T h e N a t u re o f R e m n a n t M o u n d s o n t h e M a rg i n of Chryse Planitia

¹School of Physical Sciences, The Open University, Milton Keynes, UK Joe McNEIL¹ Matt BALME¹ Peter FAWDON¹ Angela COE² ²School of Environment, Earth and Ecosystem Sciences, The Open University, Milton Keynes, UK

[email protected] @joseph_mcneil Abstract #1948 INTRODUCTION Figure 1 • There are thousands of kilometre-scale mounds distributed around the southern margin of Chryse Planitia, a ~1090 km diameter MOLA quasi-circular basin that lies north of the dichotomy boundary (Pan et al., 2019, Figure 1A-C). projection of study area and • Large numbers of mounds are present in Oxia Colles/Oxia Planum (Figure 1A), Hypanis Vallis (Figure 1B), and atop the Simud Valles sub-study areas MAWRTH / Ares Vallis channel islands (Figure 1C).

C.P. IA STUDY AREA • Despite being a widespread geomorphological feature in this important highland-lowland transitional region, their origin VALLIS and stratigraphic position are poorly understood. 1A: OX • This study has investigated the morphology and distribution of these landforms in order to understand their place in the stratigraphy and their role in the geological history of the area.

• Mounds have been identified and digitised using a combination of CTX, HRSC, and MOLA data, and classified based on their • M o u n d CSproportions morphologies. Heights have been determined from the intersection of their high and low points with individual MOLA shot data. Figure 2: MOLA globe showing global context of are extremely Chryse Planitia (C.P.), and the location of Figure 1. consistent across ISTIall three study areas. M O U N D C L A S S I F I C A T I O N • On average, 77.8% of mounds are hills, are H i l l s M e s a s C o m p o u n d M o u n d s T i e r e d M o u n d s PSP_009313_2065 B B: Mawrth-like clay-rich material 7.1% (white), with high-albedo layer STATmesas, 14.8% are compound cut by WNW-ESE fault (red mounds, and 0.3% have tiers. B arrow), disconformably overlain 0.7% C • Oxia has the by dark capping unit (far left, (HiRISE/CTX) 100m right). largest number of OXIA CTX CTX CTX HiRISE 7.3% Representative Image Representative 0 1 2 0 1 2 0 0.6 1.2 0 1 2 tiered mounds, km Figure 3 km Figure 4 km Figure 5 km Figure 6A PSP_009313_2065 78.4% Schematic C C: Mawrth-like clay-rich material with most Cross Section (white) containing repetitive occurring in the Area HYPANIS OUTFLOW OXIA HYPANIS OUTFLOW OXIA HYPANIS OUTFLOW OXIA HYPANIS OUTFLOW OXIA high and low albedo layering, northern areas. N = 10,168 Count 2712 570 7973 231 55 743 528 116 1381 6 0 71 topped by a ~10 m thick dark OUTFLOW % in capping unit (upper right). 15.6% study 78.0 76.9 78.4 6.6 7.4 7.3 15.2 15.7 13.6 0.2 0.0 0.7 100m • The channel islands, area 7.4% despite having • Mounds were categorised based on the morphology of their highest points, which are either smooth/peak-topped (hills, Figure 3), flat-topped (mesas, Figure 4), multi- fewer examples, 76.9% tiered (tiered mounds, Figure 6), or any combination of these (compound mounds, Figure 5). The distribution of these types are visualised in Figure 1, and their populations retains a markedly N = 741 are graphed in the statistics section. similar proportion 0.2%

• In total, 14,368 mounds were digitised and categorised using the above classification scheme. 1C: OUTFLOW ISLAND STUDY AREA of mounds. HYPANIS ARES V KEY 15.2% Hill 6.6% • Hypanis study area Mesa also shows similar 78.0% ALLIS Compound TO CHRYSE PLANITIA proportions. Tiered N = 3,477

MORPHOMETRICS • The elevations of 2236 mounds were calculated using MOLA shot data. 13.6%

• Mound height increases with area, up to a maximum of ~500 m, where they plateau (Figure 8), suggesting that 500 m was the original maximum thickness of the layer that these mounds eroded from.

LEDERBERG CHRYSE • Mesas and tiered mounds are typically the tallest and most extensive, with compound mounds CRATER CHAOS intermediate, and hills usually being the smallest examples. This implies that hills are a more advanced erosional state and likely eroded from antecedent mesa and compound stages.

MAGONG A: Heights of Mounds vs Area of Mounds, all areas B: Elevation of Base of Mound vs Height of Mound, all areas CRATER 500.00 500.00 S NA VALLI RI B U R I E D C R A T E R S AB S • Mounds are often associated with curvilinear wrinkle ridges. These features

HYPANISVALLES sometimes demarcate the rims of buried impact craters, suggesting that 50.00 1B: HYPANIS STUDY AREA mounds could be related to an underlying surface or structure. Height (m) Height (m) STRATIGRAPHY • Wrinkle ridges typically do not cut through mounds (Figure 9). Nonetheless, in some 5.00 areas they can be seen to truncate mounds, suggesting that the most recent • The nature of the relationship between mounds and the surrounding plains is ambiguous due tectonic episode post-dates the deposition and erosion of the mounds (Figure 10). 0.50 to extensive regolith cover at the mound margins. 0.50 5.00 50.00 0.01 0.10 1.00 10.00 100.00 −2500 −3000 −3500 −4000 Area (km²) • All three sub-study areas yield an approximate age of 4.1 Ga, the maximum Elevation at Base of Slope (m) • Despite this, a few examples throughout the Oxia Study Area (Figure 1A) reveal that at least some of Hills Figure 8: A: log-log scatter plot depicting the relationship between the heights of the mounds Oxia (1A) Mesas the mounds are embayed by the dark plains material (Figure 7). depositional age of the mound-forming material. Hypanis (1B) and area of the mounds. B: Scatter plot depicting the heights of the mounds (log-scale), and Compound Mounds Outflow (1C) the elevation of the base of the mounds (a proxy for their location on the regional slope). Tiered Mounds • Some mounds show intense fractures (Figure 6A, B) that do not appear to propagate into the • This age likely represents a pre-Noachian surface, upon which the mounds were surrounding plains, suggesting that the mounds predate the plains, although this could be due to deposited. Subsequent erosion, likely in the mid-to-late Noachian, created the P R E L I M I N A R Y C O N C L U S I O N S extensive regolith cover. isolated mounds we see today. • The different morphologies of mounds are likely to represent different erosional states. Hills are likely to • High albedo layers (e.g. Figure 7C) can be traced between mounds up to 100 km away, suggesting • This surface must be younger than the Chryse-forming impact, suggesting this be derived from compound mounds, which in turn are likely to be derived from mesas. that the mounds were once part of a more contiguous unit. impact occurred > 4.1 Ga, in the pre-Noachian. • Mound morphometries are diverse, and their heights converge towards elevations of 500 m above the • Mawrth-like stratigraphy is common in the top tier of tiered mounds in the northern section of the Cumulative Crater Size Distribution Plot surrounding plains, suggesting that this was the thickness of the layer from which the mounds eroded. 10-2 Oxia Study Area (Figure 6A, B), implying that the Mawrth phyllosilicates extended into the northern -- -2 34 Craters, N(1)=6.48x10 ² km ² lowlands early in Mars’ history. 16 Craters, N(1)=7.76x10-- ² km ² • The dark plains material embays the mounds, and is therefore younger. -- +0.05 5 Craters, N(1)=7.48x10 ² km ² 4.1 -0.08 Ga 10-3 Channel Island • Crater counting the buried craters that the mounds delineate reveals that the maximum depositional age A B C Study Area

01 2 4 of the mounds is approximately 4.1 Ga, which is the age of the buried surface they are deposited on. This +0.03 Hypanis km 4.1 -0.04 Ga Study Area age is also therefore the minimum age of the Chryse-forming impact. 10-4

+0.02 Figure 9: CTX image of mound that is 4.1 -0.03 Ga • Where the mounds demarcate buried craters, the implied crater is consistently above the minimum Oxia not affected by the curved wrinkle simple-complex crater diameter of 3 km (Pike, 1980). Study Area 10-5 ridge that runs diagonally under it. • There are two possible explanations for this observation: simple crater margins are also delineated by Figure 10: CTX image of a north- south wrinkle ridge in Oxia Colles that mounds, but are not seen due to burial/ambiguity, or, mounds only form around complex craters. -6 10 truncates the rounded hill in the centre of the image • If the latter is true, this could suggest some relationship to fracturing associated with complex craters on Cumulative Crater Frequency, (km Frequency, Crater Cumulative ) Mars. One possibility is that impact-related hydrothermalism indurates the mound material above -7 10 fractures in large enough craters, causing those areas to be preferentially shaped into mounds during 0.1 1 10 100 1000 10000 Diameter (km) periods of erosion. 01 2 4 010 20 40 01 2 4 J McNeil acknowledges that Figure 11: Cumulative crater counts for buried craters in CTXkm CTXkm CTX km this work is made possible the three study areas (coloured). The quoted age of 4.1 Ga REFERENCES by Open University and Figure 7: A: Embayment of dark unaltered plains material (white arrow) on top and around highly eroded mound is the approximate age of the buried surface on which the Pan, L.; Quantin-Nataf, C.; Breton, S.; Michaut, C.; The impact origin and evolution of STFC funding. with bay-like features (black arrow) around its margin. B: Buried mound field (white arrows), with high-albedo mounds are deposited, and the maximum age of the Chryse Planitia on Mars revealed by buried craters, 2019, Nature Comms. 10, 425

material protruding from dark plains material (black arrows). C: Mounds with high-albedo layers (black), and clear mounds. Chronology function: Hartmann (2005), 01 2 4 Pike, R.J.; Formation of Complex Impact Craters: Evidence from Mars and Other evidence for embayment of dark plains material onto mound material (white arrow). production function: Hartmann & Daubar, 2016. km Planets, 1980, Icarus 43, 1-19