51st Lunar and Planetary Science Conference (2020) 1131.pdf Timing of Smooth-Topped Chaotic Terrains, Southern Circum-Chryse, Mars. J. Walmsley1 <[email protected]>, F. Fueten1 <ffueten@ brocku.ca>, R. Stesky2, A. P. Rossi3, 1Dept. of Earth Sciences, Brock University, St. Catharines, Ontario, Canada L2S 3A1, 2Pangaea Scientific, Brockville, Ontario, Canada K6V 5T5, 3Dept. of Physics and Earth Sciences, Jacobs University Bremen, Bremen, Germany. Introduction: Primary chaotic terrains are the larg- during collapse when compared with their surrounding est type of chaotic terrain on Mars, covering the largest near horizontal plateaus. Total volume loss calculated area and deepest collapse, producing catastrophic for each site are: Hydraotes Chaos (179,000 km3), floods which erode large outwash channels [1]. Sec- Hydraspis FFC (10,900 km3), Candor Chaos (<2,000 ondary chaotic terrains are much smaller than primary km3), Baetis Chaos (3,200 km3), and the East Chaos chaos and are confined to the floors and walls of pre- (400 km3). Estimated depth of collapse for each chaot- existing outflow channels [2]. They usually do not ic terrain was also calculated: Hydraotes Chaos (2.1 generate catastrophic floods during their formation [2], km), Hydraspis FFC (1.45 km), Candor Chaos (< 0.2 though examples of flooding do exist [3]. Fractured- km), Baetis Chaos (no collapse), and the East Chaos Floor Craters (FFCs) are a subset of primary chaotic (no collapse). Relative timing of chaos forming events terrains which are completely contained within a pre- was determined using data from previous studies, as existing crater and can produce catastrophic flooding well as from cross-cutting relationships. The results are [4]. This study focuses on four separate chaotic terrains shown in Table 1. in the area south of Chryse Planitia, including Hydrao- Discussion: The lack of preferred orientation of tes Chaos, an FFC in western Hydraspis Chaos, Can- collapse shows that the process of forming chaotic dor Chaos, and Baetis Chaos with accompanying un- terrains is surficial and locally controlled, with no named East Chaos (Figure 1). These sites were chosen underlying tectonic regional control. Volume loss based on the relative proximity to one another and estimates are similar to previous estimates [6]. Given Valles Marineris. All sites have smooth-topped blocks the estimates of water volume needed to erode the within the chaotic terrains whose surface attitude can large outwash channels north, sources of water other be measured. Hydraotes Chaos is an example of a large than the chaotic terrains are needed to explain their primary chaotic terrain [1] with several channels lead- occurrence [7]. Relative dating of the study sites shows ing from it. The crater pair in Hydraspis is an FFC [4] that there has been chaotic terrain forming events which has an outflow channel through a breached spanning Martian history from the Middle Noachian northern rim. Candor Chaos, Baetis Chaos and the East (~3.93 Ga) to the end of the Hesperian (~3.1 Ga). Chaos are all representative of secondary chaotic ter- References: [1] Ori, G.G., & Mosangini, C. (1998) rains [2] located within outflow channels and are on a JGR, 103(E10), 22,713-22,723. [2] Rodríguez, J.A. et much smaller scale than primary chaotic terrains like al. (2011) Icarus, 213, 50-194. [3] Coleman, N.M. Hydraotes Chaos. (2005) JGR, 110, E12S20. [4] Bamberg, M. et al. Methodology: Data used for this study includes all (2014) Planet. Space Sci., 98, 146-162. [5] Minin, M. CTX stereo pairs available for each area, as well as (2015) MSc Thesis, http://dr.library.brocku.ca/handle HRSC DEMs, with the MOLA global dataset being /10464/7893. [6] Carr, M.H. et al. (1987) LPS XVIII, used to fill gaps in data coverage. Attitude measure- Abstract #1079. [7] Carr, M.H. (1996). Water on Mars. ments were completed in ArcGIS using the Augmented New York: Oxford Univ. Press. [8] Carr, M.H., & Visualization of Attitude (AVA) tool [5] which pro- Head, J.W. (2010) Earth Planet. Sci. Lett., 294, 185- duces a colour-coded attitude visualization of a terrain 203. [9] Citron, R.I. et al. (2018) Nature, 555, 643-646. surface using a hue-saturation-lightness colour wheel [10] Pajola, M. et al. (2016) Icarus, 268, 355-381. [11] compensated for relative luminance, with saturation as Tanaka, K. et al. (2014). USGS Map I-3292. [12] Luc- slope. The loss in volume due to collapse of the chaotic chitta, B.K. (1999) USGS Map I-2568. [13] Gross, C. terrains was also completed in ArcGIS using a top et al. (2009) LPS XXXX, Abstract #1890. [14] Cole- bounding surface at the surrounding plateau elevations man, N., & Baker, V. (2007) LPS XXXVIII, Abstract and the current chaos floors as the bottom surface. #1046. [15] Weiss, D.K., & Head, J.W. (2017) Icarus, Results: Attitude measurements of smooth-topped 288, 120-147. [16] Warner, N.H. et al. (2013) Geolo- blocks shows no preferred orientation of collapse with- gy, 41, 675-678. [17] Fueten, F. et al. (2014) JGR, 119, in any of the study sites. Low dip values of the mesas 331-354. [18] Warner, N. et al. (2009) Earth Planet. (nearly all < 6°) indicate very minor rotation of blocks Sci. Lett., 288, 58-69. 51st Lunar and Planetary Science Conference (2020) 1131.pdf Figure 1: DEM overview of study sites (A) with accompanying DEM composites of Hydraotes Chaos (B), Hydraspis FFC (C), and Baetis Chaos with the adjacent East Chaos (D). Table 1: Major Martian events and approximate Chaos formation, all values in Ga. Absolute Ages Relative Ages Chaotic Terrain Noachian Early Noachian 4.55 Planet Formation [8] Middle Noachian 4.5- Dichotomy Formation [8] Activation of Tiu Valles and Hydraspis 4.1 Flooding [10] 4.0 Arabia Ocean Forms [9] Late Noachian Hydraspis FFC Hydraotes Forms (Upper Noachian) [1] Valles Marineris Starts to Form [8] Hydraotes Chaos Hesperian Early Hesperian 3.7 End of Heavy Bombardment [8] Major Volcanic Resurfacing [8] Flooding north from Hydraotes [11] 3.7 Tharsis mostly Accumulated [8] Candor Forms [12] 3.6 Deuteronilus Ocean Forms [9] Late Hesperian 3.33 Juventae Forms [13] ILD in Candor Forms [12] 3.33- Maja Floods [13] Maja Floods [14] Baetis Chaos 2.18 3.3 - Final Floods in Simud and Tiu Valles Valles Marineris Mostly Complete [8] 3.2 [10] 3.1 Draining of Capri-Eos Lake into Auro- Candor Drains [12;17] Candor Chaos rae [16] Amazonian Early Amazonian 2.5 Aram Crater Floods Ares Valles [18] .