Caloris Group

The Caloris group is a set of geologic units on Mercury. McCauley and others[1] have proposed the name “Caloris Group” to include the mappable units created by the impact that formed the Caloris Basin and have formally named four formations within the group, which were first recognized and named informally by Trask and Guest[2] based on imagery from the Mariner 10 spacecraft that flew by Mercury in 1974 and 1975. The extent of the formations within the group have been expanded and refined based on imagery and other data from the MESSENGER spacecraft which orbited Mercury from 2011 to 2015, and imaged parts of the planet that were in shadow at the time of the Mariner 10 encounters.[3]

Like the Imbrium and Orientale Basins on the Moon, Caloris Basin is surrounded by an extensive and well-preserved ejecta blanket[4][2][5] As on the Moon, where ejecta from the better preserved basins was used to construct a stratigraphy, the ejecta from the Caloris Basin also can be used as a marker horizon. This ejecta is recognizable to a distance of about one basin diameter in all directions. A stratigraphic and structural comparison between the Orientale and Caloris Basins has been made by McCauley.[6] McCauley and others[1] proposed a formal rock stratigraphy for the Caloris Basin. This stratigraphy is patterned after that used in and around the Orientale Basin on the Moon.[7] Crater degradation chronologies, such as the one modified from Trask,[1] and correlations between plains units on the basis of crater frequency may aid in tying much of the remainder of the surface of Mercury to the Caloris event.

Unlike the Imbrium-related stratigraphy of Shoemaker and Hackman,[8] the stratigraphy devised for Mercury is a rock rather than a time stratigraphy. It recognizes the existence of an orderly, in essence isochronous sequence of mappable units around Caloris that are similar in character to those recognized around the better preserved impact basins of the Moon such as Orientale, Imbrium, and Nectaris.

The four formations are described in order of occurrence from the rim of Caloris Basin outward:

  1. ^ a b c McCauley, J. F., Guest, J. E., Schaber, G. G., Trask, N. J., and Greeley, Ronald, 1980, Stratigraphy of the Caloris Basin, Mercury: Icarus, 1980
  2. ^ a b Trask, N. J.; Guest, J. E. (1975). "Preliminary geologic terrain map of Mercury". Journal of Geophysical Research. 80 (17): 2461–2477. doi:10.1029/jb080i017p02461.
  3. ^ Denevi, B. W., Earnst, C. M., Prockter, L. M., and Robinson, M. S., 2018. The Geologic History of Mercury. In Mercury: The View After MESSENGER edited by Sean C. Solomon, Larry R. Nittler, and Brian J. Anderson. Cambridge Planetary Science. Section 6.3.3.
  4. ^ Strom, R. G.; Trask, N. J.; Guest, J. E. (1975). "Tectonism and volcanism on Mercury". Journal of Geophysical Research. 80 (17): 2478–2507. doi:10.1029/jb080i017p02478.
  5. ^ Guest, J. E., and O’Donnell, W. P., 1977, Surface history of Mercury: A review: Vistas in Astronomy, v. 20, p. 273–300.
  6. ^ McCauley, J. F., 1977, Orientale and Caloris: Physics of the Earth and Planetary Interiors, v. 15, no. 2–3, p. 220–250.
  7. ^ Scott, D. H., McCauley, J. F., and West, M. N., 1977, Geologic map of the west side of the Moon: U.S. Geological Survey Miscellaneous Investigations Series Map I-1034, scale 1:5,000,000.
  8. ^ Shoemaker, E. M., and Hackman, R. J., 1962, Stratigraphic basis for a lunar time scale, in Kopal, Zdenek, and Mikhailov, Z. K., eds., The Moon: International Astronomical Union Symposium, 14th, Leningrad, U.S.S.R., 1960: London, Academic Press, p. 289–300.