GRAIL Gravity Observations of the Transition from Complex Craters to Peak-Ring Basinson the Moon: Implications for Crustal Structure and Impact Basin Formation

David M.H. Baker1, James W. Head1, Roger J. Phillips2, Gregory A. Neumann3, David E. Smith3,4, and Maria T. Zuber3,4

Contact: david_baker@brown.edu

Author Affiliations:1Dept. of Geological Sciences, Brown University, Providence, RI.2Southwest Research Institute, Boulder, CO.3Solar System Exploration Division, NASA Goddard Space Flight

Center, Greenbelt, MD. 4Dept. of Earth, Atmospheric and Planetary Sciences, MIT,

Cambridge, MA.

[1] Pike, R.J. (1988) Mercury, Ch.8, 165–273. [2] Zuber, M.T. et al. (2013) Science, 339, 668–671. [3] Muller, P.M. and W.L. Sjogren (1968) Science, 161, 680-684. [4] Neumann, G.A., et al. (1996) J. Geophy. Res., 101(E7), 16,841–16,843. [5] Namiki, N. et al. (2009) Science, 323, 900-905. [6] Wieczorek, M.A. and R.J. Phillips (1999) Icarus, 139, 246-259. [7] Potter, R.K. et al. (2012) Geophys. Res. Lett., 39, L18203. [8] Christeson, G.L. et al. (2009) Earth Planet. Sci. Lett. 284, 249–257. [9] Wieczorek, M.A. et al. (2013) Science, 339, 671–675. [10] Head, J.W. et al. (2010) Science, 329, 1504–1507. [11] Baker, D.M.H. et al. (2011) Icarus, 214, 377–393. [12] Baker, D.M.H. et al. (2013) Planet. Space Sci., in review. [13] Collins, G.S. et al. (2002) Icarus, 157, 24–33. [14] Head, J.W. (2010) Geophys. Res. Lett. 37, L02203. [15] Baker, D.M.H. et al. (2012) Geol. Soc. Am. Conf., no. 224-10. [16] Melosh, H.J. et al. (2013) Science, 340, 1552-1555.

References

II. Average Profiles and Measurements of Major Features

ABOVE: Average grid values within concentric rings of 5 km widths (a) were used to generate radial profiles (b) for each crater and basin. Major profile features (1-4) were measured. Korolev peak-ring basin (center in (a)) is used as an example here.

BELOW: Distribution of peak-ring basins (N=15), protobasins (N=3), complex craters >100 km (N=73), and candidate peak-ring basins (N=18) analyzed. Basemap is GRAIL Bouguer anomaly at 7 to 560 degree expansion. Irregular white lines are 0 mGal BA contours.

Central Max.

1st Zero Crossing

2nd Zero Crossing

Interior Min.

peak-ring basincomplex crater protobasin

candidate peak-ring basin

I. Data

• GRAIL JGGRAIL_660C6A gravity model.• GRAIL gravity free-air anomaly and Bouguer anomaly coefficients used to generate grids at 4

pixel/degree with spherical harmonic expansions from degrees 16 to 310.• Moho relief and crustal thickness Model 1 from Wieczorek et al. [9].• Lunar Orbiter Laser Altimeter (LOLA) gridded topography data. • Catalogs of craters and basins, including rim-crest and peak-ring diameters from [10-12].

Dirichlet-Jackson

South Pole - Aitken

Korolev

Example: Korolev Peak-Ring Basin (417 km)

rim crest

peak ring

Introduction:• There is a progression of crater morphologies with increasing size on planetary bodies [e.g., 1]: simple crater (bowl-shaped) [ complex crater (central peak) [ protobasin (central peak + peak ring) [ peak-ring basin (peak ring) [ multi-ring basin (>2 rings).• Here, we are concerned with the transition from complex craters to basins on the Moon, as there is

still debate as to the processes forming basin rings.

Goals: • To characterize the evolution in gravity signatures in the transition from complex craters to peak-

ring basins on the Moon using new GRAIL gravity data [2].• Use GRAIL gravity to interpret and understand crustal structure in the crater to basin transition. • Present implications for basin and mascon formation on the Moon and planetary bodies.

Previous Work on Large Lunar Basins:• Free-AirAnomaly(FAA): “Mascon” basins have central positve free-air anomalies [3,4]. Central

FAAs also ringed by concentric negative then positive anomalies.• BouguerAnomaly(BA):Basins have central positive Bouguer anomalies surrounded by negative

BA annuli [4,5].• GeneralInterpretations: Central (super-isostatic) Moho uplift associated with central crustal thin-

ning. Annulus of thickened crust surrounding this central uplift [4,6]. • Numericalsimulations of impacts show Moho uplift and annular crustal thickening [e.g., 7].• Chicxulub peak-ring/multi-ring basin on Earth (~200 km diameter) also shows ~2 km of Moho

uplift and a slight annular crustal bulge [8].

What insights do smaller lunar peak-ring basins give us? Read on!...

Free-Air Anomalies(a) 9/15 peak-ring basins show positive central anomalies. Pro-tobasins are more irregular and most similar to complex craters.

(b) Central max. increases with crater size.(c) Interior min. has no trend with crater size.

(d) Complex craters do not show positive central anoma-lies. Profiles strongly corre-lated with topography.

Bouguer Anomalies(a) All peak-ring basins show pos. central anomalies confined within peak ring and neg. annuli between peak ring and 1.5R.. Protobasins are more irregular.

(b) Central max. increases lin-early with crater size starting at diameters ~150 km.(c) Interior min. increases in magnitude with crater size.

(d) Complex craters generally do not show positive central anomalies except in some transi-tional forms between diameters of 150-200 km.

IV. Moho Relief(a) All peak-ring basins show Moho uplift confined within peak ring and Moho downwarp-ing between peak ring and 1.5R.. Protobasins are more irregular.

(b) Central max. increases linearly with crater size.(c) Interior min. increases in magnitude with crater size.

(d) Complex craters generally do not show central mantle up-lift except for some examples in the 150-200 km diameter range.

V. Candidate Peak-Ring Basins

ABOVE• 18 candidate peak-ring basins identified.•Morphology: Have rim crests but no preserved

peak rings (due to proximity weathering, etc.).•Gravity/Moho: Have Bouguer anomaly (a) and

Moho relief (b) patterns similar to peak-ring basins on the Moon (see Sections III and IV).

RIGHT• Amundsen-Ganswindt (“A-G”, candidate peak-

ring basin, 377 km) and Schrödinger (peak-ring basin, 326 km).

•(a)Topography. A-G’s interior has been completely obscured by the Schrödinger impact event.

•(b)BouguerGravity. A-G’s subsurface structure appears to be still preserved. See its strong central positive Bouguer anomaly.

•(c)AverageBouguerAnomalyProfileof A-G. This is indistinguishable from peak-ring basins.

VII. Implications for Basin and Mascon FormationRoleof depthof transientcavity(dtc),depthof melting(dm),andcrustalthickness(h).

•BasinStructure: Central mantle uplifts and and crustal bulges are characteristic of basins from their onset; transitional craters occur between diameters of 150 to 200 km.

•Peak-RingFormation: The onset and diameters of central mantle/crustal uplift are correlated with the onset and diameters of peak rings. Peak-ring formation probably involves interaction between this uplift and the inward collapsing transient cavity [13-15].

•MasconFormation: Mascon-forming processes occur down to at least ~250 km in diameter with vestiges down to ~150 km in diameter. Use to test current models [e.g., 4, 16].

Crust

Mantle

III. Gravity Anomalies

VI. SUMMARY: Crustal Structure in the Complex Crater to Peak-Ring Basin Transition

• Irregular Bouguer anomaly patterns, with symmetrical, central positive anomalies generally absent.

• Some transitional craters with modest central Moho uplifts or uplifts of deep crustal density layers occur from 150-200 km in diameter.

• Central positive Bouguer anomalies with annuli of negative anomalies.

• Smallest basins do not have central positive free-air anomalies.

• Onset of well-defined central mantle uplift and annular “crustal bulge”.

• Strong central positive Bouguer anomalies with annuli of negative anomalies.

• Most large basins have central positive free-air anomalies.

• Tens of kilometers of central mantle uplift and 5-10 km thick crustal bulge.

Keeler (161 km)

d’Alembert (232 km)

Korolev (417 km)