Post contributed by Helle L. Skjetne, PhD candidate, Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, USA.
Chaos terrain is formed by disruption of preexisting surfaces into irregularly shaped blocks with a “chaotic” appearance (Image 1). This typically occurs through fracturing (that can be induced by a variety of mechanisms), and the subsequent evolution of these blocks can follow several paths (Image 2). These distinctive areas of broken terrains are most notably found on Jupiter’s moon Europa, Mars, and Pluto. Although chaos terrains on these bodies share some common characteristics, there are also distinct morphological differences between them (Image 1). The geologic evolution required to shape this enigmatic terrain type has not yet been fully constrained, although several chaos formation models have been proposed. We studied chaotic terrain blocks on Pluto, Europa and Mars to infer information about crustal lithology and surface layer thickness (Skjetne et al. 2020).
Image 1: Examples of chaotic terrain “blocks” (referring to each mountain-like topographic feature). Chaos on Pluto in a) Tenzing Montes and b) Al-Idrisi Montes, respectively (New Horizons image at ~315 m px–1), c) Hydraotes Chaos on Mars (Mars Odyssey THEMIS daytime infrared global mosaic at 100 m px–1), and d) Conamara Chaos on Europa (Galileo 210–220 m px–1 East and West RegMaps).
Image 1 shows regions of chaotic terrain on Pluto, Jupiter’s moon Europa, and Mars. Chaotic terrain is one of the major terrain types on Europa and may cover ~25–40% of the icy surface. Conamara Chaos is a classic example of chaotic terrain on Europa, where many large blocks surrounded by finer matrix material are contained within a relatively distinct, scarp-bounded region. The chaotic mountain ranges on Pluto occur mainly along the western margin of the nitrogen-rich ice sheet called Sputnik Planitia as an intermittent series of angular blocks. The finer matrix material surrounding the larger blocks in chaos on Pluto and Europa could be the disintegrative product of chaotic mountain blocks breaking into smaller fragments. On Mars, chaotic terrain is commonly located within outflow channels in the equatorial and mid-northern latitudes. Martian chaotic terrain features large, steep-sided mesas and smaller, crest-topped knobs separated by flat, smooth-floored valleys. Fracturing of the surrounding terrain is commonly observed. The size, orientation, and spacing of these fractures are likely directly correlated to the size and morphology of the chaotic terrain blocks within each region.
Image 2: Cartoons of chaotic mountain block profiles for Pluto, Europa, and Mars. If the blocks are completely destabilized and free from the surface below, they may rotate and translate. Block floatation may occur in a liquid or solid with sufficient density contrasts and sufficiently low viscosities at the given surface temperatures, and reach an isostatic position similar to partially or fully floating icebergs (e.g., on Pluto and Europa). Alternatively, the blocks may remain in place and the factures around them may be deepened over time by erosion (e.g., on Mars). Modified from Skjetne et al. 2020.
Image 2 illustrates how the size and height distributions of chaotic terrain blocks could provide information about the lithologic structure and thickness of the fractured units on each body. If the measured blocks are all the same height or reach a maximum height and level out (i.e. cease to increase in height with increasing diameter; Image 3), then this could yield information about the layer thickness of the fractured unit. We applied buoyancy models for floating iceberg-like blocks to estimate the thickness of the ice shell of Europa, and the root depth required to support blocks floating in isostasy in Sputnik Planitia on Pluto.
Image 3: Height and diameter of chaotic terrain blocks on Pluto, Europa, and Mars. Chaotic terrain blocks on Pluto and Mars show a positive linear relationship, whereas blocks in chaotic terrain on Europa display a “flat” height and diameter trend. Redrawn and modified from Skjetne et al. 2020.
On Europa, block heights (Image 3) were used to estimate a minimum ice shell thickness of ~1 to 4 km at the time of the block formation, if the blocks reached an isostatic position after fracturing. The tall heights of blocks on Pluto (Image 3) suggests that the larger blocks are not currently floating, as the estimated root depth of 10s of km would be too large compared to the likely maximum depth (up to ~5 km) of Sputnik Planitia basin along its perimeter. On Mars, we propose that block heights (Image 3) could be used to indicate a lithologic layer thickness if their apparent bases represent a more competent lithologic layer and their tops are covered by a cap rock.
Further Reading:
Collins, G. C., Nimmo, F., 2009. Chaotic terrain on Europa. In: Pappalardo, R. T., McKinnon, W. B., Khurana, K. K., (Eds.), Europa. Univ. Arizona Press, Tucson, pp. 259-282.
Meresse, S., Costard, F., Mangold, N., Masson, P., & Neukum, G., 2008. Formation and evolution of the chaotic terrains by subsidence and magmatism: Hydraotes chaos, Mars. Icarus, 194, pp. 487-500, doi:10.1016/j.icarus.2007.10.023.
Moore, J. M, McKinnon, W. B., Spencer, J. R., Howard, A. D., Schenk, P. M., Beyer, R. A., . . . Wilhelms, D. E., 2016. The geology of Pluto and Charon through the eyes of New Horizons. Science, 351, pp. 1284-1292, doi:10.1126/science.aad7055.
Schmidt, B., Blankenship, D., Patterson, G., Schenk, P., 2011. Active formation of ‘chaos terrain’ over shallow subsurface water on Europa. Nature, 479, pp. 502-505.
Singer, K. N., McKinnon, W. B., Schenk, P. M., 2019. Pits, Uplifts and Small Chaos Features on Europa: Evidence for Diapiric Upwelling from Morphology and Morphometry. Icarus. Under Review.
Skjetne, H. L., Singer, K. N., Hynek, B. M., Knight, K. I., Schenk, P. M., Olkin, C. B., … Ennico, K., 2020. Morphological comaprison of blocks in chaos terrains on Pluto, Europa, and Mars. Icarus, in press, doi:10.1016/j.icarus.2020.113866.
Williams, K. K., Greeley, R., 1998. Estimates of ice thickness in the Conamara Chaos region of Europa. Geophys. Research Letters, 25, pp. 4273-4276, doi:10.1029/1998GL900144.
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