Description: Available online 29 December 2012 Publication year: 2012 Source: Icarus We present the first results on solar occultations performed with the UV channel of SPICAM, on board Mars Express. From the dataset of over 900 occultations (performed between April 2004 and October 2011), about 640 atmospheric profiles of the Martian atmosphere were derived. This dataset, spanning four Martian years, allows characterization of the seasonal evolution and inter-annual comparisons of ozone and suspended particles. The dataset also includes observations of the Mars Year (MY) 28 global dust storm. In this paper the aforementioned data are analyzed with a focus on the aerosol profiles. We have mapped the seasonal behavior of the near-surface haze, revealing the typical behavior of the Martian aerosol cycle, where the season most prone to develop dust storms (southern summer) shows aerosols lofted high in the atmosphere, whereas in the polar regions the aerosols are confined near the surface. More generally, aerosols seem to remain in the lower atmosphere at high latitudes and progressively penetrate to higher altitudes towards the tropics. This prevailing trend is probably related to enhanced atmospheric circulation at tropical regions due to high insolation and/or to higher cloud formation level in a warmer atmosphere. The dataset reveals frequent aerosol layers, found above or within the persistent near-surface haze. We have observed single and multiple layers (up to three layers in one profile) and we have mapped their properties. The highest layer altitudes observed during the global dust storm in the southern hemisphere, where thick layers form high above the abundant lower atmosphere dust haze. We present results on the analyzed Ångström coefficient α and its vertical variations. We also discuss the conversion of α into particle effective radius and present some examples of the effective radius vertical behavior. Highlights ► A four-year climatology of the aerosol vertical distribution is presented. ► The seasonal evolution of aerosol vertical distribution and interannual variability are discussed. ► Almost half of the profiles reveal detached aerosol layers. ► Angström coefficients are also analysed.

Description: Available online 28 December 2012 Publication year: 2012 Source: Icarus Airless planetary bodies are covered by a dusty layer called regolith. The grain size of the regolith determines the temperature and the mechanical strength of the surface layers. Thus, knowledge of the grain size of planetary regolith helps to prepare future landing and/or sample-return missions. In this work, we present a method to determine the grain size of planetary regolith by using remote measurements of the thermal inertia. We found that small bodies in the Solar System (diameter less than ∼ 100 km) are covered by relatively coarse regolith grains with typical particle sizes in the millimeter to centimeter regime, whereas large objects possess very fine regolith with grain sizes between 10 μ m and 100 μ m. Highlights ► In this work, we presented a new method to determine the grain size of regolith particles by using remote measurements only. ► Therefore, measurements of the thermal inertia performed for the regolith of various objects in the Solar System were utilized. ► The grain size of the planetary regolith was then determined from a comparison of the derived heat conductivity (derived from the thermal inertia measurements) with a modeled heat conductivity of granular materials in vacuum. ► We determined the grain size of planetary regolith for a large number of asteroids, the Moon, the Martian moons and Mercury with diameters between 0.3 km and 4,880 km and found an anti-correlation between the regolith grain size and the diameter of the planetary body.

Description: Available online 28 December 2012 Publication year: 2012 Source: Icarus Saturn is orbited by a half dozen ice rich middle-sized moons (MSMs) of diverse geology and composition e.g. ( Smith et al., 1981; Thomas, 2010; Schenk et al., 2011 ) that comprise ∼4.4% of Saturn’s satellite mass. The rest is Titan, more massive per planet than Jupiter’s satellites combined. Jupiter has no MSMs. Disk-based models to explain these differences exist e.g. ( Sasaki et al., 2010; Canup, 2010; Mosqueira et al., 2010a; Charnoz et al., 2011 ) but have various challenges and assumptions. We introduce the hypothesis that Saturn originally had a ‘galilean’ system of moons, comparable to Jupiter’s, that collided and merged, ultimately forming Titan. Mergers liberate ice-rich spiral arms, in simulations, that self-gravitate into escaping clumps resembling Saturn’s MSMs in size and compositional diversity. We reason that MSMs were spawned in a few such collisions around Saturn, while Jupiter’s original satellites stayed locked in resonance. The dynamical validity of our scenario depends on whether some MSMs can be scattered or otherwise migrated to stable orbits following each collision, before they are accreted. If satellite formation concludes with a ‘late stage’ of giant impacts e.g. ( Ogihara and Ida, 2012 ) then MSMs could have formed originally by this mechanism. More speculatively, solar-system-wide dynamical upheaval e.g. ( Tsiganis et al., 2005; Morbidelli et al., 2009 )might have triggered final mergers, leaving behind young MSMs and a dynamically excited Titan. Highlights ► We propose that Saturn originally had a family of Galilean-like satellites that collided and merged. ► Simulations of these mergers result in the formation of escaping, clump-forming, ice-rich spiral arms. ► Clump sizes and compositions in these simulations resemble Saturn’s middle-sized moons (MSMs). ► MSM-forming collisions would have occurred over a wide range of orbital dis-tances from Saturn. ► Late collisions triggered by solar system upheaval could explain extraordinary aspects of the Saturn system

Description: Available online 25 December 2012 Publication year: 2012 Source: Icarus The vertical distribution of water vapor is a very important diagnostic to determine the physical and chemical processes that drive the Martian water cycle. Yet, very few direct measurements have been performed so far, and our knowledge of the H 2 O vertical distribution on Mars relies on General Circulation Models (GCM). The study presented here follows for the first time the evolution of water vapor profile during a Martian year. 120 profiles, obtained by the SPICAM spectrometer onboard Mars Express with the solar occultations technique, are retrieved. They cover the northern spring-summer season and the southern spring of Mars Year (MY) 29. The seasonal evolution of H 2 O mixing ratio vertical distribution reveals its strong dynamism, especially during southern spring. There are significant discrepancies with the predictions of the General Circulation Model developed at the Laboratoire de Météorologie Dynamique (LMD-GCM). The LMD-GCM underestimates the water vapor content in the middle atmosphere. The measured profiles also exhibit often abrupt temporal variations and a greater variety of shapes, with the frequent presence of detached layers. We believe that the model underestimates the strength of the coupling between water vapor and aerosols, whose slant optical depth profile is obtained by SPICAM simultaneously with H 2 O. The SPICAM measurements can be grouped according to the mutual behavior of the two profiles. Individual features are often related too. The presence of water supersaturation and of correlated aerosol-water detached layers highlights the role of water ice clouds as a favorable location for the dust-water coupling. The water vapor vertical distribution is more reactive than expected to regional perturbations, which can propagate rapidly through the atmosphere, create abrupt water vapor and aerosol upsurges and influence the large-scale vertical evolution of these two constituents. This phenomenon has been observed thrice during MY29. The Martian annual water cycle revealed by the SPICAM profiles exhibits a different behavior with respect to nadir observations. This result suggests a generally weak connection between the upper atmosphere and the lower atmospheric layers, to whom the nadir measurements are most sensitive and that are not resolved by SPICAM occultations, and hints at a significant influence of surface-atmosphere interactions on the water cycle. Highlights ► We study H2O vertical profiles in Mars atmosphere with SPICAM solar occultations. ► The measured vertical distribution differs significantly from model predictions. ► Evolution of H2O and aerosols is strongly reactive to local perturbations. ► Connection between water vapor and aerosol profiles is tighter than expected. ► Water behavior in lower atmospheric layers is detached from the upper atmosphere.

Description: Available online 22 December 2012 Publication year: 2012 Source: Icarus The Cassini mission has investigated Titan’s upper atmosphere in detail and found that, under solar irradiation, it has a well-developed ionosphere, which peaks between 1000 and 1200 km. In this paper we focus on the T40, T41, T42 and T48 Titan flybys by the Cassini spacecraft and use in situ measurements of N 2 and CH 4 densities by the Ion Neutral Mass Spectrometer (INMS) as input into a solar energy deposition model to determine electron production rates. We combine these electron production rates with estimates of the effective recombination coefficient based on available laboratory data for Titan ions’ dissociative recombination rates and electron temperatures derived from the Langmuir probe (LP) to predict electron number densities in Titan’s upper atmosphere, assuming photochemical equilibrium and loss of electrons exclusively through dissociative recombination with molecular ions. We then compare these predicted electron number densities with those observed in Titan’s upper atmosphere by the LP. The assumption of photochemical equilibrium is supported by a reasonable agreement between the altitudes where the electron densities are observed to peak and where the electron production rates are calculated to peak (roughly corresponding to the unit optical depth for HeII photons at 30.38 nm). We find, however, that the predicted electron number densities are nearly a factor of two higher than those observed throughout the altitude range between 1050 and 1200 km (where we have made estimates of the effective recombination coefficient). There are different possible reasons for this discrepancy; one possibility is that there may be important loss processes of free electrons other than dissociative recombination in Titan’s upper atmosphere. Highlights ► We model the electron density in Titan’s dayside ionosphere using Cassini data. ► We obtain a factor of about 2 higher densities than observed by the Langmuir probe. ► We discuss possible reasons for the discrepancy found. ► Dissociative recombination is probably not the only important electron loss process.

Description: Available online 20 December 2012 Publication year: 2012 Source: Icarus We present evidence that the failure to account for proper motion is responsible for most of the regional biases seen in the astrometry of asteroids produced by the Pan-STARRS sky survey, as well as the 0.02 arcsec northward bias seen in our own Apophis astrometry.

Description: Available online 20 December 2012 Publication year: 2012 Source: Icarus We show that the unusual behavior of dust jets seen embedded in the sunward coma of 103P/Hartley 2 originate in active regions migrating over the two lobes of the nucleus following the sun. The slowly changing orientation of the jets and their rapid changes in brightness is due to the shape and local topography of the nucleus coupled with the complex spin state. The intermittent appearance of a second jet is due to periodic deviations in the direction of the ejection of dust from the small lobe of the nucleus. The release of dust into the structures is likely due to the sublimation of H 2 O. The jets are characterized by injection speeds from the nucleus of 50 - 210 m/s, a radiation pressure parameter 0.08 〈 〈 1, and a particle life-time near 7 h. Within the jets, the average particle size decreases and the injection speed increases with distance from the nucleus. Highlights ► The dust jets in comet 103P arise from the sub-solar region of each lobe of the nucleus. ► Their changing directional properties are due to the comet’s excited rotation state. ► Their curvature and extent is due to solar radiation pressure and a finite lifetime for the particles.

Description: January 2013 Publication year: 2013 Source: Icarus, Volume 222, Issue 1 Here we present new adaptive optics observations of the Quaoar–Weywot system. With these new observations we determine an improved system orbit. Due to a 0.39 day alias that exists in available observations, four possible orbital solutions are available with periods of ∼11.6, ∼12.0, ∼12.4, and ∼12.8 days. From the possible orbital solutions, system masses of 1.3–1.5 ± 0.1 × 10 21 kg are found. These observations provide an updated density for Quaoar of 2.7–5.0 g cm −3 . In all cases, Weywot’s orbit is eccentric, with possible values ∼0.13–0.16. We present a reanalysis of the tidal orbital evolution of the Quaoar–Weywot system. We have found that Weywot has probably evolved to a state of synchronous rotation, and has likely preserved its initial inclination over the age of the Solar System. We find that for plausible values of the effective tidal dissipation factor tides produce a very slow evolution of Weywot’s eccentricity and semi-major axis. Accordingly, it appears that Weywot’s eccentricity likely did not tidally evolve to its current value from an initially circular orbit. Rather, it seems that some other mechanism has raised its eccentricity post-formation, or Weywot formed with a non-negligible eccentricity. Highlights ► We present new adaptive optics observations of the Quaoar–Weywot system. ► We determine four possible system orbits with masses between ∼1.3 and ∼1.5 × 10 21 kg. ► We confirm that Weywot is on an eccentric orbit. ► We find tidal evolution is in principle compatible with the orbit of Weywot. ► Tidal evolution simulations demonstrate that tides alone cannot account for Weywot’s high eccentricity.

Description: January 2013 Publication year: 2013 Source: Icarus, Volume 222, Issue 1 We present observations of Neptune’s 1- and 3-mm spectrum from the Combined Array for Research in Millimeter-wave Astronomy (CARMA). Radiative transfer analysis of the CO (2–1) and (1–0) rotation lines was performed to constrain the CO vertical abundance profile. We find that the data are well matched by a CO mole fraction of 0.1 - 0.1 + 0.2 parts per million (ppm) in the troposphere, and 1.1 - 0.3 + 0.2 ppm in the stratosphere. A flux of 0.5–20 × 10 8 CO molecules cm −2 s −1 to the upper stratosphere is implied. Using the Zahnle et al. (Zahnle, K., Schenk, P., Levison, H., Dones, L. [2003]. Icarus 163, 263–289) estimate for cometary impact rates at Neptune, we calculate the CO flux that could be formed from (sub)kilometer-sized comets; we find that if the diffusion rate near the tropopause is small (200 cm 2 s −1 ), these impacts could produce a flux as high as 0.5 - 0.4 + 0.8 × 10 8 CO molecules cm −2 s −1 . We also revisit the calculation of Neptune’s internal CO contribution using revised calculations for the CO → CH 4 conversion timescale in the deep atmosphere (Visscher, C., Moses, J.I. [2011]. Astrophys. J. 738, 72). We find that an upwelled CO mole fraction of 0.1 ppm implies a global O/H enrichment of at least 400, and likely more than 650, times the protosolar value. Highlights ► We observed Neptune in the CO (2–1) and (1–0) rotational lines. ► We derive new estimates of Neptunes stratospheric and tropospheric CO mole fractions. ► 0.1 ppm of CO in the troposphere implies an O/H ratio at least 400 times protosolar. ► Infall of (sub)kilometer-sized comets could supply the observed stratospheric CO.

Description: Available online 19 December 2012 Publication year: 2012 Source: Icarus The unlit side of the dense B ring of Saturn does not receive direct sunlight. Yet it cools down as the sun sets on the ring from solstice to equinox. A multi-scale thermal model that treats the heat transfer through a packed ensemble of particles is built to study the orbital and seasonal temperature variations of both lit and unlit sides of this opaque ring. Heat transfer by radiation, conduction or through contacts are considered both at the ring and the particles scale. A statistical approach shows that three simple constraints, such as the heat diffusion time through the ring, the current estimates on its thermal inertia and its optical depth, yield important constraints on its thickness, its filling factor and on particle properties such as their thermal inertia, porosity or size. The B ring is probably a few-meters-thick, with a filling factor D below 0.25, made of a population of porous but still conductive particles. Its thickness is compatible with a surface mass density ranging between 50 and 75 g/cm 2 in its densest part. Highlights ► Multi-scale thermal model of heat transfer by radiation and conduction in a dense ring. ► Separate ring and particles thermal properties. ► Constraints on the structure of the B ring: thickness, filling factor, thermal inertias and sizes