Impact of atmospheric gravity waves on the jovian ionosphere     

Daniel Barrow  Katia I. Matcheva 《Icarus》2011,211(1):609-622

We investigate the effects of atmospheric gravity waves on the vertical and horizontal structure of the ionosphere of Jupiter. The presented non-linear, two-dimensional model of the jovian ionosphere allows for spatially and temporally varying neutral wind and temperature fields and tracks the time evolution of six ionospheric species, , and . An analytical approach is used to validate the model results for linear, small-amplitude waves and to elucidate the mechanisms that leads to perturbations in the density of the main ion species, H⁺ and . We demonstrate that the long-lived H⁺ ions are perturbed directly by wave dynamics whereas short-lived ions such as are perturbed by chemical interactions with other perturbed ion species. The model is then applied using larger gravity wave amplitudes consistent with observations. Atmospheric gravity waves propagating at high altitudes create layers of enhanced electron density similar to the system of layers observed during the J0-ingress radio occultation of the Galileo spacecraft. Our best fit to the J0-ingress observation is achieved using an 82 min period forcing wave with horizontal and vertical wavelengths of 500 km and 60 km respectively, and peaks at 510 km above the 1 bar pressure level. We further investigate the effects of the wave-induced ion flux on the background ionospheric structure and demonstrate that in the presence of a gravity wave the background density profiles of the H⁺ and ions are significantly modified. We also find that the column density of has variations that can exceed 10% as the wave propagates.  相似文献   

Parametric analysis of modeled ion escape from Mars     

Curtis V. Manning  Yingjuan Ma  Christopher P. McKay 《Icarus》2011,212(1):131-137

We develop a parametric fit to the results of a detailed magnetohydrodynamic (MHD) study of the response of ion escape rates (O⁺, and ) to strongly varied solar forcing factors, as a way to efficiently extend the MHD results to different conditions. We then use this to develop a second, evolutionary model of solar forced ion escape. We treat the escape fluxes of ion species at Mars as proportional to the product of power laws of four factors - that of the EUV flux R_euv, the solar wind particle density R_ρ, its velocity (squared) R_v², and the interplanetary magnetic field pressure R_B², where forcing factors are expressed in units of the current epoch-averaged values. Our parametric model is: , where ?(i) is the escape flux of ion i. We base our study on the results of just six provided MHD model runs employing large forcing factor variations, and thus construct a successful, first-order parametric model of the MHD program. We perform a five-dimensional least squares fit of this power law model to the MHD results to derive the flux normalizations and the indices of the solar forcing factors. For O⁺, we obtain the values, 1.73 × 10²⁴ s⁻¹, 0.782, 0.251, 0.382, and 0.214, for ?₀, α, β, γ, and δ, respectively. For , the corresponding values are 1.68 × 10²⁴ s⁻¹, −0.393, 0.798, 0.967, and 0.533. For , they are 8.66 × 10²² s⁻¹, −0.427, 1.083, 1.214, and 0.690. The fit reproduces the MHD results to an average error of about 5%, suggesting that the power laws are broadly representative of the MHD model results. Our analysis of the MHD model shows that by itself an increase in R_EUV enhances O⁺ loss, but suppresses the escape of and , whereas increases in solar wind (i.e., in , and R_B², with R_euv constant) favors the escape of heavier ions more than light ions. The ratios of escaping ions detectable at Mars today can be predicted by this parametric fit as a function of the solar forcing factors. We also use the parametric model to compute escape rates over martian history. This second parametric model expresses ion escape functions of one variable (per ion), ?(i) = ?₀(i)(t/t₀)^−ξ(i). The ξ(i) are linear combinations of the epoch-averaged ion escape sensitivities, which are seen to increase with ion mass. We integrate the and oxygen ion escape rates over time, and find that in the last 3.85 Gyr, Mars would have lost about mbars of , and of water (from O⁺ and ) from ion escape.  相似文献   

Eclipsing binary Trojan asteroid Patroclus: Thermal inertia from Spitzer observations     

Michael Mueller  Franck Marchis  Joshua P. Emery  Stefano Mottola  Jérome Berthier 《Icarus》2010,205(2):505-515

We present mid-infrared observations of the binary L5-Trojan system (617) Patroclus-Menoetius before, during, and after two shadowing events, using the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope. For the first time, we effectively observe changes in asteroid surface temperature in real time, allowing the thermal inertia to be determined very directly. A new detailed binary thermophysical model is presented which accounts for the system’s known mutual orbit, arbitrary component shapes, and thermal conduction in the presence of eclipses.We obtain two local thermal-inertia values, representative of the respective shadowed areas: and . The average thermal inertia is estimated to be , potentially with significant surface heterogeneity. This first thermal-inertia measurement for a Trojan asteroid indicates a surface covered in fine regolith. Independently, we establish the presence of fine-grained (<a few μm) silicates on the surface, based on emissivity features near 10 and similar to those previously found on other Trojans.We also report V-band observations and report a lightcurve with complete rotational coverage. The lightcurve has a low amplitude of peak-to-peak, implying a roughly spherical shape for both components, and is single-periodic with a period equal to the period of the mutual orbit, indicating that the system is fully synchronized.The diameters of Patroclus and Menoetius are 106±11 and , respectively, in agreement with previous findings. Taken together with the system’s known total mass, this implies a bulk mass density of , significantly below the mass density of L4-Trojan asteroid (624) Hektor and suggesting a bulk composition dominated by water ice.All known physical properties of Patroclus, arguably the best studied Trojan asteroid, are consistent with those expected in icy objects with devolatilized surface (extinct comets), consistent with what might be implied by recent dynamical modeling in the framework of the Nice Model.  相似文献   

Heavy ion escape from Mars, influence from solar wind conditions and crustal magnetic fields     

Hans Nilsson  Niklas J.T. Edberg  Stas Barabash  Yoshifumi Futaana  Andrei Fedorov 《Icarus》2011,215(2):475-484

We have used more than 4 years of Mars Express ion data to estimate the escape of heavy ions ( and ) from Mars. To take the limited field of view of the instrument into account, the data has been binned into spatial bins and angular bins to create average distribution functions for different positions in the near Mars space. The net escape flux for the studied low solar activity period, between May 2007 and May 2011, is 2.0 ± 0.2 × 10²⁴ s⁻¹. The escape has been calculated independently for four different quadrants in the Y_MSO − Z_MSO plane, south, dusk, north and dawn. Escape is highest from the northern and dusk quadrants, 0.6 ± 0.1 × 10²⁴ s⁻¹, and smallest from the south and dawn quadrants, 0.4 ± 0.1 × 10²⁴ s⁻¹. The flux ratio of molecular ( and ) to O⁺ ions is 0.9 ± 0.1, averaged over all quadrants. The flux difference between the north and south quadrants is statistically significant, and is presumed to be due to the presence of significant crustal magnetic fields in the southern hemisphere, reducing the outflow. The difference between the dawn and dusk quadrants is likely due to the magnetic tension associated with the nominal Parker angle spiral, which should lead to higher average magnetic tension on the dusk side. The escape increases during periods of high solar wind flux and during times when co-rotating interaction regions (CIR) affect Mars. In the latter case the increase is a factor 2.4-2.9 as compared to average conditions.  相似文献   

Physical properties of (2) Pallas     

Benoît Carry  Christophe Dumas  Jérôme Berthier  Stéphane Erard  Jack D. Drummond  Marcello Fulchignoni 《Icarus》2010,205(2):460-472

Ground-based high angular-resolution images of asteroid (2) Pallas at near-infrared wavelengths have been used to determine its physical properties (shape, dimensions, spatial orientation and albedo distribution).We acquired and analyzed adaptive optics (AO) J/H/K-band observations from Keck II and the Very Large Telescope taken during four Pallas oppositions between 2003 and 2007, with spatial resolution spanning 32-88 km (image scales 13-20 km/pixel). We improve our determination of the size, shape, and pole by a novel method that combines our AO data with 51 visual light-curves spanning 34 years of observations as well as archived occultation data.The shape model of Pallas derived here reproduces well both the projected shape of Pallas on the sky (average deviation of edge profile of 0.4 pixel) and light-curve behavior (average deviation of 0.019 mag) at all the epochs considered. We resolved the pole ambiguity and found the spin-vector coordinates to be within 5° of [longitude, latitude] = [30°, −16°] in the Ecliptic J2000.0 reference frame, indicating a high obliquity of about 84°, leading to high seasonal contrast. The best triaxial-ellipsoid fit returns ellipsoidal radii of , and . From the mass of Pallas determined by gravitational perturbation on other minor bodies , [Michalak, G., 2000. Astron. Astrophys. 360, 363-374], we derive a density of significantly different from the density of C-type (1) Ceres of [Carry, B., Dumas, C., Fulchignoni, M., Merline, W.J., Berthier, J., Hestroffer, D., Fusco, T., Tamblyn, P., 2008. Astron. Astrophys. 478 (4), 235-244]. Considering the spectral similarities of Pallas and Ceres at visible and near-infrared wavelengths, this may point to fundamental differences in the interior composition or structure of these two bodies.We define a planetocentric longitude system for Pallas, following IAU guidelines. We also present the first albedo maps of Pallas covering ∼80% of the surface in K-band. These maps reveal features with diameters in the 70-180 km range and an albedo contrast of about 6% with respect to the mean surface albedo.  相似文献   

Titania's radius and an upper limit on its atmosphere from the September 8, 2001 stellar occultation     

《Icarus》2009,199(2):458-476

On September 8, 2001 around 2 h UT, the largest uranian moon, Titania, occulted Hipparcos star 106829 (alias SAO 164538, a V=7.2, K0 III star). This was the first-ever observed occultation by this satellite, a rare event as Titania subtends only 0.11 arcsec on the sky. The star's unusual brightness allowed many observers, both amateurs or professionals, to monitor this unique event, providing fifty-seven occultations chords over three continents, all reported here. Selecting the best 27 occultation chords, and assuming a circular limb, we derive Titania's radius: (1-σ error bar). This implies a density of using the value derived by Taylor [Taylor, D.B., 1998. Astron. Astrophys. 330, 362-374]. We do not detect any significant difference between equatorial and polar radii, in the limit , in agreement with Voyager limb image retrieval during the 1986 flyby. Titania's offset with respect to the DE405 + URA027 (based on GUST86 theory) ephemeris is derived: ΔαTcos(δT)=−108±13 mas and ΔδT=−62±7 mas (ICRF J2000.0 system). Most of this offset is attributable to a Uranus' barycentric offset with respect to DE405, that we estimate to be: and ΔδU=−85±25 mas at the moment of occultation. This offset is confirmed by another Titania stellar occultation observed on August 1st, 2003, which provides an offset of ΔαTcos(δT)=−127±20 mas and ΔδT=−97±13 mas for the satellite. The combined ingress and egress data do not show any significant hint for atmospheric refraction, allowing us to set surface pressure limits at the level of 10-20 nbar. More specifically, we find an upper limit of 13 nbar (1-σ level) at 70 K and 17 nbar at 80 K, for a putative isothermal CO₂ atmosphere. We also provide an upper limit of 8 nbar for a possible CH₄ atmosphere, and 22 nbar for pure N₂, again at the 1-σ level. We finally constrain the stellar size using the time-resolved star disappearance and reappearance at ingress and egress. We find an angular diameter of 0.54±0.03 mas (corresponding to projected at Titania). With a distance of 170±25 parsecs, this corresponds to a radius of 9.8±0.2 solar radii for HIP 106829, typical of a K0 III giant.  相似文献   

An analysis of Pluto occultation light curves using an atmospheric radiative-conductive model     

Angela M. Zalucha  Amanda A.S. Gulbis  Xun Zhu  J.L. Elliot 《Icarus》2011,211(1):804-818

We use a radiative-conductive model to least-squares fit Pluto stellar occultation light curve data. This model predicts atmospheric temperature based on surface temperature, surface pressure, surface radius, and CH₄ and CO mixing ratios, from which model light curves are to be calculated. The model improves upon previous techniques for deriving Pluto’s atmospheric thermal structure from stellar occultation light curves by calculating temperature (as a function of height) caused by heating and cooling by species in Pluto’s atmosphere, instead of a general assumption that temperature follows a power law with height or some other idealized function. We are able to fit for model surface radius, surface pressure, and CH₄ mixing ratio with one of the 2006 datasets and for surface pressure and CH₄ mixing ratio for other datasets from the years 1988, 2002, 2006, and 2008. It was not possible to fit for CO mixing ratio and surface temperature because the light curves are not sensitive to these parameters. We determine that the model surface radius, under the assumption of a stratosphere only (i.e. no troposphere) model in radiative-conductive balance, is . The CH₄ mixing ratio results are more scattered with time and are in the range of 1.8-9.4 × 10⁻³. The surface pressure results show an increasing trend from 1988 to 2002, although it is not as dramatic as the factor of 2 from previous studies.  相似文献   

New Horizons Alice ultraviolet observations of a stellar occultation by Jupiter’s atmosphere     

Thomas K. Greathouse  G.R. Gladstone  S.A. Stern  R.J. Vervack Jr.  M.H. Versteeg  L.A. Young  H. Throop 《Icarus》2010,208(1):293-305

The Alice ultraviolet spectrograph onboard the New Horizons spacecraft observed two occultations of the bright star χ Ophiucus by Jupiter’s atmosphere on February 22 and 23, 2007 during the approach phase of the Jupiter flyby. The ingress occultation probed the atmosphere at 32°N latitude near the dawn terminator, while egress probed 18°N latitude near the dusk terminator. A detailed analysis of both the ingress and egress occultations, including the effects of molecular hydrogen, methane, acetylene, ethylene, and ethane absorptions in the far ultraviolet (FUV), constrains the eddy diffusion coefficient at the homopause level to be  cm² s⁻¹, consistent with Voyager measurements and other analyses (Festou, M.C., Atreya, S.K., Donahue, T.M., Sandel, B.R., Shemansky, D.E., Broadfoot, A.L. [1981]. J. Geophys. Res. 86, 5717-5725; Vervack Jr., R.J., Sandel, B.R., Gladstone, G.R., McConnell, J.C., Parkinson, C.D. [1995]. Icarus 114, 163-173; Yelle, R.V., Young, L.A., Vervack Jr., R.J., Young, R., Pfister, L., Sandel, B.R. [1996]. J. Geophys. Res. 101 (E1), 2149-2162). However, the actual derived pressure level of the methane homopause for both occultations differs from that derived by [Festou et al., 1981] and [Yelle et al., 1996] from the Voyager ultraviolet occultations, suggesting possible changes in the strength of atmospheric mixing with time. We find that at 32°N latitude, the methane concentration is  cm⁻³ at 70,397 km, the methane concentration is  cm⁻³ at 70,383 km, the acetylene concentration is  cm⁻³ at 70,364 km, and the ethane concentration is  cm⁻³ at 70,360 km. At 18°N latitude, the methane concentration is  cm⁻³ at 71,345 km, the methane concentration is  cm⁻³ at 71,332 km, the acetylene concentration is cm⁻³ at 71,318 km, and the ethane concentration is  cm⁻³ at 71,315 km. We also find that the H₂ occultation light curve is best reproduced if the atmosphere remains cold in the microbar region such that the base of the thermosphere is located at a lower pressure level than that determined by in situ instruments aboard the Galileo probe (Seiff, A., Kirk, D.B., Knight, T.C.D., Young, R.E., Mihalov, J.D., Young, L.A., Milos, F.S., Schubert, G., Blanchard, R.C., Atkinson, D. [1998]. J. Geophys. Res. 103 (E10), 22857-22889) - the Sieff et al. temperature profile leads to too much absorption from H₂ at high altitudes. However, this result is highly model dependent and non-unique. The observations and analysis help constrain photochemical models of Jupiter’s atmosphere.  相似文献   

Titan haze distribution and optical properties retrieved from recent observations     

P. Rannou  T. Cours  S. Rodriguez  P. Drossart 《Icarus》2010,208(2):850-867

The VIMS instrument onboard Cassini observed the north polar region of Titan at 113° phase angle, 28 December 2006. On this spectral image, a vast polar cloud can be seen northward to 62°N, and elsewhere, the haze appears as the dominant source of scattering. Because the surface does not appear in the wavelength range between 0.3 and , this spectro-image is ideal to study airborn scatterers both in methane bands and windows. In this work, we study this image, along with another image taken at 13° phase angle. This image probe both the atmosphere and the surface from pole to pole. First, we characterise the spatial distribution of the haze layer above 100 km between 80°S and 70°N. We find a north south asymmetry with a haze opacity increasing by a factor 3 from the south pole to the equator, then a constant value up to about 30°N and a decrease of a factor 2 between 30°N and about 60°N. Beyond 60°N, we can see the influence of the north polar cloud, even in the band, but no polar haze accumulation. The fact that the north polar region is still in the polar night is a possible explanation. No haze accumulation is observed in the southern polar region. Secondly, we partly identify the origin of spectral features in the 2.8-μm methane window, which are found to be due to deuterated methane (CH₃D). This allows the analyse of this window and to retrieve the opacity of scatterers layer below 80 km (presumably made of aerosols and condensate droplets) between 35°N and 60°N. Finally, we constrained the values and the spectral behaviour of the imaginary part of the aerosol refractive index in the range between 0.3 and . To do so, we studied the 2.8-μm window with the image taken at 113° phase angle. To complete the analysis, we studied the transmission through the haze layer in the 3.4-μm band observed in solar occultation mode with VIMS, and we analysed the single scattering albedo retrieved with DISR instrument between 0.4 and . The imaginary part of the refractive index that we find for Titan aerosols follows Khare et al. (Khare, B.N. et al. [1984]. Icarus 60, 127-137) optical constant up to and becomes constant beyond this wavelength at least up to . It also has a prominent peak at and a secondary peak at , which indicates material rich in C-H bonds, with much less N-H bonds than in Khare et al. (1984) tholins.  相似文献   

A high-amplitude thermal inertia anomaly of probable magnetospheric origin on Saturn’s moon Mimas     

C.J.A. Howett  J.R. Spencer  R.E. Johnson  T.A. Hurford  M. Segura 《Icarus》2011,216(1):221-226

Spectral maps of Mimas’ daytime thermal emission show a previously unobserved thermal anomaly on Mimas’ surface. A sharp V-shaped boundary, centered at 0°N and 180°W, separates relatively warm daytime temperatures from a cooler anomalous region occupying low- to mid-latitudes on the leading hemisphere. Subsequent observations show the anomalous region is also warmer than its surroundings at night, indicating high thermal inertia. Thermal inertia in the anomalous region is , compared to < outside the anomaly. Bolometric Bond albedos are similar between the two regions, in the range 0.49-0.70. The mapped portion of the thermally anomalous region coincides in shape and location to a region of high-energy electron deposition from Saturn’s magnetosphere, which also has unusually high near-UV reflectance. It is therefore likely that high-energy electrons, which penetrate Mimas’ surface to the centimeter depths probed by diurnal temperature variations, also alter the surface texture, dramatically increasing its thermal inertia.  相似文献   

Fragment properties at the catastrophic disruption threshold: The effect of the parent body’s internal structure   总被引：1，自引：0，他引：1  

Martin Jutzi  Patrick Michel  Derek C. Richardson 《Icarus》2010,207(1):54-346

Numerical simulations of asteroid breakups, including both the fragmentation of the parent body and the gravitational interactions between the fragments, have allowed us to reproduce successfully the main properties of asteroid families formed in different regimes of impact energy, starting from a non-porous parent body. In this paper, using the same approach, we concentrate on a single regime of impact energy, the so-called catastrophic threshold usually designated by , which results in the escape of half of the target’s mass. Thanks to our recent implementation of a model of fragmentation of porous materials, we can characterize for both porous and non-porous targets with a wide range of diameters. We can then analyze the potential influence of porosity on the value of , and by computing the gravitational phase of the collision in the gravity regime, we can characterize the collisional outcome in terms of the fragment size and ejection speed distributions, which are the main outcome properties used by collisional models to study the evolutions of the different populations of small bodies. We also check the dependency of on the impact speed of the projectile.In the strength regime, which corresponds to target sizes below a few hundreds of meters, we find that porous targets are more difficult to disrupt than non-porous ones. In the gravity regime, the outcome is controlled purely by gravity and porosity in the case of porous targets. In the case of non-porous targets, the outcome also depends on strength. Indeed, decreasing the strength of non-porous targets make them easier to disrupt in this regime, while increasing the strength of porous targets has much less influence on the value of . Therefore, one cannot say that non-porous targets are systematically easier or more difficult to disrupt than porous ones, as the outcome highly depends on the assumed strength values. In the gravity regime, we also confirm that the process of gravitational reaccumulation is at the origin of the largest remnant’s mass in both cases. We then propose some power-law relationships between and both target’s size and impact speed that can be used in collisional evolution models. The resulting fragment size distributions can also be reasonably fitted by a power-law whose exponent ranges between −2.2 and −2.7 for all target diameters in both cases and independently on the impact velocity (at least in the small range investigated between 3 and 5 km/s). Then, although ejection velocities in the gravity regime tend to be higher from porous targets, they remain on the same order as the ones from non-porous targets.  相似文献   

Photochemical escape of oxygen from Mars: A comparison of the exobase approximation to a Monte Carlo method     

Jane L. Fox  Aleksander B. Ha? 《Icarus》2009,204(2):527-544

The non-thermal escape of neutral O atoms from Mars at the current epoch is largely due to dissociative recombination of :