  A key outstanding question in Martian science is 'are the polar caps gaining
or losing mass and what are the implications for past, current and future
climate?' To address this question, we use observations from the Compact
Reconnaissance Imaging Spectrometer for Mars (CRISM) of the north polar cap
during late summer for multiple Martian years, to monitor the summertime water
cycle in order to place quantitative limits on the amount of water ice
deposited and sublimed in late summer.
  We establish here for the first time the summer cycle of water ice absorption
band signatures on the north polar cap. We show that in a key region in the
interior of the north polar cap, the absorption band depths grow until Ls=120,
when they begin to shrink, until they are obscured at the end of summer by the
north polar hood. This behavior is transferable over the entire north polar
cap, where in late summer regions 'flip' from being net sublimating into net
condensation mode. This transition or 'mode flip' happens earlier for regions
closer to the pole, and later for regions close to the periphery of the cap.
  The observations and calculations presented herein estimate that on average a
water ice layer ~70 microns thick is deposited during the Ls=135-164 period.
This is far larger than the results of deposition on the south pole during
summer, where an average layer 0.6-6 microns deep has been estimated by Brown
et al. (2014).
