A Curious Lunar Spot

Jack Kramer

There is an interesting bright spot near the large crater Atlas close to the northeast limb of the moon. Small features like this are nothing special, except that this one doesn't appear on some lunar atlases. I first heard of it a few years ago via a call for observations of it from the American Lunar Society. It appears on the following image that I had taken in 2004 through a 6" refractor.

A search of various sources shows this is not a named feature, nor have I found any information about an object at this spot. The best image I've located is from the Clementine lunar mapping mission, which portrays features in the straight-on view from a spacecraft rather than foreshortened as when observed from Earth. It turns out to be a very small crater with a bright collar of ejecta and unusual ray pattern. There also appears to be an oblong upraised area extending to one side. That's probably a type of dune, which on the moon is a low ridge of loose rock formed during the ejection of material from an impact crater. That seems to fit the case here. The Clementine image prompts a mental picture of a meteoroid striking the moon at an oblique angle, scattering debris in a pattern commensurate with the angle of impact. The ejecta is very bright, indicating relatively fresh material. Considering that the crater is so small, the amount of ejecta is surprisingly large, while other small craters in the area show little or no ejecta.

Since the feature's appearance varies in different images, I began observing it regularly to determine how it shows up visually at different phases. The tiny crater was not visible at all, due to its small size and the foreshortened view that we have near the limb. Observation of the young moon under high power shows the dune as a short, thin black line. The ejecta is visible even at low power. I couldn't detect the dune later in the lunar month during gibbous moons, but the unique spray pattern of ejecta becomes brighter and more detailed at that time even in a telescope as small as four inches. By the time of full moon, the ejecta seemed to have faded somewhat. Further observation, especially under the best seeing conditions, may reveal more aspects.

Close to full phase, two neighboring features take on some added interest. The larger dark spot adjacent to the inside rim of the crater Atlas becomes very prominent at this time, appearing as a pitch black circle. (It shows up well in the Clementine image.) Apparently this is an area of especially dark lava that welled-up within the crater. Also the largest crater within adjacent Hercules becomes very bright.

This shows that as familiar as the moon's terrain may be, now and then we do come across a feature that prompts more than just a passing glance. I'd be interested to find more detail on this little spot of lunar activity.

Published in the March 2009 issue of the NightTimes