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1.
Sarah M. Milkovich James W. Head III The HRSC Co-Investigator Team 《Planetary and Space Science》2008,56(2):266-288
We examine the stratigraphy of the polar layered deposits (PLD) within the north polar cap of Mars to assess its layer continuity, correlations, cyclicity and structure and implications for the recent climate record. PLD sequences characterized using Fourier analysis and curve shape matching algorithms show that layers correlate throughout the upper part of the PLD. We tested for cyclicity and found that the uppermost ∼300 m contain a dominant wavelength layer packet of ∼30 m, interpreted to be a climate signal related to the 51 kyr precession cycle. Directly below this region we document a section of polar layered deposits ∼100 m thick without a dominant periodic signal; this is interpreted to represent a phase of low net accumulation and lag deposits formed during the last ice age, about 0.5-2 Ma ago. We further analyzed layer structure by combining these results with three-dimensional determinations of layer orientation (strike and dip) to assess the internal stratigraphy of the PLD and its implications for polar history. We show that individual layers within the PLD stratigraphy are not horizontal (no dip) but rather show broad variation in elevation with distance. Correlations suggest that the layer strikes and dips broadly follow present cap surface topography. Local variations in layer orientations in the vicinity of the troughs suggest that (1) trough structures were present at the time of layer accumulation and (2) dips may have been influenced by ice flow and/or static ice accumulation in the presence of preexisting troughs. This new information favors models in which the troughs are long-term structures of the PLD rather than (1) recently eroded into the PLD, or (2) very active and laterally migrating around the PLD. Our results strongly support the hypothesis that significant volumes of polar volatiles are mobilized and transported equator-ward during periods of increased obliquity. Our results predict that the upper ∼300 m of the north polar PLD accumulated in the last 500 ka, yielding net accumulation rates of ∼0.06 cm/yr. The presence and albedo of the no periodic signal zone suggest that polar net accumulation rates are very low and that dust rich lag deposits form during periods of sustained high obliquity. Layer sequences in the south polar and equatorial regions are examined and compared to those in the north; rhythmic sequences are observed in both regions but no direct correlations to the dominant signals of the north polar deposits have yet been found. These new techniques and observations provide a paradigm for further analysis of recent polar history (the upper kilometer of the record) and a basis for extending assessments to the lower part of the polar deposits and to other cyclic deposits in the geological record of Mars. 相似文献
2.
《Planetary and Space Science》2007,55(10):1328-1345
The planetary fourier spectrometer (PFS) for the Mars express mission (MEX) is an infrared spectrometer operating in the wavelength range from 1.2 to 45 μm by means of two spectral channels, called SWC (short wavelength channel) and LWC (long wavelength channel), covering, respectively, 1.2–5.5 and 5.5–45 μm.The middle-spring Martian north polar cap (Ls∼40°) has been observed by PFS/MEX in illuminated conditions during orbit 452. The SWC spectra are here used to study the cap composition in terms of CO2 ice, H2O ice and dust content. Significant spectral variation is noted in the cap interior, and regions of varying CO2 ice grain sizes, water frost abundance, CO2 ice cover and dust contamination can be distinguished. In addition, we correlate the infrared spectra with an image acquired during the same orbit by the OMEGA imaging spectrometer and with the altimetry from MOLA data. Many of the spectra variations correlate with heterogeneities noted in the image, although significant spectral variations are not discernible in the visible. The data have been divided into five regions with different latitude ranges and strong similarities in the spectra, and then averaged. Bi-directional reflectance models have been run with the appropriate lighting geometry and used to fit the observed data, allowing for CO2 ice and H2O ice grain sizes, dust and H2O ice contaminations in the form of intimate granular mixtures and spatial mixtures.A wide annulus of dusty water ice surrounds the recessing CO2 seasonal cap. The inner cap exhibits a layered structure with a thin CO2 layer with varying concentrations of dark dust, on top of an H2O ice underneath ground. In the best-fits, the ices beneath the top layer have been considered as spatial mixtures. The results are still very good everywhere in the spectral range, except where the CO2 ice absorption coefficients are such that even a thin layer is enough to totally absorb the incoming radiation (i.e. the band is saturated). This only happens around 3800 cm−1, inside the strong 2.7-μm CO2 ice absorption band. The effect of finite snow depth has been investigated through a layered albedo model. The thickness of the CO2 ice deposits increases with latitude, ranging from 0.5–1 g cm−2 within region II to 60–80 g cm−2 within the highest-latitude (up to 84°N) region V.Region I is at the cap edge and extends from 65°N to 72°N latitude. No CO2 ice is present in this region, which consists of relatively large grains of water ice (20 μm), highly contaminated by dust (0.15 wt%). The adjacent region II is a narrow region [76–79°N] right at the edge of the north residual polar cap. This region is very distinct in the OMEGA image, where it appears to surround the whole residual cap. The CO2 ice features are barely visible in these spectra, except for the strong saturated 2.7 μm band. It basically consists of a thin layer of 5-mm CO2 ice on top of an H2O ice layer with the same composition as region I. A third interesting region III is found all along the shoulder of the residual cap [79–81°N]. It extends over 1.5 km in altitude and over only 2° of latitude and consists of CO2 ice with a large dust content. It is an admixture of CO2 ice (3–4 mm), with several tens of ppm by mass of water ice and more than 2 ppt by mass of dust. The surface temperatures have been retrieved from the LWC spectra for each observation. We found an increase in the surface temperature in this region, indicating a spatial mixture of cold CO2 ice and warmer dust/H2O ice. Region IV is close to the top of the residual cap [81–84°N]; it is much brighter than region III, with a dust content 10 times lower than the latter. The CO2 grain size is 3 mm and strong CO2 ice features are present in the data, indicating a thicker CO2 ice layer than in region II (1–2 g cm−2). The final region V is right at the top of the residual cap (⩾84°N). It is “pure” CO2 ice (no dust) of 5 mm grain sizes, with 30 ppm by weight of water ice. The CO2 ice features are very pronounced and the 2.7 μm band is saturated. The optical thickness is close to the semi-infinite limit (30–40 g cm−2). Assuming a snowpack density of 0.5 g cm−3, we get a minimum thickness of 1–2 cm for the top-layer of regions II and III, 4–10 cm for region IV, and ⩾60–80 cm thickness for region V. These values are in close agreement with several recent results for the south seasonal polar cap.These results should provide new, useful constraints in models of the Martian climate system and volatile cycles. 相似文献
3.
4.
We numerically explore the obliquity (axial tilt) variations of a hypothetical moonless Earth. Previous work has shown that the Earth’s Moon stabilizes Earth’s obliquity such that it remains within a narrow range, between 22.1° and 24.5°. Without lunar influence, a frequency map analysis by Laskar et al. (Laskar, J., Joutel, F., Robutel, P. [1993]. Nature 361, 615–617) showed that the obliquity could vary between 0° and 85°. This has left an impression in the astrobiology community that a big moon is necessary to maintain a habitable climate on an Earth-like planet. Using a modified version of the orbital integrator mercury, we calculate the obliquity evolution for moonless Earths with various initial conditions for up to 4 Gyr. We find that while obliquity varies significantly more than that of the actual Earth over 100,000 year timescales, the obliquity remains within a constrained range, typically 20–25° in extent, for timescales of hundreds of millions of years. None of our Solar System integrations in which planetary orbits behave in a typical manner show obliquity accessing more than 65% of the full range allowed by frequency-map analysis. The obliquities of moonless Earths that rotate in the retrograde direction are more stable than those of prograde rotators. The total obliquity range explored for moonless Earths with rotation periods less than 12 h is much less than that for slower-rotating moonless Earths. A large moon thus does not seem to be needed to stabilize the obliquity of an Earth-like planet on timescales relevant to the development of advanced life. 相似文献
5.
《New Astronomy》2007,12(7):556-561
The results of a long-term UBV photometric monitoring of the red supergiant (RSG) star V424 Lac are presented. V424 Lac shows multiperiodic brightness variations which can be attributed to pulsational oscillations. A much longer period (P = 1601 d), that allows us to classify this star as a long secondary period variable star (LSPV) has been also detected. The B − V and U − B color variations related to the long secondary period (LSP) are similar to those related to the shorter periods, supporting the pulsational nature of LSP. The long period brightness variation of V424 Lac is accompanied by a near-UV (NUV) excess, which was spectroscopically detected in a previous study [Massey, P., Plez, B., Levesque, E.M., et al., 2005. ApJ 634, 1286] and which is now found to be variable from photometry. On the basis of the results found for V424 Lac, the NUV excess recently found in a number of RSGs may be due not solely to circumstellar dust but may also have a contribution from a still undetected LSP variability. 相似文献
6.
We investigated gullies and their relationships to the atmospherically derived dust–ice mantle and aeolian features in the northwestern part of the Argyre basin. A detailed morphologic map of the Argyre study region allowed us to constrain the stratigraphic relationships and relative ages of gullies. In addition, we investigated the morphologic characteristics and orientations of all gullies in the Argyre study region. Maximum absolute ages for gullies were determined with crater size–frequency distribution measurements of the dust–ice mantle, which is the source material of gullies in the study area. Gullies only evolve from this mantle probably by melting of its ice content. Two different morphologies of pristine and degraded gullies were identified, mostly occurring on pole- and equatorward-facing slopes, respectively. We conclude that the morphologies and orientations were initiated either by a more rapid and extensive erosion of equatorward-facing gullies or by at least two generations of gullies with generally older gullies on equatorward-facing slopes and younger ones on pole-facing slopes. Different intensities of solar insolation on equator- and pole-facing slopes might be responsible for the different development of pristine and degraded gullies. Gullies in the study area generally have ages ?20 Ma. Some uncratered (and thus very young) aeolian dunes are superposed by a few gullies in some locations, indicating another even younger generation of gullies with an upper limit absolute model age of about <500 ka. 相似文献
7.
Over the past years the processes involved in the growth of planetesimals have extensively been studied in the laboratory. Based on these experiments, a dust-aggregate collision model was developed upon which computer simulations were based to evaluate how big protoplanetary dust aggregates can grow and to analyze which kinds of collisions are relevant in the solar nebula and are worth further studies in the laboratory. The sticking threshold velocity of millimeter-sized dust aggregates is one such critical value that have so far only theoretically been derived, as the relevant velocities could not be reached in the laboratory. We developed a microgravity experiment that allows us for the first time to study free collisions of mm-sized dust aggregates down to velocities of ～0.1 cm s?1 to assess this part of the protoplanetary dust evolution model. Here, we present the results of 125 free collisions between dust aggregates of 0.5–2 mm diameter. Seven collisions with velocities between 0.2 and 3 cm s?1 led to sticking, suggesting a transition from perfect sticking to perfect bouncing with a certain sticking probability instead of a sharp velocity threshold. We developed a model to explain the physical processes involved in dust-aggregate sticking, derived dynamical material properties of the dust aggregates from the results of the collisions, and deduced the velocity below which dust aggregates always stick. For millimeter-sized porous dust aggregates this velocity is 8 × 10?5 m s?1. 相似文献
8.
Explaining the redox imbalance between the H and O escape fluxes at Mars by the oxidation of methane
From a comparison between the different observations of Martian methane existing today, including the new TES methane maps (Fonti and Marzo, 2010), we show that all sets of data are globally consistent with each other, and that a well definite seasonal cycle of methane has been at work for at least 10 yr. With a simple model of the balance between the loss fluxes of H and O, using up-to-date values of the escape fluxes, we show that the long-standing enigma of the imbalance between H and O escape fluxes may be solved by assuming that the missing sink of oxygen is the oxidation of methane. If no H2 is released together with CH4, a good agreement is found between the present CH4 flux and the value imposed by the balance between H and O escape fluxes, an average over the last ≈103 yr. If H2 is released together with CH4, as expected if CH4 originates in serpentinization, the average level of CH4 during the last 103 yr should have been at least ten times lower than the present one. The lack of present H2 release could suggest a long-term storage of methane in the subsurface under the form of clathrates, whereas H2 has been lost to the atmosphere shortly after being produced. We suggest that the thin layer of CO2 ice covering the permanent southern polar cap could result from the release of methane since the end of the last obliquity transition (time scale: 1 Myr), at an average rate of 0.1 Mt yr?1, consistent with the values derived from: (i) the present observations of methane (time scale: 10 yr), (ii) the estimate from the observed imbalance between the H and O escape fluxes (time scale: 1 kyr). If so, the present release of methane from subsurface clathrates would have acted at a similar rate since at least 3 Myr. 相似文献
9.
This work is a continuation of our previous paper about brightening of Comet 17P/Holmes (Kossacki, K.J., Szutowicz, S. [2010]. Icarus 207, 320–340). In that paper we presented results of simulations indicating that the nonuniform crystallization of amorphous water ice itself is probably not sufficient for an explosion. In the present work we investigate the possibility that the explosion is caused by a rapid sublimation of the CO ice leading to the rise of gas pressure above the tensile strength of the nucleus. We simulated evolution of a model nucleus in the orbit of Comet 17P/Holmes. The nucleus is composed of water ice, carbon monoxide ice and dust and has the shape of an elongated ellipsoid. The simulations include crystallization of amorphous ice in the nucleus, changes of the dust mantle thickness, and sublimation of the CO ice. In our model CO is mantling grains composed of dust and amorphous water ice. Orientation of the nuclear spin axis in space is the same as derived in Moreno et al. (Moreno, F., Ortiz, J.L., Santos-Sanz, P., Morales, N., Vidal-Nunez, M.J., Lara, L.M., Gutierrez, P.J. [2008]. Astrophys. J. 677, L63–L66) for Comet Holmes during recent brightening event. Hence, the angle between the orbital and the equatorial planes of the comet is I = 95°, and the cometocentric solar longitude at perihelion is Φ = 210°. The calculations are performed for the south pole being the sub-solar point close to time of the outburst. Our computations indicate, that the CO pressure within the comet nucleus can rise to high values. When the layer between the dust mantle and the crystallization front of the amorphous water ice is very fine grained, few microns in radius, the CO pressure within the nucleus can exceed 10 kPa. This value is the lowest estimate for the tensile strength of the nucleus of Comet Holmes (Reach, W.T., Vaubaillon, J., Lisse, C.M., Holloway, M., Rho, J. [2010]. Icarus 208, 276–292). Hence, when the gas pressure reaches this value the nucleus may explode. 相似文献
10.
Hubble Space Telescope/Wide Field and Planetary Camera 2 (HST/WFPC2) images of Io obtained between 1995 and 2007 between 0.24 and 0.42 μm led to the detection of the Pele plume in reflected sunlight in 1995 and 1999; imaging of the Pele plume via absorption of jovian light in 1996 and 1999; detection of the Prometheus-type Pillan plume in reflected sunlight in 1997; and detection of the 2007 Pele-type Tvashtar plume eruption in reflected sunlight and via absorption of jovian light. Based on a detailed analysis of these observations we characterize and compare the gas and dust properties of each of the detected plumes. In each case, the brightness of the plumes in reflected sunlight is less at 0.26 μm than at 0.33 μm. Mie scattering analysis of the wavelength dependence of each plume’s reflectance signature suggests that range of particle sizes within the plumes is quite narrow. Assuming a normal distribution of particle sizes, the range of mean particle sizes is ～0.035–0.12 μm for the 1997 Pillan eruption, ～0.05–0.08 μm for the 1999 Pele and 2007 Tvasthar plumes, and ～0.05–0.11 μm for the 1995 Pele plume, and in each case the standard deviation in the particle size distribution is <15%. The Mie analysis also suggests that the 2007 Tvashtar eruption released ～109 g of sulfur dust, the 1999 Pele eruption released ～109 g of SO2 dust, the 1997 Pillan eruption released ～1010 g of SO2 dust, and the 1995 Pele plume may have released ～1010 g of SO2 dust. Analysis of the plume absorption signatures recorded in the F255W filter bandpass (0.24–0.28 μm) indicates that the opacity of the 2007 Tvashtar plume was 2× that of the 1996 and 1999 Pele plume eruptions. While the sulfur dust density estimated for the Tvashtar from the reflected sunlight data could have produced 61% of the observed plume opacity, <10% of the 1999 Pele F255W plume opacity could have resulted from the SO2 dust detected in the eruption. Accounting for the remaining F255W opacity level of the Pele and Tvasthar plumes based on SO2 and S2 gas absorption, the SO2 and S2 gas density inferred for each plume is almost equivalent corresponding to ～2–6 × 1016 cm?2 and 3–5 × 1015 cm?2, respectively, producing SO2 and S2 gas resurfacing rates ～0.04–0.2 cm yr?1 and 0.007–0.01 cm yr?1; and SO2 and S2 gas masses ～1–4 × 1010 g and ～2–3 × 109 g; for a total dust to gas ratio in the plumes ～10?1–10?2. The 2007 Tvashtar plume was detected by HST at ～380 ± 40 km in both reflected sunlight and absorbed jovian light; in 1999, the detected Pele plume altitude was 500 km in absorbed jovian light, but in reflected sunlight the detected height was ～2× lower. Thus, for the 1999 Pele plume, similar to the 1979 Voyager Pele plume observations, the most efficient dust reflections occurred in the region closest to the plume vent. The 0.33–0.42 μm brightness of the 1997 Pillan plume was 10–20× greater than the Pele or Tvashtar plumes, exceeding by a factor of 3 the average brightness levels observed within 200 km of 1979 Loki eruption vent. But, the 0.26 μm brightness of the 1997 Pillan plume in reflected sunlight was significantly lower than would be predicted by the dust scattering model. Presuming that the 0.26 μm brightness of the 1997 Pillan plume was attenuated by the eruption plume’s gas component, then an SO2 gas density ～3–6 × 1018 cm?2 is inferred from the data (for S2/SO2 ratios ?4%), comparable to the 0.3–2 × 1018 cm?2 SO2 density detected at Loki in 1979 (Pearl, J.C. et al. [1979]. Nature 280, 755; Lellouch et al., 1992), and producing an SO2 gas mass ～3–8 × 1011 g and an SO2 resurfacing rate ～8–23 cm yr?1. These results confirm the connection between high (?1017 cm?2) SO2 gas content and plumes that scatter strongly at nearly blue wavelengths, and it validates the occurrence of high density SO2 gas eruptions on Io. Noting that the SO2 gas content inferred from a spectrum of the 2003 Pillan plume was significantly lower ～2 × 1016 cm?2 (Jessup, K.L., Spencer, J., Yelle, R. [2007]. Icarus 192, 24–40); and that the Pillan caldera was flooded with fresh SO2 frost/slush just prior to the 1997 Pillan plume eruption (Geissler, P., McEwen, A., Phillips, C., Keszthelyi, L., Spencer, J. [2004a]. Icarus 169, 29–64; Phillips, C.B. [2000]. Voyager and Galileo SSI Views of Volcanic Resurfacing on Io and the Search for Geologic Activity at Europa. Ph.D. Thesis, Univ. of Ariz., Tucson); we propose that the density of SO2 gas released by this volcano is directly linked to the local SO2 frost abundance at the time of eruption. 相似文献
11.
We present a study of water vapour in the Venus troposphere obtained by modelling specific water vapour absorption bands within the 1.18 μm window. We compare the results with the normal technique of obtaining the abundance by matching the peak of the 1.18 μm window. Ground-based infrared imaging spectroscopy of the night side of Venus was obtained with the Anglo-Australian Telescope and IRIS2 instrument with a spectral resolving power of R ～ 2400. The spectra have been fitted with modelled spectra simulated using the radiative transfer model VSTAR. We find a best fit abundance of 31 ppmv (?6 +9 ppmv), which is in agreement with recent results by Bézard et al. (Bézard, B., Fedorova, A., Bertaux, J.-L., Rodin, A., Korablev, O. [2011]. Icarus, 216, 173–183) using VEX/SPICAV (R ～ 1700) and contrary to prior results by Bézard et al. (Bézard, B., de Bergh, C., Crisp, D., Maillard, J.P. [1990]. Nature, 345, 508–511) of 44 ppmv (±9 ppmv) using VEX/VIRTIS-M (R ～ 200) data analyses. Comparison studies are made between water vapour abundances determined from the peak of the 1.18 μm window and abundances determined from different water vapour absorption features within the near infrared window. We find that water vapour abundances determined over the peak of the 1. 18 μm window results in plots with less scatter than those of the individual water vapour features and that analyses conducted over some individual water vapour features are more sensitive to variation in water vapour than those over the peak of the 1. 18 μm window. No evidence for horizontal spatial variations across the night side of the disk are found within the limits of our data with the exception of a possible small decrease in water vapour from the equator to the north pole. We present spectral ratios that show water vapour absorption from within the lowest 4 km of the Venus atmosphere only, and discuss the possible existence of a decreasing water vapour concentration towards the surface. 相似文献
12.
L. Soret J.-C. Gérard F. Montmessin G. Piccioni P. Drossart J.-L. Bertaux 《Icarus》2012,217(2):849-855
The Visible and Infra-Red Thermal Imaging Spectrometer (VIRTIS) instrument on board the Venus Express spacecraft has measured the O2(a1Δ) nightglow distribution at 1.27 μm in the Venus mesosphere for more than two years. Nadir observations have been used to create a statistical map of the emission on Venus nightside. It appears that the statistical 1.6 MR maximum of the emission is located around the antisolar point. Limb observations provide information on the altitude and on the shape of the emission layer. We combine nadir observations essentially covering the southern hemisphere, corrected for the thermal emission of the lower atmosphere, with limb profiles of the northern hemisphere to generate a global map of the Venus nightside emission at 1.27 μm. Given all the O2(a1Δ) intensity profiles, O2(a1Δ) and O density profiles have been calculated and three-dimensional maps of metastable molecular and atomic oxygen densities have been generated. This global O density nightside distribution improves that available from the VTS3 model, which was based on measurements made above 145 km. The O2(a1Δ) hemispheric average density is 2.1 × 109 cm?3, with a maximum value of 6.5 × 109 cm?3 at 99.2 km. The O density profiles have been derived from the nightglow data using CO2 profiles from the empirical VTS3 model or from SPICAV stellar occultations. The O hemispheric average density is 1.9 × 1011 cm?3 in both cases, with a mean altitude of the peak located at 106.1 km and 103.4 km, respectively. These results tend to confirm the modeled values of 2.8 × 1011 cm?3 at 104 km and 2.0 × 1011 cm?3 at 110 km obtained by Brecht et al. [Brecht, A., Bougher, S.W., Gérard, J.-C., Parkinson, C.D., Rafkin, S., Foster, B., 2011a. J. Geophys. Res., in press] and Krasnopolsky [Krasnopolsky, V.A., 2010. Icarus 207, 17–27], respectively. Comparing the oxygen density map derived from the O2(a1Δ) nightglow observations, it appears that the morphology is very different and that the densities obtained in this study are about three times higher than those predicted by the VTS3 model. 相似文献
13.
Observations of Venus using the ultraviolet filter of the Venus Monitoring Camera (VMC) on ESA’s Venus Express Spacecraft (VEX) provide the best opportunity for study of the spatial and temporal distribution of the venusian unknown ultraviolet absorber since the Pioneer Venus (PV) mission. We compare the results of two sets of 125 radiative transfer models of the upper atmosphere of Venus to each pixel in a subset of VMC UV channel images. We use a quantitative best fit criterion based upon the notion that the distribution of the unknown absorber should be independent of the illumination and observing geometry. We use the product of the cosines of the incidence and emission angles and search for absorber distributions that are uncorrelated with this geometric parameter, finding that two models can describe the vertical distribution of the unknown absorber. One model is a well-mixed vertical profile above a pressure level of roughly 120 mb (～63 km). This is consistent with the altitude of photochemical formation of sulfuric acid. The second model describes it as a thin layer of pure UV absorber at a pressure level roughly around 24 mb (～71 km) and this altitude is consistent with the top of upper cloud deck. We find that the average abundance of unknown absorber in the equatorial region is 0.21 ± 0.04 optical depth and it decreases in the polar region to 0.08 ± 0.05 optical depth at 365 nm. 相似文献
14.
L.H. Ma 《New Astronomy》2009,14(1):1-3
Long-term variations of solar activity significantly affect terrestrial phenomena. Studies have shown cyclic components in solar activity and geophysical phenomena (e.g., the Schwabe, Hale, Gleissberg, and Suess cycles, and a cycle of about 2300 years). In this paper, the wavelet technique is employed to investigate the Gleissberg cycle in solar variations during 5000 BC–1995 AD. Analysis shows time-variable characteristics in the Gleissberg solar cycle over the period; no obvious correlation between the Gleissberg and Suess cycles has been found. 相似文献
15.
M.R. Sanad 《New Astronomy》2010,15(8):688-694
We present ultraviolet spectra of the AE Aqr binary system taken by the International Ultraviolet Explorer (IUE) during the period from 1978 to 1993, to accomplish a large scale study of what happens to the ultraviolet fluxes of different emission lines during different orbital phases. Five profiles of AE Aqr binary system show variations of line fluxes at different orbital phases are presented. We concentrated on studying N V emission line at 1240 Å, O I emission line at 1306 Å, He II emission line at 1640 Å, N III emission line at 1749 Å and Si III emission line at 1892 Å, produced in the line emitting gas (Eracleous et al., 1994, Eracleous and Horne, 1996), by calculating the line fluxes of these spectral lines. Our results show that there are spectral variations of line fluxes for the aforementioned emission lines at different times, similar to the light curves found for AE Aqr binary system by using ASCA, XMM – Newton and Chandra X-ray observations (Mauche, 2006). We attributed these spectral variations to the variations of the mass-transfer rate (Ikhsanov et al., 2004) and to the collisions between the compact blobs and the fluffy blobs, where the collisions are supersonic, shocking the gas, causing heating, then expansion (Eracleous et al., 1994, Eracleous and Horne, 1996, Horne, 2009). 相似文献
16.
D. Ulusen J.G. Luhmann Y.J. Ma K.E. Mandt J.H. Waite M.K. Dougherty J.E. Wahlund C.T. Russell T.E. Cravens N.J.T. Edberg K. Agren 《Icarus》2012,217(1):43-54
Recent papers suggest the significant variability of conditions in Saturn’s magnetosphere at the orbit of Titan. Because of this variability, it was expected that models would generally have a difficult time regularly comparing to data from the Titan flybys. However, we find that in contrast to this expectation, it appears that there is underlying organization of the interaction features roughly above ～1800 km (1.7 Rt) altitude by the average external field due to Saturn’s dipole moment. In this study, we analyze Cassini’s plasma and magnetic field data collected at 9 Titan encounters during which the external field is close to the ideal southward direction and compare these observations to the results from a 2-fluid (1 ion, 1 electron) 7-species MHD model simulations obtained under noon SLT conditions. Our comparative analysis shows that under noon SLT conditions the Titan plasma interaction can be viewed in two layers: an outer layer between 6400 and 1800 km where interaction features observed in the magnetic field are in basic agreement with a purely southward external field interaction and an inner layer below 1800 km where the magnetic field measurements show strong variations and deviate from the model predictions. Thus the basic features inferred from the Voyager 1 flyby seem to be generally present above ～1800 km in spite of the ongoing external variations from SLT excursions, time variability and magnetospheric current systems as long as a significant southward external field component is present. At around ～1800 km kinetic effects (such as mass loading and heavy ion pickup) and below 1800 km ionospheric effects (such as drag of ionospheric plasma due to coupling with neutral winds and/or magnetic memory of Titan’s ionosphere) complicate what is observed. 相似文献
17.
S. Keihm F. Tosi L. Kamp F. Capaccioni S. Gulkis D. Grassi M. Hofstadter G. Filacchione S. Lee S. Giuppi M. Janssen M. Capria 《Icarus》2012,221(1):395-404
The European Space Agency’s Rosetta spacecraft is the first Solar System mission to include instrumentation capable of measuring planetary thermal fluxes at both near-IR (VIRTIS) and submillimeter–millimeter (smm–mm, MIRO) wavelengths. Its primary mission is a 1 year reconnaissance of Comet 67P/Churyumov–Gerasimenko beginning in 2014. During a 2010 close fly-by of Asteroid 21 Lutetia, the VIRTIS and MIRO instruments provided complementary data that have been analyzed to produce a consistent model of Lutetia’s surface layer thermal and electrical properties, including a physical model of self-heating. VIRTIS dayside measurements provided highly resolved 1 K accuracy surface temperatures that required a low thermal inertia, I < 30 J/(K m2 s0.5). MIRO smm and mm measurements of polar night thermal fluxes produced constraints on Lutetia’s subsurface thermal properties to depths comparable to the seasonal thermal wave, yielding a model of I < 20 J/(K m2 s0.5) in the upper few centimeters, increasing with depth in a manner very similar to that of Earth’s Moon. Subsequent MIRO-based model predictions of the dayside surface temperatures reveal negative offsets of ～5–30 K from the higher VIRTIS-measurements. By adding surface roughness in the form of 50% fractional coverage of hemispherical mini-craters to the MIRO-based thermal model, sufficient self-heating is produced to largely remove the offsets relative to the VIRTIS measurements and also reproduce the thermal limb brightening features (relative to a smooth surface model) seen by VIRTIS. The Lutetia physical property constraints provided by the VIRTIS and MIRO data sets demonstrate the unique diagnostic capabilities of combined infrared and submillimeter/millimeter thermal flux measurements. 相似文献
18.
The geologic history of planetary surfaces is most effectively determined by joining geologic mapping and crater counting which provides an iterative, qualitative and quantitative method for defining relative ages and absolute model ages. Based on this approach, we present spatial and temporal details regarding the evolution of the Martian northern plains and surrounding regions.The highland–lowland boundary (HLB) formed during the pre-Noachian and was subsequently modified through various processes. The Nepenthes Mensae unit along the northern margins of the cratered highlands, was formed by HLB scarp-erosion, deposition of sedimentary and volcanic materials, and dissection by surface runoff between 3.81 and 3.65 Ga. Ages for giant polygons in Utopia and Acidalia Planitiae are 3.75 Ga and likely reflect the age of buried basement rocks. These buried lowland surfaces are comparable in age to those located closer to the HLB, where a much thinner, post-HLB deposit is mapped. The emplacement of the most extensive lowland surfaces ended between 3.75 and 3.4 Ga, based on densities of craters generally in diameter. Results from the polygonal terrain support the existence of a major lowland depocenter shortly after the pre-Noachian formation of the northern lowlands. In general, northern plains surfaces show gradually younger ages at lower elevations, consistent local to regional unit emplacement and resurfacing between 3.6 and 2.6 Ga. Elevation levels and morphology are not necessarily related, and variations in ages within the mapped units are found, especially in units formed and modified by multiple geological processes. Regardless, most of the youngest units in the northern lowlands are considered to be lavas, polar ice, or thick mantle deposits, arguing against the ocean theory during the Amazonian Period (younger than about 3.15 Ga).All ages measured in the closest vicinity of the steep dichotomy escarpment are also 3.7 Ga or older. The formation ages of volcanic flanks at the HLB (e.g., Alba Mons (3.6–3.4 Ga) and the last fan at Apollinaris Mons, 3.71 Ga) may give additional temporal constraint for the possible existence of any kind of Martian ocean before about 3.7 Ga. It seems to reflect the termination of a large-scale, precipitation-based hydrological cycle and major geologic processes related to such cycling. 相似文献
19.
Sub-millimeter 12CO (346 GHz) and 13CO (330 GHz) line absorptions, formed in the mesosphere and lower thermosphere of Venus (70–120 km), have been mapped across the nightside Venus disk during 2001–2009 inferior conjunctions, employing the James Clerk Maxwell Telescope (JCMT). Radiative transfer analysis of these thermal line absorptions supports temperature and CO mixing profile retrievals, as well as Doppler wind fields (described in the companion paper, Clancy et al., 2012). Temporal sampling over the hourly, daily, weekly and interannual timescales was obtained over 2001–2009. On timescales inferred as several weeks, we observe changes between very distinctive CO and temperature nightside distributions. Retrieved nightside CO, temperature distributions for January 2006 and August 2007 observations display strong local time, latitudinal gradients consistent with early morning (2–3 am), low-to-mid latitude (0–40NS) peaks of 100–200% in CO and 20–30 K in temperature. The temperature increases are most pronounced above 100 km altitudes, whereas CO variations extend from 105 km (top altitude of retrieval) down to below 80 km in the mesosphere. In contrast, the 2004 and 2009 periods of observation display modest temperature (5–10 K) and CO (30–60%) increases, that are centered on antisolar (midnight) local times and equatorial latitudes. Doppler wind derived global (zonal and should be SSAS) circulations from the same data do not exhibit variations correlated with these CO, temperature short-term variations. However, large-scale residual wind fields not fit by the zonal, SSAS circulations are observed in concert with the strong temperature, CO gradients observed in 2006 and 2007 (Clancy et al., 2010). These short term variations in nightside CO, temperature distributions may also be related to observed nightside variations in O2 airglow (Hueso, H., Sánchez-Lavega, A., Piccioni, G., Drossart, P., Gérard, J.C., Khatuntsev, I., Zasova, L., Migliorini, A. [2008]. J. Geophys. Res. 113, E00B02. doi:10.1029/2008JE003081) and upper mesospheric SO and SO2 layers (Sandor, B.J., Clancy, R.T., Moriarty-Schieven, G.H., Mills, F.P. [2010]. Icarus 208, 49–60).The retrieved temperature profiles also exhibit 20 K long-term (2001–2009) variations in nightside (whole disk) average mesospheric (80–95 km) temperatures, similar to 1982–1991 variations identified in previous millimeter CO line observations (Clancy et al., 1991). Global average diurnal variations in lower thermospheric temperatures and mesospheric CO abundances decreased by a factor-of-two between 2000–2002 versus 2007–2009 periods of combined dayside and nightside observations. The infrequency and still limited temporal extent of the observations make it difficult to assign specific timescales to such longer term variations, which may be associated with longer term variations observed for cloud top SO2 (Esposito, L.W., Bertaux, J.-L., Krasnopolsky, V., Moroz, V.I., Zasova, L.V. [1997]. Chemistry of lower atmosphere and clouds. In: Bougher, S.W., Hunten, D.M., Phillips, R.J. (Eds.), VENUS II, 1362pp) and mesospheric water vapor (Sandor, B.J., Clancy, R.T. [2005]. Icarus 177, 129–143) abundances. 相似文献
20.
Canada’s Near-Earth Object Surveillance Satellite (NEOSSat), set to launch in early 2012, will search for and track Near-Earth Objects (NEOs), tuning its search to best detect objects with a < 1.0 AU. In order to construct an optimal pointing strategy for NEOSSat, we needed more detailed information in the a < 1.0 AU region than the best current model (Bottke, W.F., Morbidelli, A., Jedicke, R., Petit, J.M., Levison, H.F., Michel, P., Metcalfe, T.S. [2002]. Icarus 156, 399–433) provides. We present here the NEOSSat-1.0 NEO orbital distribution model with larger statistics that permit finer resolution and less uncertainty, especially in the a < 1.0 AU region. We find that Amors = 30.1 ± 0.8%, Apollos = 63.3 ± 0.4%, Atens = 5.0 ± 0.3%, Atiras (0.718 < Q < 0.983 AU) = 1.38 ± 0.04%, and Vatiras (0.307 < Q < 0.718 AU) = 0.22 ± 0.03% of the steady-state NEO population. Vatiras are a previously undiscussed NEO population clearly defined in our integrations, whose orbits lie completely interior to that of Venus. Our integrations also uncovered the unexpected production of retrograde orbits from main-belt asteroid sources; this retrograde NEA population makes up ?0.1% of the steady-state NEO population. The relative NEO impact rate onto Mercury, Venus, and Earth, as well as the normalized distribution of impact speeds, was calculated from the NEOSSat-1.0 orbital model under the assumption of a steady-state. The new model predicts a slightly higher Mercury impact flux. 相似文献