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Snowy dunes on Mars.

Over the winter, snow and ice cover Martian dunes…and unlike on Earth, this snow and ice is carbon dioxide… a.k.a. dry ice. When the Sun starts shining on it in the spring, the ice on the smooth surface of the dunes cracks and escaping gas carries dark sand out from the dune below, often creating beautiful patterns. On the rough surface between the dunes, frost is trapped behind small sheltered ridges. Seen by our Mars Reconnaissance Orbiter, this image was taken over the Northern hemisphere of the Red Planet. Credit: NASA/JPL/University of Arizona
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Astronomers took this image as they were observing an extraordinary exploding star – a supernova – near the galaxy’s central yellow core! The star rapidly evolved from a supernova containing very little hydrogen to one that is hydrogen-rich — in just one year. This rarely observed metamorphosis was luminous at high energies and provides unique insight into the poorly understood final phases of massive stars. By studying similar galaxies we hold a scientific mirror up to our own, allowing us to build a better understanding of our galactic environment, which we cannot always observe, and of galactic behavior and evolution as a whole. Credit: ESA/Hubble & NASA/D. Milisavljevic (Perdue University)