AN INVESTIGATION OF MORPHOMETRIC RELATIONSHIPS OF MARTIAN CRATERS 21-30 KM IN DIAMETER

Crater morphometry (i.e. quantitative relationships between impact crater landforms) is generally well-understood for many solid solar system bodies. Previous works have focused on examining these relationships using the entire crater population for a given object (e.g. planet). This work focuses on how active geologic processes have affected crater morphometry over time. Unlike Earth, where dynamic phenomena have altered, obscured, and even obliterated most impact craters, Mars provides a laboratory for observing the effects of erosion and weathering in a preserved crater population.

In this study, we have isolated 3 crater populations associated with each major martian geologic epoch (Noachian, Hesperian, and Amazonian), measured the dimensions of crater landforms, and are deriving morphometric relationships for each in an effort to determine how active processes degrade impact craters over geologic timescales. This study measured landforms in complex craters ranging from 21-30 km in diameter, well beyond the simple-complex transition for Mars and the postulated transition to peak pit/peak ring structures and compliments the work of (Brusnahan & Milam, this vol.). Craters were selected using the Barlow Catalog of Large Martian Impacts. Craters without central peaks or non-circular rims in this range were excluded. This resulted in the selection of 244 craters, ~86% of which resided in Noachian-aged geologic units, ~9% Hesperian, and ~5% Amazonian. We utilized 128 ppd gridded Mars Orbiter Laser Altimeter (MOLA) data to measure rim diameters (D), central peak diameters (D_cp), and central peak heights (H_cp). Preliminary results show that D_cp=0.24D for the Noachian crater population, similar to globally-determined averages for all solar system bodies (D_cp=0.23(+0.03)D), an unexpected result for such a weathered crater population. For the same population, H_cp=0.0005D^1.97, a relationship can be made to lunar crater morphometry: H_cp = 0.0006D^1.97, which shows a higher degree of degradation on Noachian craters as opposed to lunar craters. Ongoing work to discern additional morphometric relationships and to derive those in the Hesperian and Amazonian crater populations are expected to reveal systematic morphometric changes over time.