Color of the Lassell Massif

Color composite WAC image of the Lassell Massif.
A shock of color on the Moon! The Lassell Massif (a so-called "red spot") in Mare Nubium appears orange in the center of this WAC color composite image. The image was derived by assigning red to 689 nm, green to 415 nm, and blue to 321 nm images. The image was enlarged by a factor of two for viewing here (the original data was sampled to 400 m per pixel); north is up. Image is 190 km across. [NASA/GSFC/Arizona State University].

The Lassell Massif is a mountainous complex of ancient lava flows (around 4 billion years old), volcanic craters, and possibly a blanketing of pyroclastic material (ash). Lassell Massif, along with several similar landforms, were noted in the early 1970's for their anomalous reddish color compared to the rather uniform color of the highlands. These anomalously reddish landforms are still prominent today in WAC images.

Oblique NAC view of the Lassell Massif (looking east-to-west), north is to the right. Prominent features of the massif are labeled. You can read more about the morphology of the massif in this previous article. This mountainous complex is roughly 25 km (east-to-west) and 46 km (north-to-south) [NASA/GSFC/Arizona State University].

In a color composite WAC image (red=689 nm, green=415 nm, and blue=321 nm), the Lassell Massif has a distinctive orange color (see above);  the southern half of the massif is more UV-absorbing than the northern half, making it redder (read more about the colors of the Moon in previous posts). The massif is surrounded by mare flows, but is itself too rugged and too poor in mafic minerals to be made up of mare basalt.

Clementine color maps of FeO and TiO2 abundance
FeO and TiO2 abundance maps derived from Clementine multispectral data (from Ashley et al. 2016).

A recent paper by Ashely et al. in the journal Icarus correlated the orange WAC signature to the morphology of the massif. The authors also noted a distinct silica (SiO2) signature associated with the massif using thermal infrared spectroscopy (LRO Diviner instrument) around the 8-micron part of the electromagnetic spectrum. So, the massif is UV-absorbing, but low in mafic minerals, and rich in silica – this sounds a lot like rhyolite, a silicic volcanic rock made up of feldspars and quartz.

While most of Lassell Massif is highly reflective (and poor in mafic minerals), some small patches exposed in superposed crater walls are low in reflectance. These small deposits may be glassy materials eroding from the steep wall of the caldera and falling across the silicic lava flows that compose most of the volcano. Alternatively, they may be thin deposits of basalt.

Low-reflectance materials exposed in the Lassell Massif (read more in this previous article). NAC M1116585481R, image is ~1.8 km wide [NASA/GSFC/Arizona State University].

An oblique view from the other side of Lassell Massif (looking west-to-east) may help clarify the geologic interpretations. Lassell C is located about 5 km west of the massif. Either or both Lassell G and a 2-km impact crater (just left of Lassell G in the image below) could be the sources of deposits in the interior of Lassell C.

From where did the hummocky materials in the floor of Lassell C (9 km diameter) originate? Perhaps as a landslide caused by the 2 km diameter crater that formed on the steep slope just east of Lassell C (view is from west-to-east), or as effusive or pyroclastic flows from Lassell G? Detail from LROC NAC M1111904689LR [NASA/GSFC/Arizona State University].

Below is a NAC analglyph centered on Lassell C crater which lies 5 km west of Lassell Massif. This crater is not a primary volcanic feature, but its interior may be partially covered with materials that slid off the Lassell massif. Or, these deposits may be pyroclastic ash from the Lassell caldera. The full size NAC analglyph is also available here.

Lassell C anaglyph
NAC anaglpyh of Lassell C [NASA/GSFC/Arizona State University].

The Lassell Massif and surrounding regions have a complex volcanic history and returning samples of these landforms would go a long way to improving our understanding the Moon and 'ground-truthing' our ideas about what the massif is composed of and when it formed.

Zoom into the full resolution west-to-east looking oblique (M1111904689LR) and explore the area on your own!

The full paper was recently published in the LRO special issue of Icarus:

Ashley, J. W., M. S. Robinson, J. D. Stopar, T. D. Glotch, B. R. Hawke, C. H. van der Bogert, H. Hiesinger, S. J. Lawrence, B. L. Jolliff, B. T. Greenhagen, T. A. Giguere, D. A. Paige (2016) The Lassell Massif – A silicic lunar volcano, Icarus 273: 248-261. (doi:10.1016/j.icarus.2015.12.036)

More colorful views of the Moon:

Southside, Aristarchus Crater

Chang'e 3 Lander and Rover From Above

Color of the Moon

Swirls Across the Moon

Published by J. Stopar on 12 September 2016