Regular solar observer & correspondent Harry Roberts reports on lunar crater Vitello, on the southern shores of Mare Humorum.

Scattered around the “shoreline” of Mare Humorum are several unusual lunar craters.  The most conspicuous is gigantic Gassendi to the north, and opposite it on the southern “shore” is Doppelmayer, looking like a half size copy of Gassendi.  Both are shallow craters with a chain of central peaks and a network of rilles and ridges on their floors. Both give the impression that their floors are convex.  Close to Doppelmayer we find Vitello, looking like a quarter size version of Gassendi, a family of crater “clones”.

When I happened across Vitello I was struck by its similarity to Gassendi, and most eye-catching of all was a near circular bright rille that enclosed the brilliant chain of central peaks: definitely a crater to be sketched.  I have oriented the sketch to give an astronauts-eye view into the crater from overhead; I prefer this view for craters towards the lunar limb where the vertical relief aids interpretation of the landforms.

Vitello has the looks of a crater three times its size, but in fact is only 42 km across.  The bright rille arises at a small crater or vent just inside the northern rim and, after meandering south for 10km, sweeps around the crater floor in a near perfect curve to terminate near the central peaks.  How did such a feature arise?  Viewing the rille under high power gave the impression that the bright wall of the rille was higher on the inside (central) face, i.e. that the rille was also a scarp or uplift feature.  Re-examination under “earlier” lighting may make this clearer.

Wood tells us that these flat-floored craters surrounding Humorum are floor-fractured craters (FFC’s, Charles Wood “Modern Moon” p85).  He suggests the craters floors have been uplifted by lava from below.  The Clementine probe revealed a strong MASCON (mass concentration) under Humorum, and it can be seen that the mare has undergone several episodes of lava flooding. A close look at Mare Humorum shows that most of the larger craters around its margin (excluding Vitello) dip downwards on the side facing the mare, so presumably Mare Humorum is a concave formation, and that lava has partly submerged the craters rims.

Other features inside Vitello include a terrace and scarp inside the SW rim, and some lesser rilles on the west side of the floor.  A bright ray-like patch crosses the floor behind the central mountains.  And on the eastern rim several bright peaks seemed to be the source of a pair of bright rays on the east side of the crater.  Also two small bright patches inside the crater may be mountains or tiny fresh craters.

Orbiter images of Vitello show that the central rille is not as neat as it seemed in the eyepiece.  I missed other branches and complexities in the feature; but it may be that the uplifted face of the rille gave the impression of a uniform feature, while the overhead Orbiter image did not did show the scarp feature at all.  Several dark patches on Vitello’s floor may be the ejecta fields of volcanic vents.

Outside the crater on the north side is a tapering ridge of what looks like impact debris, on which I could see a small fresh crater.  On the east side a segment of sharp concentric ridge gives Vitello the look of a multi-ring  basin.  On the west side a debris feature seems to be crossed by numerous short ridges.

Vitello was Erazmus Witelo[1], 1225 – 1290, a Polish mathematician and physicist who worked in Padua, Italy.  The Moon is covered with craters of all kinds, and Vitello is one of the stranger ones.  Take a close look at Mare Humorum, and see the shallow fractured floor craters that surround it. It’s an extraordinary sight.

Enjoy moon watching.

[1] Rukl, A. “Atlas of the Moon”.  Sky Publishing Corp.  2004.  Map 62.

Harry Roberts is a Sun and Moon observer, a regular contributor to the Sydney Observatory blog and a member of the Sydney City Skywatchers.