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Avani Soares

Location of Photo:

Canoas, Brazil

Date/Time of photo:



C14 Edge + ASI 290MC + IR Pas 685


Rupes Recta, Rima Birt and Ancient Thebit There is an image to be analyzed with affection! In a small area, we have a diversity of interesting formations, let's take advantage and bring you the reader curiosities about some. The absolute best example of a lunar fault is found along the east coast of Mare Nubium - it's the straight wall or Rupes Recta. This lunar feature is known for a long hairline that never fails to impress; even casual lunar observers can enjoy a magnificent view. The view is best when the sun rises and the terminator is a little west (as in the attached photo), in the center of Mare Nubium. Thus, the wall casts a shadow that extends along its entire length of about 120 km. This dramatically demonstrates that the lunar surface must be lower on the western side of the fault than on the eastern side. Several authors have reported that the wall is around 250-300 meters high, but the latest measurements through the projected shadow suggest that there may be parts as much as 450m above the bottom of the western basin. Despite appearances, the straight wall is not a cliff, although it is relatively steep, it rises above the plain at an angle of slightly more than 20°. The narrow part of the wall, (traditionally called the Railway by British observers) is now officially known as the Rupes Recta and ends in the south against a bunch of short ridge segments which in the 17th century, the selenographer Christian Huygens likened to the hilt of a sword ( 1), with the straight wall corresponding to the blade of the sword (2). If you expand your view, you will notice the ruined circular walls of an ancient unnamed crater, which had its floor flooded with lava and which my friend Charles Wood calls "Ancient Thebit", and Thebit itself (5) with 57 km wide is located almost in the center of the photo. The 200 km wide eastern edge of this ancient Thebit (indicated by the yellow circle, 3) is well defined, but its western edge is marked only by wrinkled arc-shaped grooves. Although some researchers have noted that Rupes Recta is roughly radial to the Imbrium basin and therefore perhaps related to it, it is very clear that it is much more closely related to ancient Thebit. Wood believes that ancient Thebit was formed on the rim of Mare Nubium by a massive impact, and the western wall of the crater was eventually buried by lava flows that flooded the basin. This is essentially the same sequence that occurred in the Sinus Iridum basin in Mare Imbrium. A good part of the floor of the great Thebit gave way to accommodate the sinking of the basin, this is what probably gave rise to what we currently know as Rupes Recta. Finally, if you look carefully in good viewing conditions, you will see a small parallel channel to the west of Rupes Recta, and to the north of the bright crater Birt (7). Each end of this channel, known as Rima Birt (4), ends in a small cave called Birt "E"(8) and "F" (9). Rima Birt is a challenge for amateur observers and scientists trying to explain why it is there. Because the small cave at the north end of the channel is close to the rim of Greater Thebit, we can speculate that the fractures associated with the rim formed an easy path for lava to erupt onto the lunar surface, producing a dome collapse, a lava channel, and perhaps a small deposit of gas-driven ash and debris. The difference in terrain altitude of about 50m in height between Birt E and Birt F helps to confirm this hypothesis, and makes it more evident that the lava flowing from Birt E found an easy path to penetrate Birt F. In order not to make the text tiresome, let's talk about the beautiful Arzachel crater (6) at a later opportunity. Source: Charles Wood - Sky & Telescope Dobins, Parker, Capen - Observing and Photographing the Solar System Adaptation and text: Avani Soares