Compositional variability on the surface of 4 Vesta revealed through GRaND measurements of high‐energy gamma rays     

Patrick N. Peplowski  David J. Lawrence  Thomas H. Prettyman  Naoyuki Yamashita  Dave Bazell  William C. Feldman  Lucille Le Corre  Timothy J. McCoy  Vishnu Reddy  Robert C. Reedy  Chris T. Russell  Michael J. Toplis 《Meteoritics & planetary science》2013,48(11):2252-2270

Measurements of the high‐energy gamma‐ray flux emanating from asteroid 4 Vesta by the Dawn Gamma‐Ray and Neutron Detector (GRaND) have revealed variability in the near‐surface elemental composition of the Vestan surface. These observations are consistent with the presence of large (≥8 × 10⁴ km²) regions with distinct, HED‐like elemental compositions. The results agree broadly with other global measurements, such as the macroscopic neutron absorption cross section and spectral reflectance‐derived mineralogic maps. Two distinct regions with eucrite‐like elemental compositions have been identified, the first located primarily within the Lucaria and Marcia quadrangles and the second within Oppia quadrangle. The former region is collocated with some of the oldest, most heavily cratered terrain on Vesta. The interior of the 500 km diameter Rheasilvia impact basin is found to have a composition that is consistent with diogenite‐like material. Taken together, these observations support the hypothesis that Vesta's original crust was composed of basaltic outflows in the form of eucritic‐like material and that the Rheasilvia‐basin‐forming impact exposed lower‐crustal, diogenite‐like material. These measurements also constrain the maximum amount of mesosiderite‐like material to <10% for each 15 × 15° surface element.  相似文献   

Characteristics of Saturn's polar atmosphere and auroral electrons derived from HST/STIS, FUSE and Cassini/UVIS spectra     

J. Gustin  J.-C. Gérard  P.D. Feldman  G. Holsclaw 《Icarus》2009,200(1):176-187

Ultraviolet (UV) spectra of Saturn's aurora obtained with the Hubble Space Telescope Imaging Spectrograph (STIS), the Cassini Ultraviolet Imaging Spectrograph (UVIS) and the Far Ultraviolet Spectroscopic Explorer (FUSE) have been analyzed. Comparisons between the observed spectra and synthetic models of electron-excited H₂ have been used to determine various auroral characteristics. Far ultraviolet (FUV: 1200-1700 Å) STIS and UVIS spectra exhibit, below 1400 Å, weak absorption due to methane, with a vertical column ranging between 1.4×¹⁰¹⁵ and . Using the low-latitude Moses et al. [Moses, J.I., Bézard, B., Lellouch, E., Feuchtgruber, H., Gladstone, G.R., Allen, M., 2000. Icarus, 143, 244-298] atmospheric model of Saturn and an electron energy-H₂ column relationship, these methane columns are converted into the mean energy of the primary precipitating electrons, estimated to lie in the range 10-18 keV. This result is confirmed by the study of self-absorption with UVIS and FUSE extreme ultraviolet (EUV: 900-1200 Å) spectra. Below 1200 Å, it is seen that transitions connecting to the v^″<2 vibrational levels of the H₂ electronic ground state are partially self-absorbed by H₂ molecules overlying the auroral emission. Because of its low spectral resolution (∼5.5 Å), the UVIS EUV spectrum we analyzed does not allow us to unequivocally determine reasonable ranges of temperatures and H₂ columns. On the other hand, the high spectral resolution (∼0.2 Å) of the FUSE LiF1a and LiF2a EUV spectra we examined resolve the H₂ rotational lines and makes it possible to determine the H₂ temperature. The modeled spectrum best fitting the FUSE LiF1a observation reveals a temperature of 500 K and self-absorption by a H₂ vertical column of . When converted to energy of precipitating electrons, this H₂ column corresponds to primary electrons of ∼10 keV. The model that best fits the LiF2a spectrum is characterized by a temperature of 400 K and is not self-absorbed, making this segment ideal to determine the H₂ temperature at the altitude of the auroral emission. The latter value is in agreement with temperatures obtained from infrared polar spectra. Self-absorption is detectable in the LiF2a segment for H₂ columns exceeding , which sets the maximum mean energy determined from the FUSE observations to ∼15 keV. The total electron energy range of 10-18 keV deduced from FUV and EUV observations places the auroral emission peak between the 0.1 and 0.3 μbar pressure levels. These values should be seen as an upper limit, since most of the Voyager UVS spectra of Saturn's aurora examined by Sandel et al. [Sandel, B.R., Shemansky, D.E., Broadfoot, A.L., Holberg, J.B., Smith, G.R., 1982. Science 215, 548] do not exhibit methane absorption. The auroral H₂ emission is thus likely located above but close to the methane homopause. The H₂ auroral brightness in the 800-1700 Å bandwidth varies from 2.9 kR to 139 kR, comparable to values derived from FUV Faint Object Camera (FOC) and STIS images.  相似文献   

Helium abundance variations in the solar wind     

G. Borrini  J. T. Gosling  S. J. Bame  W. C. Feldman 《Solar physics》1983,83(2):367-378

Helium abundance variations in the solar wind have been studied using data obtained with Los Alamos plasma instrumentation on IMP 6, 7, and 8 from 1971 through 1978. For the first time, average flow characteristics have been determined as a function of helium abundance, A(He). Low and average values of A(He) are each preferentially identified with a different characteristic plasma ‘state’ these correspond to what have previously been recognized as the signatures of interplanetary magnetic field polarity reversals and high speed streams, respectively. Helium enhancements at 1 AU also can be identified with a characteristic plasma state, which includes high magnetic field intensity and low proton temperature. This is further evidence that such enhancements are a signal of coronal transient mass ejections. Long-term averages of A(He) at least partially reflect the relative frequency with which coronal streamers, holes, and transients extend their influence into the ecliptic plane at 1 AU. As a result, there is a real and pronounced solar cycle variation of solar wind H(He).  相似文献   

The Bragg Crystal Spectrometer for SOLAR-A     

J. L. Culhane  E. Hiei  G. A. Doschek  A. M. Cruise  Y. Ogawara  Y. Uchida  R. D. Bentley  C. M. Brown  J. Lang  T. Watanabe  J. A. Bowles  R. D. Deslattes  U. Feldman  A. Fludra  P. Guttridge  A. Henins  J. Lapington  J. Magraw  J. T. Mariska  J. Payne  K. J. H. Phillips  P. Sheather  K. Slater  K. Tanaka  E. Towndrow  M. W. Trow  A. Yamaguchi 《Solar physics》1991,136(1):89-104

The Bragg Crystal Spectrometer (BCS) is one of the instruments which makes up the scientific payload of the SOLAR-A mission. The spectrometer employs four bent germanium crystals, views the whole Sun and observes the resonance line complexes of H-like Fexxvi and He-like Fexxv, Caxix, and Sxv in four narrow wavelength ranges with a resolving power (/) of between 3000 and 6000. The spectrometer has approaching ten times better sensitivity than that of previous instruments thus permitting a time resolution of better than 1 s to be achieved. The principal aim is the measurement of the properties of the 10 to 50 million K plasma created in solar flares with special emphasis on the heating and dynamics of the plasma during the impulsive phase. This paper summarizes the scientific objectives of the BCS and describes the design, characteristics, and performance of the spectrometers.After the launch the name of SOLAR-A has been changed to YOHKOH.Tragically Professor K. Tanaka died on January 2, 1990.  相似文献   

Diagenetic mass transport in the southern San Joaquin basin, California, U.S.A.: Implications from the strontium isotopic composition of modern pore fluids     

Mark D. Feldman  S.-T. Kwon  J.R. Boles  G.R. Tilton 《Chemical Geology》1993,110(4):329-343

Three types of chemically and isotopically distinct pore fluids from the southern San Joaquin basin previously recognized by J.B. Fisher and J.R. Boles also have distinctive ⁸⁷Sr/⁸⁶Sr ratios and Sr concentrations. Meteoric fluids have stable isotopic compositions which lie on or near the meteoric water line and low chlorinities. Sr concentrations are between 0.01 and 2.6 mg l⁻¹, and ⁸⁷Sr/⁸⁶Sr ratios range from 0.7061 to 0.7078. Diagenetically modified connate marine fluids have δD-and δ¹⁸O-values more positive than −35‰ and 0‰, respectively, and have chlorinities generally comparable to seawater. Sr concentration are much higher than the meteoric group (16–198 mg l⁻¹), although the ⁸⁷Sr/⁸⁶Sr ratios (0.7070–0.7081) are not distinctive. Mixed meteoric-modified connate fluids have δD, δ¹⁸O and chlorinity intermediate between the meteoric and modified connate groups. Sr concentrations are also intermediate, between 16 and 22 mg l⁻¹, but ⁸⁷Sr/⁸⁶Sr ratios (0.7080–0.7087) are generally more radiogenic than either the meteoric or modified connate groups.

All of the fluids have ⁸⁷Sr/⁸⁶Sr ratios comparable to or lower than Tertiary seawater. Alteration of detrital plagioclase is the probable origin of the low isotopic ratios. Mass-balance calculations based on the Sr data suggest that essentially no transport of Sr occurred during diagenesis of sandstones containing modified connate pore fluids, while large amounts of Sr have been transported out of meteoric reservoirs by fluid flow. The chemically anomalous mixed meteoric-modified connate fluids contain the most radiogenic strontium in the basin. These fluids are spatially associated with major faults, and may represent clay mineral dehydration waters which have been transported upward from greater depth.

These results suggest that the three types of fluids identified by Fisher and Boles represent three distinct mass transport regimes: a largely stagnant deep-basin system containing modified connate pore fluids; an actively recharging meteoric system along the basin flanks; and a third system restricted to the southern basin which may be characterized by largescale cross-formational fluid flow, rather than dilution by meteoric waters.  相似文献   

Statistical analysis of multiple absorption spectra in QSO     

Giora Shaviv  Uri Feldman  Ben-Zion Koslovsky 《Astrophysics and Space Science》1972,19(1):159-163

A detailed statistical analysis was carried out for several quasars with multiple red-shifts systems. The most important results are (a) The statistical test of the reliability of az-system is very sensitive toz, the total number of absorption lines, the selected set of reference lines and the number of identified lines (b) The standard deviation from , the average number of randomz-systems per random spectra is larger than . Consequently, the reliability of any system depends predominately on and not on .  相似文献