There are numerous river valleys in this section of the transition zone from the Martian highlands to the northern lowlands of Acidalia Planitia. Some of these valleys show what is referred to as a dendritic pattern, imitating the shape of the uniform branching of a tree. Such flow patterns occur as a result of precipitation, in the form of rain or snow, for example. The existence of dendritic outflow systems on Mars indicates that the planet once had a different, presumably warmer and more humid, climate – at least at some point in time.
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| Dendritic river valleys and craters in Acidalia Planitia on Mars. Credit: ESA/DLR/FU Berlin (G. Neukum) |
Isolated fractures in the Martian crust are visible in part of these images. Water stored in subterranean cavities may have escaped from these fractures. Several craters 10 to 20 kilometres wide and filled with sediment are the origin of some of the river valleys. It can be assumed that the craters were once filled with water, forming lakes. There are also several smaller, very well preserved craters. These were presumably formed after the water activity ceased and thus were not filled with water and eroded, nor were sediments deposited there.
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| A large crater in Acidalia Planitia on Mars. Credit: ESA/DLR/FU Berlin (G. Neukum) |
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| Three major and many minor craters in Acidalia Planitia on Mars. Credit: ESA/DLR/FU Berlin (G. Neukum) |
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| An impressive river valley in Acidalia Planitia. Credit: ESA/DLR/FU Berlin (G. Neukum) |
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