Clark R. Chapman W. J. Merline, L. R. Ostrach, Z. Xiao, S. C. Solomon, J. W. Head III & J. L. Whitten Clark R. Chapman (SwRI), W. J. Merline (SwRI), L. R. Ostrach (ASU), Z. Xiao (U. Az., China Univ. Geo.), S. C. Solomon (Carnegie Inst. Wash.), J. W. Head III (Brown U.) & J. L. Whitten (Brown U.) 2011 Geological Society of America Annual Meeting 2011 Geological Society of America Annual Meeting Minneapolis MN, 10 October Geological Society of America Annual Meeting 2011 Geological Society of America Annual Meeting Minneapolis MN, 10 October 2011 Statistics of Morphologies of Small Primary and Secondary Craters on Mercury’s Northern Plains

Mercury’s Northern Plains (enhanced color)

Northern Plains “Ghost” Craters These regions around 80 degrees N latitude show that a previously heavily cratered surface has been flooded by voluminous lava flows

Crater Morphology Classifications Our crater classes conform to those widely used since 1960s (e.g. LPL) While “degradation state” correlating with age is the usual interpretation, craters may be formed with non- pristine morphology (e.g. secondaries and endogenic craters) Crater clusters (and chains) usually imply secondary cratering Approx. %-tage of lifetime in classes: 1=5%, 2=10%, 3=40%, 4=45%.

Extensive Northern Plains Goal: to investigate small craters in the northern plains to understand role of secondaries and degradation by volcanic processes red Extensive plains (Head et al., Science, 30 Sept. 2011), red outline green Study area in mosaic (green rectangle) purple Location of Narrow Angle Camera frame (purple “+” sign) Head et al. (2011) describe this high-latitude region (6% of Mercury’s surface) covered by flood volcanism, dating from the time of the Caloris plains, at about the end of the Late Heavy Bombardment. 180º 90ºW 90º E 50ºN 30ºN

Section of Northern Plains Mosaic We measured large, flooded ghost craters (as class 4) Box shows corner of frame counted for craters half the minimum size counted in the whole frame M M Msw righthand boundary next slide

Context of Narrow Angle Camera (NAC) Frame in North Polar Mosaic

Narrow Angle Camera frame EN M Target #2758 (~70.5º N, 321º E) ~20 km across Large, “soft” (or degraded) craters in chains and clusters Small clusters of tiny craters with fresh morphology Basic study frame North is to the right

Marked NAC Frame Large craters measured in whole frame = A Portions measured to smaller sizes B = clusters of small craters C = no clusters of small craters A A B B C C

Sections of NAC Frame: Clusters of small craters…or not C C B B

R Plot: Crater Size- Frequency Distributions Ratio of big craters to small ones varies with crater size (on the Moon) At 10’s – 100’s of meters, slope ~-4 At 10 – 60 km, slope ~ “Slope” is the exponent of a differential power-law R Plot Differential Plot Geometric saturation Empirical saturation

Total = All Crater Classes Total counts for all craters from 5 sections Curves are eyeball fits to the data points Large secondaries saturated near 1 km diameter Note split for D<40 meters indicating difference between clustered (B) and non-clustered (C)

Comparison of Northern Plains with Mariner/MSGR M1 Note that large, flooded ghost craters have similar R at D=100 km as Mariner 10 cratered terrain; slightly smaller craters (30 – 70 km) are deficient on northern plains (covered over by lava plains). R for km craters is the same on N plains as on plains inside Caloris, implying similar age near end of Late Heavy Bombardment Secondary branch begins at D<10 km. Highlands (flooded) Caloris plains dens. Secondary branch

SFDs for Crater Classes Note that the drop-off to the left from the peak progresses from ~1 km for fresh craters (classes 1 & 2) down to ~400 m for the most degraded craters (class 4)

Schematic Diagram of Crater SFDs Subject to Period of Degradation If a population of craters with a steady-state distribution of classes is then “flooded” or massively degraded, fresh craters will shift to a degraded class below a particular size…and the most degraded craters will be totally removed below a much smaller size threshold. Recratering by fresh small craters can be seen at the smallest sizes. Log Diameter Log R (spatial density) Class 1 Class 2 Class 3 Class 4 Total This could explain the observed class distributions. An early population of mainly secondary craters is inundated by lava flows ~200 m deep. Afterwards there is re-cratering by small secondaries from afar.

Scenario: Period of Massive Volcanism Ended We suggest that prior to the (last episode of) northern plains volcanic flooding, it was nearly saturated by numerous secondaries (D < 10 km) from large craters. There was then massive flooding (crater degradation) with flows > 200 m cumulative depth erasing all craters < 400 m diameter. Later, the region has been peppered by clusters of small secondaries from large, distant primaries. There has been essentially zero subsequent modification of the surface (as evidenced by the fresh appearance of these small craters). Though this analysis is based on one NAC frame, many others on N plains have similar appearance. Note the dominance of secondaries among craters <10 km diameter: rules out dating of small units from densities of primary craters.

The End