The availability of high-resolution images provided by the Narrow Angle Camera, on board of the Lunar Reconnaissance Orbiter, has enabled the study of recent and active surface processes on the Moon by revealing geological and geomorphological features with unprecedented details. Of the many locations on the Moon where recent geological structures have been identified, Taurus-Littrow Valley (Image 1) is one of the most interesting. Indeed, it was also selected as the Apollo 17 landing site because of its complex geology.
Image 1: Oblique view of the South Massif in Taurus-Littrow valley, on the Moon.
Post contributed by Alistair Blance, The Open University, UK
During an impact on Mercury’s surface, material is ejected from the forming impact crater. As Mercury has only a tenuous atmosphere, ejected material travels predominantly ballistically, creating an ejecta deposit around the crater that thins gradually with increasing distance. However, large deposits emplaced by ground-hugging flows can be found around some impact craters on Mercury (Image 1). Evidence for flow includes material being diverted around obstacles, a steep edge or distal ridge at deposit margins, and a lobate shape to several examples. Some flow deposits extend outwards around a whole crater, but often they are confined within topographic lows adjacent to the crater. To help assess the origin of these features, it is useful to compare them to similar features across the Solar System. This comparison may also indicate how differences between the planets can influence the development of flows around craters.
Image 1: Flow deposits around craters on Mercury. Deposit boundaries indicated with red triangles. (A) Flow deposit extending from the central crater into an underlying crater in the top right of the image. Steep margins with a lobate shape suggest emplacement by flow. Image taken from MESSENGER MDIS BDR Global Basemap. (B) A crater with two sections of flow deposit extending into the underlying crater in the bottom right of the image. Image taken from MESSENGER MDIS frame EW0260906588G. (C) Sketch map of the image in B. Shows the two sections of flow deposit in red, with hypothesised direction of emplacement shown with red arrows. The deposit appears to have been diverted around a central peak within the underlying crater, providing evidence for emplacement via ground-hugging flow.
Images of the lunar surface reveal layered deposits presumed to be sequences of basaltic lava flows. These sequences have been imaged since the Apollo astronauts acquired both orbital and surface photographs in the 1960s and 1970s. Apollo 15 astronauts visited Hadley Rille, a 130 km long, 200 m deep sinuous feature that was formed by flowing lava, similar to lava channels or tubes on Earth. Photographs taken by the astronauts (such as Image 1) show that the rille cut into the underlying substrate, revealing sequences of layered material. The layers are believed to be basaltic lava flows, based on outcrop morphologies and nearby samples. The thicknesses of ancient lava flows provide insight into the emplacement, dynamics, and history of volcanism on the Moon.
Image 1: Apollo 15 surface image of the interior wall of Hadley Rille (https://www.hq.nasa.gov/alsj/a15/AS15-89-12106HR.jpg). Inset highlights layered deposits presumed to be basaltic lava flows with possible intercalated regolith deposits. Outcrop is approximately 8 meters thick.
Post by Dr. O. Ruesch, European Space and Technology Center, European Space Agency (ESA), the Netherlands.
A fragmented block is referred to a cluster of fragments formed by the disruption of a parent block. The identification of such features on planetary surfaces is possible due to the minor spatial dispersion of the fragments away from the parent block. This morphology is to be distinguished from clusters of fragments formed by mass wasting like rockfall or disintegration during block rolldown. Observations of fragmented blocks have been reported on almost every rocky planetary body where images captured by orbital and surface craft resolved features in sufficient spatial detail. Despite the fact that disrupted blocks can reveal important clues on the formation process of soil (regolith) on planetary surfaces, they have started to receive attention only in recent years.
On the airless surface of the Moon, fragmented blocks display a wide range of morphologies (Images 1 and 2). In general, the configurations of the fragments can be described by a continuum from highly catastrophic to sub-catastrophic. Image 1 shows an example of a catastrophic fragmentation where the number and size of the fragments indicate that the parent block was much larger than the largest fragment. The radial pattern formed by small fragments and brighter areas is diagnostic of disruption by a meteoroid impact.
Image 1. Example of a block fragmented catastrophically near crater Copernicus on the Moon, where the largest fragmented in considerable smaller than the original parent block. LROC/NAC image M127063668LE. http://bit.ly/2mAl0CB
Post by Valentin Bickel, PhD student, Department of Earth Sciences, ETH Zurich, CH & Department Planets and Comets, Max Planck Institute for Solar System Research, GER.
One of the most intriguing objects on the surface of the Moon are the “rolling stones”, also known as lunar rockfalls or rolling boulders (Image 1). These boulders are abundant all over the Moon and have sizes that range from a couple of meters to several 10s of meters. Lunar boulders are believed to be displaced by moonquakes or impacts and can carve tracks with lengths that range from a couple of meters to several kilometers (Image 1; Xiao et al., 2013; Kumar et al., 2016). Besides their value for geomorphological analyses, these boulder tracks provide insights into the mechanical behavior and the trafficability of the lunar “soil”, the regolith (Bickel et al., 2019).
Image 1: A number of large and small boulders with tracks at the bottom of a lunar slope. The analysis of tracks provides insights about the mechanical properties of the regolith and is performed using high-resolution satellite imagery, taken by NASA’s Lunar Reconnaissance Orbiter Narrow Angle Camera (NAC). Detail of NAC Image M113934119LC.
Post contributed by Dr. Mike Sori, Lunar and Planetary Laboratory, University of Arizona
Uranus and its moons have only ever been visited by one spacecraft, Voyager 2, which flew by the system in 1986. One of its large moons, Umbriel, was found to have a mysterious bright ring 80-km-wide inside a 131-km-diameter crater named Wunda. Image 1 shows Umbriel and this annulus-shaped feature.
Image 1: Voyager 2 image 1334U2-001 showing the Uranian moon Umbriel; note the bright ring inside the crater Wunda at top of the image (which is at Umbriel’s equator).
Posted by Dr P. Senthil Kumar, National Geophysical Research Institute, Council of Scientific & Industrial Research, Hyderabad 500007, India.
Gullies are well-known geomorphic features on Earth where they are mainly formed by erosion due to flow of liquid water. They are also detected on Mars and the Moon and their origin on those bodies are under discussion (Malin and Edgett, 2000; Senthil Kumar et al., 2010). The gullies consist of alcoves (erosional features), channels (features indicating transportation) and fans or debris aprons (depositional structures). These features are clearly observed on the interior walls of impact craters on Mars and widely on the mountain slopes of Earth. Hence, geomorphologists use these features to examine the characteristics of liquid water flow either in the present or past geological records.
Image 1: (a) The Chandrayaan-1 terrain mapping camera image showing the ~7.2-km-diameter fresh crater (centred at 72º12’S, 133º12’E) emplaced in the peak-ring material of Schrödinger basin. The topographic profiles along A-A’ and B-B’ are shown in Figure 1d. A 6860-m-diameter circle fits perfectly to the crater rim from the western to the northern sides of the crater, while the crater rim recedes in other parts due to enhanced crater wall erosion. (b) The shadow-enhanced TMC image reveals the presence of arcuate ridge and the pond material on the crater floor. Note the pond is oriented toward the prominent landslide surface. (c) The TMC image showing the presence of concentric faults along the northwestern crater rim. (d) The topographic profiles along A-A’ and B-B’. The interior wall that contains the landslides (B-B’) is gentler and shallower than the interior wall with the gullies (A-A’). The ridge material is characterized by a higher topographic relief than the surrounding crater floor. The pond material has a flat surface that embays the ridge and other floor materials. See Senthil Kumar et al. (2013) for more details.
Posted by Rebecca Thomas, Department of Physical Sciences, The Open University, UK.
Recent channelized flows from vents in the Cerberus plains of Mars demonstrate the difficulties of uniquely ascribing process to landforms on other planets. The image below shows two fissures emanating from a wrinkle ridge. Both fissures appear to be sources of approximately contemporaneous channels running down onto the surrounding plains (Thomas, 2013). The channel in the west is constructive and differs from that in the east which is clearly shows several phases of incision (Image 1).
Image 1: a. Vents and channels in the Cerberus plains, Mars (156.9° E, 7.1° N); b. incised channel; c. constructed, leveed channel. (HiRISE ESP_016361_1870)
Post by Scott Mest, Planetary Science Institute, Tucson, AZ 85719, USA.
Lunar sinuous rilles (German for ‘groove’) consist of long, narrow depressions in the lunar surface that meander in a curved path across the surface and morphologically resemble terrestrial fluvial channels (Image 1). Sinuous rilles are generally up to several kilometers wide and hundreds of kilometers in length. On the Moon, sinuous rilles are found within volcanic terrains such as the extensive lunar mare. Their morphology and association with volcanic deposits suggests that they are the remains of lava channels or collapsed lava tubes.
Image 1: Part of LROC image M115429448L (resolution is 0.970 m/pixel) showing a close-up of a sinuous rille (arrows) that cuts through dark plains (p) and adjacent hilly (h) materials on the floor of Schrödinger.
Planetary Geomorphology Image of the Month 