Ideas on the Formation of Resistive Polygonal Ridges on Planet Mars
Today we have a guest post from Planet Four: Ridges volunteer, Bill Hood (geocanuck). Bill Hood is a semi-retired Canadian geologist who has spent 40+ years in the mineral exploration business as a contractor, consultant and prospector. When not wandering around in the mosquito-infested swamps of northern Ontario or the grizzly-prone mountains of the Yukon, he can be found residing in a small town near the city of Winnipeg. A self-confessed Star Trek fan, Bill occasionally argues that it is mineral exploration that will drive human exploration of space, and is rumoured to have already started Mars Palladium Corp. Bill spotted the NASA P4R news release in early 2017, and has been addicted to Mars images/geology ever since.
It appears customary on Planet Four, that after one does a talk or presentation involving Planet Four: Ridges material, a blog is in order. On January 8, 2020, I did a talk titled “Ideas on the Formation of Resistive Polygonal Ridges on Planet Mars” at a meeting of the Manitoba Mineral Society here in Canada. The Society meets monthly in the city of Winnipeg in the local planetarium building, so there’s a slight crossover with the astronomy crowd.
My presentation comprised three main parts: 1) fun facts about Mars, 2) Planet Four: Ridges and the science arising from it, and 3) my ideas on the formation of polygonal ridges. After a brief run-through of the basic geography of Mars, locations of all the landers, and some fun images of faces, structures and items that look like they could only have been constructed by beings with opposing digits or sharp teeth, I outlined how the Zooniverse Planet Four websites functioned to generate Mars data. Next, I outlined some of the science interpretations coming from this data, including the abstract for the talk by Aditya Khuller, Laura Kerber et al, at the 2018 American Geophysical Union convention, as well as a proposed paper titled “Polygonal Ridge Networks in Arabia Terra, Nili Fossae and Nilosyrtis: Evidence for Groundwater Influence”, presently in preparation by the same authors. I then summarized the basic hypothesis of this work to date, which suggests that “Nili” type polygonal ridges on Mars have resulted from burial, compaction and faulting of shallow-basin, clastic sediments, with subsequent groundwater flow and mineral deposition along these polygonal-oriented fault/fracture systems. Subsequent erosion exposed these hard, resistive ridges on Mars. Terrestrial models from the North Sea basin and resistive ridges exposed across the Middle East seemed to be an entirely plausible analogy.
But being a person of contrary character and residing in the frozen environs of rural Canada, I explained to the members of the society that I had difficulty being convinced by the proposed ridges formation argument. When I looked at Mars images, I saw glaciers and pingos and permafrost patterned ground that looked like something out of Arctic Canada, while my on-line Zooniverse friends, most residents of warmer climates, saw a world of palm trees and torrid deserts. As I told my friends in the local Mineral Society on that cold January night, it was clear that I had not just a responsibility, but a Canadian national duty, to advocate a “permafrost hypothesis” for polygonal ridges on Mars.
From my observation, it appears that the “Nili” type polygonal ridges, named for a future type locality in the Nili Fossae region of northeast Arabia Terra on Mars, comprise a range of polygonal oriented resistive ridges, as well as irregular or semi-circular ridges which enclose the margins of ridge areas. It appears that these polygonal ridges are forming in the subsurface, within the basal clastic sediments in local craters, valleys and basins, just above the unconformity at the top of the older Noachian cratered basement rocks on Mars. The similarities to ice fracture patterns and permafrost patterned ground were fairly obvious. So I presented a couple dozen slides, both from Mars and Earth, pointing out fracture pattern similarities, with the caution that there were scale differences and the obvious sub-sediment vs. sub-aerial disparities.
I concluded my talk showing a series of hypothetical cross-sections illustrating a possible process for forming these unusual polygonal ridges in the sub-surface on Mars. The basic idea I am presenting is that a sub-surface permafrost cap may have formed a confined groundwater aquifer. Evidence of sub-surface artesian flow is rather obvious in many Mars images, but the question of what triggered this flow is important. I’m suggesting that the accumulation of aeolian sediment may have formed a thermal blanket which allowed remnant geothermal heat to erode the permafrost cap, triggering artesian flow from the aquifer into basal clastic sediments above the unconformity and into overlying ice-wedge fractures in the permafrost. Having a long residence time, these groundwaters would be at maximum total dissolved solids, so mineral deposition would be significant on evaporation/sublimation.
So that’s my Planet Four blog. I’ll conclude with one takeaway, which is that if this ridge process consumed all the subsurface ice, the Nili ridge areas may not be the best places to send future Mars colonists. I can also advise that the members of the Mineral Society asked that Mars be added to the summer field trip schedule.