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In one of my weekly posts last month (dated May 14th) delving into the May image release from Mars Reconnaissance Orbiter’s (MRO) high resolution camera, I featured an image of what planetary geologists have labeled chaos terrain, a hummocky chaotic terrain that has no real parallel on Earth but is found in many places on Mars.
This month’s image MRO release included two more fascinating images of this type of terrain. In addition, the Mars Odyssey team today also released its own image of chaos terrain, showing a small part of a region dubbed Margaritifer Chaos. Below, the Mars Odyssey image is on the right, with one of the MRO images to the left. Both have been cropped, with the MRO image also reduced in resolution. The full MRO image shows what the MRO science team labels “possibly early stage chaos” on the rim of a canyon dubbed Shalbatana Vallis.
Both of these regions, as well as the chaos image featured in my May 14th post, are located in the outflow region from the solar system’s largest canyon, Marineris Valles. The overview map on the right shows their locations by the white crosses. Shalbatana Valles is the left cross, Margaritifer Chaos the right cross, and the center cross the location of my May 14th post.
Geologists believe that the random hummocky hills and mesas of chaos terrain were formed by erosion, as noted at the Mars Odyssey image site:
The term chaos is applied to regions where the surface is being eroded to form mesas. As the surface processes continue individual mesas become more isolated and take on the appearance of regions of hills. At the top of the image is Margaritifer Terra, which is being dissected into large mesas. Towards the bottom of the image the mesas have been eroded into smaller hills.
The MRO image above on the left seems to support this. This possibly young chaos appears to be eating away at the rim of Shalbatana Vallis, with mesas closer to the rim and hills farther away.
The location of so much chaos terrain at the mouth of Marineris Valles leads one to think that at least some if not most of this erosion was specifically caused by the sudden catastrophic outrush of liquid water that is believed to have formed Marineris Valles. Later erosion, as shown by the Shalbatana Vallis image above, might have come from the slow drainage of any remaining underground water-ice.
The problem is that chaos terrain is found in other places on Mars, and in places that do not really fit this scenario. The second MRO image of chaos terrain released this month, cropped and reduced in resolution and shown on the right, illustrates this. The chaos terrain fills the floor of this small crater. More importantly, the crater is surrounded by smooth plains, and is no where near any major outflow channels, or any other chaos terrain. Instead, this small patch of chaos is located to the southwest of Mars’s giant volcanoes, on the vast volcanic slopes heading downward towards the transition zone between Mars’s southern cratered highlands and the northern low plains.
The chaos terrain thus might not require a catastrophic outflow, as seen at the mouth of Marineris Valles. Instead, it might simply indicate a specific geological layer under the surface that is seen in many places on Mars, and becomes evident only with erosion. Or it could be a specific erosion feature formed due to the lower Martian gravity, which will lower the density of bedrock near the surface. Erosion will thus shape geology differently than on Earth.
Or it could form from many other erosion processes, some presently unknown. For example, why did chaos terrain only form inside this crater, and not on the plains that surround it?
It all remains a mystery, but a fundamental one for Mars. When we know more about chaos terrain, we will know more about the planet’s own inner geology.