In 2008, NASA scientists working with images from the Mars Phoenix mission were baffled by an unexpected difference between what they thought they would see and what Phoenix is showed them. Among the spectacular images Phoenix relayed was a color mosaic of the terrain looking out from the lander showing "polygons" of the northern plains.
Through examination of THEMIS, MOLA, Viking, and Mariner data and images, planetary scientists have found that areas on the northern plains of Mars are divided into large polygon-shaped portions and that sets of these polygons span extensive areas of the Martian surface. Smaller polygon-shaped bodies are found elsewhere on Mars, but these are best explained by thermal contraction processes similar to those in terrestrial permafrost environments and not likely to form larger polygons.
Lorena Moscardelli and her colleagues from The University of Texas at Austin have presented a detailed comparison of the geometric features of these large Martian polygons and similar features found in deep-sea sediments here on Earth. Moscardelli and colleagues note striking similarities.
On Earth, polygon-shaped areas, with the edges formed by faults, are common in fine-grained deep-sea sediments. Some of the best examples of these polygon-fault areas are found in the North Sea and the Norwegian Sea. These are imaged using detailed, 3-D seismic surveys conducted to search for offshore oil and gas deposits. Images reproduced in this paper show that these deep-water polygons are also 1,000 meters or greater in diameter.
While the details of deep-sea polygon formation on Earth are complex, Moscardelli and her colleagues conclude that the majority of these polygons form in a common environment: sediments made up of fine-grained clays in ocean basins that are deeper than 500 meters, and when these sediments are only shallowly buried by younger sediments. A key observation -- also made recently by Michelle Cooke at the University of Massachusetts -- is that the physical mechanism of polygon formation requires a thick, wet, and mechanically weak layer of sediment.
Moscardelli and colleagues also conclude that the slope angle of the sea floor plays an important role in both the formation and preservation of these polygons. Where the seafloor slope is very gentle (slopes less than half a degree), the polygons have very regular shapes and sizes. In many locations where polygons have formed on top of buried topographic features on the seafloor, the shapes of the polygons were altered, and in some cases were broken up and disrupted where the slopes were steepest. Both observations are consistent with deformation of the soft marine sediments as they creep or flow downslope in these areas.
In the northern plains of Mars, where the surface is basically flat, the polygons have very regular shapes and sizes -- remarkably similar to the deep-sea polygons found on Earth. In places where the topography on Mars is more varied, and where there may be evidence for other sediment-transport features on the surface, areas of deformed and disrupted polygons can be found -- again similar to the disrupted polygons here on Earth.
On the basis of these striking similarities, the University of Texas team concludes that these features most likely share a common origin and were formed by similar mechanisms in a similar environment. The team argues that the Martian polygons were formed within a thick, wet, and weak layer of fine-grained sediments that were deposited in a deep-water setting, similar to the Earth polygons. Thus, these interesting geometric features may provide additional evidence for the existence of an ocean in the northern portion of Mars approximately three billion years ago.
The topographic map above from Mars Global Surveyor shows colour-coded altitudes; the blue areas are the lowest and correspond to the possible ancient ocean in the northern hemisphere.The color-coded elevation map below of the local terrain on the north side of NASA's Phoenix Mars Lander shows the contours of polygons and relationship of polygon boundaries to trenches and other features in the workspace of the lander's Robotic Arm.
The Daily Gallaxy via geosociety,org
Image credits: NASA/JPL