Photo sequence credits: NASA/JPL/Malin Space Systems
From October 1999 to August 2001 (approximately 1 Martian year=687 days), the Mars Orbital Camera captured these comparative time sequences. This yearly inventory suggested major, global climate changes with similar time scales to terrestrial ones. The first thing the sequenced images show is a changing landscape: pits have enlarged while mesas and buttes are either disappearing or have vanished altogether from 1999 to 2001.
The retreat was calibrated to around 3 meters (3.3 yards) per Martian year. If made of water ice, the retreat would be much less (not measurable from 1999-2001) so researchers at Malin Space Systems/JPL/NASA attributed the retreat to dry ice reservoirs (frozen carbon dioxide). The overlay images are taken during Martian summer, and otherwise portray what persists as nearly permanently frozen (residual) caps. Each inset picture represents a distance about 3 (American) football fields across (250 meters, 273 yards). As the cap erodes, the CO2 layers become exposed, pitted or depressed. Parts of the Southern cap never thaw (or sublime) and the same material may be in a hard freeze for thousands of years.
Perhaps the most remarkable finding about this rate of CO2 erosion, may be its effects not on the landscape itself, but on the atmosphere - which is 95% C02 already. Each layer (10 meters) may be equivalent to 1% of the present Martian atmosphere (which is 1% of the Earth's in density). Thus, these photographs demonstrate that a doubling time for Mars atmosphere may occur in as short as 200-1000 terrestrial years. While how Mars will look in the year 3000 is of interest, more dramatic would be how different Mars might have been in the year 1000. Based on the erosion and warming, the effects of this Martian ice age in the past would extrapolate to a thickening atmosphere and more future greenhouse warming that contributes to an acceleration towards something less hostile than today.