A spatially resolved high spectral resolution study of Neptune’s stratosphere

Using TEXES, the Texas Echelon cross Echelle Spectrograph, mounted on the Gemini North 8-m telescope we have mapped the spatial variation of H₂, CH₄, C₂H₂ and C₂H₆ thermal-infrared emission of Neptune. These high-spectral-resolution, spatially resolved, thermal-infrared observations of Neptune offer a unique glimpse into the state of Neptune’s stratosphere in October 2007, L_S = 275.4° just past Neptune’s southern summer solstice (L_S = 270°). We use observations of the S(1) pure rotational line of molecular hydrogen and a portion of the ν₄ band of methane to retrieve detailed information on Neptune’s stratospheric vertical and meridional thermal structure. We find global-average temperatures of 163.8 ± 0.8, 155.0 ± 0.9, and 123.8 ± 0.8 K at the 7.0 × 10⁻³-, 0.12-, and 2.1-mbar levels with no meridional variations within the errors. We then use the inferred temperatures to model the emission of C₂H₂ and C₂H₆ in order to derive stratospheric volume mixing ratios (hence forth, VMR) as a function of pressure and latitude. There is a subtle meridional variation of the C₂H₂ VMR at the 0.5-mbar level with the peak abundance found at −28° latitude, falling off to the north and south. However, the observations are consistent within error to a meridionally constant C₂H₂ VMR of at 0.5 mbar. We find that the VMR of C₂H₆ at 1-mbar peaks at the equator and falls by a factor of 1.6 at −70° latitude. However, a meridionally constant VMR of at the 1-mbar level for C₂H₆ is also statistically consistent with the retrievals. Temperature predictions from a radiative-seasonal climate model of Neptune that assumes the hydrocarbon abundances inferred in this paper are lower than the measured temperatures by 40 K at 7 × 10⁻³ mbar, 30 K at 0.12 mbar and 25 K at 2.1 mbar. The radiative-seasonal model also predicts meridional temperature variations on the order of 10 K from equator to pole, which are not observed. Assuming higher stratospheric CH₄ abundance at the equator relative to the south pole would bring the meridional trends of the inferred temperatures and radiative-seasonal model into closer agreement.We have also retrieved observations of C₂H₄ emission from Neptune’s stratosphere using TEXES on the NASA Infrared Telescope Facility (IRTF) in June 2003, L_S = 266°. Using the observations from the middle of the planet and an average of the middle three latitude temperature profiles from the 2007 observations (9.5° of L_S later, the seasonal equivalent of 9.5 Earth days within Earth’s seasonal cycle), we infer a C₂H₄ VMR of at 1.5 × 10⁻³ mbar, a value that is 3.25 times that predicted by global-average photochemical models.     

(47171) 1999 TC36, A transneptunian triple

We present new analysis of HST images of (47171) 1999 TC₃₆ that confirm it as a triple system. Fits to the point-spread function (PSF) consistently show that the apparent primary is itself composed of two similar-sized components. The two central components, A1 and A2, can be consistently identified in each of nine epochs spread over 7 years of time. In each instance, the component separation, ranging from 0.023 ± 0.002 to 0.031 ± 0.003 arcsec, is roughly one half of the Hubble Space Telescope’s diffraction limit at 606 nm. The orbit of the central pair has a semi-major axis of a ∼ 867 km with a period of P ∼ 1.9 days. These orbital parameters yield a system mass that is consistent with M_sys = 12.75 ± 0.06 × 10¹⁸ kg derived from the orbit of the more distant secondary, component B. The diameters of the three components are . The relative sizes of these components are more similar than in any other known multiple in the Solar System. Taken together, the diameters and system mass yield a bulk density of . HST photometry shows that component B is variable with an amplitude of ?0.17 ± 0.05 magnitudes. Components A1 and A2 do not show variability larger than 0.08 ± 0.03 magnitudes approximately consistent with the orientation of the mutual orbit plane and tidally distorted equilibrium shapes. The system has high specific angular momentum of J/J′ = 0.93, comparable to most of the known transneptunian binaries.     

Adsorptive fractionation of HDO on JSC MARS-1 during sublimation with implications for the regolith of Mars

A chamber was constructed to simulate the boundary between the ice table, regolith and atmosphere of Mars and to examine fractionation between H₂O and HDO during sublimation under realistic martian conditions of temperature and pressure. Thirteen experimental runs were conducted with regolith overlying the ice. The thickness and characteristic grain size of the regolith layer as well as the temperature of the underlying ice was varied. From these runs, values for the effective diffusivity, taking into account the effects of adsorption, of the regolith were derived. These effective diffusivities ranged from 1.8 × 10⁻⁴ m² s⁻¹ to 2.2 × 10⁻³ m² s⁻¹ for bare ice and from 2.4 × 10⁻¹¹ m² s⁻¹ to 2.0 × 10⁻⁹ m² s⁻¹ with an adsorptive layer present. From these, latent heats of adsorption of 8.6 ± 2.6 kJ mol⁻¹ and 9.3 ± 2.8 kJ mol⁻¹ were derived at ice-surface temperatures above 223 ± 8 K and 96 ± 28 kJ mol⁻¹ and 104 ± 31 kJ mol⁻¹ respectively for H₂O and HDO were derived at colder temperatures. For temperatures below 223 K, the effective diffusivity of HDO was found to be lower than the diffusivity of H₂O by 40% on average, suggesting that the regolith was adsorptively fractionating the sublimating gas with a fractionation factor of 1.96 ± 0.74. Applying these values to Mars predicts that adsorbed water on the regolith is enriched in HDO compared to the atmosphere, particularly where the regolith is colder. Based on current observations, the D/H ratio of the regolith may be as high as 21 ± 8 times VSMOW at 12°S and L_S = 357° if the regolith is hydrated primarily by the atmosphere, neglecting any hydration from subsurface ice.     

Mars high resolution gravity fields from MRO, Mars seasonal gravity, and other dynamical parameters

With 2 years of tracking data collection from the MRO spacecraft, there is noticeable improvement in the high frequency portion of the spherical harmonic Mars gravity field. The new JPL Mars gravity fields, MRO110B and MRO110B2, show resolution near degree 90. Additional years of MGS and Mars Odyssey tracking data result in improvement for the seasonal gravity changes which compares well to global circulation models and Odyssey neutron data and Mars rotation and precession (). Once atmospheric dust is accounted for in the spacecraft solar pressure model, solutions for Mars solar tide are consistent between data sets and show slightly larger values (k₂ = 0.164 ± 0.009, after correction for atmospheric tide) compared to previous results, further constraining core models. An additional 4 years of Mars range data improves the Mars ephemeris, determines 21 asteroid masses and bounds solar mass loss (dGM_Sun/dt < 1.6 × 10⁻¹³ GM_Sun year⁻¹).     

Mineralogy and thermal properties of V-type Asteroid 956 Elisa: Evidence for diogenitic material from the Spitzer IRS (5-35 μm) spectrum

We present the thermal infrared (5-35 μm) spectrum of 956 Elisa as measured by the Spitzer Infrared Spectrograph (“IRS”; Houck, J.R. et al. [2004]. Astrophys. J. Suppl. 154, 18-24) together with new groundbased lightcurve data and near-IR spectra. From the visible lightcurve photometry, we determine a rotation period of 16.494 ± 0.001 h, identify the rotational phase of the Spitzer observations, and estimate the visible absolute magnitude (H_V) at that rotational phase to be 12.58 ± 0.04. From radiometric analysis of the thermal flux spectrum, we find that at the time of observation 956 Elisa had a projected radius of 5.3 ± 0.4 km with a visible albedo p_V = 0.142 ± 0.022, significantly lower than that of the prototype V-type asteroid, 4 Vesta. (This corresponds to a radius of 5.2 ± 0.4 km at lightcurve mean.) Analysis with the standard thermal model (STM) results in a sub-solar temperature of 292.3 ±  2.8 K and beaming parameter η = 1.16 ± 0.05. Thermophysical modeling places a lower limit of on the thermal inertia of the asteroid’s surface layer (if the surface is very smooth) but more likely values fall between 30 and depending on the sense of rotation.The emissivity spectrum, calculated by dividing the measured thermal flux spectrum by the modeled thermal continuum, exhibits mineralogically interpretable spectral features within the 9-12 μm reststrahlen band, the 15-16.5 μm Si-O-Si stretching region, and the 16-25 μm reststrahlen region that are consistent with pyroxene of diogenitic composition: extant diogenitic pyroxenes fall within the narrow compositional range Wo_2±1En_74±2Fs_24±1. Spectral deconvolution of the 9-12 μm reststrahlen features indicates that up to ≈20% olivine may also be present, suggesting an olivine-diogenite-like mineralogy. The mid-IR spectrum is inconsistent with non-cumulate eucrite as the major component on the surface of 956 Elisa, although cumulate eucrite material may be present at abundances lower than that of the diogenite component.Analysis of new near-IR spectra of 956 Elisa with the Modified Gaussian Model (MGM; Sunshine, J.M., Pieters, C.M., Pratt, S.F. [1990]. J. Geophys. Res. 95 (May), 6955-6966) results in two pyroxene compositions: 75% magnesian low-Ca pyroxene and 25% high-Ca pyroxene. High-Ca pyroxene is not evident in the mid-IR data, but may belong to a component that is underrepresented in the mid-IR spectrum either because of its spatial distribution on the asteroid or because of its particle size. High-Ca pyroxenes that occur as exsolution lamellae may also be more evident spectrally in the NIR than in the mid-IR. In any case, we find that the mid-IR spectrum of 956 Elisa is dominated by emission from material of diogenite-like composition, which has very rarely been observed among asteroids.     

Spatially-resolved high-resolution spectroscopy of Venus 2. Variations of HDO, OCS, and SO2 at the cloud tops

While CO, HCl, and HF, that were considered in the first part of this work, have distinct absorption lines in high-resolution spectra and were detected four decades ago, the lines of HDO, OCS, and SO₂ are either very weak or blended by the telluric lines and have not been observed previously by ground-based infrared spectroscopy at the Venus cloud tops. The H₂O abundance above the Venus clouds is typically below the detection limit of ground-based IR spectroscopy. However, the large D/H ratio on Venus facilitates observations of HDO. Converted to H₂O with D/H ≈ 200, our observations at 2722 cm⁻¹ in the Venus afternoon show a H₂O mixing ratio of ∼1.2 ppm at latitudes between ±40° increasing to ±60° by a factor of 2. The observations in the early morning reveal the H₂O mixing ratio that is almost constant at 2.9 ppm within latitudes of ±75°. The measured H₂O mixing ratios refer to 74 km. The observed increase in H₂O is explained by the lack of photochemical production of sulfuric acid in the night time. The recent observations at the P-branch of OCS at 4094 cm⁻¹ confirm our detection of OCS. Four distributions of OCS along the disk of Venus at various latitudes and local times have been retrieved. Both regular and irregular components are present in the variations of OCS. The observed OCS mixing ratio at 65 km varies from ∼0.3 to 9 ppb with the mean value of ∼3 ppb. The OCS scale height is retrieved from the observed limb darkening and varies from 1 to 4 km with a mean value of half the atmospheric scale height. SO₂ at the cloud tops has been detected for the first time by means of ground-based infrared spectroscopy. The SO₂ lines look irregular in the observed spectra at 2476 cm⁻¹. The SO₂ abundances are retrieved by fitting by synthetic spectra, and two methods have been applied to determine uncertainties and detection limits in this fitting. The retrieved mean SO₂ mixing ratio of 350 ± 50 ppb at 72 km favors a significant increase in SO₂ above the clouds since the period of 1980-1995 that was observed by the SOIR occultations at Venus Express. Scale heights of OCS and SO₂ may be similar, and the SO₂/OCS ratio is ∼500 and may be rather stable at 65-70 km under varying conditions on Venus.     

Early-type W UMa contact binary system: New data on V535 Ara

This paper presents the results of spectroscopic and photometric observations of the early-type W UMa system V535 Ara. New high-resolution spectra were taken at the Mt. John University Observatory in 2007. Radial velocities and spectroscopic orbital elements of the system were determined by applying KOREL spectral disentangling. The resulting orbital elements were: a₁sini = 0.0047 ± 0.0001 AU, a₂sini = 0.0146 ± 0.0001 AU, M₁sin³i = 1.85 ± 0.01 M_⊙, and M₂sin³i = 0.59 ± 0.01 M_⊙. The components were found to be in synchronous rotation following examination of their disentangled Hγ line profiles. Four photometric data-sets (1966 BV, 1967 BV, HIPPARCOS and ASAS) were modeled using the Wilson-Devinney method. The model describes V535 Ara as an A sub-type W UMa type eclipsing binary which has a fill out factor of 0.22 in marginal contact configuration. The simultaneous solution of light and radial velocity curves gave the following absolute parameters: M₁ = 1.94 ± 0.04 M_⊙, M₂ = 0.59 ± 0.02 M_⊙, R₁ = 2.09 ± 0.03 R_⊙, R₂ = 1.23 ± 0.02R_⊙, L₁ = 18 ± 3 L_⊙ and L₂ = 6 ± 1 L_⊙. The distance to V535 Ara was calculated as 123 ± 20 pc using distance modulus with correction for interstellar extinction.     

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

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.     

Electromagnetic properties of ice-coated iron whiskers

The effect of H₂O-ice mantles on infinitely long cylindrical metallic whiskers of conductivity =10¹⁸ s^–1 is investigated using the rigorous Kerker-Matijevic formulae. Cosmologically relevant properties of uncoated metallic whiskers which have been discussed earlier are essentially unaltered.     

Low-speed impacts between rubble piles modeled as collections of polyhedra, 2

We present the results of additional calculations involving the collisions of km-scale rubble piles. In new work, we used the Open Dynamics Engine (ODE), an open-source library for the simulation of rigid-body dynamics that incorporates a sophisticated collision-detection and resolution routine. We found that using ODE resulted in a speed-up of approximately a factor of 30 compared with previous code. In this paper we report on the results of almost 1200 separate runs, the bulk of which were carried out with 1000-2000 elements. We carried out calculations with three different combinations of the coefficients of friction η and (normal) restitution ?: low (η=0,?=0.8), medium (η=0,?=0.5), and high (η=0.5,?=0.5) dissipation.For target objects of ∼1 km in radius, we found reduced critical disruption energy values in head-on collisions from 2 to 100 J kg⁻¹ depending on dissipation and impactor/target mass ratio. Monodisperse objects disrupted somewhat more easily than power-law objects in general. For oblique collisions of equal-mass objects, mildly off-center collisions (b/b₀=0.5) seemed to be as efficient or possibly more efficient at collisional disruption as head-on collisions. More oblique collisions were less efficient and the most oblique collisions we tried (b/b₀=0.866) required up to ∼200 J kg⁻¹ for high-dissipation power-law objects. For calculations with smaller numbers of elements (total impactor or 200 elements) we found that collisions were more efficient for smaller numbers of more massive elements, with values as low as for low-dissipation cases. We also analyzed our results in terms of the relations proposed by Stewart and Leinhardt [Stewart, S.T., Leinhardt, Z.M., 2009. Astrophys. J. 691, L133-L137] where where Q_R is the impact kinetic energy per unit total mass m_i+m_T. Although there is a significant amount of scatter, our results generally bear out the suggested relation.     

Photometric study of Centaur (60558) 2000 EC98 and trans-neptunian object (55637) 2002 UX25 at different phase angles

We present photometric observations of Centaur (60558) 2000 EC₉₈ and trans-neptunian object (55637) 2002 UX₂₅ at different phase angles and with different filters (mainly R but also V and B for some data). Results for 2000 EC₉₈ are: (i) a rotation period of 26.802±0.042 h if a double-peaked lightcurve is assumed, (ii) a lightcurve amplitude of 0.24±0.06 for the R band, (iii) a phase curve with H=9.03±0.01 and G=−0.39±0.08 (R filter) and H=9.55±0.04 and G=−0.50±0.35 (V filter) or a slope of (R filter) and 0.22±0.06 (V filter), (iv) the color indices B-V=0.76±0.15 and V-R=0.51±0.09 (for α=0.1-0.5°) and 0.55±0.08 (for α=1.4-1.5°). The rotation period is amongst the longest ever measured for Centaurs and TNOs. We also show that our photometry was not contaminated by any cometary activity down to magnitude ?27/arcsec². For 2002 UX₂₅ the results are: (i) a rotation period of 14.382±0.001 h or 16.782±0.003 h (if a double-peaked lightcurve is assumed) (ii) a lightcurve amplitude of 0.21±0.06 for the R band (and the 16.782 h period), (iii) a phase curve with H=3.32±0.01 and G=+0.16±0.18 or a slope of (R filter), (iv) the color indices B-V=1.12±0.26 and V-R=0.61±0.12. The phase curve reveals also a possible very narrow and bright opposition surge. Because such a narrow surge appears only for one point it needs to be confirmed.     

Absolute properties of the neglected eclipsing B-type binary HD 194495

We present the results of the high-resolution spectroscopic observations of the neglected binary system HD 194495 (B3 IV-V+B4 V). A combined analysis of three different photometric data set (Tycho B_T and V_T photometry, H_p-band data of Hipparcos and V-band data of ASAS3 photometry) and radial velocities indicates that the system has an orbital period of 4.90494 ± 0.00005 days and an inclination of 69 ± 1 degrees. This solution yields masses and radii of M₁ = 7.57 ± 0.08 M_⊙ and R₁ = 5.82 ± 0.03 R_⊙ for the primary and M₂ = 5.46 ± 0.09 M_⊙ and R₂ = 3.14 ± 0.08 R_⊙ for the secondary. Based on the position of the two stars plotted on a theoretical H-R diagram, we find that the age of the system is ?28 Myr, according to stellar evolutionary models. The spectroscopic and photometric results are in agreement with those obtained using theoretical predictions.     

Extension of atmospheric dust loading to high altitudes during the 2001 Mars dust storm: MGS TES limb observations

Mars Global Surveyor (MGS) visible (solarband bolometer) and thermal infrared (IR) spectral limb observations from the Thermal Emission Spectrometer (TES) support quantitative profile retrievals for dust opacity and particle sizes during the 2001 global dust event on Mars. The current analysis considers the behavior of dust lifted to altitudes above 30 km during the course of this storm; in terms of dust vertical mixing, particle sizes, and global distribution. TES global maps of visible (solarband) limb brightness at 60 km altitude indicate a global-scale, seasonally evolving (over 190-240° solar longitudes, L_S) longitudinal corridor of vertically extended dust loading (which may be associated with a retrograde propagating, wavenumber 1 Rossby wave). Spherical radiative transfer analysis of selected limb profiles for TES visible and thermal IR radiances provide quantitative vertical profiles of dust opacity, indicating regional conditions of altitude-increasing dust mixing ratios. Observed infrared spectral dependences and visible-to-infrared opacity ratios of dust scattering over 30-60 km altitudes indicate particle sizes characteristic of lower altitudes (cross-section weighted effective radius, ), during conditions of significant dust transport to these altitudes. Conditions of reduced dust loading at 30-60 km altitudes present smaller dust particle sizes . These observations suggest rapid meridional transport at 30-80 km altitudes, with substantial longitudinal variation, of dust lifted to these altitudes over southern hemisphere atmospheric regions characterized by extraordinary (m/s) vertical advection velocities. By L_S=230° dust loading above 50 km altitudes decreased markedly at southern latitudes, with a high altitude (60-80 km) haze of fine (likely) water ice particles appearing over 10°S-40°N latitudes.     

Close binary system GO Cyg

In this study, we present long term photometric variations of the close binary system GO Cyg. Modelling of the system shows that the primary is filling Roche lobe and the secondary of the system is almost filling its Roche lobe. The physical parameters of the system are M₁ = 3.0 ± 0.2M_⊙, M₂ = 1.3 ± 0.1M_⊙, R₁ = 2.50 ± 0.12R_⊙, R₂ = 1.75 ± 0.09R_⊙, L₁ = 64 ± 9L_⊙, L₂ = 4.9 ± 0.7L_⊙, and a = 5.5 ± 0.3R_⊙. Our results show that GO Cyg is the most massive system near contact binary (NCB). Analysis of times of the minima shows a sinusoidal variation with a period of 92.3 ± 0.5 yr due to a third body whose mass is less than 2.3M_⊙. Finally a period variation rate of −1.4 × 10⁻⁹ d/yr has been determined using all available light curves.     

Secular light curve of 2P/Encke, a comet active at aphelion

We present the secular light curve of Comet 2P/Encke in two phase spaces, the log plot, and the time plot. The main conclusions of this work are: (a) The comet shows activity at perihelion and aphelion, caused by two different active areas: Source 1, close to the south pole, active at perihelion, and Source 2, at the north pole, centered at aphelion. (b) More than 18 physical parameters are measured from the secular light curves, many of them new, and are listed in the individual plots of the comet. Specifically we find for Source 1 the location of the turn on and turn off points of activity, RON=−1.63±0.03 AU, ROFF=+1.49±0.20 AU, TON=−87±5 d, TOFF=+94±15 d, the time lag, LAG(q)=6±1 d, the total active time, TACTIVITY=181±16 d, and the amplitude of the secular light curve, ASEC(1,1)=4.8±0.1 mag. (c) From this information the photometric age and the time-age defined in Ferrín [2005a. Icarus 178, 493-516; 2006. Icarus 185, 523-543], can be calculated, and we find P-AGE = 97 ± 8 comet years and T-AGE = 103 ± 9 comet years (cy). Thus Comet 2P/Encke is an old comet entering the methuselah stage (100 cy < age). (d) The activity at aphelion (Source 2), extends for TACTIVITY=815±30 d and the amplitude of the secular light curve is ASEC(1,Q)=3.0±0.2 mag. (e) From a new phase diagram an absolute magnitude and phase coefficient for the nucleus are determined, and we find RNUC(1,1,0)=15.05±0.14, and β=0.066±0.003. From this data we find a nucleus effective diameter DEFFE=5.12(+2.5;−1.7) km. These values are not much different from previous determinations but exhibit smaller errors. (f) The activity of Source 1 is due to H₂O sublimation because it shows curvature. The activity of Source 2 might also be due to H₂O due to the circumstantial situation that the poles point to the Sun at perihelion and aphelion. (g) We found a photometric anomaly at aphelion, with minimum brightness between +393 and +413 days after perihelion that may be an indication of topography. (h) We have re-reduced the 1858 secular light curve of Kamel [1991. Icarus 93, 226-245]. There are secular changes in 7 physical parameters, and we achieve for the first time, an absolute age calibration. We find that the comet entered the inner Solar System and began sublimating in 1645±40 AD. (i) It is concluded that the secular light curve can place constraints on the pole orientation of the nucleus of some comets, and we measure the ecliptic longitude of the south pole of 2P/Encke equal to 213.2±4.5°, in excellent agreement with other determinations of this parameter, but with smaller error. (j) Using the observed absolute magnitude of 1858 and 2003 and a suitable theoretical model, the extinction date of the comet is determined. We obtain ED=2056±3 AD, implying that the comet's lifetime is 125±12 revolutions about the Sun after entering the inner Solar System.     

Gravity field and interior of Rhea from Cassini data analysis

The Cassini spacecraft encountered Rhea on November 26, 2005. Analysis of the Doppler data acquired at and around closest approach yields the mass of Rhea and the quadrupole moments of its gravity field with unprecedented accuracy. We obtained which corresponds to a density of . Our results for J₂ and C₂₂ are (7.947±0.892)×¹⁰⁻⁴ and (2.3526±0.0476)×¹⁰⁻⁴, respectively. These values are consistent with hydrostatic equilibrium. From the value of C₂₂, we infer the non-dimensional moment of inertia C/MR²=0.3721±0.0036. Our models of Rhea's interior based on the gravity data favor an almost undifferentiated satellite. A discontinuity between a core and a mantle is possible but not required by the data. Models with a constant silicate mass fraction throughout the body cannot account for the determined quadrupole coefficients. The data exclude fully differentiated models in which the core would be composed of unhydrated silicates and the mantle would be composed of pure ice. If the mantle contains 10% in mass of silicates, the core extends to 630 km in radius and has a silicate mass fraction of 40%. A continuous model in which the silicates are more concentrated toward the center of the body than in the outer layers is allowed by the gravity data but excluded by thermal evolution considerations. The one model that fits the gravity data and is self-consistent when energy transport and ice melting are qualitatively considered is an “almost undifferentiated” Rhea, in which a very large uniform core is surrounded by a relatively thin ice shell containing no rock at all.     

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

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.     

Neglected reverse Algol system: WZ Hor

This paper presents the first analysis of spectroscopic and photometric observations of the eclipsing binary star WZ Hor. Observations of the system were made at the Mt. John University Observatory in 2007. Since the light contribution of the secondary component was merely 2-3% of the total light of the system in the optical wavelengths, the radial velocity of the primary component could only be determined using the cross-correlation method. A single-lined spectroscopic orbital solution of WZ Hor was obtained, and the BVRI light curves of the system and radial velocity curve of the primary component were analysed simultaneously using the Wilson-Devinney method. The results describe WZ Hor as a reverse Algol-like binary star with a detached configuration. The following absolute parameters of the components were also derived: M₁ = 1.51 ± 0.03 M_⊙, M₂ = 0.66 ± 0.01 M_⊙, R₁ = 1.62 ± 0.02 R_⊙, R₂ = 0.66 ± 0.01 R_⊙, L₁ = 4.93 ± 0.64 L_⊙ and L₂ = 0.09 ± 0.02 L_⊙. The distance to WZ Hor was calculated as 95 ± 8 pc using distance modulus with correction for interstellar extinction, in agreement with the HIPPARCOS value.     

Radiolysis of sulfuric acid, sulfuric acid monohydrate, and sulfuric acid tetrahydrate and its relevance to Europa

We report laboratory studies on the 0.8 MeV proton irradiation of ices composed of sulfuric acid (H₂SO₄), sulfuric acid monohydrate (H₂SO₄·H₂O), and sulfuric acid tetrahydrate (H₂SO₄·4H₂O) between 10 and 180 K. Using infrared spectroscopy, we identify the main radiation products as H₂O, SO₂, (S₂O₃)_x, H₃O⁺, , and . At high radiation doses, we find that H₂SO₄ molecules are destroyed completely and that H₂SO₄·H₂O is formed on subsequent warming. This hydrate is significantly more stable to radiolytic destruction than pure H₂SO₄, falling to an equilibrium relative abundance of 50% of its original value on prolonged irradiation. Unlike either pure H₂SO₄ or H₂SO₄·H₂O, the loss of H₂SO₄·4H₂O exhibits a strong temperature dependence, as the tetrahydrate is essentially unchanged at the highest irradiation temperatures and completely destroyed at the lowest ones, which we speculate is due to a combination of radiolytic destruction and amorphization. Furthermore, at the lower temperatures it is clear that irradiation causes the tetrahydrate spectrum to transition to one that closely resembles the monohydrate spectrum. Extrapolating our results to Europa’s surface, we speculate that the variations in SO₂ concentrations observed in the chaotic terrains are a result of radiation processing of lower hydration states of sulfuric acid and that the monohydrate will remain stable on the surface over geological times, while the tetrahydrate will remain stable in the warmer regions but be destroyed in the colder regions, unless it can be reformed by other processes, such as thermal reactions induced by diurnal cycling.