This is an image showing a Martian outflow channel.
Click on image for full size
Image from: NASA

Martian Outflow Channels

The Martian surface exhibits a large number of large, tear-drop shaped outflow channels such as the one shown in this image. Most of these are found on the slopes of the Martian volcanoes. They seem to start from a hole in the ground, and quickly develop their mature form, as if whatever came out of the ground came out full steam. An examination of their structure helps scientists estimate the rate at which the fluid came out, or the channels "capacity". The channels are typically 10 m to 100 m deep (about the depth of a football field), 40 km across, and filled with rubble. Mars Pathfinder landed on a channel such as this called Ares Vallis.

The most plausible explanation for these is that they are eruptions of ground water. There is no water left in these channels today.

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