Clark R. Chapman Clark R. Chapman (SwRI),(SwRI), R.G. Strom, J.W. Head, C.I. Fassett, W.J. Merline, S.C. R.G. Strom, J.W. Head, C.I. Fassett, W.J. Merline, S.C.

Solomon, D.T. Blewett, T.R. WattersSolomon, D.T. Blewett, T.R. Watters

Geological Society of America Annual Meeting,Geological Society of America Annual Meeting,Session P4: “1Session P4: “1stst Global View of the Geology of Mercury” Global View of the Geology of Mercury”

Portland, Oregon, 20 October 2009Portland, Oregon, 20 October 2009

Geological Society of America Annual Meeting,Geological Society of America Annual Meeting,Session P4: “1Session P4: “1stst Global View of the Geology of Mercury” Global View of the Geology of Mercury”

Portland, Oregon, 20 October 2009Portland, Oregon, 20 October 2009

Cratering on MercuryCratering on Mercury

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.)

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

Mercury’s Crater PopulationsMercury’s Crater Populations

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)

Lunar Absolute Chronology. South Pole-Aitken (oldest basin), Orientale (youngest basin)

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?

Apollo/Luna samples Apollo/Luna samples have dated some basins have dated some basins and maria between 3.9 and maria between 3.9 and 3.0 Ga.and 3.0 Ga.

Mercury’s Geological History Determined from Crater Record

First Goal: Determine the relative stratigraphic history from superimposed crater densities.

Second Goal: Determine the absolute geological chronology.

Most visible lunar basins formed during the latter part of the Late Heavy Bombardment (LHB) or “Cataclysm” (Strom et al. 2006)

ApproachApproach

First, measure crater size-frequency distri-First, measure crater size-frequency distri-butions (SFDs) on various geological units.butions (SFDs) on various geological units.

Determine spatial densities of craters, Determine spatial densities of craters, emphasizing larger craters, which are less emphasizing larger craters, which are less likely to be likely to be secondariessecondaries (temporally/spatially variable)..

Interpret the Interpret the relative relative stratigraphic ages in stratigraphic ages in terms of terms of absoluteabsolute ages by applying models ages by applying models (e.g. lunar cratering chronology, modified (e.g. lunar cratering chronology, modified by differences in Moon/Mercury cratering by differences in Moon/Mercury cratering flux and other geophysical or dynamical flux and other geophysical or dynamical constraints).constraints).

Smooth Plains West of Caloris: Craters, “Hills”

~ 770 craters, ~ 770 craters, greengreen ~ 190 positive relief features (PRFs), ~ 190 positive relief features (PRFs), yellowyellow

(Small Craters)

R-Plots of SFDs for Small Craters on Four M1 Flyby Frames

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 (pinkpink)

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

This “R-Plot” is a differential size-frequency plot This “R-Plot” is a differential size-frequency plot divided by Ddivided by D-3 -3 such that the vertical axis shows log such that the vertical axis shows log of “spatial density” (vs. log diameter).of “spatial density” (vs. log diameter).

Interpretation Framework: Impactors (Strom et al., 2005)

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

Late LHB = Population 1 = Main-Belt Asteroids

As LHB declines, cratering by modern NEAs dominates = Population 2

Interpretational Framework: Cratering Components

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

Caloris Exterior Plains ~25% Younger than Interior Plains

Important resultImportant result: If exterior plains are even younger than : If exterior plains are even younger than the Caloris interior plains, then they are certainly volcanic the Caloris interior plains, then they are certainly volcanic flows. Thus the interpretation of knobby texture of the Odin flows. Thus the interpretation of knobby texture of the Odin Formation as Cayley-Plains-like Caloris ejecta is wrong.Formation as Cayley-Plains-like Caloris ejecta is wrong.

Caloris BasinCaloris Basin

“Twin” Young Basins on Mercury

Both basins ~260 km diam. Similar inner peak rings Lightly cratered floors with

circumferential extensional troughs

Similar rim morphologies

Newly Seen Basin Revealed on M3 FlybyNewly Seen Basin Revealed on M3 Flyby

Raditladi Basin Seen on M1 FlybyRaditladi Basin Seen on M1 Flyby

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)

Ejecta and Secondary Craters of Raditladi and its “Twin”; Volcanically Active Region?

Raditladi BasinRaditladi Basin

Newly Seen “Twin” BasinNewly 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 km100 km

Craters on Floor of “Twin” Basin

Craters on Floor of Rembrandt

New Basin Floor Crater Data

Issues

D

Diam. (km)

Rembrandt Raditladi floor

“Twin” outer floor

“Twin” inner floor

No secondaries, poor statistics

8 170 (40) 70 (0)

Better statistics, possible secondary contamination

5 4500 (40) 140 (<40)

Near/below resolution limit, good statistics, secondaries probably dominate

2.5 X 500 1100 350

Summary: Relative Density

0.3 0.01 0.02 0.007

Cumulative # craters > D per million sq. km.Cumulative # craters > D per million sq. km.

Caveat! Small craters may be non-uniform secondaries!Caveat! Small craters may be non-uniform secondaries!Caveat! Small craters may be non-uniform secondaries!Caveat! Small craters may be non-uniform secondaries!

Preliminary

Basins: Approx. Relative Stratigraphy

1.01.0: Highlands craters 0.50.5: Caloris rim = Rembrandt rim

[note poor statistics: same to within 50%]

0.30.3: Floor of Rembrandt 0.10.1: Floor of Caloris (volcanic)

0.080.08: Caloris exterior plains (volcanic)

0.020.02: Outer floor of “Twin” 0.010.01: Floor of Raditladi = rim of

Raditladi (is floor recent volcanism or impact melt?)

0.0070.007: Inner floor of “Twin” (unexpectedly recent volcanism)

Relative Crater DensityRelative Crater Density (varies by factor >100!)

Intercrater Plains (Strom, 1977)

Deficiency of smaller Mercurian craters due to plains volcanism

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 (?)

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

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 arrowsred arrows) If smooth plains formed ~3.75 Ga then

Raditladi’s age is <1 Ga! (green green arrowsarrows)Preferred!Preferred!

Possible Role of Vulcanoids

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 looking during spacecraft’s perihelia passages]; but their absence today wouldn’t negate their possible earlier presence

Vulcanoid belt?Vulcanoid belt?

♀♀

♂♂♂♂

☼☼☼☼

Jupiter orbit

Asteroid belt

Two Chronologies for Mercury

4.5 4 3.5 3 2.5 2 1.5 1 0.5 NOW

Formation to magma ocean/crustal solidificationFormation to magma ocean/crustal solidification

Bombardment, LHB, intercrater plains formationBombardment, LHB, intercrater plains formation

Smooth plains volcanism “Twin” plainsSmooth plains volcanism “Twin” plains

Cratering, raysCratering, raysLobate scarps, crustal shorteningLobate scarps, crustal shortening

Formation to magma ocean solidificationFormation to magma ocean solidification

Bombardment, LHBBombardment, LHB

Vulcanoid bombardment, intercrater plainsVulcanoid bombardment, intercrater plainsSmooth plains volcanism “Twin”…Smooth plains volcanism “Twin”…

Cratering, ray formationCratering, ray formation

Lobate scarps, crustal shorteningLobate scarps, crustal shortening

Classical (Lunar) ChronologyClassical (Lunar) Chronology

Vulcanoid Chronology ExampleVulcanoid Chronology Example

Age before present, Ga

CCAALLOORRIISS

CCAALLOORRIISS

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?

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