LROC: Farside Highlands volcanism

Small dome in the Compton-Belkovich "Thorium Anomaly" region (61.33 °N, 99.68 °E). Evidence indicates a volcanic origin for this and other intriguing features in this relatively small area in the highlands of the lunar farside's northern hemisphere. Solar illumination Incidence Angle is 64°, Sun is from the SSW, field of view roughly 510 meters. LROC Narrow Angle Camera (NAC) observation M139238146L, LRO orbit 5653, September 16, 2010. See the full-size LROC Featured Image HERE [NASA/GSFC/Arizona State University].

Same dome, early evening shadow, brings out small topographic features in relief. Note in the close-up view in the image following below the summit boulder shadows. Full frame field of view above is roughly 3100 meters across; LROC NAC observation M119198897L, LRO orbit 2700, January 27, 2010 [NASA/GSFC/Arizona State University].

Mark Robinson
Principle Investigator
Lunar Reconnaissance Orbiter Camera
Arizona State University

Since the domes in the Compton-Belkovich area are rounded and smooth (excepting the boulders scattered across the summit) they are not easy to spot. At sunrise and sunset (above) even smooth topography casts long shadows. In this case the Sun is 13° degrees above the horizon, showing that the slopes of the dome are steeper than 13°, an important clue to unraveling its origin. Steeper slopes often mean more viscous magma, which in turn points toward more silica rich compositions.

Full-resolution close-up of Compton-Belkovich "Dome-1" from M119198897L and the early evening shadow brings the meter-sized boulders into view [NASA/GSFC/Arizona State University].

Are you having trouble seeing the feature as a dome, that is, with positive relief? Consider the above subsampled images above from M119198897L and the wider view that includes a small crater just east of the dome. The sun is from the left. Sometimes your brain can be fooled - you see what is up as what is down. If you know the Sun direction you can "force" the ups to become downs, and vice versa. And what about the little crater at the top of the dome? Is it a volcanic vent or an impact crater? If you said “impact crater” – you are right! The top of the dome is actually about 100 m south of the small crater. The crater just happens to be near the top of the dome.

LROC Wide Angle Camera (WAC) observation M119212328M, LRO orbit 2702, January 27, 2010, and a rare example of "non-mare" silicic volcanism situated between Compton and Belkovich craters on the lunar farside, the "Compton-Belkovich High-Reflectance Feature." For scale, the small dome (and it's companion crater to the east) are indicated by the yellow arrow [NASA/GSFC/Arizona State University].

Composite image showing geochemistry from Lunar Prospector and the spike in thorium coincident with the CBHRF. (Note the nearside-farside differences in thorium seen in the inset) View the full-size context composite illustration HERE [Jolliff et al, 2011].

The Compton-Belkovich site was of special interest even before the LRO mission began. Back in 1998 the Lunar Prospector spacecraft, with its gamma-ray spectrometer, measured the global distribution of the element thorium, which has a strong peak in the gamma-ray spectrum because it is naturally radioactive. Although much of the Moon’s thorium, at least as expressed on the surface, lies mostly on the nearside, a terrain between the craters Compton and Belkovich lit up like a bull’s eye (above). For this reason, the site was selected as one of NASA’s Project Constellation sites because it is of high interest for future human or robotic exploration.

Figure 3 from "Non-Mare, Silicic Volcanism on the Moon's Far Side," Jolliff, et al., Lunar & Planetary Science Conference 2011, #2224.

This thorium “hot spot” was described by David Lawrence and the Lunar Prospector gamma-ray spectrometer team (see figure), later Jeff Gillis and coworkers noted in looking at Clementine images that a high albedo feature was located near the center of the thorium bull’s eye (see WAC context image above). However, since then, the origin of the hot spot and the nature of the deposits was not known until LRO imaged the site with the LROC Narrow Angle Cameras. Those images revealed numerous volcanic features, some large, and some small, like the little dome seen in today’s featured image. Between the Lunar Prospector and Diviner geochemistry and the LROC images, we are able to determine that these domes are examples of silicic (rich in silica relative to basalt) volcanism. An amazing discovery - the only silicic volcanism on the farside.

Be sure to return and read future posts about other features in this intriguing and unusual volcanic terrain on the Moon. What a fabulous place this would be for astronaut geologists to explore, do field geology, and collect samples for analysis!

In the meantime explore the full resolution NAC and see what volcanic features you can find!