Life on Mars
Astronomers strongly suspect that Mars currently has no liquid water, diminishing the chances for life there now. But running water and a denser atmosphere in the past could have conceivably fostered conditions suitable for the emergence of life. And there is some evidence that water did flow on Mars long ago, presumably before the planet entered its current ice age less than a billion years ago.
1.The word “diminishing” in the passage is closest in meaning to
A ending
B spoiling
C changing
D reducing
Perhaps ancient fossils of long-dead Martian life – simple bacterial life possibly enduring prior to the arrival of the numbing cold that likely prohibited sustained life as we know it – might show paleontological evidence for rudimentary life. If a severe ice age had locked Earth into a deep freeze a billion years ago, the only evidence that life arose on our planet would be microscopic remains of fossilized microbes – and humans would not be here to examine it.
Surprisingly, one place to look for Martian fossils is right here on Earth. Paleontologists agree that a small fraction of meteorites found on Earth’s surface have actually come from the Moon and from Mars. These meteorites were apparently blasted off these bodies long ago during an impact of some sort, thrown into space violently enough to escape their parent bodies, and eventually captured by Earth’s gravity, ultimately to fall to the ground. The most fascinating of these rocks are surely a dozen or so from the Red Planet (Mars) – their trapped gases match exactly those present in Mars’s atmosphere- and one of them may harbor fossil evidence for past life.
2.According to paragraph 3, scientists believe all of the following about meteorites from the Moon and Mars found on Earth’s surface EXCEPT:
A Earth’s gravity caused them to fall to the surface.
B They left their parent bodies quite recently.
C Impacts knocked them off their parent bodies.
D They make up only a small proportion of the meteorites found on Earth.
3.According to paragraph 3, why do planetologists believe that some of the meteorites found on Earth are from Mars?
A The fossil evidence in the rocks appears to match previously known early life on Mars.
B The red coloration of the rocks is much like the present coloration of Mars.
C The rocks appear to have originated in an explosive impact known to have occurred on Mars.
D The gases in the rocks are the same gases currently found in Mars’s atmosphere.
Based on estimates of the cosmic-ray exposure it received while drifting toward Earth, the meteorite catalogued as ALH84001 was ejected from the Martian surface about sixteen million years ago. The blackened rock itself, some four billion years old and about the size and weight of a grapefruit, was found in 1984 in the Alan Hills of Antarctica, a place where pristine meteorites often just sit atop the icy wastes of the barren, frozen landscape. Upon breaking it open and examining closely its cracks and crevices, scientists could see rounded, brownish globules of carbonate matter no larger than the period at the end of this sentence. Because carbonates form only in the presence of water, these small globules imply carbon dioxide gas and liquid water near ground level at some time in Mars’s history. This matches the inferences drawn earlier from orbital photos of valleys and tributaries apparently carved by water when the Martian climate was wetter and warmer.
4.According to paragraph 4, why do the globules in a particular Martian meteorite suggest the possibility of life?
A Dating of the globules based on cosmic-ray exposure suggests they originated when
life would have been most likely on Mars.
B The color and size of the globules suggest they came from early water-based life.
C The carbonate in the globules could only have formed if water were present on Mars.
D The globules contain traces of water that have been preserved by the frozen temperatures of Antarctica.
Claims that the ALH84001 rock contains traces of primitive Martian life stem mainly from our knowledge of bacteria on Earth. Terrestrial bacteria do produce structures similar to the Martian globules, and they also manufacture iron-rich chemical crystals that the rock displays in tiny, teardrop-shaped crystals embedded in places where the carbonate has dissolved.
Furthermore, the rock contains traces of PAHs (polycyclic aromatic hydrocarbons), often found among the decay products of terrestrial plants and other Earth organisms. None of these data would individually indicate life if found on Earth, but all of them collectively make the case stronger for life on Mars. Even so, as is often said in science, extraordinary claims require extraordinary evidence”
5.What does the author imply with the phrase, “extraordinary claims require extraordinary evidence”?
A Data that individually seems to support the idea of life on Mars actually weakens this idea when considered collectively with other data.
B Scientists believe that traces of Martian life would be very different from structures found on Earth.
C Scientists would require stronger evidence than that obtained from ALH84001 to prove that life once existed on Mars.
D Scientists believe that the traces of PAHs found in ALH84001 provide extraordinary evidence of life on Mars.
6.According to paragraph 5, all of the following have been interpreted as evidence of life in ALH84001 EXCEPT
A similarities between the globules in ALH84001 and structures produced by bacteria on Earth
B the presence of iron-rich crystals in ALH84001 that resemble crystals that are made by bacteria on Earth
C the presence of chemicals in ALH84001 that are associated with the decay of Earth organisms
D the presence of tiny, tear-shaped markings in ALH84001
A final piece of Martian evidence is the most dramatic – and the most controversial. On very small scales seen only through a powerful microscope, elongated and egg-shaped structures are discernable inside the carbonate globules of ALH84001. And these are what some scientists have taken to be fossils of primitive organisms. Outwardly, photomicrographic images reveal curved, wormlike structures clearly resembling bacteria on Earth. But scale is a crucial part of any interpretation. The minute structures are only about half a micron across, or some ten times smaller (hence a thousand times less voluminous) than ancient bacterial cells found fossilized on Earth. Many biologists argue that such minute bags of chemicals are simply too small to have functioned as life as we know it. What is more, the Martian rock contains no evidence of amino acids, cell walls, semipermeable membranes, or any kind of internal cavities for bodily fluids – all of which properties attend even the oldest and most primitive fossils found on Earth.
7.According to paragraph 6, what is one reason for thinking that the elongated, egg-shaped structures in carbonate globules are not fossilized organisms?
A The structures are not shaped like bacteria on Earth.
B Bacteria would not have been able to function inside the carbonate globules where the structures appear.
C They contain combinations of chemicals unknown on Earth.
D The structures are much smaller than cells of ancient bacteria on Earth.
8.In paragraph 6, why does the author state that amino acids, cell walls, and semipermeable membranes are properties of primitive fossils found on Earth?
A To cast further doubt on the evidence for Martian life
B To contrast Earth’s primitive life with that of Mars
C To describe the unique properties of Earth’s fossils
D To identify some of the properties needed for life to develop
9.Look at the four squares that indicate where the following sentence could be added to the passage
Close study of the rock began soon after its discovery.
Where would the sentence best fit？Click on a square sentence to the passage.
Based on estimates of the cosmic-ray exposure it received while drifting
toward Earth, the meteorite catalogued as ALH84001 was ejected from the Martian surface about sixteen million years ago. ⬛ The blackened rock itself, some four billion years old and about the size and weight of a grapefruit, was found in 1984 in the Alan Hills of Antarctica, a place where pristine meteorites often just sit atop the icy wastes of the barren, frozen landscape. ⬛Upon breaking it open and examining closely its cracks and crevices, scientists could see rounded, brownish globules of carbonate matter no larger than the period at the end of this sentence. ⬛Because carbonates form only in the presence of water, these small globules imply carbon dioxide gas and liquid water near ground level at some time in Mars’s history. ⬛This matches the inferences drawn earlier from orbital photos of valleys and tributaries apparently carved by water when the Martian climate was wetter and warmer.
10.Although it is unlikely that life exists on Mars today, there may have been enough liquid water before the Martian ice age to sustain primitive life.
Select 3 answers
A If Mars’s atmosphere was once warm and wet enough for life to develop, the best evidence of living organisms could be fossilized bacteria preserved in meteorites.
B Some meteorites have been found on Earth that scientists believe to have originated on Mars before its ice age, and one of these could indicate whether some simple life forms had developed.
C Because Martian meteorites were exposed to cosmic rays before falling to Earth, any sings of Martian life that they may have contained were probably destroyed.
D Places on Earth, like Antarctica, that have frozen conditions similar to those on Mars when its meteorites were ejected provide scientists with sources of evidence of ancient Martian life.
EMany scientists believe that the most convincing evidence that life once existed on Mars is the liquid water inside globules of carbonate matter recovered from Martian meteorites.
FAlthough a Martian meteorite was found to have chemicals and structures like those produced by simple life forms on Earth, it contains no evidence of ancient Martian life.
