Department of Terrestrial Magnetism

The Mars Orbiter Laser Altimeter (MOLA), an instrument on the Mars Global Surveyor spacecraft, has measured the topography, surface roughness, and 1.064‐μm reflectivity of Mars and the heights of volatile and dust clouds. This paper discusses the function of the MOLA instrument and the acquisition, processing, and correction of observations to produce global data sets. The altimeter measurements have been converted to both gridded and spherical harmonic models for the topography and shape of Mars that have vertical and radial accuracies of ∼1 m with respect to the planet's center of mass. The current global topographic grid has a resolution of 1/64° in latitude × 1/32° in longitude (1×2 km2at the equator). Reconstruction of the locations of incident laser pulses on the Martian surface appears to be at the 100‐m spatial accuracy level and results in 2 orders of magnitude improvement in the global geodetic grid of Mars. Global maps of optical pulse width indicative of 100‐m‐scale surface roughness and 1.064‐μm reflectivity with an accuracy of 5% have also been obtained.

The Galactic Legacy Infrared Mid‐Plane Survey Extraordinaire (GLIMPSE), a Space Infrared Telescope Facility ( SIRTF ) Legacy Science Program, will be a fully sampled, confusion‐limited infrared survey of 2/3 of the inner Galactic disk with a pixel resolution of ∼1 2 using the Infrared Array Camera at 3.6, 4.5, 5.8, and 8.0 μm. The survey will cover Galactic latitudes |b|≤1° and longitudes |l| = 10°–65° (both sides of the Galactic center). The survey area contains the outer ends of the Galactic bar, the Galactic molecular ring, and the inner spiral arms. The GLIMPSE team will process these data to produce a point‐source catalog, a point‐source data archive, and a set of mosaicked images. We summarize our observing strategy, give details of our data products, and summarize some of the principal science questions that will be addressed using GLIMPSE data. Up‐to‐date documentation, survey progress, and information on complementary data sets are available on the GLIMPSE Web site.

The recent discoveries of extrasolar giant planets, coupled with refined models of the compositions of Jupiter and Saturn, prompt a reexamination of theories of giant planet formation. An alternative to the favored core accretion hypothesis is examined here; gravitational instability in the outer solar nebula leading to giant planet formation. Three-dimensional hydrodynamic calculations of protoplanetary disks show that giant gaseous protoplanets can form with locally isothermal or adiabatic disk thermodynamics. Gravitational instability appears to be capable of forming giant planets with modest cores of ice and rock faster than the core accretion mechanism can.

We present new analytical data of major and trace elements for the geological MPI‐DING glasses KL2‐G, ML3B‐G, StHs6/80‐G, GOR128‐G, GOR132‐G, BM90/21‐G, T1‐G, and ATHO‐G. Different analytical methods were used to obtain a large spectrum of major and trace element data, in particular, EPMA, SIMS, LA‐ICPMS, and isotope dilution by TIMS and ICPMS. Altogether, more than 60 qualified geochemical laboratories worldwide contributed to the analyses, allowing us to present new reference and information values and their uncertainties (at 95% confidence level) for up to 74 elements. We complied with the recommendations for the certification of geological reference materials by the International Association of Geoanalysts (IAG). The reference values were derived from the results of 16 independent techniques, including definitive (isotope dilution) and comparative bulk (e.g., INAA, ICPMS, SSMS) and microanalytical (e.g., LA‐ICPMS, SIMS, EPMA) methods. Agreement between two or more independent methods and the use of definitive methods provided traceability to the fullest extent possible. We also present new and recently published data for the isotopic compositions of H, B, Li, O, Ca, Sr, Nd, Hf, and Pb. The results were mainly obtained by high‐precision bulk techniques, such as TIMS and MC‐ICPMS. In addition, LA‐ICPMS and SIMS isotope data of B, Li, and Pb are presented.

The Stardust spacecraft collected thousands of particles from comet 81P/Wild 2 and returned them to Earth for laboratory study. The preliminary examination of these samples shows that the nonvolatile portion of the comet is an unequilibrated assortment of materials that have both presolar and solar system origin. The comet contains an abundance of silicate grains that are much larger than predictions of interstellar grain models, and many of these are high-temperature minerals that appear to have formed in the inner regions of the solar nebula. Their presence in a comet proves that the formation of the solar system included mixing on the grandest scales.

We analyze 8 years of precise radial velocity measurements from the Keck Planet Search, characterizing the detection threshold, selection effects, and completeness of the survey. We first carry out a systematic search for planets, by assessing the false-alarm probability associated with Keplerian orbit fits to the data. This allows us to understand the detection threshold for each star in terms of the number and time baseline of the observations, and the underlying "noise" from measurement errors, intrinsic stellar jitter, or additional low-mass planets. We show that all planets with orbital periods P < 2000 days, velocity amplitudes K > 20 m s -1 , and eccentricities e ≲ 0.6 have been announced, and we summarize the candidates at lower amplitudes and longer orbital periods. For the remaining stars, we calculate upper limits on the velocity amplitude of a companion. For orbital periods less than the duration of the observations, these are typically 10 m s -1 and increase ∝ P 2 for longer periods. We then use the nondetections to derive completeness corrections at low amplitudes and long orbital periods and discuss the resulting distribution of minimum mass and orbital period. We give the fraction of stars with a planet as a function of minimum mass and orbital period and extrapolate to long-period orbits and low planet masses. A power-law fit for planet masses >0.3 M J and periods < 2000 days gives a mass-period distribution dN = CM α P β d ln Md ln P with α = -0.31 ± 0.2, β = 0.26 ± 0.1, and the normalization constant C such that 10.5% of solar type stars have a planet with mass in the range 0.3–10 M J and orbital period 2–2000 days. The orbital period distribution shows an increase in the planet fraction by a factor of ≈5 for orbital periods ≳300 days. Extrapolation gives 17%–20% of stars having gas giant planets within 20 AU. Finally, we constrain the occurrence rate of planets orbiting M dwarfs compared to FGK dwarfs, taking into account differences in detectability.

The steady state kinetics of lead metabolism were studied in five healthy men with stable isotope tracers. Subjects lived in a metabolic unit and ate constant low lead diets. Their intake was supplemented each day with 79--204 mug of enriched lead-204 as nitrate which was ingested with meals for 1--124 days. The concentration and isotopic composition of lead was determined serially in blood, urine, feces, and diet and less commonly in hair, nails, sweat, bone, and alimentary tract secretions by isotopic dilution, mass spectrometric analysis. The data suggest a three compartmental model for lead metabolism. The first compartment encompasses blood and is 1.5--2.2 times larger than the blood mass. It contains approximately 1.7--2.0 mg of lead and has a mean life of 35 days. This pool is in direct communication with ingested lead, urinary lead, and pools two and three. The second compartment is largely composed of soft tissue, contains about 0.3--0.9 mg of lead, and has a mean life of approximately 40 days. This pool gives rise to lead in hair, nails, sweat, and salivary, gastric, pancreatic, and biliary secretions. Pool three resides primarily in the skeleton, contains the vast quantity of body lead, and has a very slow mean life. Bones appear to differ in their rates of lead turnover. Within the relatively small changes in blood lead observed in the present study, the transfer coefficients between the pools remained constant.

A graphical procedure is described for rapid calculation of discordant uraniumlead ages resulting from multiple episodes of uranium‐lead fractionation. A proof of the validity of this graphical procedure is given. The graphical procedure is extended to permit the calculation of the effect of the presence of primary radiogenic lead and of constant loss of intermediate daughter products.

Organics found in comet 81P/Wild 2 samples show a heterogeneous and unequilibrated distribution in abundance and composition. Some organics are similar, but not identical, to those in interplanetary dust particles and carbonaceous meteorites. A class of aromatic-poor organic material is also present. The organics are rich in oxygen and nitrogen compared with meteoritic organics. Aromatic compounds are present, but the samples tend to be relatively poorer in aromatics than are meteorites and interplanetary dust particles. The presence of deuterium and nitrogen-15 excesses suggest that some organics have an interstellar/protostellar heritage. Although the variable extent of modification of these materials by impact capture is not yet fully constrained, a diverse suite of organic compounds is present and identifiable within the returned samples.

We present high-resolution Hα rotation curves for a sample of 15 dwarf and low surface brightness galaxies. From these we derive limits on the slopes of the central mass distributions, using both a direct inversion of the rotation curves and detailed mass models. Assuming that the density distributions of dark matter halos follow a power law at small radii, ρ( r ) ∝ r -α , we find inner slopes in the range 0 ≲ α ≲ 1 for most galaxies. Thus, even with the relatively high spatial resolution of the Hα rotation curves presented here, the inner slopes are poorly constrained. In general halos with constant density cores (α = 0) provide somewhat better fits, but the majority of our galaxies (~75%) are also consistent with α = 1, as long as the R -band stellar mass-to-light ratios are smaller than about 2. Halos with α = 1.5, however, are ruled out in virtually every case. In order to investigate the robustness of these results we discuss and model several possible causes of systematic errors, including noncircular motions, galaxy inclination, slit width, seeing, and slit alignment errors. Taking the associated uncertainties into account, we conclude that even for the ~25% of the cases where α = 1 seems inconsistent with the rotation curves, we cannot rule out cusp slopes this steep. Inclusion of literature samples similar to the one presented here leads to the same conclusion when the possibility of systematic errors is taken into account. In the ongoing debate on whether the rotation curves of dwarf and low surface brightness galaxies are consistent with predictions for a cold dark matter universe, we argue that our sample and the literature samples discussed in this paper provide insufficient evidence to rule out halos with α = 1. At the same time, we note that none of the galaxies in these samples require halos with steep cusps, as most are equally well or better explained by halos with constant density cores.

Annual means from continuous registration of cosmic-ray ionization at four stations from 1937 to 1952 show a variation of nearly four per cent, which is similar at all stations and which is negatively correlated with sunspot numbers. This variation in cosmic-ray intensity is quite similar for the annual means of all days, international magnetic quiet days, and international magnetic disturbed days, which indicates that it is not due to transient decreases accompanying some magnetic storms. Although the cosmic-ray intensity at some stations is affected by meteorological conditions, it is shown that on the average the cosmic-ray changes observed at Huancayo agree well with those at other stations. From an analysis of the variability of daily means at Huancayo and a sample comparison with Simpson's neutron data, it is concluded that the cosmic-ray ionization at Huancayo is very little affected by meteorological effects. Through a comparison with Neher's balloon observations, evidence is provided to indicate the reliability of cosmic-ray results at Huancayo over long periods of time. The relation between cosmic-ray decreases and some measures of geomagnetic activity is indicated, and it is shown that the major transient decreases in cosmic-ray intensity occur during magnetic disturbance. Graphs are included which depict the daily means of cosmic-ray intensity at Huancayo for all available data, 1937–1953.

Doppler spectroscopy has uncovered or confirmed all the known planets orbiting nearby stars. Two main techniques are used to obtain precision Doppler measurements at optical wavelengths. The first approach is the gas cell method, which consists of least-squares matching of the spectrum of iodine imprinted on the spectrum of the star. The second method relies on the construction of a stabilized spectrograph externally calibrated in wavelength. The most precise stabilized spectrometer in operation is the High Accuracy Radial velocity Planet Searcher (HARPS), operated by the European Southern Observatory in La Silla Observatory, Chile. The Doppler measurements obtained with HARPS are typically obtained using the cross-correlation function (CCF) technique. This technique consists of multiplying the stellar spectrum by a weighted binary mask and finding the minimum of the product as a function of the Doppler shift. It is known that CCF is suboptimal in exploiting the Doppler information in the stellar spectrum. Here we describe an algorithm to obtain precision radial velocity measurements using least-squares matching of each observed spectrum to a high signal-to-noise ratio template derived from the same observations. This algorithm is implemented in our software HARPS-TERRA (Template-Enhanced Radial velocity Re-analysis Application). New radial velocity measurements on a representative sample of stars observed by HARPS are used to illustrate the benefits of the proposed method. We show that, compared with CCF, template matching provides a significant improvement in accuracy, especially when applied to M dwarfs.

Context. Since low-mass stars have low luminosities, orbits at which liquid water can exist on Earth-sized planets are relatively close-in, which produces Doppler signals that are detectable using state-of-the-art Doppler spectroscopy.

We report the detection of Kepler-47, a system consisting of two planets orbiting around an eclipsing pair of stars. The inner and outer planets have radii 3.0 and 4.6 times that of Earth, respectively. The binary star consists of a Sun-like star and a companion roughly one-third its size, orbiting each other every 7.45 days. With an orbital period of 49.5 days, 18 transits of the inner planet have been observed, allowing a detailed characterization of its orbit and those of the stars. The outer planet's orbital period is 303.2 days, and although the planet is not Earth-like, it resides within the classical "habitable zone," where liquid water could exist on an Earth-like planet. With its two known planets, Kepler-47 establishes that close binary stars can host complete planetary systems.

Abstract— We present a model for the thermal processing of particles in shock waves typical of the solar nebula. This shock model improves on existing models in that the dissociation and recombination of H2 and the evaporation of particles are accounted for in their effects on the mass, momentum and energy fluxes. Also, besides thermal exchange with the gas and gas‐drag heating, particles can be heated by absorbing the thermal radiation emitted by other particles. The flow of radiation is calculated using the equations of radiative transfer in a slab geometry. We compute the thermal histories of particles as they encounter and pass through the shock. We apply this shock model to the melting and cooling of chondrules in the solar nebula. We constrain the combinations of shock speed and gas density needed for chondrules to reach melting temperatures, and show that these are consistent with shock waves generated by gravitational instabilities in the protoplanetary disk. After their melting, cooling rates of chondrules in the range 10–1000 K h−1 are naturally reproduced by the shock model. Chondrules are kept warm by the reservoir of hot shocked gas, which cools only as fast as the dust grains and chondrules themselves can radiate away the gas's energy. We predict a positive correlation between the concentration of chondrules in a region and the cooling rates of chondrules in that region. This correlation is supported by the unusually high frequency of (rapidly cooled) barred chondrules among compound chondrules, which must have collided preferentially in regions of high chondrule density. We discuss these and other compelling consistencies between the meteoritic record and the shock wave model of chondrule formation.

P‐wave and S‐wave delay times from the broadband data of the southern Africa seismic experiment have been inverted to obtain three‐dimensional images of velocity perturbations in the mantle beneath southern Africa. High velocity mantle roots appear to extend to depths of at least 250 km, and locally to depths of 300 km beneath the Kaapvaal and Zimbabwe cratons. Thick roots are confined to the Archean cratons, with no evidence for similar structures beneath the adjacent Proterozoic mobile belts. The Kaapvaal craton was modified ca. 2.05 Ga by the Bushveld magmatic event, which affected a broad swath of cratonic mantle beneath and to the west of the exposed Bushveld Complex. The mantle beneath the extended Bushveld province is characterized by seismic velocities lower than those observed in regions of undisturbed cratonic mantle. The mantle beneath the Limpopo Belt, an Archean collisional zone sandwiched between the Kaapvaal and Zimbabwe cratons, exhibits a cratonic signature.

From gravity and topography data collected by the Mars Global Surveyor spacecraft we calculate gravity/topography admittances and correlations in the spectral domain and compare them to those predicted from models of lithospheric flexure. On the basis of these comparisons we estimate the thickness of the Martian elastic lithosphere ( T e ) required to support the observed topographic load since the time of loading. We convert T e to estimates of heat flux and thermal gradient in the lithosphere through a consideration of the response of an elastic/plastic shell. In regions of high topography on Mars (e.g., the Tharsis rise and associated shield volcanoes), the mass‐sheet (small‐amplitude) approximation for the calculation of gravity from topography is inadequate. A correction that accounts for finite‐amplitude topography tends to increase the amplitude of the predicted gravity signal at spacecraft altitudes. Proper implementation of this correction requires the use of radii from the center of mass (collectively known as the planetary “shape”) in lieu of “topography” referenced to a gravitational equipotential. Anomalously dense surface layers or buried excess masses are not required to explain the observed admittances for the Tharsis Montes or Olympus Mons volcanoes when this correction is applied. Derived T e values generally decrease with increasing age of the lithospheric load, in a manner consistent with a rapid decline of mantle heat flux during the Noachian and more modest rates of decline during subsequent epochs.

A detailed petrological and geochemical study of low-temperature peridotite xenoliths from Kimberley and northern Lesotho is presented to constrain the processes that led to the magmaphile element depletion of the Kaapvaal cratonic lithospheric mantle and its subsequent re-enrichment in Si and incompatible trace elements. Whole-rocks and minerals have been characterized for Re–Os isotope compositions, and major and trace element concentrations, and garnet and clinopyroxene for Lu–Hf and Sm–Nd isotope compositions. Most samples are characterized by Archaean Os model ages, low Al, Fe and Ca contents, high Mg/Fe, low Re/Os, very low (< 0·1 × chondrite) heavy rare earth element (HREE) concentrations and a decoupling between Nd and Hf isotope ratios. These features are most consistent with initial melting at ∼3·2 Ga followed by metasomatism by hydrous fluids, which may have also caused additional melting to produce a harzburgitic residue. The low HREE abundances of the peridotites require that extensive melting occurred in the spinel stability field, possibly preceded by some melting in the presence of garnet. Fractional melting models suggest that ∼30% melting in the spinel field or ∼20% melting in the garnet field followed by ∼20% spinel-facies melting are required to explain the most melt-depleted samples. Garnet Nd–Hf isotope characteristics indicate metasomatic trace element enrichment during the Archaean. We therefore suggest a model including shallow ridge melting, followed by metasomatism of the Kaapvaal upper mantle in subduction zones surrounding cratonic nuclei, probably during amalgamation of smaller pre-existing terranes in the Late Archaean (∼2·9 Ga). The fluid-metasomatized residua have subsequently undergone localized silicate melt infiltration that led to clinopyroxene ± garnet enrichment. Calculated equilibrium liquids for clinopyroxene and their Hf–Nd isotope compositions suggest that most diopside in the xenoliths crystallized from an infiltrating kimberlite-like melt, either during Group II kimberlite magmatism at ∼200–110 Ma (Kimberley), or shortly prior to eruption of the host kimberlite around 90 Ma (northern Lesotho).

We present a coupled thermal‐magmatic model for the evolution of Mars' mantle and crust that may be consistent with estimates of the average crustal thickness and crustal growth rate. By coupling a simple parameterized model of mantle convection to a batch‐melting model for peridotite, we can investigate potential conditions and evolutionary paths of the crust and mantle in a coupled thermal‐magmatic system. On the basis of recent geophysical and geochemical studies, we constrain our models to have average crustal thicknesses between 50 and 100 km that were mostly formed by 4 Ga. Our nominal model is an attempt to satisfy these constraints with a relatively simple set of conditions. Key elements of this model are the inclusion of the energetics of melting, a wet (weak) mantle rheology, self‐consistent fractionation of heat‐producing elements to the crust, and a near‐chondritic abundance of those elements. The latent heat of melting mantle material is a small (percent level) contributor to the total planetary energy budget over 4.5 Gyr but is crucial for constraining the thermal and magmatic history of Mars. Our nominal model predicts an average crustal thickness of ∼62 km that was 73% emplaced by 4 Ga. However, if Mars had a primary crust enriched in heat‐producing elements, consistent with SNC meteorite geochemistry, then our models predict a considerably diminished amount of post 4 Ga crustal emplacement relative to the nominal model. The importance of a wet mantle in satisfying the basic constraints of Mars' thermal and crustal evolution suggests (independently from traditional geomorphology or meteorite geochemistry arguments) that early Mars had a wet environment. Extraction of water from the mantle of a one‐plate planet such as Mars is found to be extremely inefficient, such that 90–95% of all water present in the mantle after the initial degassing event should still reside there currently. Yet extraction of even 5% of a modestly wet mantle (∼36 ppm water) would result in a significant amount (6.4 m equivalent global layer) of water available to influence the early surface and climate evolution of the planet.

Uncertainty in the metal-abundance dependence of the Cepheid variable period-luminosity (PL) relation remains one of the outstanding sources of systematic error in the extragalactic distance scale and in the Hubble constant. To test for such a metallicity dependence, we have used the Wide Field Planetary Camera 2 (WFPC2) on the Hubble Space Telescope ( HST ) to observe Cepheids that span a range in oxygen abundance of 0.7 ± 0.15 dex in two fields in the nearby spiral galaxy M101. A differential analysis of the PL relations in V and I in the two fields yields a marginally significant change in the inferred distance modulus on metal abundance, with δ( m - M ) 0 /δ[O/H] = -0.24 ± 0.16 mag dex -1 . The trend is in the theoretically predicted sense that metal-rich Cepheids appear brighter and closer than metal-poor stars. External comparisons of Cepheid distances with those derived from three other distance indicators, in particular from the tip of the red giant branch method, further constrain the magnitude of any Z -dependence of the PL relation at V and I . The overall effects of any metallicity dependence on the distance scale derived with HST will be of the order of a few percent or less for most applications, though distances to individual galaxies at the extremes of the metal abundance range may be affected at the 10% level.