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Mordor Macula

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Mordor Macula
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An enhanced-color image of Mordor Macula on Charon's north pole. The ridge McCaffrey Dorsum curves around the upper center, whilst the crater Dorothy is located at the upper right
Feature typeDark region
LocationNorth pole of Charon
Diameter~375 km[1]
DiscovererNew Horizons
NamingUnofficial
EponymLand of Mordor, from Tolkien mythology

Mordor Macula /ˈmɔːrdɔːr ˈmækjʊlə/ is the informal name for a large dark area about 475 km in diameter near the north pole of Charon, Pluto's largest moon.[2] It is named after the black land called Mordor in J.R.R. Tolkien's The Lord of the Rings.

Geography

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Mordor Macula is located at Charon's north pole. Mordor Macula's dark "core" is roughly 375 kilometers in diameter, though a surrounding lighter halo extends further into the highlands of Oz Terra, beginning to appear north of 45°N and trending darker with increasing latitude.[1][3]

The boundary of Mordor Macula, though ill-defined, is roughly circular; two major disruptions to this circular boundary are observed, however. One, named McCaffrey Dorsum,[4] is a curvilinear ridge 1-2 kilometers high and stretching between ~150° and ~0°W between 72° and 77°N. McCaffrey Dorsum appears to act as a barrier to the spread or distribution of dark material. The pole-facing slopes of the ridge contain some of the darkest terrain observed on Charon, whilst the equator-facing slopes show little deposition of the dark material. The second disruption is Dorothy, the largest-known impact basin on Charon. The floor of Dorothy is significantly lighter, appearing to carve an arc into the dark core of Mordor Macula. An unnamed ~80 km wide impact crater within Mordor Macula also has a bright floor. Broadly, the distribution of Mordor Macula's dark material seems only loosely influenced by local topography.[1]

Geology

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Mordor Macula is located at Charon's north pole.

The terrain Mordor Macula occupies appears to be rather dramatic, with numerous scarps and irregular depressions. In the direction of the 90°E meridian, Mordor Macula is roughly at the same altitude as Oz Terra. In the direction of the 90°W meridian by contrast, Mordor Macula's terrain gently slopes downwards to ~2 km below Charon's mean elevation before encountering the McCaffrey Dorsum. Beyond the ridge is a massive deep canyon, Caleuche Chasma.[1] Due to Charon's current geological inactivity, Mordor Macula has been relatively unchanged since its formation.[5]

The dark material is reddish-brown in color and has been likened to the dark equatorial belt of Pluto. Pluto's dark equatorial terrain is covered in deposits of tholins, a generic term for a tar-like mixture of radiation-processed organic compounds. As such, the dark reddish-brown coloration of Mordor Macula is likely caused by tholins as well. However, spectral observations of Charon's north pole is still dominated by signatures of water ice, so the dark deposits must be thin and relatively sparse. This is supported by the inconsistent correlation between the distribution of dark material and local topography, indicating that the material is not a thick and discrete deposit.[6][7][1]

Origin

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The origin of Mordor Macula is not completely understood. It may be a deposit of frozen gases captured from Pluto's escaping atmosphere. A leading hypothesis is that nitrogen and methane escape from Pluto's atmosphere and are then deposited into the cold poles of Charon, where scattered ultraviolet light then transforms the molecules into tholins, a generic term for a tar-like organic slurry. Despite the fact that during Pluto winter the Northern hemisphere goes without sunlight for more than 100 years, enough radiation makes it to Charon's surface to form the red tholins.[8] The extreme orbital eccentricity and axial tilt of Pluto and Charon's orbits drive a large sublimation event during Plutonian spring, where methane and other gases from Pluto's surface may escape the planet's gravity and condense onto Charon's surface, an effect called "cold trapping".[9] This hypothesis implies that a similar red spot should exist on Charon's south pole as well—and indirect evidence suggests this is true.[10]

It is also possible that the Mordor Macula has a cryovolcanic origin.[5] Observations of other Kuiper belt objects (KBOs) show similar features as Mordor Macula, making the atmospheric transfer model less likely. A second hypothesis of Mordor Macula's origin states that cryovolcanism in Vulcan Planitia pumped enough methane into the paleoatmosphere from Charon's subsurface to create Mordor Macula. Methane is first released into the atmosphere via cryovolcanism, where most of the gas migrates to Charon's north and south poles, freezing and becoming trapped there as ice. Some of the gas will reach Charon's escape velocity and be lost to space. After a long period of time, solar radiation irradiated the trapped methane ice and produced tholins, creating the dark deposits observed today.[11]

See also

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References

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  1. ^ a b c d e Schenk, Paul Michael; Beyer, Ross A.; McKinnon, William B.; Moore, Jeffrey M.; Spencer, John R.; White, Oliver L.; Singer, Kelsi; Umurhan, Orkan M.; Nimmo, Francis; Lauer, Tod R.; Grundy, William M.; Robbins, Stuart; Stern, S. Alan; Weaver, Harold A.; Young, Leslie A.; Smith, K. Ennico; Olkin, Cathy (2018). "Breaking up is hard to do: Global cartography and topography of Pluto's mid-sized icy Moon Charon from New Horizons". Icarus. 315: 124–145. Bibcode:2018Icar..315..124S. doi:10.1016/j.icarus.2018.06.010. ISSN 0019-1035. S2CID 125833113.
  2. ^ Nemiroff, Robert; Bonnell, Jerry (17 July 2015). "APOD: 2015 July 17 – Charon". Astronomy Picture of the Day. Retrieved 17 July 2015.
  3. ^ Grundy, W. M.; Binzel, R. P.; Buratti, B. J.; et al. (March 2016). "Surface compositions across Pluto and Charon". Science. 351 (6279): aad9189. arXiv:1604.05368. Bibcode:2016Sci...351.9189G. doi:10.1126/science.aad9189. PMID 26989260. aad9189 (2016).
  4. ^ "Charon Nomenclature" (PDF). Gazetteer of Planetary Nomenclature. 3 March 2021.
  5. ^ a b Menten, Stephanie; Sori, Michael; Bramson, Ali. A Cryovolcanic Origin for Mordor Macula on Charon. AGU Fall Meeting 2021, held in New Orleans, LA, 13–17 December 2021, id. P45C-2449. Bibcode:2021AGUFM.P45C2449M.
  6. ^ Grundy, W. M.; Cruikshank, D. P.; Gladstone, G. R.; et al. (November 2016). "The formation of Charon's red poles from seasonally cold-trapped volatiles". Nature. 539 (7627): 65–68 (2016). arXiv:1903.03724. Bibcode:2016Natur.539...65G. doi:10.1038/nature19340. PMID 27626378.
  7. ^ Fayolle, M.; Quirico, E.; Schmitt, B.; et al. (October 2021). "Testing tholins as analogues of the dark reddish material covering Pluto's Cthulhu region". Icarus. 367. Bibcode:2021Icar..36714574F. doi:10.1016/j.icarus.2021.114574. 114574.
  8. ^ Grundy, William M. (1 January 2020). Prialnik, Dina; Barucci, M. Antonietta; Young, Leslie A. (eds.). The Trans-Neptunian Solar System. Elsevier. pp. 291–305. Bibcode:2020tnss.book..291G. doi:10.1016/b978-0-12-816490-7.00013-8. ISBN 978-0-12-816490-7. Retrieved 9 May 2024.
  9. ^ Teolis, Ben; Raut, Ujjwal; Kammer, Joshua A.; Gimar, Caleb J.; Howett, Carly J. A.; Gladstone, G. Randall; Retherford, Kurt D. (28 April 2022). "Extreme Exospheric Dynamics at Charon: Implications for the Red Spot". Geophysical Research Letters. 49 (8). Bibcode:2022GeoRL..4997580T. doi:10.1029/2021GL097580. ISSN 0094-8276.
  10. ^ Rothery, David (14 September 2016). "Mordor mystery: scientists solve puzzle of the strange, dark north pole on Pluto's moon Charon". The Conversation. Retrieved 10 April 2020.
  11. ^ Menten, Stephanie M.; Sori, Michael M.; Bramson, Ali M. (9 August 2022). "Endogenically sourced volatiles on Charon and other Kuiper belt objects". Nature Communications. 13 (1): 4457. Bibcode:2022NatCo..13.4457M. doi:10.1038/s41467-022-31846-8. ISSN 2041-1723. PMC 9363412. PMID 35945207.
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