The Moon is, and always has been a favourite observing target of mine. So when last weekend (30.05.09) brought some beautiful clear skies with it; I took the opportunity to explore some new lunar locations as well as revisit some old ones. There was so much to see that I couldn’t possibly fit it all into this blog post. However two features that really stood out for me were: Rima Ariadaeus and Rima Hyginus; both impressive rilles that lie near the west coast of the Mare Tranquillitatis.
The word rille is used to describe the various depressions, or trenches that can be seen scarring the surface of the Moon. The term was used by early observers to characterize such lunar objects and was probably introduced by the German astronomer Johann Schröter in about 1800. More recently the latin word rima (meaning fissure) is often used interchangeably with the word rille to describe the same kind of feature.
Rilles can be divided into three categories: linear, arcuate and sinuous. Those of the first type are thought to be grabens: extended slabs of crust that have collapsed between parallel faults. They are comparatively straight, flat-floored and are sometimes associated with crater chains. In comparison arcuate rilles have smooth curves and are often observed near the edges of the lunar maria. They are believed to have formed when the lava that established the maria cooled, contracted and sank. The sinuous variety seem to be similar to the channels that lava flows on Earth produce, but these lunar valleys are far more rambling; perhaps because the ancient lavas that spread across the Moon were more fluid than those found on Earth.
The shadowy laceration of Rima Ariadaeus, a 300 Km long graben, offered a wonderful view through the five inch refractor. At around 3.85 to 3.2 billion years old this lunar fault formed some time between the late, or early Imbrian epochs. A violent period characterized by colossal impacts and vast volcanic eruptions that formed the lunar maria that we see today.
Conversely Rima Hyginus proved just a little harder to find, but only because I had chosen to look for it when low light conditions were not at there optimum. This hybrid rille, also an Imbrian formation, evolved through a fault that was subsequently altered by volcanic activity. Closer inspection of this lunar feature reveals that it ramifies by adjoining Rima Ariadaeus via a shallow groove towards the north east.
Both of these rilles made for wonderful viewing and testify to the Moon’s tumultous past. So if you haven’t done so already I would thoroughly recommend that you take in an observation of these two fascinating lunar objects. A minimum aperture of around 90mm is almost certainly needed, and I would advise searching for them both around six days after a new Moon, or five days after a full Moon. Just be careful that you don’t get stuck in a groove!