Moonwatch – Mare Marginis

The 'Marginal Sea', Mare Marginis, is a tough one to spot, lying right on the edge of the lunar limb

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Mare Marginis, or the ‘Marginal Sea’, is a feature that is not well-known, and not particularly easy to observe. It lies at the edge of the Earth-turned hemisphere, east of Mare Crisium, so that it is very foreshortened. I have found that the best guides to it are the craters Hansen and Alhazen, both around 40km in diameter and both fairly regular in shape; Mare Marginis lies limbward of the pair (there are two smaller named craters between Hansen and the Mare, the 10km Sabatier and the 8km Theiler). To see the area even reasonably well you have to wait for the best conditions of libration – that is, when Mare Crisium appears at its maximum distance away from the limb.

There are several unusual features about Mare Marginis, not shared by other limb seas such as Mare Smythii. It is decidedly irregular in outline, and is not associated with a mascon (an area of concentrated mass). It gives the impression of being rather thin, so that it marks a relatively depressed area of the highlands where the mare lava were only just able to reach the surface. On the floor are some small features, some circular and others elliptical, which have been interpreted as impact craters buried in a shallow layer of lava. There are some curious bright ‘swirls’ possibly of the same type as the famous Reiner Gamma on the floor of Oceanus Procellarum.

Incidentally, it is worth noting that Mare Marginis is antipodal to the Orientale impact basin, and it has been suggested that there may be some connection; certainly the whole antipodal region seems to have been affected by the huge impact that produced Orientale. This is widely believed to have been the last of the major impacts, so that Marginis might also be young by lunar standards – but there have also been suggestions that it might be pre-Nectarian. In fact, we are still not certain about its age.

There are some larger, lava-flooded craters in the area, whose floors lie below the level of the surrounding uplands – another indication that the lavas were close to the surface. The much more prominent Al-Biruni lies to the north of Marginis, with Goddard to the northwest and Ibn Yunus to the southeast. The dark-floored Goddard, 89km in diameter, adjoins Mare Marginis, but is so foreshortened that it is hard to identify even when libration conditions are ideal.

Obviously, Mare Marginis is one of the very first formations to vanish after full Moon, and it is not surprising that early selenographers missed it. It was overlooked even by Beer and Madler, so that the name is more ‘modern’. In general, not very much attention is paid to it, but it is certainly worth taking the trouble to locate this strange, rather challenging limb sea.

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This article first appeared in the April 2010 issue of Sky at Night Magazine