Lunar Prospector Magnetometer Research Articles

A small lunar swirl and its implications for the formation of the Reiner Gamma magnetic anomaly

The Moon does not currently possess a dynamo, but its crust contains numerous magnetic anomalies detected from orbit. The geologic origins of these anomalies are still unknown, including the archetypal Reiner Gamma magnetic anomaly. To gain insight, we study a small magnetic anomaly, herein called the octopus, which is possibly associated with Reiner Gamma. The octopus has curving bright albedo patterns characteristic of features known as swirls. We use high cadence 9 Hz Lunar Prospector magnetometer data, along with constraints provided by this swirl's albedo pattern, to perform inversions for the swirl's magnetic source body characteristics. We use three different inversion methods, and they all return similar results. We also estimate the depth of magnetization from characteristics of the horizontal component of the magnetic field and the albedo pattern. We find that performing inversion for source body properties at small swirls has advantages compared to larger anomalies, or anomalies without albedo markings. We find the octopus is magnetized in the same direction as the main Reiner Gamma anomaly (within 1σ uncertainties), suggesting they formed contemporaneously. The large spatial distance between these coeval anomalies and their inferred shallow source body depths are compatible with formation in a hot ejecta deposit that cooled in the presence of a dynamo field, as suggested by Hood et al. (2001). However, a key remaining enigma is why the northeastern Reiner Gamma “tail” formation has a magnetization direction ∼60° different from the main body and octopus.

Origin of strong lunar magnetic anomalies: Further mapping and examinations of LROC imagery in regions antipodal to young large impact basins

The existence of magnetization signatures and landform modification antipodal to young lunar impact basins is investigated further by (a) producing more detailed regional crustal magnetic field maps at low altitudes using Lunar Prospector magnetometer data; and (b) examining Lunar Reconnaissance Orbiter Wide Angle Camera imagery. Of the eight youngest lunar basins, five are found to have concentrations of relatively strong magnetic anomalies centered within 10° of their antipodes. This includes the polar Schrödinger basin, which is one of the three youngest basins and has not previously been investigated in this context. Unusual terrain is also extensively present near the antipodes of the two largest basins (Orientale and Imbrium) while less pronounced manifestations of this terrain may be present near the antipodes of Serenitatis and Schrödinger. The area near the Imbrium antipode is characterized by enhanced surface thorium abundances, which may be a consequence of antipodal deposition of ejecta from Imbrium. The remaining three basins either have antipodal regions that have been heavily modified by later events (Hertzsprung and Bailly) or are not clearly recognized to be a true basin (Sikorsky‐Rittenhouse). The most probable source of the Descartes anomaly, which is the strongest isolated magnetic anomaly, is the hilly and furrowed Descartes terrain near the Apollo 16 landing site, which has been inferred to consist of basin ejecta, probably from Imbrium according to one recent sample study. A model for the origin of both the modified landforms and the magnetization signatures near lunar basin antipodes involving shock effects of converging ejecta impacts is discussed.

Magnetic field direction and lunar swirl morphology: Insights from Airy and Reiner Gamma

Many of the Moon's crustal magnetic anomalies are accompanied by high albedo features known as swirls. A leading hypothesis suggests that swirls are formed where crustal magnetic anomalies, acting as mini magnetospheres, shield portions of the surface from the darkening effects of solar wind ion bombardment, thereby leaving patches that appear bright compared with their surroundings. If this hypothesis is correct, then magnetic field direction should influence swirl morphology. Using Lunar Prospector magnetometer data and Clementine reflectance mosaics, we find evidence that bright regions correspond with dominantly horizontal magnetic fields at Reiner Gamma and that vertical magnetic fields are associated with the intraswirl dark lane at Airy. We use a genetic search algorithm to model the distributions of magnetic source material at both anomalies, and we show that source models constrained by the observed albedo pattern (i.e., strongly horizontal surface fields in bright areas, vertical surface fields in dark lanes) produce fields that are consistent with the Lunar Prospector magnetometer measurements. These findings support the solar wind deflection hypothesis and may help to explain not only the general form of swirls, but also the finer aspects of their morphology. Our source models may also be used to make quantitative predictions of the near surface magnetic field, which must ultimately be tested with very low altitude spacecraft measurements. If our predictions are correct, our models could have implications for the structure of the underlying magnetic material and the nature of the magnetizing field.

Magnetic signature of the lunar South Pole‐Aitken basin: Character, origin, and age

A new magnetic map of the Moon, based on Lunar Prospector magnetometer observations, sheds light on the origin of the South Pole‐Aitken basin (SPA), the largest and oldest of the recognized lunar basins. A set of WNW‐trending linear to arcuate magnetic features, evident in both the radial and scalar observations, covers much of a 1000 km wide region centered on the NW portion of SPA. The source bodies are not at the surface because the magnetic features show no first‐order correspondence to any surface topographic or structural feature. Patchy mare basalts of possible late Imbrian‐age are emplaced within SPA and are inferred to have been emplaced through dikes, directly from mantle sources. We infer that the magnetic features represent dike swarms that served as feeders for these mare basalts, as evident from the location of the Thomson/Mare Ingenii, Van de Graaff, and Leeuwenhoek mare basalts on the two largest magnetic features in the region. Modeling suggests that the dike zone is between 25 and 50 km wide at the surface, and dike magnetization contrasts are in the range of 0.2 A/m. We theorize that the basaltic dikes were emplaced in the lunar crust when a long‐lived dynamo was active. Based on pressure, temperature, and stress conditions prevalent in the lunar crust, dikes are expected to be a dominantly subsurface phenomenon, consistent with the observations reported here.

Lunar swirls: Examining crustal magnetic anomalies and space weathering trends

[1] We have used multispectral images from Clementine and data from Lunar Prospector's magnetometer to conduct a survey of lunar crustal magnetic anomalies, prominent lunar swirls, and lesser known swirl markings to provide new information on the nature of swirls and their association with magnetic anomalies. We find that all swirls and swirl-like albedo patterns are associated with areas of magnetized crust, but not all areas of magnetized crust are colocated with swirl-like albedo anomalies. All observed swirls exhibit spectral characteristics similar to immature material and generally have slightly lower FeO values compared with their surroundings as determined with a multispectral iron-mapping method. We discuss these results in relation to the various hypotheses for swirl formation. The comet impact hypothesis for lunar swirls would not predict a difference in the spectrally determined FeO content between swirls and nearby ordinary surfaces. The compositional difference could be explained as a consequence of (1) magnetic shielding of the surface from the solar wind, which could produce anomalous space weathering (little darkening with limited reddening) and potentially alter the predictions of the multispectral iron-mapping algorithm while the compositional contrast could be enhanced by delivery of lower-FeO ejecta from outside the swirl; and (2) accumulation of fine plagioclase-rich dust moving under the influence of electric fields induced by solar wind interactions with a magnetic anomaly. Therefore, we cannot at present clearly distinguish between the solar wind shielding and electrostatic dust accumulation models for swirl formation. We describe future measurements that could contribute to solution of the puzzle of swirl origin.

A global model of the internal magnetic field of the Moon based on Lunar Prospector magnetometer observations

A preliminary model of the internal magnetic field of the Moon is developed using a novel, correlative technique on the low-altitude Lunar Prospector magnetic field observations. Subsequent to the removal of a simple model of the external field, an internal dipole model is developed for each pole-to-pole half-orbit. This internal dipole model exploits Lunar Prospector's orbit geometry and incorporates radial and theta vector component data from immediately adjacent passes into the model. These adjacent passes are closely separated in space and time and are thus characteristic of a particular lunar regime (wake, solar wind, magnetotail, magnetosheath) or regimes. Each dipole model thus represents the correlative parts of three adjacent passes, and provides an analytic means of continuing the data to a constant surface of 30 km above the mean lunar radius. The altitude-normalized radial field from the wake and tail regimes is used to build a model in which 99.2% of the 360 by 360 bins covering the lunar surface are filled. This global model of the radial magnetic field is used to construct a degree 178 spherical harmonic model of the field via the Driscoll and Healy sampling theorem. Terms below about degree 150 are robust, and polar regions are considered to be the least reliable. The model resolves additional detail in the low magnetic field regions of the Imbrium and Orientale basins, and also in the four anomaly clusters antipodal to the large lunar basins. The model will be of use in understanding the sources of the internal field, and as a first step in modeling the interaction of the internal field with the solar wind.

Equivalent source mapping of the lunar crustal magnetic field using ABIC

Abstract An objective scheme is presented for estimating the lunar crustal magnetic field from the LMAG (Lunar MAGnetometer) data of the SELENE (“KAGUYA”) spacecraft. Our scheme improves the equivalent source method in three respects. The first improvement is that the source calculation is performed simultaneously with detrending. The second is that a great number of magnetic charges (magnetic monopoles) are used as the equivalent sources. The third is that the distribution of the magnetic charges is detremined by the damped least squares method, and the optimum smoothness is determined objectively by minimizing Akaike’s Bayesian Information Criterion (ABIC). For testing the scheme, we apply it to the Lunar Prospector magnetometer data in the region centered at the Reiner Gamma magnetic anomaly. The magnetic field map at an altitude of 20 km is stably drawn from datasets for different altitudes (18 km and 34 km). The ABIC minimizing criterion successfully controls the smoothness due to the numerical damping and extracts as much information as possible from the given data. This scheme will help produce a coherent lunar magnetic anomaly map by integrating the observations from various altitudes of the SELENE and previous missions.

A preliminary global map of the vector lunar crustal magnetic field based on Lunar Prospector magnetometer data

Previous processing of the Lunar Prospector magnetometer (LP‐MAG) data has yielded ∼40% coverage of the Moon. Here, new mapping of the low‐altitude LP‐MAG data is reported with the goal of producing the first global vector map of the lunar crustal magnetic field. By considering all data regardless of the external plasma environment and using less restrictive editing criteria, 2360 partial and complete passes have been identified that can be used to investigate the lunar crustal magnetic anomalies. The cleanest global coverage is provided using 329 low‐altitude nightside and terminator passes. An inverse power method has been used to continue the final mapping data to constant altitude. Using the 329 optimal passes, global maps of the lunar crustal magnetic field are constructed at 30 and 40 km. Consistent with previous studies: (1) the largest concentrations of anomalies are mapped antipodal to the Crisium, Serenitatis, Imbrium, and Orientale basins and (2) isolated anomalies at Reiner Gamma, Rima Sirsalis, Descartes, and Airy are mapped. Anomalies previously unmapped by the LP‐MAG experiment include (1) isolated anomalies near the craters Abel and Hartwig, (2) weak magnetization within the Nectarian‐aged Crisium and Moscoviense basins, and (3) a relatively weak anomaly in an area dominated by crater chains associated with the formation of Nectaris. Future work with the new low‐altitude data set is discussed and will include determining whether the lunar anomalies are capable of deflecting the solar wind and investigating directions of magnetization to evaluate a possible former core dynamo.

A magnetic anomaly associated with an albedo feature near Airy crater in the lunar nearside highlands

We describe a strong crustal magnetic anomaly, recently identified in Lunar Prospector magnetometer data, that is associated with a previously unreported albedo feature near the crater Airy in the lunar nearside highlands. Other workers have demonstrated a correlation between magnetic anomalies and the enigmatic bright markings known as lunar swirls. We have used Earth‐based telescopic spectra and Clementine multispectral images to investigate the compositional and optical maturity characteristics of the Airy swirl. The Airy albedo feature does not exhibit the complex sinuous structure of well‐known swirls such as the Reiner Gamma Formation, but does possess a bright loop and central dark lane. Another strong magnetic anomaly, in the Apollo 16/Descartes region, corresponds to a simple diffuse bright albedo spot. On this basis we suggest that a continuum of swirl morphologies exists on the Moon, with the Airy feature representing an intermediate or incipient swirl form.

Whistler waves observed near lunar crustal magnetic sources

We present Lunar Prospector Magnetometer (LP MAG) observations of monochromatic circularly polarized low frequency waves at frequencies of 0.4–4 Hz near the Moon. We observe such waves on about 6.6% of LP orbits in the solar wind, outside of the lunar wake. About a third of them occur on orbits when we also observe external magnetic enhancements (“limb shocks”), and most are clearly associated with lunar crustal magnetic sources, indicating an origin related to solar wind interaction with lunar magnetic fields. The inferred propagation vectors and polarizations of these waves are consistent with the expected characteristics of whistler‐mode waves. These observations could indicate either upstream whistler waves produced at shock surfaces above lunar crustal magnetic sources, or phase‐standing whistler wakes formed by direct solar wind interaction with lunar crustal magnetic fields.

Mini‐magnetosphere over the Reiner Gamma magnetic anomaly region on the Moon

We show presence of a mini‐magnetosphere above the Reiner Gamma magnetic anomaly (RGA) region in the solar wind, using Lunar Prospector magnetometer (MAG) measurement data. RGA is one of the strongest magnetic anomalies on the Moon. Two magnetic anomalies are found from six MAG datasets at 17–40 km altitudes in the lunar wake or the geomagnetic tail lobe and are well explained by a two‐dipole model. When RGA was exposed to the solar wind plasma, two MAG datasets were obtained at 27–29 km altitudes. Although the magnetic anomalies survived against the plasma pressure, they were heavily distorted in comparison with the magnetic field of the two‐dipole model. Flow directions and dynamic pressures of the solar wind plasma at those periods indicate that the distortions were caused by forming a mini‐magnetosphere over the RGA region in the solar wind.

Correlation of a strong lunar magnetic anomaly with a high‐albedo region of the Descartes mountains

Mapping and model simulations of Lunar Prospector magnetometer measurements show that the source of the strongest known magnetic anomaly on the lunar near side (42 nanoTeslas at 18.6 km altitude) coincides approximately with a high‐albedo region of the Descartes mountains centered 60 km south‐southeast of the Apollo 16 landing site. The Descartes mountains represent primary ejecta from one or more basin‐forming events (Imbrium and/or Nectaris), supporting the hypothesis that basin ejecta materials emplaced >3.8 Gyr ago are the main sources of lunar magnetic anomalies. The higher albedo of the surface at this location is consistent with a significant role for solar wind ions in the optical maturation (or “space weathering”) of the lunar surface.

Initial mapping and interpretation of lunar crustal magnetic anomalies using Lunar Prospector magnetometer data

Maps of relatively strong crustal magnetic field anomalies detected at low altitudes with the magnetometer instrument on Lunar Prospector are presented. On the lunar nearside, relatively strong anomalies are mapped over the Reiner Gamma Formation on western Oceanus Procellarum and over the Rima Sirsalis rille on the southwestern border of Oceanus Procellarum. The main Rima Sirsalis anomaly does not correlate well with the rille itself but is centered over an Imbrian‐aged smooth plains unit interpreted as primary or secondary basin ejecta. The stronger Reiner Gamma anomalies correlate with the locations of both the main Reiner Gamma albedo marking and its northeastward extension. Both the Rima Sirsalis and the Reiner Gamma anomalies are extended in directions approximately radial to the center of the Imbrium basin. This alignment suggests that Imbrium basin ejecta materials (lying in many cases beneath the visible mare surface) are the sources of the nearside anomalies. If so, then the albedo markings associated with the stronger Reiner Gamma anomalies may be consistent with a model involving magnetic shielding of freshly exposed mare materials from the solar wind ion bombardment. Two regions of extensive magnetic anomalies are mapped in regions centered on the Ingenii basin on the south central farside and near the crater Gerasimovic on the southeastern farside. These regions are approximately antipodal to the Imbrium and Crisium basins, respectively. The Imbrium antipode anomaly group is the most areally extensive on the Moon, while the largest anomaly in the Crisium antipode group is the strongest detected by the Lunar Prospector magnetometer. A consideration of the expected antipodal effects of basin‐forming impacts as well as a combination of sample data and orbital measurements on the nearside leads to the conclusion that the most probable sources of magnetic anomalies in these two regions are ejecta materials from the respective impacts. In both regions the strongest individual anomalies correlate with swirl‐like albedo markings of the Reiner Gamma class visible on available orbital photography.

Mapping of crustal magnetic anomalies on the lunar near side by the Lunar Prospector electron reflectometer

Lunar Prospector (LP) electron reflectometer measurements show that surface fields are generally weak in the large mare basalt filled impact basins on the near side but are stronger over highland terranes, especially those lying antipodal to young large impact basins. Between the Imbrium and Nectaris basins, many anomalies correlate with the Cayley and Descartes Formations. Statistical analyses show that the most strongly magnetic nearside terranes are Cayley‐type light plains, terra materials, and pre‐Imbrian craters. Light plains and terrae include basin impact ejecta as a major component, suggesting that magnetization effects from basin‐forming impacts were involved in their formation. The magnetization of pre‐Imbrian craters, however, may be evidence of early thermal remanence. Relatively strong, small‐scale magnetic anomalies are present over the Reiner Gamma feature on western Oceanus Procellarum and over the Rima Sirsalis rille on the southwestern border of Procellarum. Both Apollo subsatellite and LP data show that the latter anomaly is nearly aligned with the rille, though LP magnetometer and reflectometer data show that the anomaly peak is actually centered over a light plains unit. This anomaly and the Reiner Gamma anomaly are approximately radially aligned with the center of Imbrium, suggesting an association with ejecta from this basin.

Reply [to “Comment on “Initial measurements of the lunar induced magnetic dipole moment using Lunar Prospector Magnetometer data” by Hood et al.”

Reply [to “Comment on “Initial measurements of the lunar induced magnetic dipole moment using Lunar Prospector Magnetometer data” by Hood et al.”

Comment on “Initial measurements of the lunar induced magnetic dipole moment using Lunar Prospector Magnetometer data” by Hood et al.

Comment on “Initial measurements of the lunar induced magnetic dipole moment using Lunar Prospector Magnetometer data” by Hood et al.

Comment on “Initial measurements of the Lunar induced magnetic dipole moment using Lunar prospector magnetometer data” by Hood et al.

Comment on “Initial measurements of the Lunar induced magnetic dipole moment using Lunar prospector magnetometer data” by Hood et al.

Initial measurements of the lunar induced magnetic dipole moment using Lunar Prospector Magnetometer data

Twenty‐one orbits of Lunar Prospector magnetometer data obtained during an extended passage of the Moon through a lobe of the geomagnetic tail in April 1998 are applied to estimate the residual lunar induced magnetic dipole moment. Editing and averaging of individual orbit segments yields a negative induced moment with amplitude −2.4 ±1.6 × 1022 Gauss‐cm³ per Gauss of applied field. Assuming that the induced field is caused entirely by electrical currents near the surface of a highly electrically conducting metallic core, the preferred core radius is 340±90 km. For an iron‐rich composition, such a core would represent 1 to 3% of the lunar mass.