Clark R. Chapman Southwest Research Institute, Boulder CO, USA with assistance from

1. Size-Frequency Distributions for Very Small Craters on Mars Clark R. Chapman Southwest Research Institute, Boulder CO, USA with assistance from Brian Enke, Bill Merline, Peter Tamblyn Mars Crater Consortium Meeting, Flagstaff AZ, 29-30 September 2009

2. Small Craters (D = 1 m to 1 km) • Primaries formed by projectiles ~10 cm to 200 m • Significant vs. dominant contribution by secondaries • Break-up of meteoroids in Martian atmosphere • Complications: endogenic pits, exhumation • Morphologies (classes below) sensitive to processes operating on a vertical scale down to tens of cm

3. Methodology: Locality “N” • Presented with this image, a crater analyst faces a daunting task. • We apply our “automated assistant,” which finds 672 possible craters (purple circles) within a few minutes using A.I. template matching. • The analyst deletes false positives, corrects positions and diameters for errors, and then searches for and measures missed craters.

4. Nested images in homogeneous units attain adequate statistics over a wide range of crater sizes

5. “N” 64, 16 04, 01

6. Study region “N” is in cluster of probable secondaries • This part of Mars is lightly cratered by craters 1 to 100 km diameter, but this locality is saturated by craters <20 m diameter • For scale, Eddie Crater is ~85 km diameter

7. Locality “N”: Secondary Crater Cluster? • Remnant population of highly degraded craters a few hundred m in diameter. • Re-cratered by steep SFD of fresh (class 1+2) secondaries down to 20 m diameter, where saturation sets in…but above the equilibrium line! • Meter-scale craters roll over.

8. Variety among SFDs for “Total” Craters “Total” = all classes • Most localities (but not N & O) are nearly saturated with craters ~100 m diameter. • Fewer craters 500 m to 1 km in all localities except for O. • K follows empirical saturation line from 3 m to 150 m diam. • E is increasingly deficient in smaller craters <40 m diam. • All other localities (except E & K) show a deficiency at an intermediate diameter, ranging from ~10 m (for F, H, O) to ~40 m (for N). • Steeply rising branch of small craters (secondaries?) to empirical saturation line for F, G, H & O; but not N, which ex-ceeds the line, then rolls over.

9. Interpretational Framework for Crater Degradation Classes Theory for interpreting degradation classes was developed in the pre-Viking era and applied to Martian craters 5 to 100s of km in diameter. Now we can apply it to craters meters to km in diameter. • From pre-R-plot days; also degradation classes f/s/m/h = 1/2/3/4. • (a) shows incomplete-ness due to resolution: freshest craters seen to smallest size. • (c) opposite sequence for size-dependent degradation; (b) as seen in Jones’ (1974) data for large craters on Mars.

10. Modeling how time-variable erosion affects crater morphologies Obliteration time history Total fresh slight moderate heavily …degraded (Chapman, 1974)

11. Locality “E”: Strange Mounds Eroding Small Craters • Whole locality has mounds (degraded dunes?) • Parallel SFDs for classes for small craters => degradation and cratering in equilibrium. • Degradation by crater overlap and mound-formation process is very effective at degrading small craters (note a few small, fresh craters)

12. Locality “F”: Erosion Episode with Modest Re-Cratering • Context image shows faults, but they pre-date processes affecting small craters. • Period of degradation erased craters <30 m diameter, left those >400 m unscathed. • Modest re-cratering by craters <30 m diameter approaches equilibrium at 2 m, but very smallest craters show some degradation.

13. Locality “G”: Degradation Episode and Re-Cratering • Similar to “F” • Abruptly ending erosion of craters 10s to 100s m diam. • Considerable subsequent re-cratering (to empirical saturation level); degradation of meter-scale craters by overlap and sand dunes (?).

14. Locality “H”: Degradation Ends, Very Modest Re-Cratering • Lineaments show “fabric” at largest & smallest scales in this possibly exhumed S hemisphere terrain. • Largest craters degraded. • Very modest re-cratering by craters <20 m diam.

15. Locality “K”: Degradation by Crater Saturation (all Scales) • Context: rough plains within southern cratered terrain • SFD follows saturation line. • Degradation by crater overlap

16. Locality “O”: Good that Opportunity Avoided It! Methodology Issues: Sun angle too high; albedo features confusing; can’t distinguish secondaries from endogenic pits. Opportunity went south: it never would have survived pits going east. • Perhaps large-crater erosion, then small-crater re-cratering …but endogenic pits are confused with impact craters

17. Conclusions • Variety of small-scale processes cause cratering and degradation of craters • Global-scale inferences will be confused by isolated high-res studies • There is little correlation between Mariner/ Viking geology and what’s happening on a human scale • Statistical studies of degradation classes can reveal relevant processes • “Automated assistant” can accelerate productivity of a human analyst