image by Clementine orbiter and processed by Paolo Amoroso.
Frigoris is the forgotten mare. Its not conspicuously placed, isn’t circular nor is it bounded by a mountain rim. It is about 1500 km long, 200 km wide and includes mare rocks of different hues. The lighter parts of the mare are draped by crater ejecta - look at the crater Aristoteles (mid-right) which excavated highland rocks (red) from beneath the mare and dumped the debris over the yellow-hued mare. The eastern (right) end of Frigoris is redder than its central part showing that the eastern end is more widely sprinkled with highland ejecta - is it older as well? Probably. According to counts of superposed impact craters (the more, the older the surface), eastern Frigoris is about 3.5 to 3.8 billion years old, while central Frigoris is slightly younger at 3.4 to 3.6 b.y. Mare Frigoris has few of the small volcanic features common on many other maria. There are very few domes, sinuous rilles or volcanic dark halo craters. Mare ridges do occur - in the eastern part of the mare they are orthogonal (at right angles), near Plato they tend to run east-west, and they seem to be more north-south in the western portions, but no modern map of the ridges exist. Maybe someone can compile one?
Both of these images were acquired by the Clementine spacecraft in 1994. The bottom image was taken through a single filter and the top is a false color image made by combining images obtained through 3 different color filters. Red is highland material, yellow is mare and blue is very fresh material of either type - notice blue Archytas and the small fresh crater northeast of Plato. Paolo captured these images from the very useful Map-A-Planet web site of the US Geological Survey. I found that he had posted them on the LPOD Photo Gallery - I never know where an LPOD will come from!
Rükl plates 2-6
Paolo’s lunar website
Yesterday's LPOD: A Stained Death?
Tomorrow's LPOD: Schroeter & Pickering & Keene