49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083) 1916.pdf DISTRIBUTION, STRATIGRAPHY, AND LAYER THICKNESSES OF INTRA-CRATER DEPOSITS IN WESTERN ARABIA TERRA, MARS. G. Schmidt1, M. Pondrelli1, F. Salese1 and A. Rossi2, 1IRSPS, Universita “G.D’Annunzio”, Pescara, Italy, [email protected], [email protected], 2Department of Earth and Plan- etary Science, Jacobs University, Bremen, Germany Introduction: Arabia Terra is a regional dichoto- crater diameter and deposit thickness. The thickest my boundary between the high and lowlands of north- deposits (>900 m) appear in craters deeper than 1,700 ern Mars known for its densely cratered terrain and m, however many craters of similar depth contain thin extensive distribution of water-altered deposits (Fig. 1). deposits of <200 m. Deposits in red and blue groups By analyzing the intra-crater deposits' stratigraphy and are thicker and their layers more continuous. Veneer mineralogy, as well as surveying their geographical deposits retain abundant minor cratering on their sur- distribution, the depositional history of Arabia Terra is faces and display no significant erosional features. The constrained. 485 craters were observed within a 1,307 veneer is flat and lacks any sedimentary structures. by 1,748 km area of western Arabia Terra, bounded by Discussion: Distinct changes in deposit thickness, the Oxia Palus quadrangle (MC-11), with the aim to layering, and composition of intra-crater deposits were identify and characterize potential water-altered depos- observed across Arabia Terra from the boundary with its (Fig. 2). Several distinct varieties of deposits were Meridiani Planum in the southeast to Chryse Panitia in found and distinguished by their mineralogy, albedo, the northwest. Deposits in the southeast are thicker and thermal-inertia, layering, and erosional landforms. In more sulfate dominant and deposits in the northwest general, deposits appear either as a thin veneer or a are thinner and more clay dominant. Elevation and bulky mass that is commonly layered. geographic relationships between deposit distribution Methodology: A CTX mosiac registered to a suggests multiple upwelling sources of varying depth HRSC composite DTM of the MC-11 quadrangle and intensity at different intervals. A scenario involv- forms the base data for the study. Small-scale land- ing a complicated multi-depositional environment of forms and layering were examined within HiRISE im- various periods of upwelling groundwater and/or mul- ages. MOLA data was used in areas surrounding MC- tiple durations of a fluctuating regional water table 11. Elevation measurements were compiled into six followed by late volcanism is proposed for the deposi- graphs (Fig. 4) and made using HRSC data (50m/pix). tional history of Arabia Terra. In HiRISE stereo pairs, layer thicknesses were obtained Observations indicate that clays generally reside by measuring elevation and distance between each lay- at lower elevations, which agrees with previous studies er along a line parallel to slope. Multiple transects were [4]. Although a regional water table may explain the measured for each HiRISE image. A total of 402 layer apparent regional facies change, it does not readily thickness measurements were taken along 51 transects. explain why there are so many empty craters in close Results: Of the 67 craters determined to contain proximity to craters with deposits. Separate sources of bulk deposits (light blue in Figure 2), 43 within MC-11 upwelling groundwater may explain this apparent spon- were put into three groups based on geographic posi- taneity of deposition. Deposition in green craters (Fig. tion, elevation, and deposit similarities (Fig. 3), to 3) could be related to outflow from Mawrth Vallis, compare their deposits thickness, elevation, layering, since extensive clays have been identified on the plat- mineralogy, crater depth, diameter, and layer thick- eau adjacent to the valley [3]. However Oyama, one of nesses (Fig. 4). Blue craters represent the boundary of the prominent clay bearing craters in this group, is a Arabia Terra and Meridiani Planum, an area where kilometer deeper than Mawrth Vallis and instead could extensive sulfates have been detected previously [2, 4]. be related to the proximity of the hypothesized Ocea- Red craters represent the middle area of western Arabia nus Borealis [1] . Crater size (diameter, depth) and the Terra and what appear to be the headwaters of Mawrth existence of a central peak do not appear to influence Vallis. Green craters represent the boundary of Arabia the likelihood of there being a deposit within a crater. Terra and Chryse Panitia, a boundary previously hy- The veneer deposits are interpreted to be volcanic pothesized as an ancient shoreline [1]. material deposited from late regional volcanism. It is Blue craters have the highest average deposit thick- possible that this late volcanism represents the last sig- ness of 485 m. Red craters have an average deposit nificant regional event. thickness of 333 m. Green craters have the lowest av- References: [1] Baker et al., (1991), Nature, 352, erage deposit thickness of 160 m and are composed of 589-594. [2] Poulet et al., (2008), Icarus, 195, 106- predominantly clays (Figs. 4). No trends were observed 130. [3] Bishop et al., (2008), Science, 321, 5890, 830- between crater depths and deposit thickness or between 833. [4] Flahaut et al., (2015), Icarus, 248, 269-288. 49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083) 1916.pdf Elevation (m) N Figure 2: Combined HRSC and MOLA -5,000 0 5,000 10,000 15,000 Study Area shaded relief mosiac of western Arabia Terra. Craters are grouped by color based on what is in their interiors. N Figure 1: Study location Meridiani Planum 250 km Impact Deposits: Impact breccias and melts of various morphologies and extent. Generally massive and often layered. Deposits with Resolvable Layering Crater Depth vs Deposit Thickness 35 N 3500 30 3000 25 2500 ) ty (m X si 2000 n20 th e p D e Oyama r D e r 1500 t te a15 ater Depth (m) Y’ r ater Density ra C C Cr Cr 1000 10 Becquerel McLaughlin 500 5 0 0 0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 Yes1 No2 Deposit Thickness (m) Interior Peak/Bludge Elevaon of Crater Floor vs Deposit Thickness 30 0 -1000 Trouvelot 25 Y -2000 20 -3000 ty si n e D -4000 r 15 e at ater Density r Elevaon (m) C -5000 Cr 10 Z’ -6000 5 -7000 X’ -8000 0 0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 Yes1 No2 Deposit Thickness (m) Danielson Layer Thickness vs Elevaon Diameter vs Deposit Thickness Z -2900 Lower Mawrth Craters n=15 Crommelin 180000 160000 -3400 Upper Mawrth Craters n=6 140000 250 km Danielson n=109 120000 Meridiani Planum Boundary n=22 -3900 Becquerel n=173 100000 80000 -4400 Elevaon (m) X X’ Diameter (m) Oyama n=83 60000 Oyama Trouvelot Danielson -4900 40000 McLaughlin n=37 20000 -5400 0 0 5 10 15 20 25 0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 Layer Thickness (m) Deposit Thickness (m) Y Y’ Trouvelot Becquerel Figure 4 (above): Graphs of crater depth, floor elevation, and diameter against deposit thickness, as well as graphs showing the ratios of deposits with resolvable layering, interior peaks, and layer thicknesses. Z Z’ Crommelin Danielson Figure 3 (left): Map marked by white dashed polygon in Figure 2. Craters with bulk deposits were put into three groups: Blue, Red, and Green. Cross-sections made using HRSC topography. .