The landscape of the north polar layered deposits of Mars (NPLD) is dominated by a pinwheel array of enigmatic spiral troughs1. The troughs have intrigued planetary scientists since the Mariner 9 spacecraft returned the first close-up image in 1972, but conclusive evidence of their origin has remained elusive. Debate continues regarding all aspects of the troughs, including the possibility that they have migrated2,3,4,5, their age in relation to the current NPLD surface6, and whether they are fundamentally erosional6,7 or constructional2,4 features. The troughs are probably related to climatic processes2,8, yet the nature of this relationship has remained a mystery. Previous data characterizing only the exposed NLPD surface were insufficient to test these hypotheses. Here we show that the central spiral troughs initiated after deposition of three-quarters of the NPLD, quickly reached a stable morphology and migrated approximately 65 kilometres poleward and 600 metres in altitude over the past two million years or so. Our radar stratigraphy rules out hypotheses of erosional incision post-dating deposition6,7,9,10, and instead largely validates an early hypothesis for constructional trough migration2,3,4,5 with wind transport and atmospheric deposition as dominant processes. These results provide hard constraints for palaeo-climate models and a new context for evaluating imagery, spectral data, and now radar sounding data, the better to understand the link between orbital parameters and climate, the role of climate in shaping the polar ice of Mars, and eventually, the age of the polar deposits themselves8,11,12,13.
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This work was supported by NASA grant NAG5-12693 to J.H. and by the Gayle White Fellowship at the University of Texas Institute for Geophysics. We especially thank D. Young, S. Kempf, C. Brothers and S. Christian for listening to ideas and for help preparing the data and images. We thank A. Howard for insight and comments, and the SHARAD Operations Center for their continuous support in data acquisition. We dedicate this paper to the memory of SHARAD team member Ali Safaenili, who provided the focused radargrams we use. This is UTIG contribution 2187.
This file contains Supplementary Notes S1-S5, Supplementary Figures S1-S5 with legends and References.
About this article
Journal of Geophysical Research: Planets (2019)