This year is the United Nation’s International Year of Light and Light-based Technologies, and there are celebrations, events, and programs on-going for the duration of 2015. The purpose of this initiative, quoting the Year of Light’s webpage, is to
promote improved public and political understanding of the central role of light in the modern world while also celebrating noteworthy anniversaries in 2015—from the first studies of optics 1,000 years ago to discoveries in optical communications that power the Internet today.
Light is actually one of the important parts of the process that creates the fans and blotches that we’re asking you to map in the classification interface. The entire process is solar powered. The fans and blotches that spot the surface of the South Pole in the spring and summer are the direct result of sunlight warming and sublimating a slab of carbon dioxide ice.
When the south pole is in darkness during the winter sols, the atmosphere condenses out to form a slab of carbon dioxide ice mixed with the atmospheric dust. This ice sheet is semi-translucent so you see down to the surface below that’s it’s covering. When the sun returns to the south pole starting in the early spring, sunlight penetrates through to the base heating the regolith below. The ice at the base of the sheet sublimates turning from solid ice to gas. With carbon dioxide gas trapped between the dirt and the ice sheet, it catches some of the loose dirt and soil particles. The gas exploits weaknesses in the ice sheet, breaking out at the surface as geysers or jets.
The dirt and soil is brought up to the surface, and we think that the prevailing winds then blow the particles into the dark fans you see in the images. If there isn’t any wind or it is not bowing very hard you get the blotches instead. The fans and blotches appear dark, even though they’re really the same color as the material below due to the fact that you’re viewing the surface through tinted glasses (the ice sheet is semi-translucent because of the dust). When the ice sheet has sublimated away, the fans and blotches basically disappear blending back in with the soil.
The sols on the south pole are now getting shorter and shorter and the HiRISE seasonal monitoring campaign has ended. The sunlight is waning and soon the cycle will start anew, with the ice sheet forming as the south pole is shrouded in darkness. Around July 2016, the sun will back and the new season of the HiRISE monitoring campaign will begin again as the fans and blotches reappear at the top of the thawing ice cap.