Rimae Sirsalis is the longest rille on the Moon, at over 400km in length. Unlike many lunar rilles, Rimae Sirsalis passes almost exclusively over highland terrain. The feature is best viewed approximately 1-2 days before the Full Moon (although the exact time depends upon libration). My overview image of the area (below) is from September of this year, while the close-up annotated view was taken in December of last year (next post due to file size limitations). The Full Moon is increasing declination right now, so if you live in the Northern Hemisphere and happen to get lucky with winter weather, the next few lunar cycles are a good time to view this feature.
This upcoming week will be the next available time to view Rimae Sirsalis, and the feature will be nearly completely visible starting on November 21, 2018 at 00:00UT (although the very western-most portion will still be in shadow at that time, as will the crater Darwin). On November 21, 2018 at 12:00UT, the longitude of the terminator will be -73W, which is very similar to that in my images. And it just so happens that there will also be a libration to the West of over 5 degrees at that time, which will be helpful. The easiest way to find Rimae Sirsalis is to locate the large lava-filled Grimaldi basin, and then travel to the South and East. Another landmark to help orient yourself is the dark lava-filled crater Cruger.
Note how the main rille of Rimae Sirsalis passes over several older craters, while more recent craters have impacted upon the rille. Especially noteworthy is where the rille traverses the crater De Vico A, and descends the southwestern wall of the crater, cuts across the floor, and then ascends the northeastern wall. In contrast, crater De Vico AA is much younger and impacts upon the rille. Craters Sirsalis F and J also impact upon the rille, and nearly wipe out any trace of it for about 30km. In several locations, older rilles can be observed to intersect the main rille, including the Rimae Darwin system near the southwestern terminus.
Background Information and Current Research:
The precise origins of Rimae Sirsalis are unknown, although it is currently thought to have formed via dike propagation, in which vertical sheets of magma (dikes), exert their influence on the surface. It is notable that the rille is not associated with mare lava flows in the way that many sinuous rilles are, as it travels over the highlands with no obvious lava flows surrounding it. Many straight rilles are associated with stress fractures, either from differential cooling, or from large impacts. Mare Orientale is only about 1000km away from Rimae Sirsalis, although the rille is not oriented either radially or tangentially to the Mare, so it is unlikely to be associated with the Orientale impact. Additionally, the age doesn’t seem to be consistent with that hypothesis, as Rimae Sirsalis is likely older than the Oriental impact. Interestingly, the rille is radial to the Imbrium basin, and so could potentially be affiliated with that impact, although whether this is the case remains uncertain.
Another intriguing aspect of Rimae Sirsalis is a strong magnetic anomaly associated with the feature. The Moon’s surface is largely non-magnetic, and today the Moon has no magnetic field. However, subsatellites released by Apollo 15 and 16 measured localized magnetic fields over a variety of lunar features (including Rimae Sirsalis) that exceeded the average levels by several orders of magnitude. The origins of these magnetic anomalies have remained an area of active research, but current research suggests that the Moon at one time had a magnetic field. This ancient magnetic field was about as strong as the Earth’s magnetic field today, but disappeared several billon years ago. When molten rock containing iron cools in the presence of a magnetic field, it can become magnetized. This means that the magnetic material associated with (or beneath) Rimae Sirsalis cooled in the presence of the ancient lunar magnetic field, but does not clarify whether the origin of the material was from an impactor (such as Imbrium) or subterranean dikes. Further lunar exploration will be required to resolve these questions.
References for further reading on lunar magnetic anomalies and ancient lunar magnetic fields are below (unfortunately some of these are abstracts only without a subscription, but the abstracts are still interesting):