Estimate of the magnetic field of Mars based on the magnetic characteristics of the Yamato 000593 nakhlite

Abstract— Yamato 000593, a nakhlite, was analyzed in terms of its magnetic record and magnetomineralogy. The natural remanent magnetization (NRM: 3.55–6.07 times 10^?5 Am²/kg) was thermally demagnetized at ～320 °C, and it was unstable against alternating field demagnetization. Based on analyses of thermomagnetic curves, the temperature dependence of hysteresis parameters, and microscopic observations, the magnetic minerals mainly consist of magnetite (0.68 wt% of the sample, including ～5% Fe₂TiO₄) of less than 100 μm in size, associated with minor amounts of monoclinic pyrrhotite (<0.069 wt% of the sample) and goethite. Thermal demagnetization of NRM at ～330 °C is explained due to an offset of magnetization of antipodal NRM components of magnetite, whereas it is not due to a pyrrhotite Curie point. Large magnetite grains show exsolution texture with ilmenite laths, and are cut by silicate (including goethite) veins that formed along cracks. Numerous single‐domain (SD) and pseudo‐single‐domain (PSD) magnetite grains are scattered in the mesostasis and adjacent olivine grains. Moderate coercive forces of H_C = 6.8 mT and H_RC = 31.1 mT suggest that Yamato 000593 is fundamentally able to carry a stable NRM; however, NRM was found to be unstable. Accordingly, the meteorite was possibly crystallized at 1.3 Ga under an extremely weak or absent magnetic field, or was demagnetized by impact shock at 12 Ma (ejection age) on Mars. This finding differs from the results of previous paleomagnetic studies of SNC (shergottites, nakhlites, chassignites, and orthopyroxenite) Martian meteorites. The significant dipole magnetic field resulting from the molten metallic core was probably absent during the Amazonian Epoch (after 1.8 Ga) on Mars.     

Aspects of the validation of magnetic remanence in meteorites

Abstract— Meteorite magnetic records constitute physical evidence of processes acting during early solar system evolution. Consequently, the validation of these records is important in meteorite research. The first step in the validation process should be the REM value. The REM value is the ratio of natural remanence (NRM) to saturation remanent magnetization imparted by a 1 T magnetic field (SIRM). The REM values range over 3 to 4 orders of magnitude for stony meteorites and for chondrules from Allende (C3V‐S1), Bjurböle (L4‐S1), and Chainpur (LL3‐S1) meteorites. The REM values computed from published NRM and SIRM data identify many orders of magnitude range in the REM values including REM values >100 × 10^?3. These data suggest a dependence for the NRM intensity on the curatorial location from which the sample was obtained. Any earth rock acquiring thermoremanent magnetization (TRM) in the geomagnetic field has a restricted range in REM mostly between 5 and 50 × 10^?3, the exception being the mineral hematite in the multidomain size range. The only terrestrial samples with REM much greater than 100 × 10^?3 are those struck by lightning. The REM value provides a physical basis for recognition between valid records and those that “might be contaminated.” The isothermal remanence acquisition (RA) curve is presented as a contamination curve that allows an indication of the level of magnetic field contamination required to give the computed “REM” (RM/SIRM) value. In the case of the Bjurböle and Chainpur chondrules, with REM values >100 × 10^?3, the RA curve indicates that unrealistically large contamination magnetic fields would be required to give REM values greater than 100 × 10^?3. This would suggest contamination other than by a hand magnet that is normally available to an experimenter. This would require an explanation that would involve large magnetic fields during chondrule formation, or some extraordinary remanence acquisition mechanism that remains to be described. Magnetic contamination experiments, using ～80 and ～40 mT magnets, demonstrate that the “REM” values and extent of modification of the magnetic vector record are mineralogy dependent, and this is mostly related to the amount and characteristics of the mineral tetrataenite. The complexity of the meteorite records suggest validation of the record as a first step. The REM value is the first physical statement that can be made in this validation.     

Metal phases in ordinary chondrites: Magnetic hysteresis properties and implications for thermal history

Magnetic properties are sensitive proxies to characterize FeNi metal phases in meteorites. We present a data set of magnetic hysteresis properties of 91 ordinary chondrite falls. We show that hysteresis properties are distinctive of individual meteorites while homogeneous among meteorite subsamples. Except for the most primitive chondrites, these properties can be explained by a mixture of multidomain kamacite that dominates the induced magnetism and tetrataenite (both in the cloudy zone as single‐domain grains, and as larger multidomain grains in plessite and in the rim of zoned taenite) dominates the remanent magnetism, in agreement with previous microscopic magnetic observations. The bulk metal contents derived from magnetic measurements are in agreement with those estimated previously from chemical analyses. We evidence a decreasing metal content with increasing petrologic type in ordinary chondrites, compatible with oxidation of metal during thermal metamorphism. Types 5 and 6 ordinary chondrites have higher tetrataenite content than type 4 chondrites. This is compatible with lower cooling rates in the 650–450 °C interval for higher petrographic types (consistent with an onion‐shell model), but is more likely the result of the oxidation of ordinary chondrites with increasing metamorphism. In equilibrated chondrites, shock‐related transient heating events above approximately 500 °C result in the disordering of tetrataenite and associated drastic change in magnetic properties. As a good indicator of the amount of tetrataenite, hysteresis properties are a very sensitive proxy of the thermal history of ordinary chondrites, revealing low cooling rates during thermal metamorphism and high cooling rates (e.g., following shock reheating or excavation after thermal metamorphism). Our data strengthen the view that the poor magnetic recording properties of multidomain kamacite and the secondary origin of tetrataenite make equilibrated ordinary chondrites challenging targets for paleomagnetic study.     

Natural remanent magnetization studies of a layered Breccia Boulder from the lunar highland region

The average directions of natural remanent magnetization (NRM) of three texturally distinct layers (72215, 72255, and 72275) of a 2 m-sized breccia boulder were found to be the same, while the directions of their stable components of NRM were found to be widely divergent. One clast from 72275 yielded a stable NRM direction which was different from that of the matrix. Approximate paleointensity measurements showed that 72255 and 72275 could have obtained their stable remanence from an ancient magnetic field of the same magnitude. However, 72215 probably was magnetized by a magnetic field of a different intensity. We concluded that the coincident NRM directions owe their origin to a secondary imprint of less stable magnetization imparted during the assembly of the boulder at moderate temperatures (～ 450°C) on the South Massif. The stable directions, on the other hand, date from the last, higher-temperature (～ 770°C) magnetizing event experienced by the mineral and lithic components while they were part of the immature pre-Serenitatis regolith.     

443 Eros: Problems with the meteorite magnetism record in attempting an asteroid match

Abstract— The magnetometer experiment (MAG) onboard the Near‐Earth Asteroid Rendezvous (NEAR)‐Shoemaker spacecraft detected no global scale magnetization and established a maximum magnetization of 2.1 times 10^?6 Am² kg^?1 for asteroid 433 Eros. This is in sharp contrast with the estimated magnetization of other S‐class asteroids (Gaspra, ?2.4 times 10^?2 Am² kg^?1; Braille, ?2.8 times 10^?2 Am² kg^?1) and is below published values for all types of ordinary chondrites. This includes the L/LL types considered to most closely match 433 Eros based on preliminary interpretations of NEAR remote geochemical experiments. The ordinary chondrite meteorite magnetization intensity data was reviewed in order to assess the reasonableness of an asteroid‐meteorite match based on magnetic property measurements. Natural remanent magnetization (NRM) intensities for the ordinary chondrite meteorites show at least a 2 order of magnitude range within each of the H, L, and LL groups, all well above the 2.1 times 10^?6 Am² kg^?1 level for 433 Eros. The REM values (ratio of the NRM to the SIRM (saturation remanent magnetization)) range over 3 orders of magnitude for all chondrite groups indicating no clear relationship between NRM and the amount of magnetic material. Levels of magnetic noise in chondrite meteorites can be as much as 70% or more of the NRM. Consequently, published values of the NRM should be considered suspect unless careful evaluation of the noise sources is done. NASA Goddard SFC studies of per unit mass intensities in large (>10 000 g) and small (down to <1 g) samples from the same meteorite demonstrate magnetic intensity decreases as size increases. This would appear to be explained by demagnetization due to magnetic vector randomness at unknown scale sizes in the larger samples. This would then argue for some level of demagnetization of large objects such as an asteroid. The possibility that 433 Eros is an LL chondrite cannot be discounted.     

Magnetic properties of Martian meteorites: Implications for an ancient Martian magnetic field

Abstract— The magnetic properties of samples of seven Martian meteorites (EET 79001, Zagami, Nakhla, Lafayette, Governador Valadares, Chassigny and ALH 84001) have been investigated. All possess a weak, very stable primary natural remanent magnetization (NRM), and some have less stable secondary components. In some cases, the latter are associated with magnetic contamination of the samples, imparted since their recovery, and with viscous magnetization, acquired during exposure of the meteorites to the geomagnetic field since they fell. The magnetic properties are carried by a small content (<1%) of titanomagnetite and, in ALH 84001, possibly by magnetite as well. The most likely source of the primary NRM is a thermoremanent magnetization acquired when the meteorite material last cooled from a high temperature in the presence of a magnetic field. Current evidence is that this was 1.3 Ga ago for the nakhlites and Chassigny and 180 Ma for shergottites: the time of the last relevant cooling of ALH 84001 is not presently known. Preliminary estimates of the strength of the magnetizing field are in the range 0.5–5 üT, which is at least an order of magnitude greater than the present field. It is tentatively concluded that the magnetic field was generated by a dynamo process in a Martian core with appropriate structure and properties.     

Light noble gases and cosmogenic radionuclides in Estherville,Budulan, and other mesosiderites: Implications for exposure histories and production rates

Abstract— We report measurements of ²⁶AI, ¹⁰Be, ⁴¹Ca, and ³⁶Cl in the silicate and metal phases of 11 mesosiderites, including several specimens each of Budulan and Estherville, of the brecciated meteorite Bencubbin, and of the iron meteorite Udei Station. Average production rate ratios (atom/atom) for metal phase samples from Estherville and Budulan are ²⁶Al/¹⁰Be = 0.77 ± 0.02; ³⁶Cl/¹⁰Be = 5.3 ± 0.2. For a larger set of meteorites that includes iron meteorites and other mesosiderites, we find ²⁶Al/¹⁰Be = 0.72 ± 0.01 and ³⁶Cl/¹⁰Be = 4.5 ± 0.2. The average ⁴¹Ca/³⁶Cl production rate ratio is 1.10 ± 0.04 for metal separates from Estherville and four small iron falls. The ⁴¹Ca activities in dpm/(kg Ca) of various silicate separates from Budulan and Estherville span nearly a factor of 4, from <400 to >1600, indicating preatmospheric radii of >30 cm. After allowance for composition, the activities of ²⁶Al and ¹⁰Be (dpm/kg silicate) are similar to values measured in most ordinary chondrites and appear to depend only weakly on bulk Fe content. Unless shielding effects are larger than suggested by the ³⁶Cl and ⁴¹Ca activities of the metal phases, matrix effects are unimportant for ¹⁰Be and minor for ²⁶Al. Noble gas concentrations and isotopic abundances are reported for samples of Barea, Emery, Mincy, Morristown, and Marjalahti. New estimates of ³⁶Cl/³⁶Ar exposure ages for the metal phases agree well with published values. Neon‐21 production rates for mesosiderite silicates calculated from these ages and from measured ²¹Ne contents are consistently higher than predicted for L chondrites despite the fact that the mesosiderite silicates have lower Mg contents than L chondrites. We suggest that the elevation of the ²¹Ne production rate in mesosiderite silicates reflects a “matrix effect,” that is, the influence of the higher Fe content of mesosiderites, which acts to enhance the flux of low‐energy secondary particles and hence the ²¹Ne production from Mg. As ¹⁰Be production is relatively insensitive to this matrix effect, ¹⁰Be/²¹Ne ages give erroneously low production rates and high exposure ages. By coincidence, standard ²²Ne/²¹Ne based “shielding” corrections give fairly reliable ²¹Ne production rates in the mesosiderite silicates.     

The magnetic effects of brecciation and shock in meteorites: II. The ureilites and evidence for strong nebular magnetic fields

We have examined the magnetic characteristics of representative ureilites, with a view to identify the magnetic effects of shock and to isolate a primary component of the natural remanent magnetization (NRM). As a group, the ureilites show remarkably uniform patterns of magnetic behavior, attesting to a common genesis and history. However, a clearly observed gradation in magnetic properties of the ureilites studied with shock level, parallels their classification based on petrologic and chemical fractionation shock-related trends.The ureilite meteorites possess a strong and directionally stable NRM. Laboratory thermal modelling of this presumably primordial NRM preserved in Goalpara and Kenna produced reliable paleointensity estimates of order 1 Oe, thus providing evidence for strong early, nebular magnetic fields. This paleofield strength is compatible with values obtained previously from carbonaceous chondrites and supports isotopic evidence for a contemporary origin of these two groups of meteorites in the same nebular region. The mechanism for recording nebular fields, manifestly different in carbonaceous chondrite vs. ureilite meteorites, is thus relatively unimportant: violent collisional shock in ureilites seems to have only partially altered an original magnetization, by preferential removal of its least stable portion.     

Mass‐dependent fractionation of nickel isotopes in meteoritic metal

Abstract— We measured nickel isotopes via multicollector inductively coupled plasma mass spectrometry (MC‐ICPMS) in the bulk metal from 36 meteorites, including chondrites, pallasites, and irons (magmatic and non‐magmatic). The Ni isotopes in these meteorites are mass fractionated; the fractionation spans an overall range of ～0.4‰ amu^?1. The ranges of Ni isotopic compositions (relative to the SRM 986 Ni isotopic standard) in metal from iron meteorites (～0.0 to ～0.3‰ amu^?1) and chondrites (～0.0 to ～0.2‰ amu^?1) are similar, whereas the range in pallasite metal (～–0.1 to 0.0‰ amu^?1) appears distinct. The fractionation of Ni isotopes within a suite of fourteen IIIAB irons (～0.0 to ～0.3‰ amu^?1) spans the entire range measured in all magmatic irons. However, the degree of Ni isotopic fractionation in these samples does not correlate with their Ni content, suggesting that core crystallization did not fractionate Ni isotopes in a systematic way. We also measured the Ni and Fe isotopes in adjacent kamacite and taenite from the Toluca IAB iron meteorite. Nickel isotopes show clearly resolvable fractionation between these two phases; kamacite is heavier relative to taenite by ～0.4‰ amu^?1. In contrast, the Fe isotopes do not show a resolvable fractionation between kamacite and taenite. The observed isotopic compositions of kamacite and taenite can be understood in terms of kinetic fractionation due to diffusion of Ni during cooling of the Fe‐Ni alloy and the development of the Widmanstätten pattern.     

Acquisition of shock remanent magnetization for demagnetized samples in a weak magnetic field (7 μT) by shock pressures 5–20 GPa without plasma‐induced magnetization

Abstract— Demagnetized samples of cobalt precipitates in a copper matrix were shocked to 5, 10, and 20 GPa in a weak magnetic field of 7.7 μT to elucidate the origins of the natural remanent magnetization of meteorites and the magnetic anomalies of impact craters on the moon and Mars. The samples placed in the target acquired shock remanent magnetization (SRM) whose intensity increased up to 21.3 times compared with the demagnetized state, but SRM intensity and shock intensity were not correlated. The SRM direction was in most cases approximately perpendicular to the shock direction. The samples placed 4.8 mm from the impacted surface did not acquire significant magnetization, suggesting no plasma‐induced remanent magnetization (PIRM) up to 20 GPa. When the samples were divided into 8 sub‐samples, the SRM intensities of sub‐samples increased up to 40 times compared with bulk ones and their directions were scattered. Higher coercive force grains were magnetized perpendicular to the shock direction for shocks of 5 and 10 GPa, but at 20 GPa the directions were less systematically oriented. These results suggest that the proposed plasma‐induced magnetization of impactites should be reconsidered.     

The New Peruvian Meteorite Carancas: Mössbauer Spectroscopy and X-Ray Diffraction Studies

The Carancas meteorite fell on 15 September 2007 approximately 10 km south of Desaguadero, near Lake Titicaca, Peru, producing bright lights, clouds of dust in the sky and intense detonations. The Carancas meteorite is classified as a H4–5 ordinary chondrite with shock stage S3 and a degree of weathering W0. The Carancas meteorite is characterized by well defined chondrules composed either of olivine or pyroxene. The Mössbauer spectra show an overlapping of paramagnetic and magnetic phases. The spectra show two quadrupole doublets associated to olivine and pyroxene; and two magnetic sextets, associated with the primary phases kamacite/taenite and Troilite (Fe²⁺). Metal particles were extracted from the bulk powdered samples exhibit only kamacite and small amounts of the intergrowth tetrataenite/antitaenite. X-Ray diffractogram shows the primary phases olivine, pyroxene, troilite, kamacite, diopside and albite. Iron oxides has not been detected by Mössbauer spectroscopy or XRD as can be expected for a meteorite immediately recovered after its fall.     

Magnetic properties of asteroids from meteorite data — Implications for magnetic anomaly detections

Magnetic measurements of meteorites suggest that small bodies (e.g. asteroids) in the Solar System have small but distinct magnetic fields produced by the bulk remanent magnetisation (NRM) of the body. Here we report calculations of magnetic fields of small bodies, assuming that they can be approximated as homogeneously magnetised spheres with dipole moments derived from NRM data on known meteorites. The magnetic fields are compared with the field of the asteroid 951 Gaspra measured by spacecraft Galileo in 1991 (Kivelson et al., 1993). The result of this comparison suggests that the field of Gaspra could be caused by an L-, H- or E-chondritic or a pallasite body. The spectral reflectance data on Gaspra suggest, however, that it is a basaltic achondrite. The problem can be resolved if Gaspra is a differentiated body, its surface material being closer to that of basaltic achondrites, and the bulk closer to ordinary chondrites or pallasites. We also present magnetic anomaly profiles along the surface of Mars such as would be measured with a magnetometer installed on a Rover-type vehicle by assuming that the main sources of the surface anomalies are the NRMs of the boulders on the Martian surface. The NRM values are taken from the data measured on SNC meteorites. The results suggest large oscillations in magnetic field intensity at the Martian surface.     

Interest and design of magnetic properties measurements on planetary and asteroidal landers

Intrinsic magnetic properties, like susceptibility, provide a precise determination of the iron phases with a penetration depth not available with other chemical and mineralogical sensing tools, thus allowing to unravel space weathering effects. Systematic measurements of meteorites demonstrate that susceptibility measurements on asteroid surface could be a very efficient way to assign a meteorite class to a given asteroid. Another application could be the characterization of the highly magnetic Martian regolith. On the other hand, natural remanent magnetization (NRM) measurements are crucial to interpret magnetic field anomalies such as those found on Mars and Moon, and likely to be found on Mercury. NRM gives also access to past magnetic fields and extinct planetary dynamo. Rugged, light and low consumption systems already exist for such measurements on Earth and we present a scheme to integrate both magnetic susceptibility (using a LC oscillator) and NRM (using a 3 axis fluxgate or a gradiometer) to offer a versatile instrument package for any mission involving a lander. For the LC oscillator calibration of the geometric factor is presented. The fluxgate can be used both for making local magnetic anomaly maps, thus investigating subsurface structures, and for evaluating NRM of individual boulders.     

Atom‐probe tomography and transmission electron microscopy of the kamacite–taenite interface in the fast‐cooled Bristol IVA iron meteorite

We report the first combined atom‐probe tomography (APT) and transmission electron microscopy (TEM) study of a kamacite–tetrataenite (K–T) interface region within an iron meteorite, Bristol (IVA). Ten APT nanotips were prepared from the K–T interface with focused ion beam scanning electron microscopy (FIB‐SEM) and then studied using TEM followed by APT. Near the K‐T interface, we found 3.8 ± 0.5 wt% Ni in kamacite and 53.4 ± 0.5 wt% Ni in tetrataenite. High‐Ni precipitate regions of the cloudy zone (CZ) have 50.4 ± 0.8 wt% Ni. A region near the CZ and martensite interface has <10 nm sized Ni‐rich precipitates with 38.4 ± 0.7 wt% Ni present within a low‐Ni matrix having 25.5 ± 0.6 wt% Ni. We found that Cu is predominantly concentrated in tetrataenite, whereas Co, P, and Cr are concentrated in kamacite. Phosphorus is preferentially concentrated along the K‐T interface. This study is the first precise measurement of the phase composition at high spatial resolution and in 3‐D of the K‐T interface region in a IVA iron meteorite and furthers our knowledge of the phase composition changes in a fast‐cooled iron meteorite below 400 °C. We demonstrate that APT in conjunction with TEM is a useful approach to study the major, minor, and trace elemental composition of nanoscale features within fast‐cooled iron meteorites.     

Shock magnetism in fine particle iron

Abstract— All solid solar system bodies have been affected by impact to varying degrees, and, thus, magnetic records in these bodies may have been modified by shock events. Shock events may have overprinted all primordial magnetic records in meteorites. Shock metamorphism stages ranging from very little to extreme, when melting takes place, have been identified in meteorites. We are examining the creation and destruction of magnetic remanence associated with shock. In this paper, we develop a preliminary framework for understanding the magnetic properties of fine‐grained Fe particles (20–110 nm), which carry most of the remanent magnetization in lunar samples and, by extension, the kamacite phase in meteorite samples. Initial experiments on shock effects due to a first‐order shock‐induced crystallographic transformation are described. The first characterization of pre‐ and postshock magnetic properties for sized Fe particles and the first characterization of the transformation remanent magnetization (TMRM) associated with the face‐centered‐cubic (fcc) to body‐centered‐cubic (bcc) transformation in fine particle Fe spheres are described. This is equivalent to the 13 GPa transitions in bcc Fe. We show that the TMRM is in the same direction as the ambient magnetic field present during the shock, but is deflected from the field direction by 30–45° and that the remanence intensity is 1–2 orders of magnitude less than expected for thermoremanent magnetization (TRM) acquired during cooling through the Curie temperature. Isothermal remanence acquisition curves (RA) reveal the increasing magnetic hardness due to shock. Magnetic hysteresis loops are used to characterize the particle size and the shock‐induced magnetic anisotropy. Thermal demagnetization experiments describe the probable presence of particle size effects and the effects associated with recovery‐recrystallization due to the annealing that takes place during the thermomagnetic experiment. These observations have implications for paleofield determinations and the recognition of thermal unblocking. A TMRM mechanism could produce a shock overprint in a meteorite and might impart a significant directional feature in an asteroid magnetic signature.     

Matching Martian crustal magnetization and magnetic properties of Martian meteorites

Abstract— Magnetic properties of 26 (of 32) unpaired Martian meteorites (SNCs) are synthesized to further constrain the lithology carrying Martian magnetic crustal sources. Magnetic properties of ultramafic cumulates (i.e., Chassigny, Allan Hills [ALH] 84001) and lherzolitic shergottites (ALH 77005, Lewis Cliff [LEW] 88516) are one or two orders of magnitude too weak to account for the crustal magnetizations, assuming magnetization in an Earth‐like field. Nakhlites and some basaltic shergottites, which are the most magnetic SNCs, show the right intensity. Titanomagnetite is the magnetic carrier in the nakhlites (7 meteorites), whereas in most basaltic shergottites (11 meteorites) it is pyrrhotite. Dhofar (Dho) 378, Los Angeles, and NWA 480/1460 and 2046 are anomalous basaltic shergottites, as their magnetism is mainly due to titanomagnetite. Pyrrhotite should be among the candidate minerals for the magnetized Noachian crust.     

The Bosumtwi meteorite impact structure,Ghana: A magnetic model

Abstract— A magnetic model is proposed for the Bosumtwi meteorite impact structure in Ghana, Africa. This relatively young (～1.07 Ma) structure with a diameter of ～10.5 km is exposed within early Proterozoic Birimian—Tarkwaian rocks. The central part of the structure is buried under postimpact lake sediments, and because of lack of drill cores, geophysics is the only way to reveal its internal structure. To study the structure below and beyond the lake, a high‐resolution, low altitude (～70 m) airborne geophysical survey across the structure was conducted, which included measurements of the total magnetic field, electromagnetic data, and gamma radiation. The magnetic data show a circumferential magnetic halo outside the lakeshore, ～12 km in diameter. The central‐north part of the lake reveals a central negative magnetic anomaly with smaller positive side‐anomalies north and south of it, which is typical for magnetized bodies at shallow latitudes. A few weaker negative magnetic anomalies exist in the eastern and western part of the lake. Together with the northern one, they seem to encircle a central uplift. Our model shows that the magnetic anomaly of the structure is presumably produced by one or several relatively strongly remanently magnetized impact‐melt rock or melt‐rich suevite bodies. Petrophysical measurements show a clear difference between the physical properties of preimpact target rocks and impactites. Suevites have a higher magnetization and have low densities and high porosities compared to the target rocks. In suevites, the remanent magnetization dominates over induced magnetization (Koenigsberger ratio > 3). Preliminary palaeomagnetic results reveal that the normally magnetized remanence component in suevites was acquired during the Jaramillo normal polarity epoch. This interpretation is consistent with the modelling results that also require a normal polarity magnetization for the magnetic body beneath the lake. The reverse polarity remanence component, superimposed on the normal component, is probably acquired during subsequent reverse polarity events.     

Magnetic properties of asteroids from meteorite data — Implications for magnetic anomaly detections

Magnetic measurements of meteorites suggest that small bodies (e.g. asteroids) in the Solar System have small but distinct magnetic fields produced by the bulk remanent magnetisation (NRM) of the body. Here we report calculations of magnetic fields of small bodies, assuming that they can be approximated as homogeneously magnetised spheres with dipole moments derived from NRM data on known meteorites. The magnetic fields are compared with the field of the asteroid 951 Gaspra measured by spacecraft Galileo in 1991 (Kivelson et al., 1993). The result of this comparison suggests that the field of Gaspra could be caused by an L-, H- or E-chondritic or a pallasite body. The spectral reflectance data on Gaspra suggest, however, that it is a basaltic achondrite. The problem can be resolved if Gaspra is a differentiated body, its surface material being closer to that of basaltic achondrites, and the bulk closer to ordinary chondrites or pallasites. We also present magnetic anomaly profiles along the surface of Mars such as would be measured with a magnetometer installed on a Rover-type vehicle by assuming that the main sources of the surface anomalies are the NRMs of the boulders on the Martian surface. The NRM values are taken from the data measured on SNC meteorites. The results suggest large oscillations in magnetic field intensity at the Martian surface.     

The magnetic effects of brecciation and shock in meteorites: III. The achondrites

We present results of a magnetic survey of achondritic meteorites, representing the aubrites (A), diogenites (D), Irowardites (H), and eucrites (E) groups and relate their magnetic behavior to respective class characteristics and models of origin.Magnetic susceptibility (x) values cluster well within each group and decrease systematically between groups (from 2 to 0.1×10^–3GOe^–1 cm^–3), with the average metal contents, (from 1 to <0.1 wt%) in the above order. The natural remanent magnetization (NRM) values range broadly within each group, but group averages decrease roughly as above. However, the considerable within-sample and intra-group variability in NRM level and its demagnetization characteristics attest to inhomogeneous and localized brecciation effects. Although petrological-chemical studies resolve a primary component of magmatic differentiation on the planetoid of origin, no clear magnetic record of such event has been preserved. The magnetization of achondrites is mainly the product of their complex, multi-stage impact brecciation and metamorphism history, in accord with other lines of evidence.The magnetic behavior of achondrites is remarkably similar to that characteristic of lunar breccias and impact-melt rocks and reinforces their analogous mode of genesis, as brought out by chemical and petrographic analyses.     

Vaca Muerta mesosiderite strewnfield

Abstract A field investigation is presented of the strewnfield of the mesosiderite Vaca Muerta, originally found in 1861. The area, 11.5 km long, 2.1 km wide, is located about 60 km southeast of Taltal, Chile, in the Atacama Desert. It has yielded 80 meteorites with a total mass exceeding 3782 kg. Most fragments were found in an undisturbed state, but some had been broken by prospectors. The present studies, in connection with historical records, indicate that the original mass of Vaca Muerta exceeded 6 metric tons. One impact feature, somewhat modified by man, consists of a 10.5-m diameter, 1.7-m deep hole, without an uplifted rim. Small masses were scattered up to 85 m from the hole.