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US researchers have found traces of an anc ient lake on Mars recently, increasing hopes of discovering evidence that billions of years ago the Red planet hosted life. The lake, which dates back some 3.4 billion years, appears to have covered as much as 80 square miles and was up to 1,500 feet deep, said the team from the U  niversity of Colorado."This is the first clear evidence of shorelines on the surface of Mars,"said Boulder' s research associate, Gaetano Di Achille, in a study published in the latest edition of Geophysical Research Letters. "The identification of the shorelines and accompanying geological evidence allows us to calculate the size and volume of the lake, which appears to have formed about 3.4 billion years ago." Analysi  s of the images has shown the water carved out the canyon (溪谷) in which it was found, which then opened out into a valley depositing (使淤积) sediment (沉积) which formed a delta (三角洲). "Finding shorelines is a great discovery to us," said assistant professor Brian Hynek, adding it showed the lake existed at a time when Mars was thought to have been cold and dry. Scientists believe the oldest surfaces on Mars formed during the wet and warm era known as the Noachan epoch, about 4.1 billion to 3.7 billion years ago. The newly discovered lake is believed to date from the Hesperian era and postdates the end of the warm and wet period on Mars by 300 million years, according to the study. Scientists believe deltas next to the lake may well hold secrets about past life on Mars as such places on Earth have become the natural deposits of organic carbon and other markers of life. 1. The size of the lake found on Mars is ______. A. 80 square miles B. 1,500 square miles C. 3.4 billion square miles  D. 300 million square miles 2. When the lake existed, the weather on Mars was ____. A. hot and wet B. wet and warm C. cold and dry D. cold and wet 3. Why do scientists think deltas near the lake may hold secrets about past life on Mars? A. Because similar places on Earth have become natural deposits of marker  s of life. B. Because someone has put secrets about past life on Mars there. C. Because past life has been found in other deltas on Mars. D. Because some people are said to have already seen life marks there. 4. Which of the following statements is NOT true according to the passage? A. The Noachan epoch was a wet and warm era. B. The lake might have existed 3.4 billion years. C. The discovery can' t fully prove that the Mars once hosted life. D. The lake traces prove that there exists life on Mars now. |