Moonwatch
Lacus Mortis
THE RATHER MORBIDLY named Lacus Mortis, or Lake of Death, lies in the northeast quadrant of the Moon, 243km to the west of the distinctive 70km-crater Hercules. It covers an area of 34,000km2, which is about one and a half times the area of Wales. But despite its rather dark name, the Lacus Mortis region is full of fascinating features to take your time over and study well.
It’s an interesting feature that could be a crater or simply a depressed region of the Moon flooded with lava. Its shape appears to be polygonal rather than circular or elliptical, and the straight runs of the boundary wall are a distinctive characteristic. The floor of the region is relatively smooth save for some impressive rilles in the western half.
The smoothness is disturbed by the imposing presence of 40km Bürg, a rather splendid, terraced crater that sits slightly southeast of centre, inside Lacus Mortis. Ejecta from Bürg dominates the eastern half of the lake, with ejecta ridges appearing to connect to the lake’s boundary. At certain illuminations it looks like Bürg sits at a centre of a delicate lava web crossing the lake’s floor.
Bürg itself is a classic crater. It has raised terraced walls that tower to a height of around 2km. These lead down to a smallish flat floor that is dominated by an impressive central mountain complex. There are several interesting rilles visible to the west and northwest of Bürg, known as Rimae Bürg. A prominent east-west rille appears to be a graben, a region of the lunar surface that has dropped, forming something that resembles a road across the surface of Lacus Mortis.
A more north-south oriented rille appears to cross the eastwest one, close to a point marked by a pair of 2km craterlets. The north-south rille changes in nature as it heads south, appearing as a ridge rather than a crack. An old name for this feature was Rupes Bürg, or the Bürg Fault. Many smaller and, consequently, harder-to-see rilles can also be seen in the northern part of Lacus Mortis when observing conditions are good.
If you have a high-resolution imaging setup and fancy a challenge, there’s a pair of tiny 1.5km features marked on our main image. These are thought to be volcanoes located close to where Rimae Bürg touches the edge of Lacus Mortis – see if you can image them.
To the south of the lake lie two similar-sized craters that look distinctly different to Bürg. The western one is 45kmwide Plana. Its rugged wall surrounds a lava-filled floor that’s relatively flat except for the pockmarks of small craterlets and a central mountain. A round, 14kmdiameter crater known as Plana C touches Plana’s wall to the north and falls within the boundary of Lacus Mortis.
Next to Plana is the 43kmwide Mason. Both craters are separated by a small, rugged mountainous region that casts dramatic shadows across the floor of Plana when the terminator is close during the Moon’s waxing phase, or across the floor of Mason when the terminator is close during the waning phase.
“It could be a crater or simply a depressed region flooded with lava”