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Compton–Belkovich Thorium Anomaly

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Template:Infobox moon hot spot data The Compton–Belkovich Thorium Anomaly is a hot-spot (volcanic complex) on the Moon. It is on the far side of the Moon and was found by a gamma-ray spectrometer in 1998.[1] It is an area of concentrated thorium, a radioactive element.[2] Lunar rock samples from the Apollo missions reveal that most lunar volcanism occurred around 3 to 4 billion years ago, but could have been as recent as 1 billion years ago due to the unknown history of the moon's far side.[3]

Description

The Compton–Belkovich Thorium Anomaly was found in 1999 by the Gamma Ray Spectrometer (GRS) instrument on board the Lunar Prospector (LP) and subsequently identified as a hotspot, located around 61°06′N 99°30′E / 61.1°N 99.5°E / 61.1; 99.5.[2] The estimated thorium concentration reaches 5.3 µg/g (5.3 micrograms per gram) while the surrounding highland basalts only contain between 0 and 2 µg/g. Compared to the Earth's thorium concentration of 0.06 µg/g, the Compton–Belkovich's is very high.[4] It has unusually high reflectance, identified by a visible imaging study that was carried out later by the Clementine spacecraft in a Clementine Visible Images study. [5] High resolution images from the Lunar Reconnaissance Orbiter LRO made it possible to analyze the surface features of the Compton–Belkovich Thorium Anomaly in 2011.[6]

The Compton–Belkovich Thorium Anomaly's location on the Moon.

Location

The anomaly is between the Bel'kovich crater, which is 214 kilometres (133 mi) wide and the Compton crater, which is 162 kilometres (101 mi) wide.[4][6] The region as a whole is 32 kilometres (20 mi) wide and 18 kilometres (11 mi) long.[7]

The center of the region is a volcanic complex, 25 kilometres (16 mi) to 35 kilometres (22 mi) across, between the Bel'kovich and the Compton craters. It is 900 kilometres (560 mi) from the extent of the northeastern Procellarum KREEP Terrane (an area which has high abundances of KREEP which is a geochemical component of some lunar rocks).[1][8]

Volcanic slope

Volcanic features provide information about the composition of the lava that formed the Compton–Belkovich Thorium Anomaly. On average, many volcanoes on the Moon have slopes of lower than 7 degrees. However, the Compton–Belkovich Thorium Anomaly has a slope which reaches at the highest 25 degrees. This suggests that the region was formed by more viscous lava.[3]

Formation

A direct analysis of Apollo program samples revealed that most lunar volcanism occurred around 3 to 4 billion years ago. However, volcanic activity on the unsampled lunar back side could have occurred as recently as 1 billion years ago.[5] The smoothness of the surface associated with the anomaly indicates that it could possibly have been formed in a more recent event.[9]

As the lava cooled it would have crystallized to produce a silicate structure and incompatible elements such as thorium would have been excluded from the process and formed thorium-rich pockets in the remaining liquid rock.[5][10] The eruption associated with the thorium anomaly could have created the elevated features to the west and the low and broad area to the east.[11] The latest possible eruptions of lava would have made domes with steeper slopes, and also would have caused small bulges, as they would barely reach the surface.[1][7][5][12]

References

Citations
  1. ^ a b c "LPI (usra.edu) - Meetings" (PDF). Lunar and Planetary Institute (LPI). Retrieved May 7, 2012.
  2. ^ a b Lawrence, D. J.; Elphic, R. C.; Feldman, W. C.; Gasnault, O.; Genetay, I.; Maurice, S.; Prettyman, T. H. "Small-Area Thorium Enhancements on the Lunar Surface". Harvard University. Retrieved March 2002. {{cite journal}}: Check date values in: |accessdate= (help); Cite journal requires |journal= (help)
  3. ^ a b Jolliff, B. L.; Wiseman, S. A.; Lawrence, S. J.; Tran, T. N.; LROC Science Team (2010). "Scientific Return from Systematic Imaging of the Constellation Exploration Sites: Compton–Belkovich Example". 41st Lunar and Planetary Science Conference. 41: 2412. Bibcode:2010LPI....41.2412J.
  4. ^ a b Lawrence, D. J. (2003). "Small-area thorium features on the lunar surface". Journal of Geophysical Research. 108. doi:10.1029/2003JE002050.
  5. ^ a b c d Bradley L. Jolliff, Sandra A. Wiseman, Samuel J. Lawrence, Thanh N. Tran, Mark S. Robinson, Hiroyuki Sato, B. Ray Hawke, Frank Scholten, Jürgen Oberst, Harald Hiesinger, Carolyn H. van der Bogert, Benjamin T. Greenhagen, Timothy D. Glotch and David A. Paige (24 July 2010). "Non-mare silicic volcanism on the lunar farside at Compton–Belkovich - Nature". Nature Geoscience. doi:10.1038/ngeo1212. {{cite journal}}: |access-date= requires |url= (help)CS1 maint: multiple names: authors list (link)
  6. ^ a b "Rare Volcanoes Discovered On Far Side of the Moon". Space.com. Retrieved May 10, 2012.
  7. ^ a b "Unique volcanic complex discovered on Moon's far side - WUSTL (edu)". WUSTL (Education - Edu) - News. July 24, 2011. Retrieved March 7, 2012.
  8. ^ Lawrence, D. J.; Elphic, R. C.; Feldman, W. C.; Gasnault, O.; Genetay, I.; Maurice, S.; Prettyman, T. H. (2002). "Small-Area Thorium Enhancements on the Lunar Surface". 33rd Annual Lunar and Planetary Science Conference. 33: 1970. Bibcode:2002LPI....33.1970L.
  9. ^ Lawrence, D. J.; Feldman, W. C.; Barraclough, B. L.; Binder, A. B.; Elphic, R. C.; Maurice, S.; Miller, M. C.; Prettyman, T. H. (2000). "Thorium abundances on the lunar surface". Journal of Geophysical Research. 105: 20307. doi:10.1029/1999JE001177.
  10. ^ Warren, Paul H. (2001). "Compositional structure within the lunar crust as constrained by Lunar Prospector thorium data". Geophysical Research Letters. 28 (13): 2565. doi:10.1029/2000GL012739.
  11. ^ Jolliff, Bradley L.; Wiseman, Sandra A.; Lawrence, Samuel J.; Tran, Thanh N.; Robinson, Mark S.; Sato, Hiroyuki; Hawke, B. Ray; Scholten, Frank; Oberst, Jürgen (2011). "Non-mare silicic volcanism on the lunar farside at Compton–Belkovich". Nature Geoscience. 4 (8): 566. doi:10.1038/ngeo1212.
  12. ^ Jolliff, B. L.; Wiseman, S. A.; Lawrence, S. J.; Tran, T. N. "Scientific Return from Systematic Imaging of the Constellation Exploration Sites: Compton–Belkovich Example". Harvard University. Retrieved May 13, 2012. {{cite journal}}: Cite journal requires |journal= (help)
Further Reading
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