The dynamic neptunian ring arcs: evidence for a gradual disappearance of Liberté and resonant jump of courage

We present Adaptive Optics observations of Neptune's ring system at 1.6 and 2.2 μm, taken with the 10-m W.M. Keck II telescope in July 2002 and October 2003. We recovered the full Adams and Le Verrier rings for the first time since the Voyager era (1989), and show that the overall appearance of these rings did not change much, except for the ring arcs. Both the location and intensity of all arcs changed drastically relative to trailing arc Fraternité, which has a mean orbital motion of 820.1118 ± 0.0001 deg/day, equal to that of Nicholson et al.'s (1995, Icarus 113, 295-330) solution 2. Our data suggest that all arcs may have decayed over the last decade, while Liberté, in 2003, may be on the verge of disappearing completely. The observed changes in the relative intensities and locations of all arcs further indicate that material is migrating between resonance sites; leading arc Courage, for example, has jumped ∼8°, or, when adopting Namouni and Porco's (2002, Nature 417, 45-47) CER (corotation eccentricity resonance) theory, it advanced by one full corotation potential maximum. Overall, our observations reveal a system that is surprisingly dynamic, and no comprehensive theory exists as of yet that can explain all the observed intricacies.     

Titan's 5-micron lightcurve

We report on repeated mid-resolution (R∼2000) spectroscopic observations of Titan, acquired between November 2002 and January 2003 with ISAAC at the ESO/VLT and covering the 4.84-5.05 μm range. These observations, which sample four different positions of Titan around Saturn, clearly indicate a variability of the 5-μm continuum albedo, with Titan's geometric albedo decreasing by ∼40% from Titan's leading side to the trailing side. This Titan 5-μm “lightcurve” appears to be in phase with the other near-infrared lightcurves. This can be understood in terms of a surface model in which water ice coexists in minor and variable proportions (13-25%, if pure) with a second, dark, component.     

MAWD observations revisited: seasonal behavior of water vapor in the martian atmosphere

Viking/MAWD experiment and more recent MGC/TES observations have provided to date the most detailed information about the annual atmospheric water cycle on Mars. Their data agree in major details but still reveal some disagreements. These disagreements turn out to be most significant in the perihelion season and especially during the major dust storms. We consider the potential influence of aerosol scattering on 1.38 μm water retrieval under various types of observation geometry. In order to obtain new retrievals of water vapor abundance from MAWD data, we apply radiative transfer calculations. The resulting seasonal and spatial distribution of water turns out to be more consistent with TES results, implying a remarkable stability of the martian seasonal water cycle. Mapping data corresponding to particular seasons reveals a distinct wave structure in the global distribution of the water column. We interpret it as a manifestation of a strong control over the water cycle on Mars from the atmospheric circulation.     

Retrievals of jovian tropospheric phosphine from Cassini/CIRS

On December 30th, 2000, the Cassini-Huygens spacecraft reached the perijove milestone on its continuing journey to the Saturnian System. During an extended six-month encounter, the Composite Infrared Spectrometer (CIRS) returned spectra of the jovian atmosphere, rings and satellites from 10-1400 cm⁻¹ (1000-7 μm) at a programmable spectral resolution of 0.5 to 15 cm⁻¹. The improved spectral resolution of CIRS over previous IR instrument-missions to Jupiter, the extended spectral range, and higher signal-to-noise performance provide significant advantages over previous data sets.CIRS global observations of the mid-infrared spectrum of Jupiter at medium resolution (2.5 cm⁻¹) have been analysed both with a radiance differencing scheme and an optimal estimation retrieval model to retrieve the spatial variation of phosphine and ammonia fractional scale height in the troposphere between 60° S and 60° N at a spatial resolution of 6°. The ammonia fractional scale height appears to be high over the Equatorial Zone (EZ) but low over the North Equatorial Belt (NEB) and South Equatorial Belt (SEB) indicating rapid uplift or strong vertical mixing in the EZ. The abundance of phosphine shows a similar strong latitudinal variation which generally matches that of the ammonia fractional scale height. However while the ammonia fractional scale height distribution is to a first order symmetric in latitude, the phosphine distribution shows a North/South asymmetry at mid latitudes with higher amounts detected at 40° N than 40° S. In addition the data show that while the ammonia fractional scale height at this spatial resolution appears to be low over the Great Red Spot (GRS), indicating reduced vertical mixing above the ∼500 mb level, the abundance of phosphine at deeper levels may be enhanced at the northern edge of the GRS indicating upwelling.     

Optical characterization of laser ablated silicates

We perform an optical characterization of UV laser ablated silicates (olivine, pyroxene), starting from their reflectance spectra in the 0.3-2.5 μm spectral range. The goal is to provide useful tools to model space weathering effects on surfaces of asteroids and TNOs (trans-neptunian objects). We determine that the reddening and darkening spectral trend is compatible with the Hapke's space weathering model, using the optical constants of metallic iron in a silicate matrix. This result is supported by new magnetic susceptibility measurements on laser ablated orthopyroxene. We also investigate the potential contribution of formation of amorphous silicates in the process. Applying our results to silicate-rich surfaces in the Solar System, we investigate the possibility of a weathered olivine component on the surface of Centaur 5145 Pholus. Inclusion of this component slightly decreases the amount of complex organics and water ice from those previously estimated. Thus, the current Pholus spectrum is consistent with the presence of either unweathered or weathered olivine, or potentially both materials.     

MMIV: de SGR 1806–20 Anno Mirabili

On 27th December 2004 SGR 1806–20, one of the most active Soft γ-ray Repeaters (SGRs), displayed an extremely rare event, also known as giant flare, during which up to 10⁴⁷ ergs were released in the ∼1–1000 keV range in less than 1 s. Before and after the giant flare we carried out IR observations by using adaptive optics (NAOS-CONICA) mounted on VLT which provided images of unprecedented quality (FWHM better than 0.1″). We discovered the likely IR counterpart to SGR 1806–20 based on positional coincidence with the VLA uncertainty region and flux variability of a factor of about 2 correlated with that at higher energies. Moreover, by analysing the Rossi-XTE/PCA data we have discovered rapid Quasi-Periodic Oscillations (QPOs) in the pulsating tail of the 27th December 2004 giant flare of SGR 1806–20. QPOs at ∼92.5 Hz are detected in a 50 s interval starting 170 s after the onset of the giant flare. These QPOs appear to be associated with increased emission by a relatively hard unpulsed component and are seen only over phases of the 7.56 s spin period pulsations away from the main peak. QPOs at ∼18 and ∼30 Hz are also detected ∼200–300 s after the onset of the giant flare. This is the first time that QPOs are unambiguously detected in the flux of a Soft Gamma-ray Repeater, or any other isolated neutron star. We interpret the highest QPOs in terms of the coupling of toroidal seismic modes with Alfvén waves propagating along magnetospheric field lines. The lowest frequency QPO might instead provide indirect evidence on the strength of the internal magnetic field of the neutron star.      

The distribution of water ice in the interior of Comet Tempel 1

The Deep Impact flyby spacecraft includes a 1.05 to 4.8 μm infrared (IR) spectrometer. Although ice was not observed on the surface in the impact region, strong absorptions near 3 μm due to water ice are detected in IR measurements of the ejecta from the impact event. Absorptions from water ice occur throughout the IR dataset beginning three seconds after impact through the end of observations, ∼45 min after impact. Spatially and temporally resolved IR spectra of the ejecta are analyzed in conjunction with laboratory impact experiments. The results imply an internal stratigraphy for Tempel 1 consisting of devolatilized materials transitioning to unaltered components at a depth of approximately one meter. At greater depths, which are thermally isolated from the surface, water ice is present. Up to depths of 10 to 20 m, the maximum depths excavated by the impact, these pristine materials consist of very fine grained (∼1±1 μm) water ice particles, which are free from refractory impurities.     

A survey of debris trails from short-period comets

We observed 34 comets using the 24 μm camera on the Spitzer Space Telescope. Each image contains the nucleus and covers at least ¹⁰⁶ km of each comet's orbit. Debris trails due to mm-sized or larger particles were found along the orbits of 27 comets; 4 comets had small-particle dust tails and a viewing geometry that made debris trails impossible to distinguish; and only 3 had no debris trail despite favorable observing conditions. There are now 30 Jupiter-family comets with known debris trails, of which 22 are reported in this paper for the first time. The detection rate is >80%, indicating that debris trails are a generic feature of short-period comets. By comparison to orbital calculations for particles of a range of sizes ejected over 2 yr prior to observation, we find that particles comprising 4 debris trails are typically mm-sized while the remainder of the debris trails require particles larger than this. The lower-limit masses of the debris trails are typically ¹⁰¹¹ g, and the median mass loss rate is 2 kg/s. The mass-loss rate in trail particles is comparable to that inferred from OH production rates and larger than that inferred from visible-light scattering in comae.     

832 Karin: Absence of rotational spectral variations

832 Karin is the largest member of the young Karin cluster that formed 5.75±0.05 Myr ago in the outer main belt. Surprisingly, recent near-IR spectroscopy measurements [Sasaki, T., Sasaki, S., Watanabe, J., Sekiguchi, T., Yoshida, F., Kawakita, H., Fuse, T., Takato, N., Dermawan, B., Ito, T., 2004. Astrophys. J. 615 (2), L161-L164] revealed that Karin's surface shows different colors as a function of rotational phase. It was interpreted that 832 Karin shows us the reddish space-weathered exterior surface of the parent body as well as an interior face, which has not had time to become space-weathered. This result is at odds with recent results including seismic and geomorphic modeling, modeling of the Karin cluster formation and measurements of the space weathering rate. Consequently, we aimed to confirm/infirm this surprising result by sampling Karin's spectrum well throughout its rotation. Here, we present new visible (0.45-0.95 μm) and near-infrared (0.7-2.5 μm) spectroscopic observations of 832 Karin obtained in January and April 2006, covering most of Karin's longitudes. In the visible range, we find that Karin shows no rotational spectral variations. Similarly, we find that Karin exhibits very little (to none) spectral variations with rotation in the near-IR range. Our results imply that 832 Karin has a homogeneous surface, in terms of composition and surface age. Our results also imply that the impact that generated the family refreshed entirely Karin's surface, and probably the surfaces of all members.     

Thermal behavior of horizontally mixed surfaces on Mars

Current methods for deriving thermal inertia from spacecraft observations of planetary brightness temperature generally assume that surface properties are uniform for any given observation or co-located set of observations. As a result of this assumption and the nonlinear relationship between temperature and thermal inertia, sub-pixel horizontal heterogeneity may yield different apparent thermal inertia at different times of day or seasons. We examine the effects of horizontal heterogeneity on Mars by modeling the thermal behavior of various idealized mixed surfaces containing differing proportions of either dust, sand, duricrust, and rock or slope facets at different angles and azimuths. Latitudinal effects on mixed-surface thermal behavior are also investigated. We find large (several 100 J m⁻² K⁻¹ s^−1/2) diurnal and seasonal variations in apparent thermal inertia even for small (∼10%) admixtures of materials with moderately contrasting thermal properties or slope angles. Together with similar results for layered surfaces [Mellon, M.T., Putzig, N.E., 2007. Lunar Planet. Sci. XXXVIII. Abstract 2184], this work shows that the effects of heterogeneity on the thermal behavior of the martian surface are substantial and may be expected to result in large variations in apparent thermal inertia as derived from spacecraft instruments. While our results caution against the over-interpretation of thermal inertia taken from median or average maps or derived from single temperature measurements, they also suggest the possibility of using a suite of apparent thermal inertia values derived from single observations over a range of times of day and seasons to constrain the heterogeneity of the martian surface.