共查询到20条相似文献,搜索用时 31 毫秒
1.
Kivelson MG 《Science (New York, N.Y.)》2006,311(5766):1391-1392
Instruments on the Cassini spacecraft reveal that a heat source within Saturn's moon Enceladus powers a great plume of water ice particles and dust grains, a geyser that jets outward from the south polar regions and most likely serves as the dominant source of Saturn's E ring. The interaction of flowing magnetospheric plasma with the plume modifies the particle and field environment of Enceladus. The structure of Saturn's magnetosphere, the extended region of space threaded by magnetic-field lines linked to the planet, is shaped by the ion source at Enceladus, and magnetospheric dynamics may be affected by the rate at which fresh ions are created. 相似文献
2.
Wahlund JE Boström R Gustafsson G Gurnett DA Kurth WS Pedersen A Averkamp TF Hospodarsky GB Persoon AM Canu P Neubauer FM Dougherty MK Eriksson AI Morooka MW Gill R André M Eliasson L Müller-Wodarg I 《Science (New York, N.Y.)》2005,308(5724):986-989
The Cassini Radio and Plasma Wave Science (RPWS) Langmuir probe (LP) sensor observed the cold plasma environment around Titan during the first two flybys. The data show that conditions in Saturn's magnetosphere affect the structure and dynamics deep in the ionosphere of Titan. The maximum measured ionospheric electron number density reached 3800 per cubic centimeter near closest approach, and a complex chemistry was indicated. The electron temperature profiles are consistent with electron heat conduction from the hotter Titan wake. The ionospheric escape flux was estimated to be 10(25) ions per second. 相似文献
3.
Spahn F Schmidt J Albers N Hörning M Makuch M Seiss M Kempf S Srama R Dikarev V Helfert S Moragas-Klostermeyer G Krivov AV Sremcevic M Tuzzolino AJ Economou T Grün E 《Science (New York, N.Y.)》2006,311(5766):1416-1418
During Cassini's close flyby of Enceladus on 14 July 2005, the High Rate Detector of the Cosmic Dust Analyzer registered micron-sized dust particles enveloping this satellite. The dust impact rate peaked about 1 minute before the closest approach of the spacecraft to the moon. This asymmetric signature is consistent with a locally enhanced dust production in the south polar region of Enceladus. Other Cassini experiments revealed evidence for geophysical activities near Enceladus' south pole: a high surface temperature and a release of water gas. Production or release of dust particles related to these processes may provide the dominant source of Saturn's E ring. 相似文献
4.
Hansen CJ Esposito L Stewart AI Colwell J Hendrix A Pryor W Shemansky D West R 《Science (New York, N.Y.)》2006,311(5766):1422-1425
The Cassini spacecraft flew close to Saturn's small moon Enceladus three times in 2005. Cassini's UltraViolet Imaging Spectrograph observed stellar occultations on two flybys and confirmed the existence, composition, and regionally confined nature of a water vapor plume in the south polar region of Enceladus. This plume provides an adequate amount of water to resupply losses from Saturn's E ring and to be the dominant source of the neutral OH and atomic oxygen that fill the Saturnian system. 相似文献
5.
Krimigis SM Mitchell DG Hamilton DC Krupp N Livi S Roelof EC Dandouras J Armstrong TP Mauk BH Paranicas C Brandt PC Bolton S Cheng AF Choo T Gloeckler G Hayes J Hsieh KC Ip WH Jaskulek S Keath EP Kirsch E Kusterer M Lagg A Lanzerotti LJ Lavallee D Manweiler J McEntire RW Rasmuss W Saur J Turner FS Williams DJ Woch J 《Science (New York, N.Y.)》2005,307(5713):1270-1273
The Magnetospheric Imaging Instrument (MIMI) onboard the Cassini spacecraft observed the saturnian magnetosphere from January 2004 until Saturn orbit insertion (SOI) on 1 July 2004. The MIMI sensors observed frequent energetic particle activity in interplanetary space for several months before SOI. When the imaging sensor was switched to its energetic neutral atom (ENA) operating mode on 20 February 2004, at approximately 10(3) times Saturn's radius RS (0.43 astronomical units), a weak but persistent signal was observed from the magnetosphere. About 10 days before SOI, the magnetosphere exhibited a day-night asymmetry that varied with an approximately 11-hour periodicity. Once Cassini entered the magnetosphere, in situ measurements showed high concentrations of H+, H2+, O+, OH+, and H2O+ and low concentrations of N+. The radial dependence of ion intensity profiles implies neutral gas densities sufficient to produce high loss rates of trapped ions from the middle and inner magnetosphere. ENA imaging has revealed a radiation belt that resides inward of the D ring and is probably the result of double charge exchange between the main radiation belt and the upper layers of Saturn's exosphere. 相似文献
6.
Mitchell DG Brandt PC Roelof EC Dandouras J Krimigis SM Mauk BH 《Science (New York, N.Y.)》2005,308(5724):989-992
The Cassini Magnetospheric Imaging Instrument (MIMI) observed the interaction of Saturn's largest moon, Titan, with Saturn's magnetosphere during two close flybys of Titan on 26 October and 13 December 2004. The MIMI Ion and Neutral Camera (INCA) continuously imaged the energetic neutral atoms (ENAs) generated by charge exchange reactions between the energetic, singly ionized trapped magnetospheric ions and the outer atmosphere, or exosphere, of Titan. The images reveal a halo of variable ENA emission about Titan's nearly collisionless outer atmosphere that fades at larger distances as the exospheric density decays exponentially. The altitude of the emissions varies, and they are not symmetrical about the moon, reflecting the complexity of the interactions between Titan's upper atmosphere and Saturn's space environment. 相似文献
7.
Porco CC Helfenstein P Thomas PC Ingersoll AP Wisdom J West R Neukum G Denk T Wagner R Roatsch T Kieffer S Turtle E McEwen A Johnson TV Rathbun J Veverka J Wilson D Perry J Spitale J Brahic A Burns JA Delgenio AD Dones L Murray CD Squyres S 《Science (New York, N.Y.)》2006,311(5766):1393-1401
Cassini has identified a geologically active province at the south pole of Saturn's moon Enceladus. In images acquired by the Imaging Science Subsystem (ISS), this region is circumscribed by a chain of folded ridges and troughs at approximately 55 degrees S latitude. The terrain southward of this boundary is distinguished by its albedo and color contrasts, elevated temperatures, extreme geologic youth, and narrow tectonic rifts that exhibit coarse-grained ice and coincide with the hottest temperatures measured in the region. Jets of fine icy particles that supply Saturn's E ring emanate from this province, carried aloft by water vapor probably venting from subsurface reservoirs of liquid water. The shape of Enceladus suggests a possible intense heating epoch in the past by capture into a 1:4 secondary spin/orbit resonance. 相似文献
8.
Spencer JR Pearl JC Segura M Flasar FM Mamoutkine A Romani P Buratti BJ Hendrix AR Spilker LJ Lopes RM 《Science (New York, N.Y.)》2006,311(5766):1401-1405
The Cassini spacecraft completed three close flybys of Saturn's enigmatic moon Enceladus between February and July 2005. On the third and closest flyby, on 14 July 2005, multiple Cassini instruments detected evidence for ongoing endogenic activity in a region centered on Enceladus' south pole. The polar region is the source of a plume of gas and dust, which probably emanates from prominent warm troughs seen on the surface. Cassini's Composite Infrared Spectrometer (CIRS) detected 3 to 7 gigawatts of thermal emission from the south polar troughs at temperatures up to 145 kelvin or higher, making Enceladus only the third known solid planetary body-after Earth and Io-that is sufficiently geologically active for its internal heat to be detected by remote sensing. If the plume is generated by the sublimation of water ice and if the sublimation source is visible to CIRS, then sublimation temperatures of at least 180 kelvin are required. 相似文献
9.
Dougherty MK Khurana KK Neubauer FM Russell CT Saur J Leisner JS Burton ME 《Science (New York, N.Y.)》2006,311(5766):1406-1409
The Cassini magnetometer has detected the interaction of the magnetospheric plasma of Saturn with an atmospheric plume at the icy moon Enceladus. This unanticipated finding, made on a distant flyby, was subsequently confirmed during two follow-on flybys, one very close to Enceladus. The magnetometer data are consistent with local outgassing activity via a plume from the surface of the moon near its south pole, as confirmed by other Cassini instruments. 相似文献
10.
Cassini Imaging Science: initial results on Saturn's atmosphere 总被引:1,自引:0,他引:1
Porco CC Baker E Barbara J Beurle K Brahic A Burns JA Charnoz S Cooper N Dawson DD Del Genio AD Denk T Dones L Dyudina U Evans MW Giese B Grazier K Helfenstein P Ingersoll AP Jacobson RA Johnson TV McEwen A Murray CD Neukum G Owen WM Perry J Roatsch T Spitale J Squyres S Thomas P Tiscareno M Turtle E Vasavada AR Veverka J Wagner R West R 《Science (New York, N.Y.)》2005,307(5713):1243-1247
The Cassini Imaging Science Subsystem (ISS) began observing Saturn in early February 2004. From analysis of cloud motions through early October 2004, we report vertical wind shear in Saturn's equatorial jet and a maximum wind speed of approximately 375 meters per second, a value that differs from both Hubble Space Telescope and Voyager values. We also report a particularly active narrow southern mid-latitude region in which dark ovals are observed both to merge with each other and to arise from the eruptions of large, bright storms. Bright storm eruptions are correlated with Saturn's electrostatic discharges, which are thought to originate from lightning. 相似文献
11.
Brown RH Clark RN Buratti BJ Cruikshank DP Barnes JW Mastrapa RM Bauer J Newman S Momary T Baines KH Bellucci G Capaccioni F Cerroni P Combes M Coradini A Drossart P Formisano V Jaumann R Langevin Y Matson DL McCord TB Nelson RM Nicholson PD Sicardy B Sotin C 《Science (New York, N.Y.)》2006,311(5766):1425-1428
Observations of Saturn's satellite Enceladus using Cassini's Visual and Infrared Mapping Spectrometer instrument were obtained during three flybys of Enceladus in 2005. Enceladus' surface is composed mostly of nearly pure water ice except near its south pole, where there are light organics, CO2, and amorphous and crystalline water ice, particularly in the region dubbed the "tiger stripes." An upper limit of 5 precipitable nanometers is derived for CO in the atmospheric column above Enceladus, and 2% for NH3 in global surface deposits. Upper limits of 140 kelvin (for a filled pixel) are derived for the temperatures in the tiger stripes. 相似文献
12.
Krimigis SM Armstrong TP Axford WI Bostrom CO Gloeckler G Keath EP Lanzerotti LJ Carbary JF Hamilton DC Roelof EC 《Science (New York, N.Y.)》1981,212(4491):225-231
The low-energy charged particle instrument on Voyager 1 measured low-energy electrons and ions (energies >/= 26 and >/= 40 kiloelectron volts, respectively) in Saturn's magnetosphere. The first-order ion anisotropies on the dayside are generally in the corotation direction with the amplitude decreasing with decreasing distance to the planet. The ion pitch-angle distributions generally peak at 90 degrees , whereas the electron distributions tend to have field-aligned bidirectional maxima outside the L shell of Rhea. A large decrease in particle fluxes is seen near the L shell of Titan, while selective particle absorption (least affecting the lowest energy ions) is observed at the L shells of Rhea, Dione, and Tethys. The phase space density of ions with values of the first invariant in the range approximately 300 to 1000 million electron volts per gauss is consistent with a source in the outer magnetosphere. The ion population at higher energies (>/= 200 kiloelectron volts per nucleon) consists primarily of protons, molecular hydrogen, and helium. Spectra of all ion species exhibit an energy cutoff at energies >/= 2 million electron volts. The proton-to-helium ratio at equal energy per nucleon is larger (up to approximately 5 x 10(3)) than seen in other magnetospheres and is consistent with a local (nonsolar wind) proton source. In contrast to the magnetospheres of Jupiter and Earth, there are no lobe regions essentially devoid of particles in Saturn's nighttime magnetosphere. Electron pitch-angle distributions are generally bidirectional andfield-aligned, indicating closed field lines at high latitudes. Ions in this region are generally moving toward Saturn, while in the magnetosheath they exhibit strong antisunward streaming which is inconsistent with purely convective flows. Fluxes of magnetospheric ions downstream from the bow shock are present over distances >/= 200 Saturn radii from the planet. Novel features identified in the Saturnian magnetosphere include a mantle of low-energy particles extending inward from the dayside magnetopause to approximately 17 Saturn radii, at least two intensity dropouts occurring approximately 11 hours apart in the nighttime magnetosphere, and a pervasive population of energetic molecular hydrogen. 相似文献
13.
Lorenz RD Stiles BW Kirk RL Allison MD Del Marmo PP Iess L Lunine JI Ostro SJ Hensley S 《Science (New York, N.Y.)》2008,319(5870):1649-1651
Cassini radar observations of Saturn's moon Titan over several years show that its rotational period is changing and is different from its orbital period. The present-day rotation period difference from synchronous spin leads to a shift of approximately 0.36 degrees per year in apparent longitude and is consistent with seasonal exchange of angular momentum between the surface and Titan's dense superrotating atmosphere, but only if Titan's crust is decoupled from the core by an internal water ocean like that on Europa. 相似文献
14.
Tyler GL Eshleman VR Anderson JD Levy GS Lindal GF Wood GE Croft TA 《Science (New York, N.Y.)》1981,212(4491):201-206
Voyager 1 radio occultation measurements of Titan's equatorial atmosphere successfully probed to the surface, which is provisionally placed at a radius of 2570 kilometers. Derived scale heights plus other experimental and theoretical results indicate that molecular nitrogen is the predominant atmospheric constituent. The surface pressure and temperature appear to be about 1.6 bars and 93 K, respectively. The main clouds are probably methane ice, although some condensation of nitrogen cannot be ruled out. Solar abundance arguments suggest and the measurements allow large quantities of surface methane near its triple-point temperature, so that the three phases of methane could play roles in the atmosphere and on the surface of Titan similar to those of water on Earth. Radio occultation measurements of Saturn's atmosphere near 75 degrees south latitude reached a maximum pressure of 1.4 bars, where the temperature is about 156 K. The minimum temperature is about 91 K near the 60-millibar pressure level. The measured part of the polar ionosphere of Saturn has a peak electron concentration of 2.3 x 10(4) per cubic centimeter at an altitude of 2500 kilometers above the 1-bar level in the atmosphere, and a plasma scale height at the top of the ionosphere of 560 kilometers. Attenuation of monochromatic radiation at a wavelength of 3.6 centimeters propagating obliquely through Saturn's rings is consistent with traditional values for the normal optical depth of the rings, but the near-forward scattering of this radiation by the rings indicates effective scattering particles with larger than expected diameters of 10, 8, and 2 meters in the A ring, the outer Cassini division, and the C ring, respectively. Preliminary analysis of the radio tracking data yields new values for the masses of Rhea and Titan of 4.4 +/- 0.3 x 10(-6) and 236.64 +/- 0.08 x 10(-6) times the mass of Saturn. Corresponding values for the mean densities of these objects are 1.33 +/- 0.10 and about 1.89 grams per cubic centimeter. The density of Rhea is consistent with a solar-composition mix of anhydrous rock and volatiles, while Titan is apparently enriched in silicates relative to the solar composition. 相似文献
15.
Waite JH Cravens TE Ip WH Kasprzak WT Luhmann JG McNutt RL Niemann HB Yelle RV Mueller-Wodarg I Ledvina SA Scherer S 《Science (New York, N.Y.)》2005,307(5713):1260-1262
Ions were detected in the vicinity of Saturn's A ring by the Ion and Neutral Mass Spectrometer (INMS) instrument onboard the Cassini Orbiter during the spacecraft's passage over the rings. The INMS saw signatures of molecular and atomic oxygen ions and of protons, thus demonstrating the existence of an ionosphere associated with the A ring. A likely explanation for these ions is photoionization by solar ultraviolet radiation of neutral O2 molecules associated with a tenuous ring atmosphere. INMS neutral measurements made during the ring encounter are dominated by a background signal. 相似文献
16.
Saturn's diffuse E ring spans the region between 3 and 8 saturnian radii (R(s)), has its peak brightness near the orbit of the satellite Enceladus (3.95 R(s)), and is thought to be composed primarily of icy particles 1.0 +/- 0.3 micrometers in radius. Such particles are shown to move periodically along highly elliptical paths that cross the orbits of several saturnian satellites; the resulting energetic collisions of E ring particles with embedded satellites are capable of sustaining the E ring at its current optical depth. With several reasonable assumptions, this model naturally selects Enceladus as the primary source of ring material and may also provide mechanisms that explain the generation of the unusual amount of submicrometer dust in the neighboring F and G rings, the excess of OH molecules observed within the E ring, and the orbital brightness variations of nearby satellites. 相似文献
17.
Krimigis SM Armstrong TP Axford WI Bostrom CO Gloeckler G Keath EP Lanzerotti LJ Carbary JF Hamilton DC Roelof EC 《Science (New York, N.Y.)》1982,215(4532):571-577
The low-energy charged particle instrument on Voyager 2 measured low-energy electrons and ions (energies greater, similar 22 and greater, similar 28 kiloelectron volts, respectively) in Saturn's magnetosphere. The magnetosphere structure and particle population were modified from those observed during the Voyager 1 encounter in November 1980 but in a manner consistent with the same global morphology. Major results include the following. (i) A region containing an extremely hot ( approximately 30 to 50 kiloelectron volts) plasma was identified and extends from the orbit of Tethys outward past the orbit of Rhea. (ii) The low-energy ion mantle found by Voyager 1 to extend approximately 7 Saturn radii inside the dayside magnetosphere was again observed on Voyager 2, but it was considerably hotter ( approximately 30 kiloelectron volts), and there was an indication of a cooler ( < 20 kiloelectron volts) ion mantle on the nightside. (iii) At energies greater, similar 200 kiloelectron volts per nucleon, H(1), H(2), and H(3) (molecular hydrogen), helium, carbon, and oxygen are important constituents in the Saturnian magnetosphere. The presence of both H(2) and H(3) suggests that the Saturnian ionosphere feeds plasma into the magnetosphere, but relative abundances of the energetic helium, carbon, and oxygen ions are consistent with a solar wind origin. (iv) Low-energy ( approximately 22 to approximately 60 kiloelectron volts) electron flux enhancements observed between the L shells of Rhea and Tethys by Voyager 2 on the dayside were absent during the Voyager 1 encounter. (v) Persistent asymmetric pitch-angle distributions of electrons of 60 to 200 kiloelectron volts occur in the outer magnetosphere in conjunction with the hot ion plasma torus. (vi) The spacecraft passed within approximately 1.1 degrees in longitude of the Tethys flux tube outbound and observed it to be empty of energetic ions and electrons; the microsignature of Enceladus inbound was also observed. (vii) There are large fluxes of electrons of approximately 1.5 million electron volts and smaller fluxes of electrons of approximately 10 million electron volts and of protons greater, similar 54 million electron volts inside the orbits of Enceladus and Mimas; all were sharply peaked perpendicular to the local magnetic field. (viii) In general, observed satellite absorption signatures were not located at positions predicted on the basis of dipole magnetic field models. 相似文献
18.
Teolis BD Jones GH Miles PF Tokar RL Magee BA Waite JH Roussos E Young DT Crary FJ Coates AJ Johnson RE Tseng WL Baragiola RA 《Science (New York, N.Y.)》2010,330(6012):1813-1815
The flyby measurements of the Cassini spacecraft at Saturn's moon Rhea reveal a tenuous oxygen (O(2))-carbon dioxide (CO(2)) atmosphere. The atmosphere appears to be sustained by chemical decomposition of the surface water ice under irradiation from Saturn's magnetospheric plasma. This in situ detection of an oxidizing atmosphere is consistent with remote observations of other icy bodies, such as Jupiter's moons Europa and Ganymede, and suggestive of a reservoir of radiolytic O(2) locked within Rhea's ice. The presence of CO(2) suggests radiolysis reactions between surface oxidants and organics or sputtering and/or outgassing of CO(2) endogenic to Rhea's ice. Observations of outflowing positive and negative ions give evidence for pickup ionization as a major atmospheric loss mechanism. 相似文献
19.
Nitrogen ions in charge states N(5+) and N(6+) have been accelerated in the Princeton Particle Accelerator to 4 and 7.4 billion electron volts (Gev), respectively. An external N(5+) beam of 1 x 10(6) particles per second has been obtained and focused to a 6-millimeter-diameter spot. The N(6+) beam was about 2 x 10(5) particles per second. The total charge-changing collision cross section of N(5+) in water vapor was determined as a function of ion energy. The improvement in vacuum necessary to increase the N(5+) beam at least tenfold was calculated. The N(6+) total cross section is probably smaller than that of N(5+) at the higher energies. 相似文献
20.
Waite JH Combi MR Ip WH Cravens TE McNutt RL Kasprzak W Yelle R Luhmann J Niemann H Gell D Magee B Fletcher G Lunine J Tseng WL 《Science (New York, N.Y.)》2006,311(5766):1419-1422
The Cassini spacecraft passed within 168.2 kilometers of the surface above the southern hemisphere at 19:55:22 universal time coordinated on 14 July 2005 during its closest approach to Enceladus. Before and after this time, a substantial atmospheric plume and coma were observed, detectable in the Ion and Neutral Mass Spectrometer (INMS) data set out to a distance of over 4000 kilometers from Enceladus. INMS data indicate that the atmospheric plume and coma are dominated by water, with significant amounts of carbon dioxide, an unidentified species with a mass-to-charge ratio of 28 daltons (either carbon monoxide or molecular nitrogen), and methane. Trace quantities (<1%) of acetylene and propane also appear to be present. Ammonia is present at a level that does not exceed 0.5%. The radial and angular distributions of the gas density near the closest approach, as well as other independent evidence, suggest a significant contribution to the plume from a source centered near the south polar cap, as distinct from a separately measured more uniform and possibly global source observed on the outbound leg of the flyby. 相似文献