Mars Gives Birth To Twins And Triplets

The 27.5-hour-old crescent floats on its side in the western sky last night around 6:30 p.m. The faint outline of the earthlit moon is also visible. Details: 200mm lens at f/2.8, ISO 800 and 1/50" exposure. Photo: Bob King

It wasn’t so much how low the thin crescent moon was last night that made it tricky to see, but how faint. I looked off to the west 25 minutes after sunset and saw Jupiter, but where was the moon? After a bit more searching I finally found it, thin as shaved ice in a cold cup of blue sky.

Shadowed craters on a 2-day-old moon in 2009. Photo: Bob King

Binoculars enhanced the view and made the crescent look crumbly or broken. Segments of brightly lit moon alternated with dark spots created by shadow-filled craters and shadows cast by peaks and crater walls.

The sun comes in at a very low angle on a day-old crescent moon just as it does at sunrise here on Earth, touching only the tops of trees and buildings while leaving streets and valleys in shadow. This shadow effect is more dramatic at extreme crescent phases, because then we see the moon at a glancing angle rather than face on. Shadows stack up across our line of sight “chopping up” the edgy crescent.

Notice the the uneven play of light along the length of last night's crescent moon created by alternating areas of sunlit surface and dark shadows. Details: 400mm lens at f/5.6. Photo: Bob King

Bathed in pink twilight light, the moon grew slightly brighter as twilight deepened, but unless you looked for it, you wouldn’t have known it was there. A beautiful if evanescent sight.  I do hope some of you were able to see it. Tonight a brighter, slightly thicker moon will lie to the right of Jupiter in twilight.

A remarkable pair of "conjoined twin" craters on Mars. Credit: NASA/JPL/University of Arizona

Mars has been in the news recently with new photos returned of rare double and triple craters by NASA’s Mars Reconnaisance Orbiter (MRO). The symmetry of these two side-by-side impacts tells us that the impacts must have happened nearly simultaneously, otherwise one would undoubtedly overlap the other. The twins share a common wall and their impact debris forms neat plumes of surface deposits above and below them.

Multiple pictures of the 135-mile-long asteroid 216 Kleopatra photographed by reflected radio waves. Credit: Stephen Ostro et al. (JPL), Arecibo Radio Telescope, NSF, NASA

Scientists believe the meteorite that created the two broke into two large pieces when it entered Mars atmosphere. With little time to separate before hitting the surface, the two struck almost simultaneously side by side, excavating nearly identical craters. While it’s unusual enough for this to happen, what’s even odder is that the object must have split into two nearly equal parts on its way down.

Perhaps it was dogbone-shaped like the asteroid 216 Kleopatra. Scientists think its weird shape is the result of a collision between two separate bodies that stuck together over time. During atmosphere entry on Mars, a similar though smaller “dogbone” may have been severed in two by powerful air pressures.

The floors of each of this triplet crater are covered in windblown sand dunes. Credit: NASA/JPL/U. of Arizona

Fond of twins but looking for something even more extraordinary? How about triplets? This MRO photo was released earlier this week and shows not one, not two but three craters formed by the breakup of a large meteorite/asteroid when it struck the planet sometime in the distant past. Anyone one for quadruplets?

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