Fresh crater found on lunar images

From San Francisco, at a meeting of the American Geophysical Union

Scientists analyzing images of the moon’s surface taken from lunar orbit believe they’ve identified the crater that formed when a small asteroid slammed into the moon almost 5 decades ago.

Early in the evening of Nov. 14, 1953, an amateur astronomer in Oklahoma observed and photographed a bright flash–believed to be the impact of an extraterrestrial object–on the moon’s surface. Although the camera’s exposure was set for 0.5 second, the flash probably lasted about 1 s, says Bonnie J. Buratti, a planetary astronomer at NASA’s Jet Propulsion Laboratory in Pasadena, Calif.

Using the brightness of the flash and its estimated duration, she and Lane L. Johnson of Pomona College in Claremont, Calif., estimate that the energy of the impact was equivalent to that released by exploding 500,000 tons of TNT. Only about 0.3 percent of the kinetic energy was converted into visible light, Buratti notes. Much of the remainder vaporized rocks, generated huge seismic waves, and blasted a crater.

The pockmark formed by the 1953 impact isn’t large enough to be resolved by earthbound telescopes. Images of the impact’s area garnered by Clementine, a Department of Defense–designed probe that mapped the moon in 1994, show a bright, bluish halo about 1.5 kilometers across. The images don’t show a crater inside the halo because the photos were snapped when the sun was high in the lunar sky and shadows were minimal. Nevertheless, the blue tinge suggests its material is freshly excavated, says Buratti.

The researchers estimate the crater to be about 300 meters across and 60 m deep. The stony asteroid that smacked the moon was therefore probably about 20 m in diameter. Objects that size probably strike Earth or the moon once every decade or so.

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