Space Photos This Week: Sun Circles, Mars Rock, More

Triple Circles

Photograph by Laurent Laveder, TWAN

Captured in a 360-degree fish-eye photograph, two atmospheric circles are seen above Brittany, France, on September 2.

The smaller circle, with the sun at its center, is known as a 22-degree halo, because any given point on the circle is roughly 22 degrees from the sun. Such halos are formed by sunlight, or occasionally moonlight, reflecting off of hexagonal ice crystals suspended high in Earth's atmosphere.

The larger circle, in the center of the image, is a parhelic circle—much rarer than sundogs or 22-degree halos, according to the astrophotography group TWAN's website. Parhelic circles are also formed by light shining through atmospheric ice crystals, though in these cases the crystals have a particular vertical orientation.

"I was waiting for [a parhelic circle] since about 15 years ago," said photographer Laurent Laveder on the TWAN site, and when these circles appeared above Brittany last month, he was ready to get the picture.

Published October 8, 2010

Nickel-Iron Meteorite

Image courtesy NASA

Photographed by the NASA rover Opportunity on September 24, a Martian rock is shown in false color.

After an inspecting the rock via Opportunity's microscopic imager and alpha particle x-ray spectrometer, the rover's science team determined that the stone is a nickel-iron meteorite. Separately, they nicknamed it "Oileán Ruaidh," the Gaelic name of an Irish island.

Opportunity recently reached the halfway point in its multi-Earth-year journey to Endeavour Crater, an 12-mile (19-kilometer) drive. The 14-mile-wide (22-kilometer-wide) crater is particularly interesting to NASA scientists because of the recent detection of clay at the depression's rim. Clay forms only in the presence of water, and water on Mars could indicate the potential for life as we know it.

Published October 8, 2010

Heads Up

Image courtesy NASA/JPL-Caltech/UCLA/JHU

As reinforced by Hollywood blockbusters, the threat of a comet or asteroidhitting our planet is a causeofmuchconsternation. One way that humanity is addressing the threat is through the NASA's orbiting Wide-field Infrared Survey Explorer, or WISE. This solar system diagram depicts comets, asteroids, and planets, as detected through the new NEOWISE project to hunt for near-Earth objects in WISE data.

Black specks depict asteroids, heavily concentrated in our solar system's main asteroid belt, between Mars and Jupiter. Blue and yellow squares depict comets found by the NEOWISE mission.

The green and red dots are most worrisome: They depict near-Earth objects, thought to come within 124,000 miles (200,000 kilometers) of Earth.

On average, a 0.3-mile (0.5-kilometer) or larger asteroid impacts Earth every hundred thousand years, according to NASA, and such impacts are thought to have caused mass extinctions.

Fortunately, smaller asteroids or comets tend to burn up in our atmosphere.

(Watch Naked Science: Preventing Armageddon on the National Geographic Channel, airing at 10 a.m. ET on Sunday, October 9.)

Published October 8, 2010

Sand Trap

Image courtesy NASA/JPL/University of Arizona

Crescent-shaped sand dunes on Mars are seen in monochrome by the HiRISE instrument aboard NASA's Mars Reconnaissance Orbiter.

Like sand dunes on Earth, these mounds in the Herschel crater are formed as wind shifts sand. As dunes extend downwind, they merge, forming steep "slip faces" on the dunes' leading edges.

The crater at top right is notable for preventing dunes from migrating behind it, although it is accumulating some sand, according to the HiRISE website.

Published October 8, 2010