Sand and Stars

First, let’s talk about sinuous rilles, the most common type found on the Moon. Vallis Schröteri, below, is a beautiful example.These channels have the appearance of a meandering river that twists and turns as it flows along. Sinuous rilles arethought to have formed as the result of surface lava flows, or collapsed lava tubes. Many have crater-like structures at their point of origin. These depressions are most likely vents where the magma reached the surface from mantle source regions below. While sinuous rilles are a common lunar landform, there are similar, though typically much shorter, volcanic features on Earth that form when magma steadily emerges from a vent and forms a channelized flow.

Sinuous rilles meander in a curved path like a mature river, and are commonly thought to be the remains of collapsed lava tubes or extinct lava flows. They usually begin at an extinct volcano, then meander and sometimes split as they are followed across the surface. As of 2013, 195 sinuous rilles have been identified on the Moon.[2] Vallis Schröteri in Oceanus Procellarum is the largest sinuous rille, and Rima Hadley is the only one visited by humans, on the Apollo 15 mission. Another prominent example is Rima Herigonius.


Next, we can explore straight rilles, which are thought to have formed due to tectonic forces. These valleys are graben, which form when the surface pulls apart under extensional stresses, and a block of the crust drops down to create the valley floor. The largest rille on the Moon is a straight rille known as Rima Sirsalis. It extends about 400 km across the lunar near side in Oceanus Procellarum, the Ocean of Storms. On a good night (when the moon is waning and the topography results in large shadows being cast), you may be able to find it with a telescope at home!

Rima Cauchy, below, is another beautiful example of a straight rille.


The last type of rille is known as the arcuate rille. Typically curved formations, like a parentheses or a bow (without the arrow), these depressions are formed along the edges of maria within some basins. Arcuate rilles likely formed as the mare basalts sagged under their own weight toward the center of the basin, causing the edges to pull apart; the rille shape follows the circular shape of the basin.Rimae Sulpicius Gallus controlled feature mosaic (20°N Lat, 10°E Lon) with a total rille length of ~80 km, image width 30 km 

Arcuate rilles have a smooth curve and are found on the edges of the dark lunar maria. They are believed to have formed when the lava flows that created a mare cooled, contracted and sank. These are found all over the moon, examples can be seen near the south-western border of Mare Tranquillitatis and on the south-eastern border of Mare Humorum. Rima Sulpicius Gallus is a clear example in southwestern Mare Serenitatis.


Straight rilles follow long, linear paths and are believed to be grabens, sections of the crust that have sunk between two parallel faults. These can be readily identified when they pass through craters or mountain ranges. Vallis Alpes is by far the largest graben rille, indeed it is regarded as too large to be called a rille and is itself bisected by a linear rille; Rima Ariadaeus, west of Mare Tranquillitatis, is a clearer example.
Rilles which show more than one structure are termed hybrid rilles. Rima Hyginus in Sinus Medii is an example, initially formed through a fault and subsequently subject to volcanic activity.