A Gathering in Lacus Mortis

Boulders meet in a valley of the central peak of Bürg crater (45.0°N, 27.2°E). NAC image M111415328L; incidence angle is 51.7°. Sun is from the south-southeast, north is up, image is ~440 m across [NASA/GSFC/Arizona State University].

Clear impressions in the lunar soil show paths that boulders followed as they rolled downhill in the mountains surrounded by Lacus Mortis (the Lake of Death). If you look closely, you can see trails from both the top and bottom of this image. Also the trails are in some cases jagged -- not orderly, straight lines. What caused such striking patterns?

As is often the case, close inspection of a broader area reveals what we are seeing. Knowledge of the local topography and an understanding of the effects of gravity provide the explanation: The boulders rest in a valley present within the double central peak of Bürg crater. They rolled down from the bouldery summits of opposing slopes and accumulated along the valley floor. The largest block is approximately 23 meters in long diameter!

Note the tortuous paths of some of these boulders. These paths are the results of slow movement combined with irregular shapes (have you ever tried to roll a football in a straight line?). The straighter paths probably represent more rapid movement because the momentum of a faster-moving body helps it maintain its path of travel. Notice that at least one of them bounced along the slope, making impressions only when it touched the surface. Other boulders never made it all the way to the bottom of the slope. Still others don't seem to have clear traces in the regolith up-slope from their location. How could this be? Is it safe to say that some of these arrived more recently than others? Why or why not? Hint: How rapidly does the lunar regolith get reworked by micrometeorite impacts, and how quickly should such gardening erase traces like these?

B ürg Crater, showing the Featured Image location inside the double central peak. Image is ~100 km across [NASA/GSFC/Arizona State University].

Lacus Mortis is shown in the WAC mosaic (below), this area is visible through the eyepiece of a small, backyard telescope beginning with the late waxing crescent phase, about 6 days past new Moon. If you can find Bürg crater, then you'll see where the Featured Image is located, even though the details of the crater will be far too small to see.

WAC mosaic of the region of Lacus Mortis. Image is ~360 km across [NASA/GSFC/Arizona State University].

Explore the full NAC frame below. Another post featuring the complexities of Bürg crater can be found here. Related Featured Image posts also include Sampling Schrödinger; Tycho Central Peak Spectacular; and Boulder in Recht crater.

Published by James Ashley on 22 September 2011