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 University 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.”

　　Analysis 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.

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