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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

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lunar swirls enigma and opportunity
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

lunar swirls

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

hypotheses for the origin of swirls
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
objectives of this study
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, …
data sets
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
reiner gamma formation
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
descartes

100 km

Descartes
  • Peak strength is ~10 nT
  • Highland site
  • Smaller spatial extent of magnetic anomaly
  • Unusual diffuse bright patch
descartes 2
Descartes, 2
  • Note secondary magnetic anomaly of ~3 nT north of Apollo 16 (arrow)
  • No unusual albedo markings
near airy
near Airy
  • Strong magnetic anomaly, ~7 nT peak
  • Small spatial extent
  • Highland site
  • Bright loop with possible dark lane
near crozier
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

gerasimovich crisium antipode
Gerasimovich (Crisium Antipode)
  • Strong magnetic anomaly, ~15 nT peak
  • Moderate spatial extent
  • Highland site
  • Whispy bright patches
apollo basin serenitatis antipode
Apollo Basin(Serenitatis Antipode)
  • Strong magnetic anomaly,
  • ~10 nT peak
  • Smaller spatial extent
  • Highland site
  • Some bright whisps
mare ingenii imbrium antipode
Mare Ingenii (Imbrium Antipode)
  • Strong magnetic anomaly, ~11 nT peak
  • Mare/Highland site
  • Well developed swirls
  • Moderate spatial extent
mare moscoviense humorum antipode
Mare Moscoviense (Humorum Antipode)
  • No magnetic data available
  • Mare/Highland site
  • Moderately well developed swirls
  • Small spatial extent of albedo patches
mare marginis orientale antipode
Mare Marginis(Orientale Antipode)
  • Weak magnetic anomaly, <3 nT peak
  • Moderate spatial extent
  • Mare/Highland site
  • Moderate/well developed swirls
goddard a
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

goddard a17
Goddard A
  • Fan-shaped belt open to the east

AS16-121-19430

hopmann
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

a continuum of swirl types
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
spectral properties
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
enigma the swirl puzzle
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
the swirl puzzle 2
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.
the swirl puzzle 3
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?
opportunity swirls as a natural laboratory
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
the role of landed instruments
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

the role of landed instruments 2
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
the role of landed instruments 3
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
slide28
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