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The atmosphere of Mars was once thinner than it is today.

The river systems of Mars were once more extensive than Earth’s.

Large amounts of rain once fell on parts of Mars

The rivers of Mars began to dry up about 4 billion years ago.

remains

sites

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To emphasize the great size of the volume of water that seems to have flowed through Mars’ outflow channels

To indicate data used by scientists to estimate how long ago Mars’ outflow channels were formed

To argue that flash floods on Mars may have been powerful enough to cause tear-shaped “islands” to form

To argue that the force of flood waters on Mars was powerful enough to shape the northern volcanic plains

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