Layers inside Holden Crater in the southern hemisphere of Mars are shown in enhanced color in this image from the High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter. Holden is one of the most interesting locations for scientists investigating the history of water on Mars because the crater contains deep gullies carved by running water, as well as excellent examples of likely lake beds and alluvial fans (sediment deposited by streams) on its floor. These deposits are about 3.7 billion years old and date back to a wetter period of early Martian history. Since that time the planet has generally been very cold and dry, and water has remained frozen in the polar regions or middle latitudes. Credit: NASA/JPL-Caltech/Univ. of Arizona