Formation of the huge canyon (Valles Marineris) is also likely to be tectonic
in origin. However, details are not very well understood. It was probably
produced by horizontal stretching in response to some mantle processes.
The canyon's existence suggests that the Martian surface must have experienced
large-scale extension sometime in its history. Over the past billion years,
however, the planet's surface has been compressing due to planet-wide
cooling and contraction.
As a result, wrinkle ridges similar to those found on the Moon
can be observed (Figure 7.3). In this image, sunlight comes from
the left. Thus, it follows that the narrow curved feature next to
the upper left edge represents a rift valley, and the remaining
elongated structures are elevated ridges on the Martian surface.
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