This is in response to ectoterrestrial's message at
http://www.habitablezone.com/space/messages/25574.html

ectoterrestrial wrote:
Interesting to see only 2 pictures in the North cap region. To much frost to see anything?

Alex writes:
No, not enough light. The dearth of imagery is probably due to the fact that, for this particular mission subphase (E07-E12), the northern polar regions were increasingly in darkness. One will note that during this period Ls varied from ~206 degrees to ~319 degrees. For those who are not familiar with the term,
Ls (read "L sub s") gives the areocentric longitude of the sun in a Mars-centered fixed coordinate system. Due to Mars' high orbital eccentricity and ellipticity, Ls does not vary linearly with time. Moreover, it is an angular measurement and not a whole number. For example, Ls = 0 degrees = 360 degrees is the northern hemisphere vernal equinox; Ls = 90 degrees is the northern summer solstice; Ls = 180 degrees is the northern autumnal (southern vernal) equinox; and Ls = 270 degrees is the northern winter (southern summer) solstice. Moreover, in the current epoch, aphelion occurs at Ls = 70 degrees and perihelion occurs at Ls = 250 degrees.

Alex R. Blackwell
University of Hawaii