Lunar Swirls: Enigma and Opportunity

1. Lunar Swirls:Enigma and Opportunity David T. Blewett Johns Hopkins Applied Physics Lab with contributions from B. Ray Hawke Hawaii Institute of Geophysics & Planetology University of Hawaii Nicola C. Richmond Planetary Science Institute, Tucson, Ariz. C. G. Hughes Department of Geology & Planetary Science University of Pittsburgh Apollo 16 pan: the Al Biruni swirl Goddard SFC 2008 Apr 17

2. Reiner (29 km) Lunar Swirls • Sinuous, high albedo markings • Appear to have very little topographic relief • Dark lanes sometimes found within the bright portions • Type example is Reiner Gamma Formation in Oceanus Procellarum Clementine pseudo true color composite

3. Hypotheses for the Origin of Swirls • Abnormal space weathering caused by magnetic anomaly • e.g. Hood and Williams (1989) • Many swirls associated with magnetic anomalies, e.g., near antipodes of major impact basins • Magnetic anomaly stands off solar wind, preventing regolith from undergoing normal weathering • Swirls are generally old features • Impact by comet or a meteoroid swarm • Schultz and Srnka (1980); Pinet et al. (2000); Starukhina and Shkuratov (2004) • Scouring/plowing of regolith by meteoroids, cometary nucleus fragments or gas/dust in the coma • Magnetic anomaly possibly created by plasma effects in coma • Swirls are very young features

4. Objectives of this study • Lunar Prospector data has led to discovery of new magnetic anomalies. • Examine these for swirl-like tendencies • Compare field strength with morphology • Compare field strength to spectral properties. • Variables: mare vs. highland, field strength, spatial size, …

5. Data Sets • Clementine UVVIS image products • Lunar Prospector maps of total magnetic field strength (Richmond et al. 2005) • Magnetometer data from low-altitude portion of the mission • Magnetic anomaly values continued to a common height of 35.5 km

6. Reiner Gamma Formation • Contours of LP total magnetic field at 35.5 km alt. • Peak strength is ~7 nT • Mare site • Large spatial extent of both magnetic anomaly and albedo features

7. 100 km Descartes • Peak strength is ~10 nT • Highland site • Smaller spatial extent of magnetic anomaly • Unusual diffuse bright patch

8. Descartes, 2 • Note secondary magnetic anomaly of ~3 nT north of Apollo 16 (arrow) • No unusual albedo markings

9. near Airy • Strong magnetic anomaly, ~7 nT peak • Small spatial extent • Highland site • Bright loop with possible dark lane

10. near Crozier • Magnetic anomaly ~3 nT peak • Moderate spatial extent • Mare/Highland site • No albedo feature? Crozier, 22 km, 13.5 S, 50.8 E

11. Gerasimovich (Crisium Antipode) • Strong magnetic anomaly, ~15 nT peak • Moderate spatial extent • Highland site • Whispy bright patches

12. Apollo Basin(Serenitatis Antipode) • Strong magnetic anomaly, • ~10 nT peak • Smaller spatial extent • Highland site • Some bright whisps

13. Mare Ingenii (Imbrium Antipode) • Strong magnetic anomaly, ~11 nT peak • Mare/Highland site • Well developed swirls • Moderate spatial extent

14. Mare Moscoviense (Humorum Antipode) • No magnetic data available • Mare/Highland site • Moderately well developed swirls • Small spatial extent of albedo patches

15. Mare Marginis(Orientale Antipode) • Weak magnetic anomaly, <3 nT peak • Moderate spatial extent • Mare/Highland site • Moderate/well developed swirls

16. Goddard A • In Mare Marginis, near Orientale antipode • 11 km diameter impact crater, suggested by Schultz (1980) to have been formed by a comet nucleus LO4 18-H2

17. Goddard A • Fan-shaped belt open to the east AS16-121-19430

18. Hopmann • No magnetic data available • Mare/Highland site • Delicate moderately well developed swirl on mare fill • Airy-type loop just outside the crater • Small spatial extent Hopmann, 88 km diam, 50.8 S, 160.3 E

19. A Continuum of Swirl Types • There appears to be a progression in the morphology of albedo features associated with the magnetic anomalies. • Two endmembers • Diffuse bright spot • Fully-developed complex swirl

20. Spectral Properties • Bright portions of swirls have high UV/Vis ratios relative to the surroundings, corresponding to "bluer" color • Also have high values of the optical maturity parameter, consistent with the presence of fresher material

21. Enigma: The Swirl Puzzle • Interesting in themselves • May hold key to better understanding of space weathering • Lunar soils darken/redden with exposure through the production of nanophase metallic Fe blebs and coatings • Solar wind sputtering; implanted H may help to reduce Fe+2 to Fe0 • Is vapor deposition from micrometeorite impacts enough? • Magnetic shield would prevent solar wind implantation and sputtering, but would not screen out micrometeorites

22. The Swirl Puzzle, 2 • The Swirl – Magnetic Anomaly link is a good argument for the solar wind shielding model, but • Why do some areas with strong magnetic anomalies show little/no swirl markings? • Calculations by some authors suggest that the magnetic anomalies would not keep out the solar wind over time - leakage and saturation should occur. • It is not clear that the presence of implanted H is necessary to reduce FeO to nanophase Fe and hence produce the normal darkening and reddening effects of soil maturation.

23. The Swirl Puzzle, 3 • What are the sources of the magnetic anomalies? • Can the magnetic shielding hypothesis explain the occurrence of broad belts of swirls?

24. Opportunity:Swirls as a Natural Laboratory • The swirls offer a venue to examine key questions in several major areas of planetary science • Lunar geology: The origin of the lunar swirls • Planetary Magnetism: lunar dynamo / basin impact transient fields / comet-induced? • Remote Sensing of Airless Bodies: Space weathering complicates interpretation of remote sensing data for the Moon, Mercury, and asteroids. Swirls provide a control on one of the key variables: solar wind exposure

25. The Role of Landed Instruments • Surface magnetometer • Help to better determine the strength and depth of the source of the magnetic anomalies • Surficial: from comet impact • Shallow to several kilometers deep: magnetized basin ejecta • Source formed quickly, so transient fields could contribute • Deeper intrusions/crustal blocks • More likely that a long-lived dynamo produced the magnetization see also Richmond & Hood 2008 LPSC abstract

26. The Role of Landed Instruments, 2 • Solar Wind Spectrometer • Directly test the solar wind shielding model for the origin of the swirls • Flux, energy distribution reaching the surface • Variations with time • Mössbauer spectrometer • Abundance of nanophase Fe in the soils • UV-Vis-NIR spectrometer and/or multispectral camera • Ground truth to link the in situ measurements to orbital remote sensing

27. The Role of Landed Instruments, 3 • Best case: rover with magnetometer, solar wind spectrometer, camera, spectrometer, Mössbauer, XRF/XRD • A stationary package (such as ILN) should be targeted to at least one of the major magnetic/albedo anomalies and carry a magnetometer and solar wind spectrometer

28. The authors gratefully acknowledge financial support from NASA: Planetary Geology & Geophysics Program Discovery Data Analysis Program Ingenii swirl: R = 950/750, G = 900 nm, B = 415/750