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Star trails create arches over the horizon in a long-exposure picture of the night sky taken from Mount Kilimanjaro in Tanzania. The shot, captured in July and released this week, shows the apparent motion of the stars around Polaris, the star that's almost exactly aligned with Earth's north celestial pole. Also called the North Star, Polaris is the brightest dot in the constellation Ursa Minor. Equatorial regions, such as Kilimanjaro, are the only places on Earth where the celestial poles sit right at the horizon.
It may look like mold under a microscope, but the above image, released December 15, is actually a map of where sunlight hit the moon's south pole. Over six months, NASA's Lunar Reconnaissance Orbiter took 1,700 pictures of the moon's polar region. Each shot was then converted to a binary image: A pixel was dubbed a one if it was illuminated and a zero if it was dark. Stacking the images generated the illumination map. Understanding which parts of the moon are always in darkness may be key to planning future missions to the moon—at least one permanently dark crater, for instance, seems to hold a significant cache of water ice.
To human eyes, the icy surface of Saturn's moon Rhea would appear fairly monotone. But when viewed in a combination of infrared, ultraviolet, and green wavelengths, Rhea comes alive with color, as seen in a composite picture from NASA's Cassini orbiter released December 20. The shot shows the hemisphere of Rhea that always faces Saturn. The left side of the visible disk faces in the same direction that Rhea orbits around Saturn.
Color differences are most likely due to regional changes in surface composition or the sizes and structures of the grains in the moon's icy soil. Such changes could be driven by debris preferentially hitting certain parts of the moon. The colors could also be caused by an effect called magnetic sweeping, in which charged particles in Saturn's magnetic field sweep over Rhea and become implanted in the soil.
The moon's highs and lows are revealed in full color in a new topographic map produced by NASA's Lunar Reconnaissance Orbiter and released December 17. Using a laser pulse split into five beams, the spacecraft has made the most accurate map yet of the contours of the entire lunar landscape. The above picture shows the moon's southern hemisphere, with higher regions seen in red and low places in blue.