A paper in an upcoming special Mars-based issue of the planetary science journal Icarus has examined the small polygonal structures that cover large regions of northern Mars. The paper, by J. Korteniemi and M. A. Kreslavsky, comes to the conclusion that Mars may still be geologically active.
AmericaSpace previously reported on a paper by Dr. Lorena Moscardeli et al. that described the large, kilometer-scale polygons that cover certain regions of Mars,a and may indicate that Mars once had an ocean. The much smaller, meter-scale polygons described in the Korteniemi paper are unrelated to those.
Korteniemi and Kreslavsky used images taken by the HiRISE camera of the Mars Reconnaissance Orbiter and visually inspected terrain between 50 and 70° N. The team then visually inspected images to identify terrain types and look for small craters, on the order of 5 m in diameter. The primary goal was to determine the rate, or if they were at all, at which these small craters were smoothed out and obliterated by geological processes.
The new paper identifies two types of polygonal terrain. Type 1 terrain is smoothly undulating and has 10-20 m wide mounds surrounded by 3-10 m shallow troughs. Type 2 terrain, which is mostly isolated to the northernmost regions of the study area, shows clear, angularly connected, sharp polygon-forming fractures. The terrain is often accompanied by smaller scale polygonal structures, down to the limit of the resolution of the images used.
The two types of terrain are not mutually exclusive and there is a set of transitional terrains between them.
The most important part of the paper, however, and the evidence that Mars is still, in some sense, geologically active, comes from their analysis of impact craters in the study region.
The team carefully analyzed all possible small (on the order of 3-200 m in diameter) impact craters, coming up with about 4000 possible craters, and divided them into 7 categories. These categories ranged from “pristine,” which the paper describes as “small, fresh impact craters with pristine ejecta fields.”
At the other end of the spectrum are boulder patterns, or a collection of boulders signifying a crater underneath the surface.
In between these two extremes are sharp craters, smooth craters, circular pits, annular polygons, and annular fractures.
The team theorizes that “pristine” craters are eroded and changed by forces acting in the polygonal terrain. Craters identified in Type 2 terrain degrade an order of magnitude faster than those in Type 1 terrain. Calculations indicate that it only takes about 50 years for a 5 m crater in Type 2 terrain to be totally obliterated, and 150,000 years for a 50 m crater in Type 1 terrain to be obliterated.
Many craters south of 60 degrees N harbored polygonal fractures inside the crater walls. There seemed to be no particular logic to the placement of the fractures within the crater, or perhaps the fractures form across the entire crater. These types of craters are largely located within Type 1 terrain.
The team then analyzed the coincidence of the two types of polygonal terrain and shallow subsurface water ice. Neutron spectroscopy analysis indicates that the heaviest concentration of near-surface water ice begins at about 60° N, the same region where Type 2 terrain dominates.
This led the team to conclude that the presence of near-surface water ice strongly coincided with Type 2 terrain, at least in the northern hemisphere of Mars. The heavily cratered highlands of southern Mars have a very different topography.
Having made this observation, the team concluded that the thermal expansion and contraction of water ice near the surface caused the polygons of Type 2 terrain to change over relatively short timescales. This same effect drives the obliteration of small craters in the region. The team even went so far as to suggest that these features may be in the process of forming right now.
But the vast extent of the polygonal features, and the similarity of the base structure of both types of terrains begs for an explanation. The team hypothesized that during a period of high obliquity (Mars’ axial tilt with respect to its orbit, and indeed the shape of its orbit itself, is known to vary chaotically on the order of about 50,000 years) the subsurface water ice migrated south, causing the formation of Type 1 terrain across the entire region of interest. As the obliquity varied, the subsurface ice migrated northward and continued acting on the northernmost regions of the terrain, further modifying the terrain into Type 2 polygons and quickly obliterating small craters that form in those regions.
This evidence suggests that on some level, Mars is geologically active. The nature of the ground terrain is changing on what may be a continuing basis, encompassing today and the future. The exact source of this change is unknown, but the presence of subsurface water ice seems to be related.
These exciting results will appear in a special issue of the Icarus journal, focusing on Mars Polar Science. The exact date of the issue’s release is unknown at this time, but the paper, titled, “Patterned Ground in Martian High Northern Latitudes: Morphology and Age Constraints,” is available at the Icarus website, listed as S0019-1035(12)00405-8 and available at http://dx.doi.org/10.1016/j.icarus.2012.09.032 .