thin carbon dioxide (CO2) atmosphere "freezes out" during the winter in the northern and southern hemispheres. Scientists have even measured tiny changes in the gravity field of Mars due to the movement of carbon dioxide. In other words, the winter buildup of ice changes the gravity of the planet. Mars has seasons that are similar to Earth's because its rotational axis has a tilt close to our own Earth's (25.19° for Mars, 23.45° for Earth). The south polar cap is higher in altitude and colder than the one in the north. The residual southern ice cap is displaced; that is, it is not centered on the south pole. However, the south seasonal cap is centered near the geographic pole. Studies have shown that the off center cap is caused by much more snow falling on one side than the other. On the western hemisphere side of the south pole a low pressure system forms because the winds are changed by the Hellas Basin. This system produces more snow. On the other side, there is less snow and more frost. Snow tends to reflect more sunlight in the summer, so not much melts or sublimates (Mars climate causes snow to go directly from a solid to a gas). Frost, on the other hand has a rougher surface and tends to trap more sunlight, resulting in more sublimation. In other words, areas with more of the rougher frost are warmer. Research published in April 2011, described a large deposit of frozen carbon dioxide near the south pole. Most of this deposit probably enters Mars' atmosphere when the planet's tilt increases. When this occurs, the atmosphere thickens, winds get stronger, and larger areas on the surface can support liquid water. Analysis of data showed that if these deposits were all changed into gas, the atmospheric pressure on Mars would double. There are three layers of these deposits; each is capped with a 30-meter layer of water ice that prevents the CO2 from sublimating into the atmosphere. In sublimation a solid material goes directly into a gas phase. These three layers are linked to periods when the atmosphere collapsed when the climate changed. A large field of eskers exist around the south pole, called the Dorsa Argentea Formation, it is believed to be the remains of a giant ice sheet. This large polar ice sheet is believed to have covered about 1.5 million square