Differently aged intra-crater sediments next to each other on Mars

Introduction: Ice and ice-dust mixture composed deposits are frequent on Mars [1,2], and might have formed during most of its geological history. However, it is not clear in which period what locations were preferred by such accumulation. To have some insight into the age of crater filling sediments, two roughly similar sized craters at the southern hemisphere were analysed. Both are filled by smooth sediments that have been partly eroded, and these sediments seem to be different from the Concentric Crater Fills and other related middle latitude features (LVF, LDA).

 

Methods: Infilled craters at the target area have already been analysed by several authors [3-7], however in this work new or poorly analysed targets were surveyed. Using CTX images and DTM related topographic data, we analysed the Rabe crater (43.3°S 34.6°E) with 108 km diameter and 1.5 km depth, plus an unnamed crater (40.6°S 34.4°E) with 78 km diameter and 1.4 km depth. Both show relatively smooth, nearly horizontal material fill, with a large eroded depression in them. There are also some uncertain indications of possible layering at the top edges, and the youngest units in both craters are the dune coverages on their top.

 

Results: Crater occurrence based statistical age estimation provided a range of values for the surrounding area and for the various units identified in the two craters. The smooth surface of the sedimentary infill with 880 km maximal thickness in Rabe crater showed to be 3.7 (+0.08,-0.2) Ga old. The unnamed crater’s smooth surface sediment of 470 m thickness showed to be 0.63 (+0.06,-0.06) Ga old. This comparison shows that while the infill of Rabe crater formed around the Noachian/Hesperian transition period, the unnamed crater’s sediment formed in the Middle Amazonian. As these two craters were found to be about 83 km from each other, they are expected to have witnessed the same climatic and related surface conditions. The difference is difficult to explain with the current knowledge of the Martian surface processes.

 

Acknowledgement: This project was supported by the GINOP-2.3.2-15-2016-00003 project of NKFIH. The Rabe crater related research was the BSc thesis of Kondacs F., while the unnamed crater related research was the BSc thesis of Égerházi B.

 

References: [1] Head, J.W., 2007. The geology of Mars: new insights and outstanding questions, in: Chapman, M. (Ed.), The Geology of Mars. Cambridge University Press, Cambridge, pp. 1–46. [2] Neukum, G., Hiller, K., 1981. Martian ages. Journal of Geophysical Research 86, 3097–3121. [3] Fenton L.K. et al 2003. Aeolian processes in Proctor Crater on Mars: Sedimentary history as analyzed from multiple data sets. JGR 108(E12), JE002015. [4] Hiesinger, H. et al. 2018. Absolute Model Ages and Stratigraphy of Neukum Crater Geologic Units. LPSC 2001. [5] Robbins, S.J., Hynek, B.M., 2012. A new global database of Mars impact craters ≥1 km: 1. Database creation, properties, and parameters. JGR 117(E5) JE003966. [6] Tirsch, D. et al. 2018. The Complex Geomorphology of Neukum Crater on Mars. 20th EGU 1959. [7] De Hon R.E. 2018. Crater-fill on Mars: Sediment –traps and Long Term Stratigraphic Record; 49th LPSC 2083.