Martian north polar cap summer water cycle

TL;DR

This study uses CRISM observations to quantify the summer water cycle on Mars' north polar cap, revealing a seasonal mode flip and significant water ice deposition, which impacts understanding of Martian climate change.

Contribution

First to quantify the summer water cycle on Mars' north polar cap using spectral data, identifying a mode flip and estimating ice deposition thickness.

Findings

Water ice absorption increases until Ls=120, then decreases.

Regions switch from sublimating to condensing in late summer.

Average ice deposition during summer is about 70 microns.

Abstract

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).