Ultraviolet spectra taken with Hubble's faint-object spectrograph provide another line of evidence for a colder, cloudier Mars, Clancy adds. The spectra show the unmistakable fingerprints of enhanced ozone at mid to low latitudes. That's intriguing because the presence of elevated ozone indicates that these layers of the atmosphere must hold little, if any, water vapor. If they did, the vapor would rapidly deplete ozone by catalyzing its conversion into oxygen. In the absence of water vapor, ozone can reside there undisturbed.
Summer comes to the Martian north pole at aphelion, when the atmosphere is relatively chilly.
This means that water vapor subliming from the northern cap can't travel far in the atmosphere before it recondenses into ice crystals. Water vapor circulates from the north pole to the south at an altitude above 5 to 10 km, but very little rises this high before freezing. In contrast, the south pole has its summer at perihelion, when the atmosphere is warmer. Because of the higher temperature, vapor subliming from the south polar cap resides in significant quantities above the critical range of 5 to 10 km. Thus, water from the south gets pumped northward, but water from the north doesn't circulate south, accounting for the larger ice cap at the north pole, Clancy theorizes.