Early lunar geology and geophysics

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2009.Includes bibliographical references.Despite a number of human and robotic missions to the Moon, there are still important unanswered questions about its early evolution, and how it came to be the object we observe today. Here we use observational, experimental, and theoretical techniques to examine three important events that took place early in lunar history and have left a lasting signature. The first event is the formation of the largest basin on the Moon, the South Pole-Aitken Basin. We develop a systematic method to define the previously unknown boundaries of this degraded structure and quantify its gross shape. We also combine a number of remote sensing data sets to constrain the origin of heat producing elements in its interior. The second event we examine is the evolution of the lunar orbit, and the coupling between the Moon's early geophysical properties and the growth of orbital eccentricity. We use analytical models for tidal deformations and orbit evolution to show that the shape of the Moon suggests its early orbit was highly eccentric. However, we are also able to explain the presently high eccentricity entirely by traditional, secular tidal growth while the early Moon was hot. The third event we examine is the magnetization of lunar samples. We perform extensive paleomagnetic measurements of an ancient, deep-seated lunar sample, and determine that a long-lived magnetic field like that of a core dynamo is the most plausible explanation for its magnetic remanence. In sum, the earliest portion of lunar history has been largely obscured by later geologic events, but a great deal can still be learned from this formative epoch.by Ian Garrick-Bethell.Ph.D

Cross plane transfer of vestibular adaptation to human centrifugation

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 2004.Includes bibliographical references (p. 101-106).Human short-radius centrifugation (SRC) is being investigated as a volume-efficient means of delivering intermittent doses of "artificial gravity" to counter the deleterious effects of long exposures to weightlessness. Rotation rates on short radius centrifuges are high to provide the needed g-loading, and therefore entail a variety of unusual vestibular stimuli when certain head movements are made. Since these movements can elicit inappropriate nystagmus, illusions of tumbling, and motion sickness, efforts have been made to adapt people to the stimuli. So far these efforts have been successful in showing that people will adapt to at least one plane of head motion, the yaw (transverse) plane, during supine head-on-axis rotation. However, astronauts must be adapted to all planes of head motion if they are to function normally on the centrifuge. If adaptation to yaw head turns transferred to some extent to pitch (sagittal) plane turns, or any other plane of motion, it would greatly simplify and hasten the adaptation process. To investigate if transfer of adaptation across planes is possible, 10 subjects in the Experimental Group performed a sufficient number of yaw plane head turns to demonstrate adaptation. Adaptation was indicated by decreases in metrics of the off-axis vestibuloocular reflex induced by the head turns, and by subjective ratings of illusory motion. A block of pitch movements was performed before and after the yaw movements, and these two pitch blocks were compared to assess how much adaptation to pitch head turns had taken place. The same procedure was followed on a subsequent day. A Control Group of 10 subjects performed only the blocks of pitch turns, and their adaptation was compared to the adaptation to pitch turns measured in the Experimental(cont.) Group. While both Control and Experimental Groups showed statistically significant signs of adaptation to pitch head turns, we failed to find any significant differences between the amounts of adaptation. If true, this result implies that adaptation to SRC may need to be performed one plane of motion at a time. Additionally, it implies that the brain and vestibular system does not build up a generalized model of SRC stimulation, but rather builds adaptation one input at a time.by Ian Garrick-Bethell.S.M

Further evidence for early lunar magnetism from troctolite 76535

The earliest history of the lunar dynamo is largely unknown and has important implications for the thermal state of the Moon and the physics of dynamo generation. The lunar sample with the oldest known paleomagnetic record is the 4.25 billion year old (Ga) troctolite 76535. Previous studies of unoriented subsamples of 76535 found evidence for a dynamo field with a paleointensity of several tens of microteslas. However, the lack of mutual subsample orientation prevented a demonstration that the magnetization was unidirectional, a key property of thermoremanent magnetization. Here we report further alternating field demagnetization on three mutually oriented subsamples of 76535, as well as new pressure remanent magnetization experiments to help rule out shock magnetization. We also describe new 40Ar/39Ar thermochronometry and cosmogenic neon measurements that better constrain the rock's thermal history. Although the rock is unbrecciated, unshocked, and slowly cooled, its demagnetization behavior is not ideal due to spurious remanence acquisition. Despite this limitation, all three subsamples record a high coercivity magnetization oriented in nearly the same direction, implying that they were magnetized by a unidirectional field on the Moon. We find no evidence for shock remanence, and our thermochronometry calculations show no significant reheating events since 4249 ± 12 million years ago (Ma). We infer a field paleointensity of approximately 20–40 μT, supporting the previous conclusion that a lunar dynamo existed at 4.25 Ga. The timing of this field supports an early dynamo powered by thermal or thermochemical core convection and/or a mechanical dynamo but marginally excludes a dynamo delayed by thermal blanketing from radiogenic element-rich magma ocean cumulates

Parallel Session 3: Lunar Geophysics & Magnetics Overview Talk

No abstract availabl

On the application of magnetic methods for the characterisation of space weathering products

Space weathering is now commonly accepted to modify the optical and magnetic properties of airless body regoliths throughout the Solar System. Although the precise formation processes are not well understood, the presence of ubiquitous sub-microscopic metallic iron (SMFe) grains in lunar soils and corresponding spectral analyses have explained both the unique optical and magnetic properties of such soils. More recently, a variety of ion irradiation, laser melting and vaporisation and impact experiments have been shown to reproduce these effects in the laboratory. Such experiments are crucial to the study of the formation of SMFe under controlled conditions. To date, more emphasis has been placed on optical analyses of laboratory samples, as these address directly the mineralogical interpretation of remote sensing data. However, the magnetic analyses performed on the Apollo and Luna samples have provided useful qualitative and quantitative evaluation of regolith metallic iron content. These techniques are reviewed here, demonstrated on pulsed laser irradiated olivine powder, and their utility for determining the quantity and size distribution of this metallic iron discussed. Ferromagnetic resonance, multi- frequency magnetic susceptibility, vibrating sample magnetometry and thermomagnetic measurements were carried out. Each showed trends expected for the conversion of paramagnetic Fe2+ in olivine to fine-grained Fe0, with some grains in the superparamagnetic size range. Although evidence for super- paramagnetic iron was found, the quantity of sub-microscopic metallic iron produced in these experiments proved insufficient to make conclusive measurements of either the quantity or size distribution of this iron. Improvements to both the experimental and analytical procedures are discussed to better enable such measurements in the future

The Tidal-Rotational Shape of the Moon and Evidence for Polar Wander

The origin of the Moon's large-scale topography is important for understanding lunar geology, lunar orbital evolution, and the Moon's orientation in the sky.  Previous hypotheses for its origin have included late accretion events, large impacts, tidal effects, and convection processes.  However, testing these hypotheses and quantifying the Moon's topography is complicated by the large basins that have formed since the crust crystallized.  Here we estimate the low-order lunar topography and gravity spherical harmonics outside these basins and show that the bulk of the degree-2 topography is consistent with a crust-building process controlled by early tidal heating throughout the Moon.  The remainder of the degree-2 topography is consistent with a frozen tidal-rotational bulge that formed later, at a semi-major axis of »32 Earth radii.  The probability of the degree-2 shape having these two separate tidal characteristics by chance is less than 1%.  We also infer that internal density contrasts eventually reoriented the Moon's polar axis 36 ± 4°, to the present configuration we observe today.  Together, these results link the geology of the near and far sides, and resolve long-standing questions about the Moon's low-order shape, gravity, and history of polar wander

The Tidal-Rotational Shape of the Moon and Evidence for Polar Wander

The origin of the Moon's large-scale topography is important for understanding lunar geology, lunar orbital evolution, and the Moon's orientation in the sky.  Previous hypotheses for its origin have included late accretion events, large impacts, tidal effects, and convection processes.  However, testing these hypotheses and quantifying the Moon's topography is complicated by the large basins that have formed since the crust crystallized.  Here we estimate the low-order lunar topography and gravity spherical harmonics outside these basins and show that the bulk of the degree-2 topography is consistent with a crust-building process controlled by early tidal heating throughout the Moon.  The remainder of the degree-2 topography is consistent with a frozen tidal-rotational bulge that formed later, at a semi-major axis of »32 Earth radii.  The probability of the degree-2 shape having these two separate tidal characteristics by chance is less than 1%.  We also infer that internal density contrasts eventually reoriented the Moon's polar axis 36 ± 4°, to the present configuration we observe today.  Together, these results link the geology of the near and far sides, and resolve long-standing questions about the Moon's low-order shape, gravity, and history of polar wander