They might look like trees on Mars, but they’re not. Groups of dark brown streaks have been photographed by the Mars Reconnaissance Orbiter on melting pinkish sand dunes covered with light frost. The above image was taken in 2008 April near the North Pole of Mars. At that time, dark sand on the interior of Martian sand dunes became more and more visible as the spring Sun melted the lighter carbon dioxide ice. When occurring near the top of a dune, dark sand may cascade down the dune leaving dark surface streaks — streaks that might appear at first to be trees standing in front of the lighter regions, but cast no shadows. Objects about 25 centimeters across are resolved on this image spanning about one kilometer. Close ups of some parts of this image show billowing plumes indicating that the sand slides were occurring even while the image was being taken.
Not a bright comet, 67P/Churyumov-Gerasimenko now sweeps slowly through planet Earth’s predawn skies near the line-up of planets along the ecliptic. Still, this composite of telescopic images follows the comet’s progress as it moves away from the Sun beyond the orbit of Mars, from late September (left) through late November (far right). Its faint but extensive coma and tails are viewed against the colorful background of stars near the eastern edge of the constellation Leo. A year ago, before its perihelion passage, the comet was less active, though. Then the Rosetta mission’s lander Philae made it’s historic landing, touching down on the surface of the comet’s nucleus.
Albert Einstein’s general theory of relativity, published 100 years ago this month, predicted the phenomenon of gravitational lensing. And that’s what gives these distant galaxies such a whimsical appearance, seen through the looking glass of X-ray and optical image data from the Chandra and Hubble space telescopes. Nicknamed the Cheshire Cat galaxy group, the group’s two large elliptical galaxies are suggestively framed by arcs. The arcs are optical images of distant background galaxies lensed by the foreground group’s total distribution of gravitational mass dominated by dark matter. In fact the two large elliptical “eye” galaxies represent the brightest members of their own galaxy groups which are merging. Their relative collisional speed of nearly 1,350 kilometers/second heats gas to millions of degrees producing the X-ray glow shown in purple hues. Curiouser about galaxy group mergers? The Cheshire Cat group grins in the constellation Ursa Major, some 4.6 billion light-years away.
Planet Earth’s horizon stretches across this recent Solar System group portrait, seen from the southern hemisphere’s Las Campanas Observatory. Taken before dawn it traces the ecliptic with a line-up familiar to November’s early morning risers. Toward the east are bright planets Venus, Mars, and Jupiter as well as Regulus, alpha star of the constellation Leo. Of course the planets are immersed in the faint glow of zodiacal light, visible from the dark site rising at an angle from the horizon. Sometimes known as the false dawn, it’s no accident the zodiacal light and planets both lie along the ecliptic. Formed in the flattened protoplanetary disk, the Solar System’s planet’s all orbit near the ecliptic plane, while dust near the plane scatters sunlight, the source of the faint zodiacal glow.
Why are there unusual pits on Pluto? The indentations were discovered during the New Horizons spacecraft’s flyby of the dwarf planet in July. The largest pits span a kilometer across and dip tens of meters into a lake of frozen nitrogen, a lake that sprawls across Sputnik Planum, part of the famous light-colored heart-shaped region named Tombaugh Regio. Although most pits in the Solar System are created by impact craters, these depressions look different — many are similarly sized, densely packed, and aligned. Rather, it is thought that something has caused these specific areas of ice to sublimate and evaporate away. In fact, the lack of overlying impact craters indicates these pits formed relatively recently. Even though the robotic New Horizons is now off to a new destination, it continues to beam back to Earth new images and data from its dramatic encounter with Pluto.
Auroras usually occur high above the clouds. The auroral glow is created when fast-moving particles ejected from the Sun impact the Earth’s magnetosphere, from which charged particles spiral along the Earth’s magnetic field to strike atoms and molecules high in the Earth’s atmosphere. An oxygen atom, for example, will glow in the green light commonly emitted by an aurora after being energized by such a collision. The lowest part of an aurora will typically occur at 100 kilometers up, while most clouds usually exist only below about 10 kilometers. The relative heights of clouds and auroras are shown clearly in the featured picture from Dyrholaey, Iceland. There, a determined astrophotographer withstood high winds and initially overcast skies in an attempt to capture aurora over a picturesque lighthouse, only to take, by chance, the featured picture along the way.
The constellation of Orion is much more than three stars in a row. It is a direction in space that is rich with impressive nebulas. To better appreciate this well-known swath of sky, an extremely long exposure was taken over many clear nights in 2013 and 2014. After 212 hours of camera time and an additional year of processing, the featured 1400-exposure collage spanning over 40 times the angular diameter of the Moon emerged. Of the many interesting details that have become visible, one that particularly draws the eye is Barnard’s Loop, the bright red circular filament arcing down from the middle. The Rosette Nebula is not the giant red nebula near the top of the image — that is a larger but lesser known nebula known as Lambda Orionis. The Rosette Nebula is visible, though: it is the red and white nebula on the upper left. The bright orange star just above the frame center is Betelgeuse, while the bright blue star on the lower right is Rigel. Other famous nebulas visible include the Witch Head Nebula, the Flame Nebula, the Fox Fur Nebula, and, if you know just where to look, the comparatively small Horsehead Nebula. About those famous three stars that cross the belt of Orion the Hunter — in this busy frame they can be hard to locate, but a discerning eye will find them just below and to the right of the image center.