Tuesday, February 5. 2013
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.
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.
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.
Explore more of Posidonius crater in the entire LROC NAC frame!