SPATIALLY AND TEMPORALLY VARIABLE EROSION OF THE LARGEST POST-NOACHIAN IMPACT CRATERS ON MARS (Invited Presentation)

Large impact craters and their continuous ejecta blankets represent instantaneous geologic events that can be dated using crater counts. Their similar original morphology and random spatial distribution allow comparisons of crater modification processes through space and time, providing new insight into the paleoclimate and climate change on Mars.

We examined all 29 Martian impact craters >90 km in diameter, between ±60° latitude, and with well-preserved ejecta that indicate a post-Noachian age (<~3.6 Ga). We estimated their relative ages using counts of superimposed craters >2 km in diameter (we counted craters >1 km, but mantling reduced counts from 1–2 km at ~40–60°N and S latitude), and we classified the large craters based on the scale of fluvial erosion. The most heavily modified craters had topographically prominent alluvial fans sourced from deep, dense fluvial dissection of parts of the crater walls. Moderately dissected craters had smaller floor deposits and sparse wall gullies visible in 100 m/pixel Thermal Emission Imaging System (THEMIS) daytime infrared imaging. Other craters were undissected at this resolution. Dissection or localized flow of mantling deposits was common in mid-latitudes.

Crater counts showed that the heavily and moderately modified craters formed during the Hesperian Period through approximately the Hesperian/Amazonian (H/A) boundary, whereas Amazonian craters had no deep fluvial dissection at the THEMIS scale. This result places the last substantial fluvial erosion on Mars around the H/A boundary, although shallow dissection may have occurred later. The most heavily modified Hesperian or H/A craters were clustered from 15–30°S and 30°W east to 30°E, whereas moderately dissected craters of this age were widespread. Undissected Hesperian craters occur in Arabia Terra, where valley networks are sparse.

Fluvial erosion does not scale with crater diameter or latitude, and some heavily or moderately modified craters have superimposed primary craters that are also fluvially eroded, suggesting gaps in time between the impacts and the subsequent erosion. For these reasons, large impacts apparently did not cause this late fluvial erosion on Mars. The spatial and temporal pattern suggests a limited epoch of erosion in the highland plateau around the H/A boundary.