Pit chains on Enceladus

Post contributed by Dr. Emily S. Martin, Research Fellow, Center for Earth and Planetary Studies, National Air & Space Museum, Smithsonian Institution.

Pit chains are linear assemblages of circular to elliptical pits and have been observed across the solar system. Pit chains have been found on Venus, Earth, Mars, Phobos, Eros, Gaspra, Ida, and Vesta. Across the solar system, pit chains may form through a variety of mechanisms including the collapse of lava tubes, karst, venting, extensional fracturing, or dilational faulting. Saturn’s tiny icy moon Enceladus is the first body of the outer solar system on which pit chains have been identified. Enceladus is only 50km in diameter and is best known for its warm south pole and its watery plume emanating from prominent ridges known as tiger stripes. The source of the plume is likely a global liquid water ocean beneath an icy shell.

Image1

Image 1: The morphology of pit chains across the solar system. a. Eros from NEAR. Image no. 135344864. b. Phobos. Image PIA10367. c. Albalonga Catena, Vesta. d. Venus. Right-look Magellan data near 13°S, 112°E. e. Kilauea Volcano, Hawaii centered at 19.3909°N 155.3076°W. Image taken 12/06/2014, acquired from Google Earth on 04/20/2016. f. Ida, modified from image PIA00332. g. Gaspra, modified from Galileo image PIA00332. h. Pit chains in north-eastern Iceland centered near 65.9902°N and 16.5301°W. Image taken on 7/27/2012, acquired from Google Earth 04/20/2016. i. Pit chains on Mars from the Mars Global Surveyor Mars Orbiter Camera, centered near 6.5398°S and 119.9703°W on the flank of Arsia Mons. Image PIA02874.

Enceladus’ pit chains are found primarily within the ancient cratered terrains of the Saturnian and anti-Saturnian hemispheres and based on cross-cutting relationships, pit chains are among the youngest features on Enceladus’ surface. The presence of pit chains in the cratered terrains suggests that these terrains are undergoing tectonic dissection, which further suggests Enceladus’ South Pole may not be the only active region on Enceladus’ surface.

Image2

Image 2: The global distribution of pit chains on Enceladus. Shaded grey regions are the tectonized terrains: tan regions are the souther curvilinear terrains and the south polar terrains. The remaining regions are cratered terrains. Pit chains are concentrated primarily within the cratered terrains, but some pit chains are observed within the trailing hemisphere tectonized terrains near 60°E. Basemap credit: NASA/JPL-Caltech/SSI.

The morphology and distribution of pit chains in parallel fracture sets makes extensional fracturing or dilational faulting the favored mechanisms for the formation of pit chains on Enceladus. These mechanisms require a layer of regolith distributed across Enceladus’ surface where pit chains are observed. Estimates of the depth of the regolith where pit chains are present range from 90-290 m deep. The regolith on Enceladus is likely sourced from both impact-generated regolith (not including the mega-regolith of the upper crust) and fall-back material from Enceladus’ south polar plume. Observations of pit chains across Enceladus signal the presence of a deep layer of regolith across the ancient cratered terrains. Regolith likely insulates Enceladus’ surface, which influences the effective surface temperature and thermal state of Enceladus’ ice shell, perhaps even helping to maintain Enceladus’ interior ocean.

Image3

Image 3: Experimentally formed pit chains (left) (modified after Fig. 4 in Ferrill et al., 2004) formed above a high angle normal fault (D is the amount of dilation in the experimental set-up), compared with pit chains on the cratered plains of Enceladus (right). As dilation proceeds in the Ferrill et al. (2004) analog work, isolated pits (a, b.) form with a regular spacing (c), and finally merge together (d) into a continuous scalloped trough (e). All of these morphological stages are also observed in the cratered plains of Enceladus, for which we infer a similar evolutionary sequence. (Observed images centered at: a. 5.714°S, 156.672°E b. 10.285°S, 163.285°E, c. 8.128°S, 163.119°E, d. 3.15°S, 158.11°E, e. 7.911°S, 158.239°E).

Further Reading:

Martin, E. S. et al., (2017), Pit Chains on Enceladus Signal the Recent Tectonic Dissection of the Ancient Cratered Terrains. Icarus, 294, 209-217.

Ferrill, D. A., et al., (2004), Dilational fault slip and pit chain formation on Mars. GSA Today 14, (10), 4-12.

Ferrill, D. A., et al., (2011), Coseismic, dilational-fault and extension-fracture related pit chain formation in Iceland: analog for pit chains on Mars, Lithosphere 3 (2), 133-142.

Wyrick, D., et al., (2004), Distribution, morphology, and origins of Martian pit crater chains. J. Geophys. Res. 109, E06005.

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