Katabatic winds on Mars

Figure: This view of the north polar region of Mars shows the icy polar cap, about 1,000 km across.  The large canyon (arc arc)  in the lower right is Chasma Boreale which is about as long as the Grand Canyon, and up to 2 km deep.  The dark spiraling bands are troughs. Credit: NASA

The north polar region of Mars contains spiraling troughs up to 10 km in width and 1 km depth.  Winds spiral out from the north pole and in many places cross the troughs at nearly right angles. (In other places, such as the large Chasma Boreale) they flow down the canyons.  By comparison with winds on earth that flow down off high terrain, the winds on Mars have been called katabatic winds. Simulations suggest that horizontal wind velocities in some places on Mars may reach 30 m/s.  In the second figure here, streaks descending the slopes of one of the spiraling canyons are taken to indicate winds pouring over the rim of the canyon. They are eroding grooves into the slope and entraining material, presumed to be a mixture of ice and dirt.  The grooves are being carved by longitudinal vortices in the boundary layer of the winds.  The spacing of the grooves--hundreds of meters--suggests that the boundary layer is hundreds of meters thick (approximately two times the spacing of the grooves). Near the base of the canyon, the winds decelerate--possibly through a hydraulic jump--and the organized structure of the vortices is disturbed.  The entrained material is being dumped out of suspension as indicated by the turbulent clouds.

Added on December 18: Here's a New York Times article about katabatic winds in the Antarctic.