Achelous (/ˌækɪˈloʊ.əs/) is a relatively young impact crater on Jupiter's largest moon Ganymede. Located nearby the similarly-sized Gula, both craters are surrounded by a distinct raised pedestal formed from ejecta blasted out by their formative impact events.[2] Achelous is named after the Greek river god Achelous, with the name being adopted by the International Astronomical Union (IAU) in 1979.[1]

Geology and characteristics

Achelous is roughly 35 kilometres (22 mi) in diameter and polygonal in shape. Rather than being perfectly round, its crater rim consists of seven straight segments. It is located within Ganymede's bright grooved terrain, with the surrounding grooves aligned in a northeast–southwest direction. The grooves appear to have influenced the shape of Achelous's rim, so that two of its seven rim segments are aligned in the direction of the grooves; the other five rim segments are oriented roughly 45° from the grooves.[3] In contrast to Gula, Achelous lacks a central peak, though its center may be occupied by a collapsed central peak or a central pit.[2]

Achelous's surrounding ejecta appears to have a distinct inner and outer layer. The inner raised pedestal extends approximately 17 kilometres (11 mi) from its crater rim, hosting a hummocky texture near the rim.[4][3] The pedestal appears to have completely covered most preexisting topographical features, as only the most prominent grooves from the surrounding terrain extend into the pedestal. The outer thinner layer of ejecta does not obscure the underlying grooves as heavily and is overall smoother than the pedestal. Beyond these two ejecta layers is a system of radially-extending secondary craters and crater chains.[5] Most of these secondary craters are 1–2 km (0.62–1.24 mi) in diameter, with the largest observed secondary crater being 2.7 km (1.7 mi) in diameter.[6][5]

A cross-sectional diagram of how single-layer ejecta and multiple-layered ejecta craters may form on Mars.

The double-layered structure of Achelous's ejecta is similar to Martian double-layered ejecta craters. On Mars, these craters are hypothesized to form either by volatile-rich subsurface material vaporizing on impact or from Mars's atmosphere suspending ejected material, fluidizing the impact ejecta. As Ganymede currently lacks a dense atmosphere, this suggests Achelous's ejecta layers were formed due to the presence of subsurface volatile material at the impact site.[5]

References

  1. ^ a b "Achelous". Gazetteer of Planetary Nomenclature. USGS Astrogeology Research Program. (Center Latitude: 61.90°, Center Longitude: 11.78°; Planetographic, +West)
  2. ^ a b "Pedestal Craters Gula and Achelous on Ganymede". NASA Jet Propulsion Laboratory (JPL). Retrieved 5 September 2024.
  3. ^ a b Baby, Namitha Rose; Kenkmann, Thomas; Stephan, Katrin; Wagner, Roland (30 January 2024). "Polygonal impact craters on Ganymede". Meteoritics and Planetary Science. 59 (3): 544–559. doi:10.1111/maps.14138.
  4. ^ Schenk, Paul M.; Ridolfi, Francis J. (28 June 2002). "Morphology and scaling of ejecta deposits on icy satellites". Geophysical Research Letters. 29 (12): 31-1 – 31-4. Bibcode:2002GeoRL..29.1590S. doi:10.1029/2001GL013512.
  5. ^ a b c Jones, Kevin B.; Head, James W.; Pappalardo, Robert T.; Moore, Jeffrey M. (July 2003). "Morphology and origin of palimpsests on Ganymede based on Galileo observations". Icarus. 164 (1): 197–212. Bibcode:2003Icar..164..197J. doi:10.1016/S0019-1035(03)00128-3.
  6. ^ Singer, Kelsi N.; McKinnon, William B.; Nowicki, L. T. (September 2013). "Secondary craters from large impacts on Europa and Ganymede: Ejecta size–velocity distributions on icy worlds, and the scaling of ejected blocks". Icarus. 226 (1): 865–884. Bibcode:2013Icar..226..865S. doi:10.1016/j.icarus.2013.06.034.


Allikas: https://en.wikipedia.org/wiki/Achelous_(crater)
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