Lat: 20.0°S, Long: 46.5°W, Length: 300 km, Depth: km, Rükl 51
USGS Digital Atlas According to the USGS Digital Atlas current IAU nomenclature extends the traditional system of Mersenius rilles to include the fractures on the floor of Mersenius, but omits the rille on the shore of Mare Humorum known as Rima Mersenius III in the System of Lunar Craters. Prior to the System of Lunar Craters the upper part of the long rille extending out the top of the frame (near Gassendi G) was called Gassendi Ir and the lower part Mersenius Ir.
- An interpretation of Rimae Mersenius by John Moore based on LAC 93 (prepared before the System of Lunar Craters had been completed). The feature labeled Mersenius III Rille on LAC 93 is the SLCs Rima Mersenius II. The SLCs Rima Mersenius III is the vague streak extending north out of Rupes Liebig and morphing back into a bright scarp as it approaches Gassendi. It touches the right-most numeral in the ""III"". The small bright crater below the ""III"" and touching this rille on the west was once called Mersenius CC.
(IAU Directions) MERSENIUS.--... A fine crater-row traverses the central part of the interior, nearly axially, and a delicate cleft crosses the N. half of the floor from the inner foot of the N.W. wall to a crater not far from the opposite side. I detected another cleft on November 11, 1883, also crossing the N. side of the floor.
South of Mersenius is the fine ring-plain Mersenius d, about 20 miles in diameter, situated on the border of the Mare; and, extending in a line from this towards Vieta are two others (a, and Cavendish d,), somewhat larger, but otherwise similar; the more westerly being connected with Cavendish by a mountain arm. One of the principal clefts of the system (all of which run roughly parallel to the N.W. side of the Mare, and extend to the Percy Mountains W. of Gassendi) crosses the floor of d, and, I believe, partially cuts into its E. wall. Another, the coarsest, abuts on a mountain arm connecting d with Mersenius, and, reappearing on the W. side, runs up to the N.E. wall of the other ring- plain, a, and, again reappearing on the W. of this, strikes across the rugged ground between a and Cavendish d, traversing its floor and border, as does also another cleft to the N. of it. Cavendish d includes a coarse cleft on its floor, running from N. to S., which I have frequently glimpsed with a 4 inch achromatic. There are two other delicate clefts running from the Gassendi region to the S.E. side of Mersenius, which are in part crater-rills.
- Named for the nearby crater (Mersenius).
- Mary Blagg's Collated List of 1913 included seven rilles (catalog entries 2168-2174) associated with Mersenius in Neison, 1876.
- The original IAU nomenclature of Blagg and Müller retained three of these as Roman-numeraled rilles (catalog entries 2168-2170) and assigned two to Liebig (catalog entries 2172-2173). Number 2174 was dropped.
- An interpretation of the NLF Mersenius rille system by the Army Map Service places all of these east of the crater, in the vicinity of Mersenius E. Mersenius IIIr was desribed as being between Mersenius Ir and IIr.
- In preparing its System of Lunar Craters, the University of Arizona seems to have rewritten the earlier nomenclature. What was formerly Gassendi Ir was added to the north end of Mersenius Ir to form a new and longer Rima Mersenius I on SLC chart F6. Rima Merenius III was redefined as a previously unnamed rille marking the western shore of Mare Humorum; and perhaps by chance Rima Mersenius II runs through Mersenius D (as did the Mersenius IIr of Blagg and Müller. The set of three rilles is labeled Rimae Mersenius.
- Rimae Mersenius "as assigned to features on the lunar surface" was re(?)-approved by the IAU in 1985 with a notation that it is at 20.0°S/45°W (the current "official" position) and "within crater". No map reference or explanation is given as to whether this an addition to or replacement of the former name.
- The current nomenclature map in the IAU-approved(?) USGS Digital Atlas does not seem to include the System of Lunar Craters' Rima Mersenius III as part of Rimae Mersenius, however it does include the fractures on the floor of Mersenius that were approved as being encompassed by the name in 1985.
- Barker, Robert. 1941. Mersenius Clefts R1, R2, R3, R4. Popular Astronomy, Vol. 49, p. 200.