Cool image time! The picture to the right, cropped and reduced to post here, was taken on January 15, 2023 by the high resolution camera on Mars Reconnaissance Orbiter (MRO). It shows an area where the ground suddenly transitions from a crazy quilt of criss-crossing hollows and ridgelines to a very flat and smooth plain.
The location is at 21 degrees south latitude, so this is in the dry equatorial regions. Though it has a small resemblance to the chaos terrain that is found in many places on Mars, mostly in the mid-latitudes where glaciers are found, the scale here is too small and the ridges and canyons are not as sharply drawn. While chaos terrain usually forms sharply defined large flat-topped mesas with steep cliffs, here the ridges are small and the slopes to the peaked tops are somewhat gentle.
The white dot in the inset on the overview map to the right indicates the location of this strange terrain. Note the white lines in the inset. These mark the location of two parallel straight and deep fractures, one of which crosses the location of this picture. You can see the mostly filled expressions of these parallel fissures just north of this rough terrain in a wider MRO context camera image, taken on December 12, 2009.
Note also the location on the edge of the volcanic Tharsis bulge, where the biggest Martian volcanoes are found.
It appears therefore that we are looking at volcanic deposits. The smooth area I am assuming is a lava flood plain. On Mars lava is thinner, less viscous, and flows more like water. It can therefore cover large areas quickly where it will solidify into a smooth plain.
The rough criss-cross terrain meanwhile is also lava, but for some reason, likely related to the nearest fracture or fault structure to the north, was churned up as it hardened. The context camera image suggests that the flood lava at this point was falling into the fracture, like a waterfall, and as it did so it hardened.
If this hypothesis is right, then the surface here is simply the frozen waves on a river and waterfall of lava. If true, ain’t that neat?
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