THEMIS-VIS observations of clouds in the martian mesosphere: Altitudes, wind speeds, and decameter-scale morphology     

T.H. McConnochie  J.F. Bell III  D. Savransky  M.J. Wolff  A.D. Toigo  H. Wang  M.I. Richardson  P.R. Christensen 《Icarus》2010,210(2):545-565

We present measurements of the altitude and eastward velocity component of mesospheric clouds in 35 imaging sequences acquired by the Mars Odyssey (ODY) spacecraft’s Thermal Emission Imaging System visible imaging subsystem (THEMIS-VIS). We measure altitude by using the parallax drift of high-altitude features, and the velocity by exploiting the time delay in the THEMIS-VIS imaging sequence.We observe two distinct classes of mesospheric clouds: equatorial mesospheric clouds observed between 0° and 180° L_s; and northern mid-latitude clouds observed only in twilight in the 200–300° L_s period. The equatorial mesospheric clouds are quite rare in the THEMIS-VIS data set. We have detected them in only five imaging sequences, out of a total of 2048 multi-band equatorial imaging sequences. All five fall between 20° south and 0° latitude, and between 260° and 295° east longitude. The mid-latitude mesospheric clouds are apparently much more common; for these we find 30 examples out of 210 northern winter mid-latitude twilight imaging sequences. The observed mid-latitude clouds are found, with only one exception, in the Acidalia region, but this is quite likely an artifact of the pattern of THEMIS-VIS image targeting. Comparing our THEMIS-VIS images with daily global maps generated from Mars Orbiter Camera Wide Angle (MOC-WA) images, we find some evidence that some mid-latitude mesospheric cloud features correspond to cloud features commonly observed by MOC-WA. Comparing the velocity of our mesospheric clouds with a GCM, we find good agreement for the northern mid-latitude class, but also find that the GCM fails to match the strong easterly winds measured for the equatorial clouds.Applying a simple radiative transfer model to some of the equatorial mesospheric clouds, we find good model fits in two different imaging sequences. By using the observed radiance contrast between cloud and cloud-free regions at multiple visible-band wavelengths, these fits simultaneously constrain the optical depths and particles sizes of the clouds. The particle sizes are constrained primarily by the relative contrasts at the available wavelengths, and are found to be quite different in the two imaging sequences: r_eff = 0.1 μm and r_eff = 1.5 μm. The optical depths (constrained by the absolute contrasts) are substantial: 0.22 and 0.5, respectively. These optical depths imply a mass density that greatly exceeds the saturated mass density of water vapor at mesospheric temperatures, and so the aerosol particles are probably composed mainly of CO₂ ice. Our simple radiative transfer model is not applicable to twilight, when the mid-latitude mesospheric clouds were observed, and so we leave the properties of these clouds as a question for further work.  相似文献   

Solar radiation pressure on (99942) Apophis     

J. ?i?ka  D. Vokrouhlický 《Icarus》2011,211(1):511-518

Near-Earth asteroid (99942) Apophis currently resides among the top positions on the list of objects with small, yet non-zero impact probability with the Earth. For that reason an unusual observational and theoretical effort has been dedicated to precisely characterize its future orbit. Here we discuss orbital perturbation of Apophis due to incident and reflected solar radiation pressure (SRP). We both revisit recent analytical estimate of the SRP effects for this body and also formulate a numerical approach allowing us to compute the SRP orbital perturbation under general assumptions. Contrary to some previous results, we show that SRP has a much smaller effect on the Apophis trajectory than does the thermal re-radiation force which produces the Yarkovsky effect. When the Yarkovsky effect becomes constrained enough in the future, our approach may be used to improve the orbit determination for this asteroid.  相似文献   

Decadal variations in a Venus general circulation model     

Helen F. Parish  Gerald Schubert  Richard L. Walterscheid  Sebastien Lebonnois 《Icarus》2011,212(1):42-65

The Community Atmosphere Model (CAM), a 3-dimensional Earth-based climate model, has been modified to simulate the dynamics of the Venus atmosphere. The most current finite volume version of CAM is used with Earth-related processes removed, parameters appropriate for Venus introduced, and some basic physics approximations adopted. A simplified Newtonian cooling approximation has been used for the radiation scheme. We use a high resolution (1° by 1° in latitude and longitude) to take account of small-scale dynamical processes that might be important on Venus. A Rayleigh friction approach is used at the lower boundary to represent surface drag, and a similar approach is implemented in the uppermost few model levels providing a ‘sponge layer’ to prevent wave reflection from the upper boundary. The simulations generate superrotation with wind velocities comparable to those measured in the Venus atmosphere by probes and around 50-60% of those measured by cloud tracking. At cloud heights and above the atmosphere is always superrotating with mid-latitude zonal jets that wax and wane on an approximate 10 year cycle. However, below the clouds, the zonal winds vary periodically on a decadal timescale between superrotation and subrotation. Both subrotating and superrotating mid-latitude jets are found in the approximate 40-60 km altitude range. The growth and decay of the sub-cloud level jets also occur on the decadal timescale. Though subrotating zonal winds are found below the clouds, the total angular momentum of the atmosphere is always in the sense of superrotation. The global relative angular momentum of the atmosphere oscillates with an amplitude of about 5% on the approximate 10 year timescale. Symmetric instability in the near surface equatorial atmosphere might be the source of the decadal oscillation in the atmospheric state. Analyses of angular momentum transport show that all the jets are built up by poleward transport by a meridional circulation while angular momentum is redistributed to lower latitudes primarily by transient eddies. Possible changes in the structure of Venus’ cloud level mid-latitude jets measured by Mariner 10, Pioneer Venus, and Venus Express suggest that a cyclic variation similar to that found in the model might occur in the real Venus atmosphere, although no subrotating winds below the cloud level have been observed to date. Venus’ atmosphere must be observed over multi-year timescales and below the clouds if we are to understand its dynamics.  相似文献   

Estimates of gravity wave drag on Mars: Indication of a possible lower thermospheric wind reversal     

Alexander S. Medvedev  Erdal Yi?it 《Icarus》2011,211(1):909-912

Spectral gravity wave parameterization suitable for planetary thermospheres applied to wind and temperature from Mars Climate Database reveals enormously strong drag incompatible with the wind distribution. It points out to a possible wind reversal in the 110-140 km layer similar to the one in the Earth’s lower thermosphere.  相似文献   

Core-collapse supernovae and the Younger Dryas/terminal Rancholabrean extinctions     

G. Robert Brakenridge 《Icarus》2011,215(1):101-106

Early predictions that some supernovae release large quantities of prompt high energy photons are now corroborated by optical identification of core-collapse supernovae associated with extragalactic GRBS (beamed γ-ray bursts) and XRFS (beamed or un-beamed X-ray flashes). Given the in-galaxy supernova frequency and GRB and XRF recurrence statistics, significant Earth-incident events during the past several million years very likely occurred and nearby events should have affected the Earth and other planetary atmospheres, including terrestrial surface solar UV, the Earth’s climate, and its ecology. The Younger Dryas Stadial (∼12,900 to 11,550 calendar yr BP) began with sharply cooler temperatures in the Earth’s northern hemisphere, regional drought, paleoecological evidence compatible with increased UV, and abrupt increases in cosmogenic ¹⁴C and ¹⁰Be in ice and marine cores and tree rings. In North America, stratigraphic and faunal sequences indicate that a major pulse of mammalian extinctions (at least 23-31 genera) began very close to 12,830 calendar yr BP and was sudden: deposits one century younger are devoid of diverse extinct fauna remains. A 10 s beamed GRB within 2 kpc of the Earth delivers 100 kJ m⁻² fluence to the Earth’s atmosphere, where it causes spallation and catalytic reactions depleting 35-50% O³, and producing excess NO_x species (which favor cooling, drought, and surface fertility), ¹⁴C, and ¹⁰Be. An un-beamed, 10⁵⁰ erg hard photon impulse at ∼250 pc produces similar terrestrial atmospheric effects. A well-characterized massive star supernova, the unusually close Vela event (d = 250 ± 30 pc; total energy of 1-2 × 10⁵¹ erg; age constrained from remnant nebula shock velocities considerations at 13,000-16,000 yr and from the pulsar characteristic age at ∼11,400 yr) may have initiated the Younger Dryas climate change, and caused the extinction of the terminal Rancholabrean fauna.  相似文献   

Saturn’s zonal wind profile in 2004-2009 from Cassini ISS images and its long-term variability     

E. García-Melendo  S. Pérez-Hoyos  R. Hueso 《Icarus》2011,215(1):62-74

Five years of Cassini Imaging Science Subsystem images, from 2004 to 2009, are analyzed in this work to retrieve global zonal wind profiles of Saturn’s northern and southern hemispheres in the methane absorbing bands at 890 and 727 nm and in their respective adjacent continuum wavelengths of 939 and 752 nm. A complete view of Saturn’s global circulation, including the equator, at two pressure levels, in the tropopause (60 mbar to 250 mbar with the MT filters) and in the upper troposphere (from ∼350 mbar to ∼500 mbar with the CB filter set), is presented. Both zonal wind profiles (available at the Supplementary Material Section), show the same structure but with significant differences in the peak of the eastward jets and the equatorial region, including a region of positive vertical shear symmetrically located around the equator between the 10° < |φ_c| < 25° where zonal velocities close to the tropopause are higher than at 500 mbar. A comparison of previously published zonal wind sets obtained by Voyager 1 and 2 (1980-1981), Hubble Space Telescope, and ground-based telescopes (1990-2004) with the present Cassini profiles (2004-2009) covering a full Saturn year shows that the shape of the zonal wind profile and intensity of the jets has remained almost unchanged except at the equator, despite the seasonal insolation cycle and the variability of Saturn’s emitted power. The major wind changes occurred at equatorial latitudes, perhaps following the Great White Spot eruption in 1990. It is not evident from our study if the seasonal insolation cycle and its associated ring shadowing influence the equatorial circulation at cloud level.  相似文献   

Monte Carlo modeling of neutral gas and dust in the coma of Comet 1P/Halley     

Martin Rubin  Valeriy M. Tenishev  Kenneth C. Hansen  Kathrin Altwegg 《Icarus》2011,213(2):655-677

The neutral gas environment of a comet is largely influenced by dissociation of parent molecules created at the surface of the comet and collisions of all the involved species. We compare the results from a kinetic model of the neutral cometary environment with measurements from the Neutral Mass Spectrometer and the Dust Impact Detection System onboard the Giotto spacecraft taken during the fly-by at Comet 1P/Halley in 1986. We also show that our model is in good agreement with contemporaneous measurements obtained by the International Ultraviolet Explorer, sounding rocket experiments, and various ground based observations.The model solves the Boltzmann equation with a Direct Simulation Monte Carlo technique (Tenishev, V., Combi, M., Davidsson, B. [2008]. Astrophys. J. 685, 659-677) by tracking trajectories of gas molecules and dust grains under the influence of the comet’s weak gravity field with momentum exchange among particles modeled in a probabilistic manner. The cometary nucleus is considered to be the source of dust and the parent species (in our model: H₂O, CO, H₂CO, CO₂, CH₃OH, C₂H₆, C₂H₄, C₂H₂, HCN, NH₃, and CH₄) in the coma. Subsequently our model also tracks the corresponding dissociation products (H, H₂, O, OH, C, CH, CH₂, CH₃, N, NH, NH₂, C₂, C₂H, C₂H₅, CN, and HCO) from the comet’s surface all the way out to 10⁶ km.As a result we are able to further constrain cometary the gas production rates of CO (13%), CO₂ (2.5%), and H₂CO (1.5%) relative to water without invoking unknown extended sources.  相似文献   

Rebuttal to the comment by Malhotra and Strom on “Constraints on the source of lunar cataclysm impactors”     

Matija ?uk  Brett J. Gladman 《Icarus》2011,216(1):363-365

?uk et al. (?uk, M. Gladman, B.J., Stewart, S.T. [2010]. Icarus 207 590-594) concluded that the the lunar cataclysm (late heavy bombardment) was recorded in lunar Imbrian era craters, and that their size distribution is different from that of main belt asteroids (which may have been the dominant pre-Imbrian impactors). This result would likely preclude the asteroid belt as the direct source of lunar cataclysm impactors. Malhotra and Strom (Malhotra, R., Strom, R.G. [2011]. Icarus) maintain that the lunar impactor population in the Imbrian era was the same as in Nectarian and pre-Nectarian periods, and this population had a size distribution identical to that of main belt asteroids. In support of this claim, they present an Imbrian size distribution made from two data sets published by Wilhelms et al. (Wilhelms, D.E., Oberbeck, V.R., Aggarwal, H.R. [1978]. Proc. Lunar Sci. Conf. 9, 3735-3762). However, these two data sets cannot be simply combined as they represent areas of different ages and therefore crater densities. Malhotra and Strom (Malhotra, R., Strom, R.G. [2011]. Icarus) differ with the main conclusion of Wilhelms et al. (Wilhelms, D.E., Oberbeck, V.R., Aggarwal, H.R. [1978]. Proc. Lunar Sci. Conf. 9, 3735-3762) that the Nectarian and Imbrian crater size distributions were different. We conclude that the available data indicate that the lunar Imbrian-era impactors had a different size distribution from the older ones, with the Imbrian impactor distribution being significantly richer in small impactors than that of older lunar impactors or current main-belt asteroids.  相似文献   

Long-term evolution of the aerosol debris cloud produced by the 2009 impact on Jupiter     

A. Sánchez-Lavega  G.S. Orton  R. Hueso  L.N. Fletcher  E. García-Melendo  I. de Pater  H.B. Hammel  A. Simon-Miller  F. Marchis  O. Mousis  J. García-Rojas  M. Cecconi  K. Noll  S. Pedraz  P. Kalas  W. Golisch  P. Sears  V. Reddy  R. Binzel  W. Grundy  J. Emery  A. Rivkin  C. Thomas  D. Trilling  K. Bjorkman  A.J. Burgasser  H. Campins  T.M. Sato  Y. Kasaba  J. Ziffer  R. Mirzoyan  H. Bouy 《Icarus》2011,214(2):462-476

We present a study of the long-term evolution of the cloud of aerosols produced in the atmosphere of Jupiter by the impact of an object on 19 July 2009 (Sánchez-Lavega, A. et al. [2010]. Astrophys. J. 715, L155-L159). The work is based on images obtained during 5 months from the impact to 31 December 2009 taken in visible continuum wavelengths and from 20 July 2009 to 28 May 2010 taken in near-infrared deep hydrogen-methane absorption bands at 2.1-2.3 μm. The impact cloud expanded zonally from ∼5000 km (July 19) to 225,000 km (29 October, about 180° in longitude), remaining meridionally localized within a latitude band from 53.5°S to 61.5°S planetographic latitude. During the first two months after its formation the site showed heterogeneous structure with 500-1000 km sized embedded spots. Later the reflectivity of the debris field became more homogeneous due to clump mergers. The cloud was mainly dispersed in longitude by the dominant zonal winds and their meridional shear, during the initial stages, localized motions may have been induced by thermal perturbation caused by the impact’s energy deposition. The tracking of individual spots within the impact cloud shows that the westward jet at 56.5°S latitude increases its eastward velocity with altitude above the tropopause by 5-10 m s⁻¹. The corresponding vertical wind shear is low, about 1 m s⁻¹ per scale height in agreement with previous thermal wind estimations. We found evidence for discrete localized meridional motions with speeds of 1-2 m s⁻¹. Two numerical models are used to simulate the observed cloud dispersion. One is a pure advection of the aerosols by the winds and their shears. The other uses the EPIC code, a nonlinear calculation of the evolution of the potential vorticity field generated by a heat pulse that simulates the impact. Both models reproduce the observed global structure of the cloud and the dominant zonal dispersion of the aerosols, but not the details of the cloud morphology. The reflectivity of the impact cloud decreased exponentially with a characteristic timescale of 15 days; we can explain this behavior with a radiative transfer model of the cloud optical depth coupled to an advection model of the cloud dispersion by the wind shears. The expected sedimentation time in the stratosphere (altitude levels 5-100 mbar) for the small aerosol particles forming the cloud is 45-200 days, thus aerosols were removed vertically over the long term following their zonal dispersion. No evidence of the cloud was detected 10 months after the impact.  相似文献   

Changes in Jupiter’s zonal velocity between 1979 and 2008     

Xylar S. Asay-Davis  Philip S. Marcus  Imke de Pater 《Icarus》2011,211(2):1215-1232

We show that the peak velocity of Jupiter’s visible-cloud-level zonal winds near 24°N (planetographic) increased from 2000 to 2008. This increase was the only change in the zonal velocity from 2000 to 2008 for latitudes between ±70° that was statistically significant and not obviously associated with visible weather. We present the first automated retrieval of fast (∼130 m s⁻¹) zonal velocities at 8°N planetographic latitude, and show that some previous retrievals incorrectly found slower zonal winds because the eastward drift of the dark projections (associated with 5-μm hot spots) “fooled” the retrieval algorithms.We determined the zonal velocity in 2000 from Cassini images from NASA’s Planetary Data System using a global method similar to previous longitude-shifting correlation methods used by others, and a new local method based on the longitudinal average of the two-dimensional velocity field. We obtained global velocities from images acquired in May 2008 with the Wide Field Planetary Camera 2 (WFPC2) on the Hubble Space Telescope (HST). Longer-term variability of the zonal winds is based on comparisons with published velocities based on 1979 Voyager 2 and 1995-1998 HST images. Fluctuations in the zonal wind speeds on the order of 10 m s⁻¹ on timescales ranging from weeks to months were found in the 1979 Voyager 2 and the 1995-1998 HST velocities. In data separated by 10 h, we find that the east-west velocity uncertainty due to longitudinal fluctuations are nearly 10 m s⁻¹, so velocity fluctuations of 10 m s⁻¹ may occur on timescales that are even smaller than 10 h. Fluctuations across such a wide range of timescales limit the accuracy of zonal wind measurements. The concept of an average zonal velocity may be ill-posed, and defining a “temporal mean” zonal velocity as the average of several zonal velocity fields spanning months or years may not be physically meaningful.At 8°N, we use our global method to find peak zonal velocities of ∼110 m s⁻¹ in 2000 and ∼130 m s⁻¹ in 2008. Zonal velocities from 2000 Cassini data produced by our local and global methods agree everywhere, except in the vicinity of 8°N. There, the local algorithm shows that the east-west velocity has large variations in longitude; vast regions exceed ∼140 m s⁻¹. Our global algorithm, and all of the velocity-extraction algorithms used in previously-published studies, found the east-west drift velocities of the visible dark projections, rather than the true zonal velocity at the visible-cloud level. Therefore, the apparent increase in zonal winds between 2000 and 2008 at 8°N is not a true change in zonal velocity.At 7.3°N, the Galileo probe found zonal velocities of 170 m s⁻¹ at the 3-bar level. If the true zonal velocity at the visible-cloud level at this latitude is ∼140 m s⁻¹ rather than ∼105 m s⁻¹, then the vertical zonal wind shear is much less than the currently accepted value.  相似文献