Meanders in Posidonius

Small portion of a S-shaped meandering rille on the floor of Posidonius Crater (31.93°N, 29.85°E, 100 km diameter) - a floor-fractured crater. The curves in the rille are very tight. LROC Narrow Angle Camera (NAC) observation M1098658474R, LRO orbit 14260, August 3, 2012; 1.45 meters resolution, from 143.74 kilometers [NASA/GSFC/Arizona State University].

Jeffrey Plescia
LROC News System

Rilles (linear to meandering troughs) and floor-fractured craters are common on the Moon and on Mars. Lunar rilles occur primarily on the open mare (for example Hadley Rille at the Apollo 15 landing site); others are restricted to craters. Floor fractured craters are an unusual type of crater typically occurring in the highlands near the mare boundary. Posidonius crater lies in the highlands just beyond the northeast margin of Mare Serenitatis. Much of the floor of Posidonius crater is covered by smooth plains considered to be mare basalts. There are suggestions that the rilles are erosional in origin and formed as massive outpourings of lava carved (and melted) the floor. The rille crosses a distance or more than 100 km linear distance across the floor; the actual distance along the floor of the crater including all of the curves is much greater.

Regional view of the western floor of Posidonius crater (inset - full size view HERE) showing its meandering pattern across the crater floor. Toward the top of the frame there is a tributary rille. Wider field of view from LROC NAC M1098658474R [NASA/GSFC/Arizona State University].

The rille that crosses the floor of Posidonius crater extends west across margin of the northern crater floor and then turns south. Perhaps coincidentally (or not) the ridge runs south adjacent to pieces of the original crater floor poking through the crater-filling mare plains. Finally, the rille turns southwest to the southwest rim; it then continues along the base of the southwest crater rim. The rille is about 50 m deep. The wavelength of the meanders is about 1 km.

Oblique view (59° off nadir) of the same rille, LROC NAC photograph from well west of the area of interest. LROC NAC M1096379115LE, spacecraft orbit 13941, July 8, 2012; general resolution 4 meters from 145.49 km over Mare Serenitatis [NASA/GSFC/Arizona State University].

Mosaic of both the left and right frames of LROC NAC oblique observation M1096379115 captures a vast portion of Posidonius in high, late afternoon relief. The rectangle represents the field of view in the image at full resolution immediately above [NASA/GSFC/Arizona State University]

The overall geology of Posidonius crater can be seen in the WAC image below. The image clearly shows the smooth plains that form the floor on the north and west side in which the rille occurs and the tilted fractured floor on the eastern side of the crater.

Posidonius crater captured in afternoon lighting. The crater is embayed by younger mare to the west, though the floor of the crater is about 300 meters higher in elevation. LROC WAC M1096283231C [NASA/GSFC/Arizona State University].

Explore more of Posidonius crater in the entire LROC NAC frame, HERE.

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