Valles Marineris, the grand valley of Mars named after the Mariner program which first took close-up images.
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Image from: USGS

Valles Marineris

Next to the Tharsis Ridge is Valles Marineris. Valles Marineris is a large system of canyons that stretches 4000 km (2500 mi) along the equator of Mars. It was first imaged in detail by Mariner 9.

As can be seen in the image, many huge ancient river channels originate from northerly canyons and extend north, toward the top of the image. The three Tharsis volcanoes (dark red spots) are visible on the leftmost (western) edge of the image. To the south are the highlands; very ancient ground, covered by many craters.

High resolution images returned by the Mars Global Surveyor spacecraft allow closer examination of this unusual canyon. These images show slopes descending steeply to the north and south in broad, debris-filled gullies with intervening rocky spurs.

Layered rocks on Earth form from sedimentary processes (such as those that formed the layered rocks now seen in Arizona's Grand Canyon) and volcanic processes (such as layering seen in the Waimea Canyon on the island of Kauai). Both origins are possible for the Martian layered rocks seen in Valles Marineris. In either case, the total thickness of the layered rocks seen in these images indicates that there may have been a complex and extremely active early history for geologic processes on Mars.

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