The Timing of Formation of Primary, Secondary, and FFC Smooth-Topped Chaotic Terrains, Southern Circum-Chryse, Mars J. Walmsley1
Introduction: Volume Loss Comparison: 1 3 Primary chaotic terrains are the largest type of chaotic terrain on Mars, N N All volumes calculated are within the previously calculated [7] maximum covering the largest area and deepest collapse, resulting in catastrophic floods volume for individual floods. This gives further support to the argument that which erode large outwash channels [1]. Secondary chaotic terrains are much Hydraotes was a large lake where water pooled before moving northward so smaller than primary chaos and are confined to the floors and walls of pre- floods would be numerous and over a long period of time (millions of years). existing outflow channels [2]. They usually do not generate catastrophic floods The excavated channels of Simud and Tiu Valles require more water than what during their formation [2], though examples of flooding do exist [3]. Fractured- could be produced by the formation of Hydraotes alone. The Hydraspis FFC 3 Floor Craters (FFCs) are a subset of primary chaotic terrains which are can also be compared with the study by [6] which suggests 45,000 km of completely contained within a pre-existing crater and can produce catastrophic material is eroded from all Hydraspis Chaos. The Hydraspis FFC makes up flooding [4]. The majority of formation mechanisms proposed for the generation approximately 1/3 of the area of Hydraspis Chaos so it is expected to produce of chaotic terrain require a subsurface volume of ice and a thermal anomaly a significant fraction of the total water (~24% compared with [6]) needed to which causes destabilisation [1,4,15,22]. This study focuses on four separate remove material from the area. [6] estimate the volume of Maja Valles eroded chaotic terrains in the area south of Chryse Planitia, including Hydraotes 100 km 20 km by water to be 33,000 km3. Baetis and the East Chaos zones would have been Chaos, an FFC in western Hydraspis Chaos, Candor Chaos, and Baetis Chaos 2 4 eroded during the draining of Juventae Chasma [19, 20, 21]. with accompanying unnamed East Chaos (Figure 1). These sites were chosen N N based on the relative proximity to one another and Valles Marineris. All sites Timing of Chaos Formation: have smooth-topped blocks within the chaotic terrains whose surface attitude Chaotic terrains are generally difficult to date directly due to the removal of can be measured. Hydraotes Chaos is an example of a large primary chaotic material leading to a scarcity of impact craters. The first area to possibly be terrain [1] with several channels leading from it. The crater pair in Hydraspis is active is Tiu Valles, linked to Hydraspis. [10] place the activation of this channel an FFC [4] which has an outflow channel through a breached northern rim. north east of Hydraotes Chaos to be in the Middle Noachian. This is most likely Candor Chaos, Baetis Chaos and the East Chaos are all representative of before Hydraotes had formed as the activation is linked to outflow from secondary chaotic terrains [2] located within outflow channels and are on a Hydraspis, suggesting that the Hydraspis FFC had formed or was in the much smaller scale than primary chaotic terrains like Hydraotes Chaos. process of forming. Major volcanic resurfacing of the planet was occurring during the end of the Noachian and start of the Hesperian [8] which would have provided sedimentation to Hydraotes prior to its collapse. Opening and flooding from Hydraotes northward most likely occurred in the Early Hesperian following its formation [11]. Flooding of Maja Valles occurred between 3.33 – 2.18 Ga[13] 30 km 30 km most likely in the Late Hesperian, suggesting that Juventae Chasma had already formed and was releasing flood waters north, carving out Baetis Chaos Figure 4: Colorized Augmented Visualization of Attitude (AVA) results using a color coded stereonet, the color wheel and the East Chaos. The linking of Valles Marineris likely occurred by the end gives the attitude of the downward projecting pole to the topographic surface, for each study site; Hydraotes Chaos (1), Hydraspis FFC (2), Candor Chaos (3), and Baetis Chaos/East Chaos (4). Produced using the DEM mosaics. of the Late Hesperian [8] while the draining of the Capri-Eos lake into Aurorae Chaos was also occurring around 3.1 Ga [16]. It is likely that Candor Chasma was also linked by this time allowing much of Valles Marineris to flush through HydraotesPlanes N Weighted Histogram CandorPlanes N Weighted Histogram HydraspisPlanes N Weighted Histogram (Strike) Peak: 3.6% (Strike) Peak: 3.5% (Strike) Peak: 3.3% Chaos Azimuth: 167.5° Chaos Azimuth: 2.5° FFC Azimuth: 177.5° into Capri-Eos and then into Aurorae. The final and possibly largest floods of Simud and Tiu Valles north of Hydraotes Chaos have been dated to 3.3 – 3.2 Ga [10], placing it very close to the possible timing of the linking and draining of Valles Marineris.
Figure 1: MOLA global dataset overview of chaotic terrain study sites (purple stars). Table 2: Table of major events on Mars, absolute age estimates and general age estimates of events. Proposed approximate formation of studied chaotic terrains in Chaotic Terrain column. Methodology: Absolute Ages Relative Chaotic Data used for this study includes all CTX stereo pairs available for each Ages Terrain Noachian area, as well as HRSC DEMs, with the MOLA global dataset being used to fill Axial N = 8663 PolarAxial N = 8552 Axial N = 8660 4.55 Ga Planet Formation [8] Early Noachian 4.55 Ga gaps in data coverage. Attitude measurements were completed in ArcGIS EastPlanes Chaos N Weighted Histogram BaetisPlanes Chaos N Weighted Histogram (Strike) Peak: 4.5% (Strike) Peak: 3.2% using the Augmented Visualization of Attitude (AVA) tool [5] which produces a (Juventae) Azimuth: 157.5° (Juventae) Azimuth: 127.5° 4.5-4.1 Ga Dichotomy Formation color-coded attitude visualization of a terrain surface using a hue-saturation- [8] lightness colour wheel compensated for relative luminance, with saturation as 4.0 Ga Arabia Ocean Forms [9] slope. The loss in volume due to collapse of the chaotic terrains was also Middle Noachian 3.93 G a completed in ArcGIS using a top bounding surface at the surrounding plateau elevations and the current chaos floors as the bottom surface. Activation of Tiu Valles and Hydraspis Hydraspis Flooding [10] Crater Pair Late Noachian 3.82 Ga Chaotic Terrain Study Sites: Hydraotes Chaos has three main channels and lacustrine terraces Hydraotes Forms (Upper Hydraotes surrounding many of the mesas. The presence of these lacustrine terraces is Axial N = 7434 Axial N = 8551 Noachian) [1] Chaos evidence that water was stable on the surface for an extended period in Valles Marineris Starts to Hydraotes Chaos. Form [8] Figure 5: Rose diagrams of strike for all points within each respective chaotic terrain that have dips from 17°- 40°. Hesperian Hydraspis FFC’s northeastern crater has a central peak which is an Due to computational limitations a histogram of all strike/dip value combinations with dips between 17-40 were used (only includes non-zero entries). 3.7 Ga End of Heavy Early Major Volcanic Resurfacing uncommon feature of FFCs. The central peak is surrounded by small mesas, in Bombardment [8] Hesperian [8] most chaotic terrains smaller blocks tend to be located on the outskirts of the 3.7 Ga chaos. 3.7 Ga Tharsis mostly Flooding north from Hydraotes Candor Chaos has large Interior Layered Deposits (ILDs) along the north, Results: Accumulated [8] [11] east, and western edges of the chaos. The ILD would have formed in an Terraces are only observed in two of the chaotic terrains studied here. The terraces Candor Forms [12] isolated water-filled basin. do form at a similar elevation relative to the basin floors suggesting a similar water 3.6 Ga Deuteronilus Ocean Late Hesperian 3.6 Ga depth in both chaotic terrains during terrace formation.The thickness of the original ice Forms Baetis and the East Chaos have a complex history of outflow channel [9] activation surrounding them which all drain northward into Chryse Planitia. The layer is controlled by the amount of water/ice available and by the size of the sink to fill. 3.33 Ga Juventae Forms [13] ILD in Candor Forms [12] elevation of the top surface of the mesas within Baetis and the East Chaos are The widest and deepest chaotic terrain examined here is Hydraotes Chaos, making it 3.33 - 2.18 Maja Floods [13] Maja Floods [14] Baetis Chaos at or higher than the plateau area around them. the largest sink and thus the largest ice sheet by volume. Of the chaotic terrains that Ga collapsed, Hydraotes Chaos also has the most mesa showing above the surface. Block 3.3 - 3.2 Final Floods in Simud 37°W 33°W 73°W 72°W thickness measurements are based on what is visible above the surface. The mesas Ga and Tiu Valles [10] N 3.3 - 3.0 ICC Stabilizes [15] Valles Marineris Mostly 3°N 1 23 are most likely thicker than this. N Ga Complete [8] 3.1 Ga Draining of Capr i-Eos Candor Drains [12,17] Candor Lake into Aurorae [16] Chaos Table 1: Elevation data extracted from DEM mosaics for respective chaotic terrains. Calculated volume loss for each chaotic terrain is also shown. Amazonian 3.0 Ga Early Amazonian 3.0 Ga 2.5 Ga Aram Crater Floods into 7°S Results Hydraotes Hydraspis Candor Baetis & East Ares Valles [18] Chaos Crater Pair Chaos Chaos Highlights: Mesa height ≥ 1.4 km ≥ 0.7 km ≥ 0.2 km Baetis: ≥ 1.5 ŸThe formation of the chaotic terrains studied here are spread over a period of km nearly a billion years of Martian history (Middle Noachian to the end of the East: ≥ 0.5 km Late Hesperian). 1°S Total collapse depth 2.1 km 1.45 km Unknown, N/A ŸFormation of these chaotic terrains and the lakes/floods associated with them 29°W 28°W 60° W estimated shows that water was available and stable on the surface at least episodically N from the Noachian to the beginning of the Amazonian in the region around 2 4 < 0.2 km N Valles Marineris. Terrace elevations -4,161 m (Avg.) -3,244 m N/A N/A ŸChaotic terrains are not the sole source of flooding for the excavation of the (Avg.) larger outflow channels. 3°N Area of Chaotic Terrain 50,000 km2 7,700 km2 3,600 Baetis: 3,052 0° N References: [1] Ori, G.G., & Mosangini, C. (1998) JGR, 103(E10), 22,713-22,723. [2] Rodríguez, J.A. et km2 km2 al. (2011) Icarus, 213, 50-194. [3] Coleman, N.M. (2005) JGR, 110, E12S20. [4] Bamberg, M. et al. (2014) Planet. Space Sci., 98, 146-162. [5] Minin, M. (2015) MSc Thesis, http://dr.library.brocku.ca/handle East: 865 km2 /10464/7893. [6] Carr, M.H. et al. (1987) LPS XVIII, Abstract #1079. [7] Carr, M.H. (1996). Water on Mars. New York: Oxford Univ. Press. [8] Carr, M.H., & Head, J.W. (2010) Earth Planet. Sci. Lett., 294, 185-203. 3 Total Volume Loss 179,000 km 10,900 < 2,000 Baetis: 3,200 [9] Citron, R.I. et al. (2018) Nature, 555, 643-646. [10] Pajola, M. et al. (2016) Icarus, 268, 355-381. [11] km3 km3 km3 Tanaka, K. et al. (2014). 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