons, and burned leftover ammunition. A gun battle erupted as the two sides crossed at the North Bridge. In a stunning turn of events, the minutemen held the bridge, forcing the Redcoats to retreat.

As the Redcoats exited Concord on their way back to Boston, hundreds or even thousands of farmers and other colonial workers had positioned themselves behind trees, rocks, in pastures, under bridges, and in places they could not be seen, firing at them as they marched on. Desperate Redcoats, frustrated by an enemy they could not see, and in revenge for the brutal scalping of one of their dead, sometimes pillaged houses and communities along the way, killing dozens of colonists. Colonial militias, however, continued to snipe away at the lines of Redcoats. By the time they finally had made it back to Boston, 73 Redcoats were killed and 174 were wounded. The American Revolution had officially begun.

11. When did British troops begin their occupation of Boston?

Meteorite Impact and Dinosaur Extinction

There is increasing evidence that the impacts of meteorites have had important effects on Earth, particularly in the field of biological evolution. Such impacts continue to pose a natural hazard to life on Earth. Twice in the twentieth century, large meteorite objects are known to have collided with Earth.

If an impact is large enough, it can disturb the environment of the entire Earth and cause an ecological catastrophe. The best-documented such impact took place 65 million years ago at the end of the Cretaceous period of geological history. This break in Earth's history is marked by a mass extinction, when as many as half the species on the planet became extinct. While there are a dozen or more mass extinctions in the geological record, the Cretaceous mass extinction has always intrigued paleontologists because it marks the end of the age of the dinosaurs. For tens of millions of years, those great creatures had flourished. Then, suddenly, they disappeared.

The body that impacted Earth at the end of the Cretaceous period was a meteorite with a mass of more than a trillion tons and a diameter of at least 10 kilometers. Scientists first identified this impact in 1980 from the worldwide layer of sediment deposited from the dust cloud that enveloped the planet after the impact. This sediment layer is enriched in the rare metal iridium and other elements that are relatively abundant in a meteorite but very rare in the crust of Earth. Even diluted by the terrestrial material excavated from the crater, this component of meteorites is easily identified. By 1990 geologists had located the impact site itself in the Yucatan region of Mexico. The crater, now deeply buried in sediment, was originally about 200 kilometers in diameter.

This impact released an enormous amount of energy, excavating a crater about twice as large as the lunar crater Tycho. The explosion lifted about 100 trillion tons of dust into the atmosphere, as can be determined by measuring the thickness of the sediment layer formed when this dust settled to the surface. Such a quantity of material would have blocked the sunlight completely from reaching the surface, plunging Earth into a period of cold and darkness that lasted at least several months. The explosion is also calculated to have produced vast quantities of nitric acid and melted rock that sprayed out over much of Earth, starting widespread fires that must have consumed most terrestrial forests and grassland. Presumably, those environmental disasters could have been responsible for the mass extinction, including the death of the dinosaurs.

Several other mass extinctions in the geological record have been tentatively identified with large impacts, but none is so dramatic as the Cretaceous event. But even without such specific documentation, it is clear that impacts of this size do occur and that their results can be catastrophic. What is a catastrophe for one group of living things, however, may create opportunities for another group. Following each mass extinction, there is a sudden evolutionary burst as new species develop to fill the ecological niches opened by the event.

Impacts by meteorites represent one mechanism that could cause global catastrophes and seriously influence the evolution of life all over the planet. According to some estimates, the majority of all extinctions of species may be due to such impacts. Such a perspective fundamentally changes our view of biological evolution. The standard criterion for the survival of a species is its success in competing with other species and adapting to slowly changing environments. Yet an equally important criterion is the ability of a species to survive random global ecological catastrophes due to impacts.

Earth is a target in a cosmic shooting gallery, subject to random violent events that were unsuspected a few decades ago. In 1991 the United States Congress asked NASA to investigate the hazard posed today by large impacts on Earth. The group conducting the study concluded from a detailed analysis that impacts from meteorites can indeed be hazardous. Although there is always some risk that a large impact could occur, careful study shows that this risk is quite small.

21. The word “pose” on line 3 is closest in meaning to

RUNNING WATER ON MARS?

Photographic evidence suggests that liquid water once existed in great quantity on the surface of Mars. Two types of flow features are seen: runoff channels and outflow channels. Runoff channels are found in the southern highlands. These flow features are extensive systems—sometimes hundreds of kilometers in total length—of interconnecting, twisting channels that seem to merge into larger, wider channels. They bear a strong resemblance to river systems on Earth, and geologists think that they are dried-up beds of long-gone rivers that once carried rainfall on Mars from the mountains down into the valleys. Runoff channels on Mars speak of a time 4 billion years ago (the age of the Martian highlands), when the atmosphere was thicker, the surface warmer, and liquid water widespread.

Outflow channels are probably relics of catastrophic flooding on Mars long ago. They appear only in equatorial regions and generally do not form extensive interconnected networks. Instead, they are probably the paths taken by huge volumes of water draining from the southern highlands into the northern plains. The onrushing water arising from these flash floods likely also formed the odd teardrop-shaped “islands” (resembling the miniature versions seen in the wet sand of our beaches at low tide) that have been found on the plains close to the ends of the outflow channels. Judging from the width and depth of the channels, the flow rates must have been truly enormous—perhaps as much as a hundred times greater than the 10⁵ tons per second carried by the great Amazon river. Flooding shaped the outflow channels approximately 3 billion years ago, about the same time as the northern volcanic plains formed.

Some scientists speculate that Mars may have enjoyed an extended early period during which rivers, lakes, and perhaps even oceans adorned its surface. A 2003 Mars Global Surveyor image shows what mission specialists think may be a delta—a fan-shaped network of channels and sediments where a river once flowed into a larger body of water, in this case a lake filling a crater in the southern highlands.

Other researchers go even further, suggesting that the data provide evidence for large open expanses of water on the early Martian surface. A computer-generated view of the Martian north polar region shows the extent of what may have been an ancient ocean covering much of the northern lowlands. The Hellas Basin, which measures some 3,000 kilometers across and has a floor that lies nearly 9 kilometers below the basin’s rim, is another candidate for an ancient Martian sea.

These ideas remain controversial. Proponents point to features such as the terraced “beaches” shown in one image, which could conceivably have been left behind as a lake or ocean evaporated and the shoreline receded. But detractors maintain that the terraces could also have been created by geological activity, perhaps related to the geologic forces that depressed the Northern Hemisphere far below the level of the south, in which case they have nothing whatever to do with Martian water. Furthermore, Mars Global Surveyor data released in 2003 seem to indicate that the Martian surface contains too few carbonate rock layers—layers containing compounds of carbon and oxygen—that should have been formed in abundance in an ancient ocean. Their absence supports the picture of a cold, dry Mars that never experienced the extended mild period required to form lakes and oceans. However, more recent data imply that at least some parts of the planet did in fact experience long periods in the past during which liquid water existed on the surface.

Aside from some small-scale gullies (channels) found since 2000, which are inconclusive, astronomers have no direct evidence for liquid water anywhere on the surface of Mars today, and the amount of water vapor in the Martian atmosphere is tiny. Yet even setting aside the unproven hints of ancient oceans, the extent of the outflow channels suggests that a huge total volume of water existed on Mars in the past. Where did all the water go? The answer may be that virtually all the water on Mars is now locked in the permafrost layer under the surface, with more contained in the planet’s polar caps.

31. The word “merge” in line 5 is closest in meaning to