Clark R. Chapman (SwRI),

1. Clark R. Chapman (SwRI), R.G. Strom, J.W. Head, C.I. Fassett, W.J. Merline, S.C. Solomon, D.T. Blewett, T.R. Watters Cratering on Mercury Geological Society of America Annual Meeting, Session P4: “1st Global View of the Geology of Mercury” Portland, Oregon, 20 October 2009

2. Origins of Craters on the Moon & Mercury • Primary impact cratering • High-velocity comets(sun-grazers, Jup.-family, long-period) • Near-Earth, Aten, and Inter-Earth asteroids • Ancient, possibly depleted, impactor populations(accretionary remnants, Late Heavy Bombardment, vulcanoids) • Secondary cratering(<8 km diameter, + basin secondaries) • Endogenic craters(volcanism, etc.) Mercury’s Crater Populations • Basins: dozens of multi-hundred km peak-ring and multi-ring basins tentatively identified by Mariner 10 (lower bound due to 45% coverage and high sun) • Highlands craters: like heavily cratered terrains on the Moon, but fewer craters <40 km diameter (due to embayment by widespread “intercrater plains,” which may simply be older “smooth plains”) • Lighter cratering of younger “smooth plains.” 2 alternatives for plains: • Basin ejecta plains (like Cayley plains on the Moon) • Volcanic lava flows (preferred origin, based on analysis of 3 MESSENGER flybys) • Secondary craters: chains and clusters of small craters (<8 km diameter) associated with large craters and basins

3. Stratigraphy/Chronology • Stratigraphy/relative age-dating • Cross-cutting relationships • Spatial densities of primary craters (absolute ages relative to cratering rate) • Absolute chronology • On the Moon, crater densities calibrated by dated samples with specific geologic associations with counting surfaces • On Mercury, it is difficult and indirect • Classic approach: assume cratering rate changed with time just as on the Moon and that sources were the same as on the Moon (with minor adjustments, e.g. for higher vel.) • Direct approach: use known impact rates of asteroids/comets (only good to factor of 2 and only for recent epochs)

4. Lunar Absolute Chronology. South Pole-Aitken (oldest basin), Orientale (youngest basin) Apollo/Luna samples have dated some basins and maria between 3.9 and 3.0 Ga. • South-Pole Aitken is relatively old and very large. Is its age 4.3 or 4.0 Ga? • Orientale is the youngest basin. But is its age 3.72 or 3.84 Ga?

5. Mercury’s Geological History Determined from Crater Record Most visible lunar basins formed during the latter part of the Late Heavy Bombardment (LHB) or “Cataclysm”(Strom et al. 2006) • First Goal: Determine the relative stratigraphic history from superimposed crater densities. • Second Goal: Determine the absolute geological chronology. Approach First, measure crater size-frequency distri-butions (SFDs) on various geological units. Determine spatial densities of craters, emphasizing larger craters, which are less likely to be secondaries(temporally/spatially variable). Interpret the relative stratigraphic ages in terms of absoluteages by applying models (e.g. lunar cratering chronology, modified by differences in Moon/Mercury cratering flux and other geophysical or dynamical constraints).

6. Smooth Plains West of Caloris: Craters, “Hills” (Small Craters) • ~ 770 craters, green • ~ 190 positive relief features (PRFs), yellow

7. R-Plots of SFDs for Small Craters on Four M1 Flyby Frames This “R-Plot” is a differential size-frequency plot divided by D-3 such that the vertical axis shows log of “spatial density” (vs. log diameter). • Statistics are poor at D>10 km, but cratered terrain is oldest, with order-of-magnitude more craters than on floor of the Raditladi basin • Slopes of SFDs for craters <10 km vary regionally; perhaps due to varying contributions of the very steep SFD for secondaries (pink) • Craters reach empirical saturation densities at large diameters in heavily cratered terrain and at diameters < a few km in the heavily cratered terrain and in a region rich in secondary craters • Note extreme youth of Raditladi double-ring basin

8. Interpretation Framework: Impactors (Strom et al., 2005) Late LHB = Population 1 = Main-Belt Asteroids As LHB declines, cratering by modern NEAs dominates = Population 2 • Shape of main-belt asteroid SFD matches lunar highland craters • Shape of NEA SFD matches lunar maria craters • Size-selective processes bring NEAs from main belt to Earth/Moon • A solely gravitational process bringing main-belt asteroids into Earth-crossing orbits could produce highland SFD (e.g. resonance sweeping) • The “Nice Model” could produce a comet shower followed by an asteroid shower yielding the LHB Pop. 1 Pop. 2

9. Interpretational Framework: Cratering Components

10. Caloris Basin Cratering Stratigraphy • Caloris mountains on rim (measured by Caleb Fassett) show old, Pop. 1 signature • Crater density much higher than on plains • SFD shape resembles Pop. 1 on highlands of Moon and Mercury • Hence interior plains must have later volcanic origin, cannot be contemporaneous impact melt (other evidence) • Interior plains have low density, flat Pop. 2-dominated signature …so they formed mainly after the LHB had ended

11. Caloris Exterior Plains ~25% Younger than Interior Plains Important result: If exterior plains are even younger than the Caloris interior plains, then they are certainly volcanic flows. Thus the interpretation of knobby texture of the Odin Formation as Cayley-Plains-like Caloris ejecta is wrong. Caloris Basin

12. “Twin” Young Basins on Mercury • Both basins ~260 km diam. • Similar inner peak rings • Lightly cratered floors with circumferential extensional troughs • Similar rim morphologies Raditladi Basin Seen on M1 Flyby Newly Seen Basin Revealed on M3 Flyby

13. A Closer Look at the Newly Seen “Twin” Basin • Compare very low crater density inside peak ring with slightly higher crater density between peak ring and rim • Lighter colored interior floor has breached peak ring on the bottom • Both basins have fairly young ejecta blankets and many surround-ing secondary craters (next slide)

14. Ejecta and Secondary Craters of Raditladi and its “Twin”; Volcanically Active Region? Raditladi Basin Newly Seen “Twin” Basin Note “orange” color within peak ring, like other young volcanic plains on Mercury. Also note the proximity of “Twin” basin to what may be a large volcanic vent (in the very bright region northeast of the basin). 100 km

15. Craters on Floor of “Twin” Basin

16. Craters on Floor of Rembrandt

17. New Basin Floor Crater Data Preliminary Caveat! Small craters may be non-uniform secondaries! Cumulative # craters > D per million sq. km.

18. Basins: Approx. Relative Stratigraphy Relative Crater Density(varies by factor >100!) • 1.0: Highlands craters • 0.5: Caloris rim = Rembrandt rim [note poor statistics: same to within 50%] • 0.3: Floor of Rembrandt • 0.1: Floor of Caloris(volcanic) • 0.08: Caloris exterior plains (volcanic) • 0.02: Outer floor of “Twin” • 0.01: Floor of Raditladi = rim of Raditladi(is floor recent volcanism or impact melt?) • 0.007: Inner floor of “Twin”(unexpectedly recent volcanism)

19. Intercrater Plains (Strom, 1977) Deficiency of smaller Mercurian craters due to plains volcanism

20. Intercrater Plains (Strom, 2009) • M1 approach mosaic • Mostly intercrater plains • Deficiency on Mercury <30 km diam. relative to Moon due to “flooding” of smaller craters by plains-forming volcanism (?)

21. Thicker Intercrater Plains (Strom, 2009) • M2 departure mosaic • Deficiency of craters <100 km diam. suggests thicker intercrater plains volcanism erased larger craters than in M1 approach mosaic

22. Mercury’s Absolute Chronology: Raditladi Example (applying lunar chronology) • Sequence: Heavily cratered highlands → Intercrater plains → Caloris basin → Smooth plains → Raditladi basin/plains → “Twin” interior floor • If lunar chronology applies, then • If smooth plains formed early (3.9 Ga), then Raditladi is 3.8 Ga (red arrows) • If smooth plains formed ~3.75 Ga then Raditladi’s age is <1 Ga! (green arrows) Preferred!

23. Possible Role of Vulcanoids Vulcanoid belt? • Zone interior to Mercury’s orbit is dynamically stable (like asteroid belt, Trojans, Kuiper Belt) • If planetesimals originally accreted there, mutual collisions may (or may not) have destroyed them • If they survived, Yarkovsky drift of >1 km bodies to impact Mercury could have taken several Gyr (Vokroulichy et al., 2000),cratering Mercury (alone) long after the LHB • That would compress Mercury’s geological chronology toward the present (e.g. thrust-faulting might be still ongoing) • Telescopic searches during last 25 years have not yet set stringent limits on current population of vulcanoids [MESSENGER is lookingduring spacecraft’s perihelia passages]; but their absence today wouldn’t negate their possible earlier presence ♂ ♀ ☼ Jupiter orbit Asteroid belt

24. Two Chronologies for Mercury Age before present, Ga 4.5 4 3.5 3 2.5 2 1.5 1 0.5 NOW Formation to magma ocean/crustal solidification CALORIS Bombardment, LHB, intercrater plains formation Smooth plains volcanism “Twin” plains Cratering, rays Lobate scarps, crustal shortening Classical (Lunar) Chronology Vulcanoid Chronology Example Formation to magma ocean solidification CALORIS Bombardment, LHB Vulcanoid bombardment, intercrater plains Smooth plains volcanism “Twin”… Cratering, ray formation Lobate scarps, crustal shortening

25. Some Important Cratering Issues • Are current production functions (and those in the past) the same on Mercury and the Moon? • What are relationships between “Class 1” fresh craters, rayed craters, and straigraphically young craters? • Are Mercury’s secondaries unusual? Why? • Are basins saturated, as Mariner 10 suggested? • Are intercrater plains simply older smooth plains? • Are there independent clues about absolute chronology?

26. Conclusion: We must wait for orbital mission for good stratigraphic studies • Mariner 10 imaged 45% of surface? (I don’t think so.) • MESSENGER has almost completed coverage? Not YET for robust geological analysis Mariner 10 Image & Shaded Relief MESSENGER image