spring. Typically 500 meters wide and 1 meter deep, the spiders may undergo observable changes in just a few days. One model for understanding the formation of the spiders says that sunlight heats dust grains in the ice. The warm dust grains settle by melting through the ice while the holes are annealed behind them. As a result, the ice becomes fairly clear. Sunlight then reaches the dark bottom of the slab of ice and changes the solid carbon dioxide ice into a gas which flows toward higher regions that open to the surface. The gas rushes out carrying dark dust with it. Winds at the surface will blow the escaping gas and dust into dark fans that we observe with orbiting spacecraft. The physics of this model is similar to ideas put forth to explain dark plumes erupting from the surface of Triton. Research, published in January 2010 using HiRISE images, found that some of the channels in spiders grow larger as they go uphill since gas is doing the erosion. The researchers also found that the gas flows to a crack that has occurred at a weak point in the ice. As soon as the sun rises above the horizon, gas from the spiders blows out dust which is blown by wind to form a dark fan shape. Some of the dust gets trapped in the channels. Eventually frost covers all the fans and channels until the next spring when the cycle repeats. Layers Chasma Australe, a major valley, cuts across the layered deposits in the South Polar cap. On the 90 E side, the deposits rest on a major basin, called Prometheus. Some of the layers in the south pole also show polygonal fracturing in the form of rectangles. It is thought that the fractures were caused by the expansion and contraction of water ice below the surface. Polar ice cap deuterium enrichment Deuterium is a heavier isotope of hydrogen compared to the element's most common isotope, protium. This makes any celestial body's deuterium statistically much less prone to being carried into space by stellar wind compared to its protium. Evidence that Mars once had enough water to create a global ocean at least 137 m deep has been obtained from measurement of the HDO to H2O ratio over the north polar cap. In March 2015, a team of scientists published results showing that the polar cap