Last night (3-13-2008 3 UTC), I spent a few minutes observing the Moon at a Univ. of Utah Physic's Dept. public telescope night. The Moon was favorably positioned at a pre-spring equinox altitude of 45 degrees. The terminator was running through central Mare Serenatitus west of C. Plinius at about E19 long and at a low-solar angle of about 6.5 degrees. While looking along the terminator for interesting objects, I was suprised to see the dark mantle at the southeastern end of the Mare Serenitatis Basin - between Promontorium Archerusia and Mons Argaeus to show really striking contrast. It looked like there was a dark triangular kitchen knife blade above C. Plinius and north of Rimae Plinius. Initially, I took the dark patch for a shadow but then remembered mineral differences between the titanium poor lava flows at the depressed center of the mare and older more titantium rich grabens around the edges of the mare. (See LPOD 8-27-2004, http://www.lpod.org/archive/LPOD-2004-08-27.htm , LPOD 2-17-2007 http://www.lpod.org/?m=20070217 ). The dark mantle around the edges of Mare Serenitatis are well known and usually seen at the full Moon as an albedo effect shown in the 8-27-2004 LPOD. But last night under low angle illumination, the albedo effect was much more intense and triangular shaped with defined edges than normal. I wasn't sure if I was looking at darker shadow due a topography grade or a very favorable low angle illumination that enhanced the albedo darkness of the dark mantle garben just north of Rimae Plinius. See Rukl 24 and LAC I489. Or was it a combination of both? The telescope docent at the U. Physics Dept., Paul Ricketts, graciously interrupted his routine to snap some quick fits images using a DSI Pro II on a Meade 14 inch SCT. The attached processed image that I prepared is not up to modern lunar imaging standards (compare to LPOD 5-11-2007, http://www.lpod.org/?m=20070511 of C. Plinius by Simon Kidd), but it does convey the enhanced albedo of the Rimae Plinius dark mantle region that I saw on the 13th. The right-hand side is a Clementine near infra-red image from USGS Map-A-Planet. http://gallery.utahastronomy.com/main.php?g2_itemId=11038 http://gallery.utahastronomy.com/main.php?g2_itemId=11042 Other parts of the image can be taken to argue ambiguously for and against a mineral cause of the darker albedo north of the R. Plinius graben. The outline of the dark region is similar to its geologic extent seen on LAC I489. The region to the west of Dorsum Aldrovandi is made of the same geologic materials as those of the R. Plinius graben, but in the image the D. Aldrovandi region does not show a contrast difference with lighter lava flows in the center of the mare under an only slightly higher solar illumination angle. Conversely, the dark mantle deposits of the Littrow Valley fan appear darker under that same higher illumination angle. I did some post-session checking to resolve in my mind whether the heightened dark albedo was the result of 1) a shadow from topography or 2) just the low-Sun angle reflecting off the dark mantle surface or 3) some combination of the two. The USGS shaded relief map elevation values for this area were ambiguous, as where the ULCN 2005 control points plotted using LTVT. I checked a USGS DEM created from the ULCN points and plotted a profile north south across the Rima Plinius graben between C. Plinius and C. Dawes. The DEM profile showed a consistent 400 meter slope over 200 kilometers with a ledge corresponding to the graben shown north of C. Plinius and south of Dorsa Nicol in Simon Kidd's high-resolution high-detail image (LPOD 5-11-2007). Comparing the three references (my image, the USGS DEM, and Kidd's image), I decided that the enchanced dark albedo that I saw could probably be attributed to mostly a topographic shadow. In any event, the next time the terminator is around East 19 degrees at a lunar age of 5.4 days, look for this enhanced albedo effect in the southeastern Serenitatis Basin. It is really quite striking and it's dramatic effect - well-beyond the normal contrast of the Plinius dark mantle region normally seen at full Moon - is not properly conveyed by the attached image. - Canopus56 Direct image links: Unlabeled http://gallery.utahastronomy.com/main.php?g2_view=core.DownloadItem&g2_itemI... Labeled http://gallery.utahastronomy.com/main.php?g2_view=core.DownloadItem&g2_itemI... ____________________________________________________________________________________ Never miss a thing. Make Yahoo your home page. http://www.yahoo.com/r/hs