Correlation of Martian surface heights with latitude of polar hood boundaries

Abstract

A COMPARISON of Earth-based photography with topographic data from Mariner 9 reveals that the winter polar hoods on Mars tend to extend further towards the equator at lower topographic heights than at higher topographic heights. The correlation between latitude of the hood boundary and topographic heights is shown in Figs 1 and 2. In Fig. 1, contour lines derived from a map by Christensen¹ have been superimposed on the Lowell Observatory map of Martian Albedo Features and Topography². To the author's knowledge, the Christensen map is the only published map that includes contour lines up to 65°N. The dashed lines represent the mean boundary of the polar hoods during twelve apparitions between 1905 and 1958, as determined from a study of Lowell Observatory's historical photographic collection. The southern hood boundary is in close agreement with a map by Capen³ that was made from measurements of more recent apparitions (1962–68), while the northern boundary agrees well with Mariner imaging⁴. In Fig. 2, the latitude of the hood boundaries (at 10° intervals in longitude) is plotted as a function of topographic height. The dashed line was fitted to the southern latitude points by least squares. The north hood points were then shifted along the ordinate until their average value fell on the dashed line. An equally good, if not better, correlation is found if one plots the latitude of the boundary of the southern hood against atmospheric pressure from the work of Conrath et al.⁵. The hood is found to extend farther north over areas of higher pressure than over areas of lower pressure. Comparable atmospheric pressure data are not available for the Northern Hemisphere.