Click for full image.
Cool image time! The photo to the right, rotated, cropped, and reduced to post here, shows what resembles closely what in Earth caves are called boxwork, polygonal ridges sticking out from the bedrock and usually indicating cracks filled with harder material that resist erosion.
Taken by the high resolution camera on Mars Reconnaissance Orbiter (MRO) on March 23, 2021, what makes this boxwork especially interesting is its size and location. On Earth cave boxwork generally ranges from a few inches to a few feet across. Not only do these Martian ridges range from 100 feet to a half mile in length, they are located at the lowest point in Hellas Basin, the basement of Mars. In fact, this spot is as close as you can get to Mars’ Death Valley, as shown by the overview map below.
The white box indicates the location of the boxwork image. The darkest blue color indicates the deepest spot, which sits more than 26,000 feet (about five miles) below the Martian “sea level”, deeper than Mount Everest is high.
What formed the ridges? According to this 2017 paper, there are many types of such polygon ridges on Mars, likely formed from a variety of processes. However,
Hellas Basin is host to a fourth type of ridge morphology consisting of large, thick, light-toned ridges forming regular polygons at several superimposed scales. While still enigmatic, these are most likely to be the result of sediment-filled fractures.
In other words, the fractures are thought to be are filled with some sort of material that when hardened is resistant to erosion. As you can see by the image, the only place the ridges are visible are in depressions where the surrounding material has started to disappear, leaving the ridges standing above the surrounding topography.
What is the material that is eroding away? Dust? And what are the ridges made of? Lava? While both are likely, both are also pure guesses based on the available data.
The deep location, however, suggests we are looking at some of the oldest geology on Mars, billions of years old and likely giving us a glimpse of the planet from its infancy. What this is telling us is right now unfortunately very difficult to figure out, because our information is so incomplete.
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