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At least 1,009 records · Page 56Linked to original sources
PubMed1995

Signatures of the martian atmosphere in glass of the Zagami meteorite.

Isotopic signatures of nitrogen, argon, and xenon have been determined in separated millimeter-sized pockets of shock-melted glass in a recently identified lithology of the meteorite Zagami, a shergottite. The ratio of nitrogen-15 to nitrogen-14, which is at least 282 per mil larger than the terrestrial value, the ratio of xenon-129 to xenon-132 = 2.40, and the argon isotopic abundances match the signatures previously observed in the glassy lithology of the Antarctic shergottite EETA 79001. These results show that the signatures in EETA 79001 are not unique but characterize the trapped gas component in shock-melted glass of shergottites. The isotopic and elemental ratios of nitrogen, argon, and xenon closely resemble the Viking spacecraft data for the martian atmosphere and provide compelling evidence for a martian origin of the two shergottites and, by extension, of the meteorites in the shergottites-nakhlites-chassignites (SNC) group.

Argon↗
PubMed1995

Ulysses above the sun's south pole: an introduction.

Ulysses has explored the field and particle environment of the sun's polar region. The solar wind speed was fast and nearly constant above -50 degrees latitude. Compositional differences were observed in slow (low-latitude) solar wind and in fast (high-latitude) solar wind. The radial magnetic field did not change with latitude, implying that polar cap magnetic fields are transported toward the equator. The intensity of galactic cosmic rays was nearly independent of latitude. Their access to the polar region is opposed by outward-traveling, large amplitude waves in the magnetic field.

Cosmic Radiation↗
PubMed1995

Over the southern solar pole: low-energy interplanetary charged particles.

The heliosphere instrument for spectrum, composition, and anisotropy (HISCALE) recorded the fluxes of low-energy ions and electrons (> 50 kiloelectron volts) when Ulysses crossed the southern solar polar region and revealed that the large-scale structure of the heliosphere to at least approximately -75 degrees was significantly influenced by the near-equatorial heliospheric current sheet. Electrons in particular were accelerated by the current sheet-produced and poleward-propagating interplanetary reverse shock at helioradii far from the Ulysses location. At heliolatitudes higher than approximately -75 degrees on the Ulysses ascent to the pole and approximately -50 degrees on the descent, small, less regular enhancements of the lowest energy electron fluxes were measured whose relations to the current sheet were less clear. The anomalous component of low-energy (approximately 2 to 5 megaelectron volts per nucleon) oxygen flux at the highest heliolatitudes was found to be approximately 10(-8) [per square centimeter per second per steradian (per kiloelectronvolt per nucleon)]; the anomalous Ne/O ratio was approximately 0.25.

Electrons↗
PubMed1995

Observations of energetic particles with EPAC on Ulysses in polar latitudes of the heliosphere.

Measurements with the Energetic Particle Composition instrument (EPAC) aboard Ulysses show particles from near the ecliptic that were apparently accelerated by shocks associated with a corotating interaction region. The particles were detected together with the shocks and even when shocks no longer arrived at Ulysses up to -65 degrees of heliographic latitude but not beyond. Particles could have reached these latitudes along magnetic fields; such connections to the outer lower latitude heliosphere evidently do not exist above that latitude. The accelerated streams have composition similar to solar wind abundances, no dispersion, and a net inward anisotropy. The underlying composition between the recurrent stream is similar to the anomalous component of cosmic rays. The channel sensitive to high-energy protons (> 230 megaelectron volts) shows a 26-day variation of the flux superimposed on the heliospheric modulation of galactic ions.

Acceleration↗
PubMed1995

The southern high-speed stream: results from the SWICS instrument on Ulysses.

The high-speed solar wind streaming from the southern coronal hole was remarkably uniform and steady and was confined by a sharp boundary that extended to the corona and chromosphere. Charge state measurements indicate that the electron temperature in this coronal hole reached a maximum of about 1.5 million kelvin within 3 solar radii of the sun. This result, combined with the observed lack of depletion of heavy elements, suggests that an additional source of momentum is required to accelerate the polar wind.

Elements↗
PubMed1995

Water on the sun.

High-resolution infrared spectra of sunspot umbrae have been recorded with the 1-meter Fourier transform spectrometer on Kitt Peak. The spectra contain a very large number of water absorption features originating on the sun. These lines have been assigned to the pure rotation and the vibration-rotation transitions of hot water by comparison with high-temperature laboratory emission spectra.

Solar System↗
PubMed1995

Collision of comet Shoemaker-Levy 9 with Jupiter observed by the NASA infrared telescope facility.

The National Aeronautics and Space Administration (NASA) Infrared Telescope Facility was used to investigate the collision of comet Shoemaker-Levy 9 with Jupiter from 12 July to 7 August 1994. Strong thermal infrared emission lasting several minutes was observed after the impacts of fragments C, G, and R. All impacts warmed the stratosphere and some the troposphere up to several degrees. The abundance of stratospheric ammonia increased by more than 50 times. Impact-related particles extended up to a level where the atmospheric pressure measured several millibars. The north polar near-infrared aurora brightened by nearly a factor of 5 a week after the impacts.

Ammonia↗
PubMed1995

The Hubble Space Telescope (HST) observing campaign on comet Shoemaker-Levy 9.

The Hubble Space Telescope made systematic observations of the split comet P/Shoemaker-Levy 9 (SL9) (P designates a periodic comet) starting in July 1993 and continuing through mid-July 1994 when the fragments plunged into Jupiter's atmosphere. Deconvolutions of Wide Field Planetary Camera images indicate that the diameters of some fragments may have been as large as approximately 2 to 4 kilometers, assuming a geometric albedo of 4 percent, but significantly smaller values (that is, < 1 kilometer) cannot be ruled out. Most of the fragments (or nuclei) were embedded in circularly symmetric inner comae from July 1993 until late June 1994, implying that there was continuous, but weak, cometary activity. At least a few nuclei fragmented into separate, condensed objects well after the breakup of the SL9 parent body, which argues against the hypothesis that the SL9 fragments were swarms of debris with no dominant, central bodies. Spectroscopic observations taken on 14 July 1994 showed an outburst in magnesium ion emission that was followed closely by a threefold increase in continuum emission, which may have been caused by the electrostatic charging and subsequent explosion of dust as the comet passed from interplanetary space into the jovian magnetosphere. No OH emission was detected, but the derived upper limit on the H2O production rate of approximately 10(27) molecules per second does not necessarily imply that the object was water-poor.

Atmosphere↗
PubMed1995

HST imaging of atmospheric phenomena created by the impact of comet Shoemaker-Levy 9.

Hubble Space Telescope (HST) images reveal major atmospheric changes created by the collision of comet Shoemaker-Levy 9 with Jupiter. Plumes rose to 3000 kilometers with ejection velocities on the order of 10 kilometers second-1; some plumes were visible in the shadow of Jupiter before rising into sunlight. During some impacts, the incoming bolide may have been detected. Impact times were on average about 8 minutes later than predicted. Atmospheric waves were seen with a wave front speed of 454 +/- 20 meters second-1. The HST images reveal impact site evolution and record the overall change in Jupiter's appearance as a result of the bombardment.

Atmosphere↗
PubMed1995

Impact debris particles in Jupiter's stratosphere.

The aftermath of the impacts of periodic comet Shoemaker-Levy 9 on Jupiter was studied with the Wide Field Planetary Camera 2 on the Hubble Space Telescope. The impact debris particles may owe their dark brown color to organic material rich in sulfur and nitrogen. The total volume of aerosol 1 day after the last impact is equal to the volume of a sphere of radius 0.5 kilometer. In the optically thick core regions, the particle mean radius is between 0.15 and 0.3 micrometer, and the aerosol is spread over many scale heights, from approximately 1 millibar to 200 millibars of pressure or more. Particle coagulation can account for the evolution of particle radius and total optical depth during the month following the impacts.

Atmosphere↗
PubMed1995

HST far-ultraviolet imaging of Jupiter during the impacts of comet Shoemaker-Levy 9.

Hubble Space Telescope far-ultraviolet images of Jupiter during the Shoemaker-Levy 9 impacts show the impact regions darkening over the 2 to 3 hours after the impact, becoming darker and more extended than at longer wavelengths, which indicates that ultraviolet-absorbing gases or aerosols are more extended, more absorbing, and at higher altitudes than the absorbers of visible light. Transient auroral emissions were observed near the magnetic conjugate point of the K impact site just after that impact. The global auroral activity was fainter than average during the impacts, and a variable auroral emission feature was observed inside the southern auroral oval preceding the impacts of fragments Q1 and Q2.

Atmosphere↗
PubMed1995

HST spectroscopic observations of Jupiter after the collision of comet Shoemaker-Levy 9.

Ultraviolet spectra obtained with the Hubble Space Telescope identified at least 10 molecules and atoms in the perturbed stratosphere near the G impact site, most never before observed in Jupiter. The large mass of sulfur-containing material, more than 10(14) grams in S2 alone, indicates that many of the sulfur-containing molecules S2, CS2, CS, H2S, and S+ may be derived from a sulfur-bearing parent molecule native to Jupiter. If so, the fragment must have penetrated at least as deep as the predicted NH4SH cloud at a pressure of approximately 1 to 2 bars. Stratospheric NH3 was also observed, which is consistent with fragment penetration below the cloud tops. Approximately 10(7) grams of neutral and ionized metals were observed in emission, including Mg II, Mg I, Si I, Fe I, and Fe II. Oxygen-containing molecules were conspicuous by their absence; upper limits for SO2, SO, CO, SiO, and H2O are derived.

Ammonia↗
PubMed1995

Response of the Io plasma torus to comet Shoemaker-Levy 9.

Spectroscopic and imaging observations of the Io plasma torus were made in June and July 1994 in conjunction with the encounter of periodic comet Shoemaker-Levy 9 with Jupiter. Characteristic emissions from sulfur and oxygen ions showed a decline of about 30 percent in the extreme ultraviolet and an increase of about 40 percent in the far ultraviolet relative to preimpact observations. Changes in the extreme ultraviolet may be indicative of small changes in the torus electron temperature as a result of quenching of electrons by dust associated with the comet passage. However, no new emission features indicative of fragment dust within the torus were detected. The characteristic torus morphology seen in ground-based imaging was typical of that observed in the past.

Atmosphere↗
PubMed1995

Auroral signature of comet Shoemaker-Levy 9 in the jovian magnetosphere.

The electrodynamic interaction of the dust and gas comae of comet Shoemaker-Levy 9 with the jovian magnetosphere was unique and different from the atmospheric effects. Early theoretical predictions of auroral-type processes on the comet magnetic field line and advanced modeling of the time-varying morphology of these lines allowed dedicated observations with the Hubble Space Telescope Wide Field Planetary Camera 2 and resulted in the detection of a bright auroral spot. In that respect, this observation of the surface signature of an externally triggered auroral process can be considered as a "magnetospheric active experiment" on Jupiter.

Extraterrestrial Environment↗
PubMed1995

The fragment R collision: W. M. Keck telescope observations of SL9.

The W. M. Keck telescope was used to observe the impact of comet Shoemaker-Levy 9 (SL9) fragment R at a wavelength of 2.3 micrometers on 21 July 1994. The data showed three outbursts. The first flash lasted about 40 seconds and was followed 1 minute after its peak by a second flash that lasted about 3 minutes. A third, longer lasting flare began 6 minutes after the first flash and lasted for 10 minutes. At its maximum brightness, the flare outshone Jupiter. The two short flashes are probably associated with the initial meteor trail and the subsequent fireball, respectively. The bright flare occurred when the impact site rotated into view. These data show that the explosion ejected material at least 1300 kilometers above the visible cloud tops. The luminosity of the impact site during the long bright flare was probably maintained by the release of gravitational potential energy, as this material fell back onto the lower atmosphere.

Atmosphere↗
PubMed1995

Seeing through the dust: martian crustal heterogeneity and links to the SNC meteorites.

Through the application of new analytical techniques to high spatial resolution imaging spectrometer data, the ferrous mineralogy of major volcanic terrains on Mars is shown to consist of significant fractions of both low- and high-calcium pyroxene. Changes in the relative abundances of these pyroxenes are observed for units of different age and morphology, even in regions with higher degrees of alteration and contamination from dust. Volcanic rocks with these characteristics are uncommon on Earth but are typical of the basaltic SNC meteorites (shergottites, nakhlites, and chassignites) thought to be from Mars. Thus, it is possible to infer, even through the veil of dust, that the SNC meteorites have mineralogic affinities to major volcanic provinces on Mars and are therefore truly representative of the heterogeneity observed on the surface of the "red planet".

Calcium↗
PubMed1992

Tektite-bearing, deep-water clastic unit at the Cretaceous-Tertiary boundary in northeastern Mexico.

The hypothesis of Cretaceous-Tertiary (K-T) boundary impact on Yucatán, Mexico, predicts that nearby sites should show evidence of proximal impact ejecta and disturbance by giant waves. An outcrop along the Arroyo el Mimbral in northeastern Mexico contains a layered clastic unit up to 3 m thick that interrupts a biostratigraphically complete pelagic-marl sequence deposited at more than 400 m water depth. The marls were found to be unsuitable for determining magnetostratigraphy, but foraminiferal biostratigraphy places the clastic unit precisely at the K-T boundary. We interpret this clastic unit as the deposit of a megawave or tsunami produced by an extraterrestrial impact. The clastic unit comprises three main subunits. (1) The basal "spherule bed" contains glass in the form of tektites and microtektites, glass spherules replaced by chlorite-smectite and calcite, and quartz grains showing probable shock features. This bed is interpreted as a channelized deposit of proximal ejecta. (2) A set of lenticular, massive, graded "laminated beds" contains intraclasts and abundant plant debris, and may be the result of megawave backwash that carried coarse debris from shallow parts of the continental margin into deeper water. (3) At the top, several thin "ripple beds" composed of fine sand are separated by clay drapes; they are interpreted as deposits of oscillating currents, perhaps a seiche. An iridium anomaly (921 +/- 23 pg/g) is observed at the top of the ripple beds. Our observations at the Mimbral locality support the hypothesis of a K-T impact on nearby Yucatán.

Animals↗