Thermal contraction crack polygons on Mars: Classification, distribution, and climate implications from HiRISE observations

摘要

Documenting the morphology and distribution of polygonally patterned ground on Mars is critical for understanding the age and origin of the Martian latitude-dependent mantle. Polygonally patterned ground on Mars is analyzed using High Resolution Imaging Science Experiment image data in order to document the variation of polygon morphology within latitude bands 30–80° in both northern and southern hemispheres. Small-scale (<～25 m diameter) polygons are classified on the basis of morphological characteristics into seven groups, which are present in both northern and southern hemispheres. Polygon morphology is shown to be consistent with thermal contraction cracking of an ice-rich mantling unit, consistent with observations of sediment wedge thermal contraction crack polygons forming in ice-cemented sediment at the Phoenix landing site. Polygon groups are distributed symmetrically in both northern and southern hemispheres, suggesting strong climate controls on polygon morphology. Northern hemisphere polygonally patterned surfaces are found to decrease in age from low to high latitude, spanning surface ages from ～1 to <0.1 Ma, suggesting more recent deposition of ice-rich material at high latitudes than at low latitudes. Six of the seven classes of polygons are interpreted to be capable of forming because of the combined effects of thermal contraction cracking and differential sublimation, suggesting that sublimation and sand wedge polygons dominate Martian high latitudes. Gully polygon systems present at midlatitudes suggest that small amounts of liquid water may have been involved in thermal contraction crack polygon processes, producing composite wedge polygons. No evidence is found for the presence of pervasive small-scale ice wedge polygons.