Spatially and Temporally Variable Degradation of Large Post-Noachian Impact Craters: a Record of Paleoclimate and Climate Change on Mars

52nd Lunar and Planetary Science Conference 2021 (LPI Contrib. No. 2548) 2729.pdf

SPATIALLY AND TEMPORALLY VARIABLE DEGRADATION OF LARGE POST-NOACHIAN IMPACT CRATERS: A RECORD OF PALEOCLIMATE AND CLIMATE CHANGE ON MARS. R. P. Irwin III1 and E. Wolfe2, 1Center for Earth and Planetary Studies, National Air and Space Museum, Smithsonian Institution, PO Box 37012, MRC 315, Washington DC 20013-7012, [email protected]. 2University of California, Berkeley, [email protected].

Introduction: The original morphology of impact 4. Negligible: no significant fluvial dissection of craters provides a baseline for evaluating secondary crater walls or minor gulling that is detectable only modifications, which serve as a record of paleoclimate in 6 m/pixel Mars Reconnaissance Orbiter CTX or and climate change. This study focuses on the Hesperian higher–resolution imaging. and Amazonian Periods on Mars, which were more 5. Mantled: buried craters that may have gullying dynamic intervals of time than was apparent from low- confined to a mid–latitude mantling deposit, such resolution Viking Orbiter imaging [1]. We completed that we cannot uniquely determine if the crater counts and classified the modification of all 29 underlying crater wall bedrock was dissected impact craters on Mars with diameters >90 km and before emplacement of the mantle. latitudes <60°N or S, finding considerable variability in Finally, we identified primary impact craters that modification as a function of age and location. had formed between some of these impacts and the Study areas: To select the 29 study areas, we used subsequent degradation, suggesting that the erosion was the Mars Odyssey Thermal Emission Imaging System not confined to the immediate aftermath of the impact. (THEMIS) daytime infrared (DIR) global mosaic at 100 Results: All seven subject craters with Amazonian m/pixel [2] to examine all impact craters >90 km in ages (error bars that did not cross the Hesperian/ diameter in the Robbins and Hynek crater database [3]. Amazonian (H/A) boundary) had negligible dissection We used craters with exposed central peaks, rugged or mantles with negligible fluvial erosion of the mantle, rims, and preserved primary ejecta morphology. Three consistent with prior interpretations of a late phase of other craters poleward of 60°N or S were not used fluvial erosion around the H/A boundary [9,10]. Hale because of thick mantling by ice and dust, and we did appears to be the youngest impact crater of its size on not use the Orcus Patera structure because of extensive Mars, with no clearly superimposed craters >1 km in volcanic burial. Tanaka et al. [4] had mapped 27 of the diameter. 29 subject craters (all but Sagan and Hale) within the The 16 large equatorial craters that formed in the Amazonian and Hesperian impact unit. Hesperian Period or around the H/A boundary had Methods: We remapped the continuous ejecta regionally variable modification. Four craters with blankets of the subject craters at 1:2,000,000 scale using heavy fluvial dissection are concentrated from 15–30°S THEMIS and Mars Reconnaissance Orbiter Context and approximately 40°W east to 30°E. In sharp contrast, Camera (CTX) mosaics [5,6]. To estimate relative ages, five craters from 30°N–15°S and from 40°W east to we deleted the buried or secondary craters from the 60°E had negligible or light fluvial erosion regardless of Robbins and Hynek [3] database within these impact age. Nicholson and Oudemans craters in the Western units, and we binned the remaining (superimposed) Hemisphere were similarly little modified. In the craters by diameter on Hartmann [7] isochron plots. Eastern Hemisphere, four craters located between Syrtis We classified the degradation of the subject craters Major and 180°E had moderate rim dissection, as follows: including Gale, as did Sharonov crater that was eroded 1. Heavy: fully dissected alcoves in crater wall by the Kasei Valles outflow channel. bedrock with sharp divides between valleys and An additional six craters formed in the Hesperian large alluvial fans detectable in the 463 m/pixel Period or around the H/A boundary in the mid latitudes Mars Orbiter Laser Altimeter (MOLA) Mission from 30–60°N and S. In that area, regional mantling Experiment Gridded Data Record (MEGDR) [8]. included more ice flow and meltwater channels from 2. Moderate: dissected crater walls with large gullies 30–45°N and S, with less from 45–60°N and S. visible in 100 m/pixel THEMIS DIR imaging, Holden [11], Bakhuysen, Du Martheray, Murray, undissected surfaces between gullies, and terminal and an unnamed crater at 2.1°S, 57.4°E have one or deposits that are too small to be clearly identifiable more superimposed primary craters with fluvial in the MOLA MEGDR. dissection, suggesting that erosion was not entirely 3. Light: crater walls with few gullies visible in the confined to a brief period after the impact. Similar THEMIS DIR. findings apply to Gale crater [10]. 52nd Lunar and Planetary Science Conference 2021 (LPI Contrib. No. 2548) 2729.pdf

Fig. 1. Map of the subject craters with 45°N, 30°N, Acknowledgments: We thank NASA for support 30°S, and 45°S latitudes shown in yellow. The red through Mars Data Analysis Program grant contacts are mapped boundaries of continuous ejecta. NNX16AJ44G. The Base 11 internship program The erosion categories are shown in the legend. supported E. Wolfe. References: [1] Carr M. H. (1981) The Surface of Discussion: Fluvial erosion appears to have waned Mars. New Haven CT, Yale Univ. Press. [2] Edwards globally after approximately the H/A boundary, but we C. S. et al. (2011) JGR, 116, E10008, https://doi.org/ have not determined if the observed erosion was 10.1029/2010JE003755. [3] Robbins S. J. and Hynek B. concurrent at all of the affected sites. For this reason, we M. (2012) JGR, 117, E05004, https://doi.org/10.1029/ can hypothesize a simple explanation for our results but 2011JE003966. [4] Tanaka K. L. et al. (2014) USGS cannot exclude more complex alternatives. Mars map I-3292, https://dx.doi.org/10.3133/sim3292. [5] appears to have experienced one or more intervals of Malin M. C. et al. (2007) JGR, 112, E05S04, late precipitation at low and mid-latitudes. From 15– doi:10.1029/2006JE002808. [6] Dickson J. L. et al. 30°S, this precipitation may have accumulated as a (2018) LPSC 49, Abstract #2480. [7] Hartmann W. K. winter snowpack that melted quickly in the spring to (2005) Icarus, 174, 294–320, doi:10.1016/j.icarus. form large alluvial fans [12]. Areas closer to the equator 2004.11.023. [8] Smith D. E. et al. (2003) Mars Global and at lower elevations had less precipitation, or it did Surveyor laser altimeter mission experiment gridded not tend to accumulate, resulting in less effective data record, MGS-M-MOLA-5-MEGDR-L3-V1.0, erosion by rain or snowmelt. The mid-latitudes from NASA Planetary Data System. [9] Grant J. A. and 30–60° N and S experienced net accumulation of ice, Wilson S. A. (2011) GRL, 38, L08201, leading to glacial flow with less meltwater production at doi:10.1029/2011GL046844. [10] Grotzinger J. P. et al. higher latitudes. In this way, redistribution of ice could (2015) Science, 350. https://doi.org/10.1126/ be global in scale, while the regional patterns of erosion science.aac7575. [11] Irwin R. P. et al. (2015) were a function of latitude and atmospheric circulation. Geomorphology, 240, 83–101. https://doi.org/10.1016/ The time elapsed between impacts and erosion is j.geomorph.2014.10.012. [12] Grant J. A. and Wilson S. inconsistent with local effects of impacts as the main A. (2012) Planet. Space Sci., 72, 44–52, doi: driver of crater degradation. 10.1016/j.pss.2012.05.020.