Astronomical Society of Coonabarabran

Catena Krafft, and the twin craters

by Harry Roberts

Watching the rapid advance of the lunar terminator towards the west limb is an exciting process, as the observer waits to see how much of far west features will be revealed by libration; the west is a source of rare lunar landforms. So I recently swept the terminator for a suitable drawing target, thinking of a view of crater Riccioli, but it was so full of complex detail, that I deferred any work to a later date. Just north of Riccioli, though, lay two shadow-filled regular craters that seemed to be coupled by a rille or valley, and the attached drawing resulted.

The two craters, almost the same diameter, lay along the 72º line of longitude, with Cardanus to the south, and Krafft to the north. The gap between them was about 50 km., roughly the same as their diameters. From the rim of Cardanus a wide, shadow filled rille emerged and ran to the rim of Krafft, being widest at the south end. At Kraft the rille entered the crater through a cleft in the rim, between the large secondary crater Krafft D on the west, and an unlettered crater (in Rukl's "Atlas") to the east. The rim of Cardanus cast dark shadows westwards, some of them associated with shadowed wrinkle-ridges. On the left side (S) of Cardanus a lit ridge or crater-chain ran southwards. At the NW side a bright spot seemed to be a small crater, emerging from the shadow.

Looking into Rukl's "Atlas" after the session (maps 17 and 28), I found that there was a small crater plotted on Cardanus' NW side. The most surprising thing was that the "rille" connecting the two main craters was called Catena Krafft.! A "catena" is a crater chain, yet the feature I saw looked more like a rima (or rille.) Downloading the Lunar Orbiter image revealed a valley with apparently several small craters along its length, it looked more like a small version of the Alpine Valley (Vallis Alpes) than a crater chain. The latter vallis is thought to be a collapse feature, so the true identity of Catena Krafft seems far from clear. Wood suggests[1] the Catena originates from Cardanus, and notes that it crosses the floor of Krafft.

The bright inner walls of both Krafft and Cardanus showed some terracing, with a possible small crater on the wall of Cardanus. The inner terrace of Krafft seemed very dark near the rille, and looked to be connected to it. I found that lunar draftsman Harold Hill had drawn both in his "Portfolio"[2] and his sketch showed the "rille" connecting smoothly to the terrace inside Krafft. My view doesn't quite agree with this, and I hope to make a "follow-up" observation at the next opportunity.

A dramatic crater ray seemed to come from the edge of Cardanus (where the "rille" emerged) and run to the northwest across Procellarum, but photos show this major ray comes from crater Glushko further south west of Cardanus (Wood[3] uses the old name Olbers A for this crater in referring to the ray, see page 186). Wood notes how this bright ray "bends" adjacent to Krafft, then passing Seleucus heads north of Aristarchus; a very strange ray indeed. Near Cardanus and close to the ray was a bright half crescent that the Orbiter image suggests was the rim of an old crater flooded by mare lava ("Cr" in fig 1.)

Curiously, Harold Hill says of the crater pair: "One of those striking meridional arrangements that we find on the Moon; they would seem to occur too frequently to be regarded as merely fortuitous"[4]. He goes on to cite O. Struve and Russell as a further example. Could Hill be implying a volcanic origin, or does he think more than chance causes crater pairings?

Take a look, and let me know what you think.

[1] Wood, C. "The Modern Moon, a Personal View." Sky Publishing 2003. P 185
[2] Hill, H. "A Portfolio of Lunar Drawings" Practical Astronomy Handbooks 1. Cambridge Uni. Press. P80-81
[3] Ibid. P 186
[4] Hill. P 80.

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