The North Polar Cap of Mars
During late 2013 and early 2014, the Northern Hemisphere of Mars faced Earth. This provided a prime opportunity for studying its North Polar Cap (NPC). The writer measured the cap size during the intervals 30° < Ls < 70° and 82° < Ls < 134°. (The areocentric longitude of the Sun from Mars is designated as Ls and it determines the seasons on that planet; essentially, the beginning of the northern spring, summer, fall and winter are at Ls = 0°, 90°, 180° and 270°). NPC latitude measurements were made with the software package WinJupos. The mean latitudes were measured for 23 four-degree intervals of Ls. Wilcoxon Signed Rank Tests at the 90% confidence level are consistent with there being no statistical difference between the NPC in 2000 and both 2013 and 2011-2012 for 30° < Ls < 70°. Based on this same test there is a statistical difference for the NPC in 2002 and 2009-2010 compared to 2000. Therefore, it is concluded that the NPC may show interannual differences. Essentially, the NPC stopped shrinking at Ls = 82°. Its mean latitude for 82° < Ls < 134° was 81.9° ± 0.3° which is equivalent to a mean radius of 480 ± 18 km. This is nearly the same value that was measured between 1905 and 1971.