Lunar Challenge

Grant Barlow

While perusing a recent copy of Nature (June 30, 1994), I came across an interesting letter in the Scientific Correspondence section on lunar crater chains. My challenge is for you to find them. They should be visible using amateur telescopes such as what we own. Here is a condensation of the letter from H. J. Melosh and E. A. Whitaker, of the Lunar and Planetary Laboratory, University of Arizona, Tucson:

"The recognition that nearly a dozen crater chains on the jovian moon Callisto may have been created by comets disrupted during the passage inside Jupiter's Roche limit has prompted a search for crater chains on our Moon that may have been created by comets similarly disrupted near the Earth. A careful search of the available global photography of the Moon yielded two chains (see table) that are not obviously secondaries from any large crater or basin, nor are endogenic. Both chains are on the lunar nearside, as expected for comets split during a close pass by the Earth, and range in length from about 50 km for the well-known Davy chain to 200-250 km for a chain near the crater Abulfeda.

Secondary crater chains on the Moon are common, but such chains are usually radial or nearly radial to a large crater or basin, occur in the vicinity of other such chains, and have raised rims with a characteristic 'chevron' imprint between craters that point back to the primary. Individual secondary craters are frequently irregular in shape. Much rarer are chains, such as the northwest arm of Rima Hyginus or a small chain near the crater Beer, that are obviously of endogenic origin. These craters exactly fill the interiors of rilles or grabens and are often elongated in the direction of the chain.

Although the Davy chain is a relatively fresh, post- Imbrian feature, the Albulfeda chain is more degraded and is assigned an Imbrian age. The disparity in the lengths of these chains might at first suggest that they do not have similar origins. However, a simple model for tidal splitting that assumes a radial breakup at perigee shows that the length of the chain is a linear function of the parent size. Because the craters in the Albulfeda chain are roughly five times larger than those in the Davy chain, it is not unreasonable to suggest that the Albulfeda chain is roughly five times longer.

The remarkably regular shape, spacing and alignment of the craters in the 47-km-long Davy crater chain is unmatched anywhere else on the lunar surface. The unimpeded continuation of the craters across the floor and up the east wall of crater Davy Y is inconsistent with an endogenic origin. The linear dunes separating many of the craters indicate nearly simultaneous impacts."

The rest of the letter is math and an explanation showing the tidal forces encountered by an object going through the Earth's Roche limit.


Observing Results:

Davy comes into view near the terminator when the Moon is about nine days old. On your atlas, Davy is shown as the small crater to the (lunar) east of the large crater Alphonsus. However, the craterlets are not in Davy, but rather, they're in the larger, highly eroded crater basin lying roughly between the craters Davy and Palisa. I can find no name or other designation for this basin; however, the preceding article refers to this as "Davy Y". On a large-scale Lick Observatory photographic atlas of the Moon, some of the craterlets are faintly visible. Within the day after it is beyond the terminator, the craterlets should show up best due to the steep angle of illumination. On a fairly steady night, using 174x and 270x in my 3" refractor, I was able to spot the craterlets only as a slightly brighter line running across the dark basin floor. The line was brightest where it meets the wall of Davy Y. In my 10" reflector at 212x, I was able to periodically glimpse several of the larger craterlets as discreet dots. Going to 425x in the 10" didn't improve the view. The overall image was steadier and sharper in the refractor, but in this case, the greater light gathering (and resolution) of the 10" made the difference. - J. Kramer