Clark R. Chapman (SwRI) & R.G. Strom (Univ. Ariz.)

1. Clark R. Chapman (SwRI)& R.G. Strom (Univ. Ariz.) with help from Caleb Fassett, Jim Head, and others on the MESSENGER Science Team Chronology of Mercury Geological Units from Crater Size Frequency Data:  A View from MESSENGER Brown-Vernadsky Microsymposium #49 “Moon and Mercury” 21-22 March 2009, The Woodlands TX, USA

2. Outline of Talk • Brief introduction • Origins for craters on Mercury and the Moon • Relative chronology (stratigraphy) and absolute ages • Crater measurement methods, R-plot • Caloris basin: rim, interior plains, exterior plains • Caloris, Rembrandt, Raditladi basins compared • Intercrater plains • Absolute chronology • Classical lunar chronology (applied to Raditladi) • Hypothetical vulcanoid alternative • Comparison of two alternative chronologies

3. 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 • Basin ejecta plains (like Cayley plains on the Moon) • Volcanic lava flows (preferred origin, based on MESSENGER flyby analysis) • Secondary craters: chains and clusters of craters associated with large craters and basins

4. Stratigraphy/Chronology • Stratigraphy/relative age-dating • Cross-cutting relationships • Spatial densities of primary craters (absolute ages wrt 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)

5. 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?

6. 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).

7. Double-Ring Basin Raditladi (~260 km diam., like Bailly or Schickard on the Moon)

8. Raditladi: Measurements of Small Craters on its Flooded Floor and Ejecta Blanket • Part of ejecta deposit • (Excluded) • Part of smooth plains on basin floor

9. Smooth Plains West of Caloris: Craters, “Hills” (Small Craters) • ~ 770 craters, green • ~ 190 positive relief features (PRFs), yellow • Cluster of PRFs on right side of image: (a) lunar Marius Hills; (b) Odin/”Cayley Plains”

10. Caloris Interior and Exterior Plains (Large Craters) • Counts of craters >8 km diameter within plains units, both inside and exterior to Caloris • Counts from best images from Mariner 10 and first MESSENGER flyby 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). MESSENGER M1 Exterior Plains Interior Plains Exterior Plains Mariner 10

11. Caloris Interior Plains ~25% Older than Exterior Plains

12. Interpretation Framework (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

13. Determining Age from the shape (slope) of the SFD as a mixture of Pop. 1 and Pop. 2 • Assume that relative proportions of early Pop. 1 and later Pop. 2 changed on Mercury as observed on the Moon • Then (to the degree you believe fits and error bars) Caloris exterior plains post-date Caloris interior plains by both (a) slope and (b) R value (crater density)

14. Caloris Basin Cratering Stratigraphy (more on this later today by Caleb Fassett) Important issue raised by these results: If exterior plains are volcanic, then interpretation of knobby texture of Odin Formation as Cayley-Plains-like Caloris ejecta may be wrong. • 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 and exterior plains have low density, flat Pop. 2-dominated signature …so they formed mainly after the LHB had ended

15. Comparison: Caloris and Rembrandt Crater Densities Rembrandt Rembrandt (patch) • Spatial densities of large craters on two basins (above ↑) are similar • Spatial densities of smaller craters (← left) are similar…and much higher than on young Raditladi • Conclusion: Caloris and Rembrandt floors are of similar age

16. Basins: Approx. Relative Stratigraphy Relative Crater Density(varies by factor of 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) • 0.075: Caloris exterior plains (volcanic) • 0.01: Floor of Raditladi = rim of Raditladi (is floor recent volcanism or impact melt?) Densities of small craters Caveat! Small craters may be non-uniform secondaries!

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

18. 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 (?)

19. 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

20. Mercury’s Absolute Chronology: Raditladi Example (applying lunar chronology) • Sequence: Heavily cratered highlands → Intercrater plains → Caloris basin → Smooth plains → Raditladi basin/plains • 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!

21. 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 20 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

22. 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 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 Cratering, ray formation Lobate scarps, crustal shortening

23. 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

24. The End

27. Mercury: Cratering Components • New data consistent with M10 view: Pop. 1 (LHB), Pop. 2 (recent Near-Earth Asteroids) • Secondary branch upturn near 8 km (vs 2 km on Moon) • Variations in R near 2 km due to proportions of Pop. 1, chains, clusters • Smooth plains are ~25% younger than plains on floor of Caloris Population 1 Sample of MSGR cratered terrains more densely cratered than Mar. 10 avg. variable 2ndry SFD ? 25% older than Caloris plains the smooth exterior plains Population 2 Secondaries